WO2019115263A1 - Two-part mirror - Google Patents

Two-part mirror Download PDF

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Publication number
WO2019115263A1
WO2019115263A1 PCT/EP2018/083288 EP2018083288W WO2019115263A1 WO 2019115263 A1 WO2019115263 A1 WO 2019115263A1 EP 2018083288 W EP2018083288 W EP 2018083288W WO 2019115263 A1 WO2019115263 A1 WO 2019115263A1
Authority
WO
WIPO (PCT)
Prior art keywords
mirror
wafer
front side
back side
process step
Prior art date
Application number
PCT/EP2018/083288
Other languages
French (fr)
Inventor
Mathias Müller
Original Assignee
Blickfeld GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Blickfeld GmbH filed Critical Blickfeld GmbH
Publication of WO2019115263A1 publication Critical patent/WO2019115263A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
    • G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
    • G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
    • G02B26/0858—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD the reflecting means being moved or deformed by piezoelectric means
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B3/00—Devices comprising flexible or deformable elements, e.g. comprising elastic tongues or membranes
    • B81B3/0064—Constitution or structural means for improving or controlling the physical properties of a device
    • B81B3/0067—Mechanical properties
    • B81B3/007—For controlling stiffness, e.g. ribs
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
    • B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
    • B81C1/00436—Shaping materials, i.e. techniques for structuring the substrate or the layers on the substrate
    • B81C1/00555—Achieving a desired geometry, i.e. controlling etch rates, anisotropy or selectivity
    • B81C1/00626—Processes for achieving a desired geometry not provided for in groups B81C1/00563 - B81C1/00619
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4817—Constructional features, e.g. arrangements of optical elements relating to scanning
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
    • G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
    • G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
    • G02B26/085—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD the reflecting means being moved or deformed by electromagnetic means
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
    • G02B26/10—Scanning systems
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B2201/00—Specific applications of microelectromechanical systems
    • B81B2201/04—Optical MEMS
    • B81B2201/042—Micromirrors, not used as optical switches
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B2203/00—Basic microelectromechanical structures
    • B81B2203/01—Suspended structures, i.e. structures allowing a movement
    • B81B2203/0118—Cantilevers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
    • B81C2201/00—Manufacture or treatment of microstructural devices or systems
    • B81C2201/01—Manufacture or treatment of microstructural devices or systems in or on a substrate
    • B81C2201/0101—Shaping material; Structuring the bulk substrate or layers on the substrate; Film patterning
    • B81C2201/0128—Processes for removing material
    • B81C2201/013—Etching
    • B81C2201/0132—Dry etching, i.e. plasma etching, barrel etching, reactive ion etching [RIE], sputter etching or ion milling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
    • B81C2201/00—Manufacture or treatment of microstructural devices or systems
    • B81C2201/01—Manufacture or treatment of microstructural devices or systems in or on a substrate
    • B81C2201/0174—Manufacture or treatment of microstructural devices or systems in or on a substrate for making multi-layered devices, film deposition or growing
    • B81C2201/0191—Transfer of a layer from a carrier wafer to a device wafer
    • B81C2201/0194—Transfer of a layer from a carrier wafer to a device wafer the layer being structured
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
    • B81C2203/00—Forming microstructural systems
    • B81C2203/03—Bonding two components
    • B81C2203/032—Gluing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
    • B81C2203/00—Forming microstructural systems
    • B81C2203/03—Bonding two components
    • B81C2203/033—Thermal bonding
    • B81C2203/036—Fusion bonding

Definitions

  • Various embodiments of the invention relate to techniques of producing a mirror, e.g., for a scan module configured for resonantly scanning light, e.g., for use by a LIDAR system.
  • Various examples of the invention specifically relate to a two-part production process in which a mirror front side and a mirror back side are sepa- rately produced and subsequently coupled.
  • Mirrors for scanning light are required in various use cases.
  • One example use case is distance measurement using light (light detection and rang- ing; LIDAR). Pulsed or continuous-wave laser light is transmitted and, after reflec- tion at an object, detected. For providing a lateral resolution, the light may be scanned using a scanning mirror.
  • Such techniques typically employ microelectromechanical systems (MEMS) ap- proaches.
  • MEMS microelectromechanical systems
  • the mirror is defined by a wafer such as a semiconductor wafer or specifically a silicon wafer.
  • the wafer is processed using one or more of the fol- lowing techniques: lithography; dry etching; wet etching; lift off; etc..
  • DRIE deep reactive ion beam etching
  • process gas/etching gas may not be easily supplied at the required high rates. This may result in concentration gradients across the wafer; thereby resulting in inhomoge- neous etching rates across the wafer. Inhomogeneous etching is highly undesira- ble, because structures positioned at different positions across the wafer are then processed differently. This makes it hard to implement a controlled process.
  • a sec- ond drawback of conventional DRIE processes employed in connection with MEMS techniques is that heat input to the system increases for increasing volumes of material being removed.
  • the cool- ing capacity - e.g., of back side cooling - may not be sufficient to limit the temper- ature increase.
  • temperature gradients can be observed across the wafer, again leading to inhomogeneous processing.
  • Conventional DRIE processes have limitations with respect to producing large mirrors being comparably light- weight.
  • a method of producing a mirror includes producing a mirror front side and produc- ing a mirror back side.
  • the mirror back side includes a frame structure.
  • the frame structure includes fins and cavities.
  • the method also includes coupling the mirror front side and the mirror back side to obtain the mirror.
  • the mirror front side and the mirror back side may be defined by one or more wafers.
  • a glass MEMS technique may be employed.
  • the frame structure may implement a web structure.
  • an area of the frame structure may cover not less than 50 % of the area of the mirror front side, optionally not less than 80 %, further optionally not less than 95 %. Thereby, structural rigidity can be provided to the mirror front side by the frame structure.
  • a mirror includes a mirror front side and a mirror back side.
  • the mirror front side includes a reflective layer.
  • the mirror back side includes a frame structure.
  • the frame structure includes fins and cavities. The mirror front side and the mirror back side are not integrally formed.
  • the mirror front side and the mirror back side may thus be two parts that are cou- pled with each other.
  • the method may further include isolating the mirror from surrounding wafer mate- rial after said coupling.
  • the mirror front side and the mirror back side may be made from at least one wafer including the surrounding wafer material.
  • the method may further include isolating the mirror from surrounding wafer mate- rial, after said coupling.
  • the method may include isolating the mirror front side from surrounding wafer material and isolating the mirror back side from surround wafer material, prior to said coupling.
  • a mirror includes a mirror front side and a mirror back side.
  • the mirror front side includes a reflective layer.
  • the mirror back side includes a frame structure.
  • the frame structure includes fins and cavities. At least one fin of the fins extends be- yond an outer circumference of the mirror front side.
  • the at least one fin extends beyond an outer circumference of the reflective layer.
  • the mirror may be a mesoscopic mirror.
  • the mirror may have a diameter not smaller than 4 millimeters, optionally not smaller than 6 millimeters, further option- ally not smaller than 8 millimeters.
  • a scan unit includes the mirror and an elastic mount.
  • the scan unit may further include an actuator configured to resonantly scan the mirror by exciting an eigenmode of the elastic mount.
  • an actuator configured to resonantly scan the mirror by exciting an eigenmode of the elastic mount.
  • a torsional eigenmode of the elastic mount may be resonantly excited.
  • a LIDAR system includes such a scan unit.
  • the LIDAR system may be configured to determine the distance to a target object, e.g., by implementing time of light measurements of laser light.
  • FIG. 1 schematically illustrates a scan unit including a mirror and an elastic mount according to various examples.
  • FIG. 2 is a perspective view of a scan unit according to various examples.
  • FIG. 3 is a further perspective view of the scan unit according to FIG. 2.
  • FIG. 4 is a further perspective view of the scan unit according to FIG. 2.
  • FIG. 5 is a flowchart of a method of producing a mirror and an elastic mount of a scan unit according to various examples.
  • FIG. 6 illustrates a process step of producing a mirror back side according to vari- ous examples.
  • FIG. 7 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 6.
  • FIG. 8 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 7.
  • FIG. 9 schematically illustrates a process step of fabricating the mirror back side following the process step of FIG. 8.
  • FIG. 10 illustrates an array of mirror back sides on wafer level according to various examples.
  • FIG. 11 schematically illustrates a process step of producing a mirror front side according to various examples.
  • FIG. 12 schematically illustrates a process step of producing the mirror front side following the process step according to FIG. 11.
  • FIG. 13 schematically illustrates a process step of producing the mirror front side following the process step according to FIG. 12.
  • FIG. 14 schematically illustrates a process step of producing the mirror front side following the process step according to FIG. 13.
  • FIG. 15 schematically illustrates a process step of producing a mirror by coupling a mirror front side and a mirror back side, wherein the process step according to FIG. 15 follows the process steps according to FIGs. 9 and 14.
  • FIG. 16 schematically illustrates a process step of producing the mirror following the process step according to FIG. 15.
  • FIG. 17 schematically illustrates the mirror obtained from the process step accord- ing to FIG. 16.
  • FIG. 18 schematically illustrates a process step of producing a mirror, wherein the process step according to FIG. 18 follows the process steps according to FIG. 9 and FIG. 11.
  • FIG. 19 schematically illustrates a process step of producing the, wherein the pro- cess step according to FIG. 19 follows the process step according according to FIG. 18.
  • FIG. 20 schematically illustrates a process step of producing the mirror following the process step of FIG. 19.
  • FIG. 21 schematically illustrates a process step of producing the mirror following the process step according to FIG. 20.
  • FIG. 22 schematically illustrates a process step of producing the mirror following the process step according to FIG. 21.
  • FIG. 23 schematically illustrates a mirror obtained from the process step according to FIG. 22.
  • FIG. 24 schematically illustrates a process step of producing a mirror back side according to various examples.
  • FIG. 25 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 24.
  • FIG. 26 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 25.
  • FIG. 27 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 26.
  • FIG. 28 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 27.
  • FIG. 29 schematically illustrates a process step of producing a mirror front side according to various examples.
  • FIG. 30 schematically illustrates a process step of producing the mirror front side following the process step of FIG. 29.
  • FIG. 31 schematically illustrates a process step of producing a mirror following the process steps of FIGs.28 and 30.
  • FIG. 32 schematically illustrates a process step of producing the mirror following the process step of FIG. 31.
  • FIG. 33 schematically illustrates a process step of producing the mirror following the process step of FIG. 32.
  • FIG. 34 schematically illustrates a process step of producing the mirror following the process step of FIG. 33.
  • FIG. 35 schematically illustrates a process step of producing the mirror following the process step of FIG. 34.
  • FIG. 36 schematically illustrates a process step of producing the mirror following the process step of FIG. 35.
  • FIG. 37 schematically illustrates a process step of producing the mirror following the process step of FIG. 36.
  • FIG. 38 schematically illustrates the mirror obtained from the process step of FIG. 37.
  • the mirrors may be used to deflect light, e.g., laser light.
  • the mirrors may be moved, e.g., by an elas- tical mount. Thereby, the light may be scanned.
  • the techniques described herein may relate to scanning mirrors.
  • the mirrors described herein may find application in various use cases.
  • Example use cases include, but are not limited to: LIDAR with lateral resolution; spectrome- tres; projectors; endoscopes; etc.
  • the techniques described herein may facilitate production of mesoscopic mirrors, i.e., mirrors having a diameter of the respective reflective surface situated at a transition regime between micromirrors (typically having diameters in the submilli- meter range) and macroscopic mirrors (typically having diameters of the respective reflective layer in the centimeter regime, e.g., for multi-facetted polygon mirrors).
  • mesoscopic mirrors i.e., mirrors having a diameter of the respective reflective surface situated at a transition regime between micromirrors (typically having diameters in the submilli- meter range) and macroscopic mirrors (typically having diameters of the respective reflective layer in the centimeter regime, e.g., for multi-facetted polygon mirrors).
  • the techniques described herein may facilitate production of large mirrors that are comparably light-weight. Specifically, the techniques described herein help to pro- cute mirrors having a web-like frame structure of low filling factor - hence, a light- weight structure providing structural rigidity.
  • Such mesoscopic mirrors may have certain advantages: first, they may be built sufficiently lightweight - in particular if compared to macroscopic mirrors - to be resonantly actuated using an elastic mirror mount. This facilitates resonant scan- ning at comparably high frequencies, e.g., above 100 Hz up to kHz. Resonant scanning by elastic deformation is frictionless and, therefore, respective scanners typically have a higher mean time between failure if compared to scanners using rotational bearings, etc..
  • mesoscopic mirrors may be small enough to be manufactured using MEMS processes on one or more wafers, or from other ap-litiste mass-production techniques.
  • such mesoscopic mirrors may be made from silicon using standard MEMS processes. Specifically if corn- pared to macroscopic mirrors this helps to facilitate automated production and to reduce production costs.
  • Other examples include ceramics, e.g., manufactured from electrophoretic deposition.
  • mesoscopic mirrors may support compara- bly large emitter apertures and/or detector apertures of the corresponding optical system. This may help to implement a high measurement signal level, e.g., by collecting a large number of photons reflected from a target object when being applied in a LIDAR use case. This, particularly, applies if compared to micro- mirrors.
  • a two-step production process is provided.
  • a mirror front side and a mirror back side can be indi- vidually produced, e.g., on different wafers or generally different proceses. Then, the mirror front side and the mirror back side are coupled to obtain the mirror.
  • Such a two-step process may provide certain advantages vis-a-vis a one-step process with the mirror front side and the mirror back side are produced in-situ. For exam- pie, in such a two-step production process, the amount of material that is required to be removed by dry etching such as DRIE may be reduced.
  • the two-step production process helps to provide the mirror with a mirror back side having a frame structure.
  • the frame structure includes fins - sometimes also referred to as ridges - and corresponding cavities between the fins.
  • the frame structure may be produced at a comparably low filling factor: the filling factor defines the area of the fins with respect to the overall area covered by the frame structure. At low filling factors, the fins occupy only a small area of the frame structure.
  • the mirrors produced by the techniques described herein may have a filling factor of the frame structure which is not larger than 20%, optionally not larger than 5%, further optionally not larger than 2%. This may be at a diameter of the reflective layer of the mirror - which is typically fully or predomi- nantly covered by the frame structure - not smaller than 4 mm, optionally not smaller than 6 mm, further optionally not smaller than 8 mm. This provides for a large aperture of the corresponding optical system.
  • the frame structure provided by the mirror back surface supports the mirror front surface and the reflective material.
  • the mirror may be accelerated quickly, as is typically the case for resonant scanning where the mirror oscillates between stop positions and therefore exhibits periodic acceleration.
  • the frame structure can provide rigidity to the mirror front side, thereby reducing dynamic deformation of the reflective layer of the mirror front side when accelerating. This provides stability to the corresponding optical system; unwanted deflection or di- vergence of the light is avoided.
  • the mass moment of inertia of the mirror back side is limited. This facilitates high resonance of the eigenmodes of the mirror being res- onantly driven.
  • different move- ment patterns of the mirror can be implemented.
  • the techniques described herein may facilitate one-dimensional or two-dimensional scanning. Scanning can relate to repetitive transmission of light at different emitter angles.
  • laser light can be scanned.
  • coherent or inco- herent laser light may be used.
  • Polarized or unpolarized laser light may be used.
  • Pulsed laser light may be used.
  • Laser pulses having a full width at half maximum in the range of femtoseconds or picoseconds or nanoseconds may be used.
  • a pulse duration in the range of 0.5 - 3 nanoseconds may be used.
  • the laser light may have a wavelength in the range of 700 - 1800 nanometers, for example, 1550 nanometers or 950 nanometers.
  • references primarily made to laser light the various techniques described herein could also be used for light originating from other light sources, e.g., wideband light sources or RGB light sources.
  • RGB light sources can relate to light sources in the visible spectrum, wherein the color space is covered by superposition of different colors - e.g., red, green or blue.
  • light may be scanned using a mirror mounted on an elastic mount.
  • the elastic mount may comprise one or more spring elements.
  • the one or more spring elements may provide a form-induced and/or material-induced elasticity and, therefore, may not be implemented flexurally rigid. Therefore, the one or more spring elements may also be referred to as support elements.
  • the mirror may be coupled to a moving end of at least one of the one or more spring elements. Torsion and/or transversal movement of the one or more spring elements can trigger rotation and/or tilt, i.e., generally deflection, of the mir- ror. Thereby, different scan angles may be implemented. Resonant movement, i.e., oscillatory movement, of the one or more spring elements is possible.
  • the one or more spring elements have a length in the range of 2 millimeters - 8 millimeters, e.g., in the range of 3 millimeters - 6 millimeters.
  • the one or more spring elements may be of linear shape, in a rest position.
  • the one or more spring elements can have a diameter of, e.g., 50 - 250 micrometers.
  • the one or more spring elements may be made from silicon. Specifically, it would be possible to use torsional spring elements that can twist, but have a comparably high stiffness for flexure. In various examples, it would be possible that the one or more spring elements are produced by MEMS techniques.
  • lithography and/or etching can be applied to a wafer to produce the one or more spring elements.
  • a DRIE pro- cess may be used.
  • Silicon on insulator (SOI) wafers can be used to define stops for etching.
  • the one or more spring elements can extend from the mirror back side.
  • the one or more spring elements may enclose a significant angle with a reflective layer on the mirror front side, e.g., in the range of 30° - 90°, optionally 45°.
  • the one or more spring elements are arranged in the plane defined by the reflective layer of the mirror front side.
  • a respective scanner corn- prising a mirror may be used for, both, emitting laser light, as well as detecting reflected laser light.
  • the detector aperture can be defined by the mirror.
  • Such techniques are sometimes referred to as spatial filtering: by means of spatial filtering, a particularly large signal to noise ratio may be achieved, because light is selectively detected for the particular direction into which laser light has been emit- ted. Thereby, background noise from other areas is suppressed. By means of the large signal to noise ratio, long range measurements become possible.
  • FIG. 1 illustrates aspects with respect to a scan unit 100.
  • the scan unit 100 corn- prises a base 141 .
  • the base 141 defines a reference coordinate system.
  • the base 141 and a light source for light 180 could be arranged at fixed positions in the reference coordinate system.
  • the scan unit 100 also includes a spring element 111.
  • the spring element 111 can be deformed for resonant scanning of a mirror 150.
  • the spring element 111 provides elasticity.
  • the spring element 111 is coupled with the mirror 150 via an interface 142.
  • the base 141 , the spring element 111 , and the interface 142 implement an elastic mount 119. By deformation of the spring element 111 , the mirror 150 is moved, thereby implementing different deflection angles 181 of the light 180.
  • the elastic mount 119 can be configured to facilitate a rotation of the mirror 150 vis-a-vis the base 141 , respectively the reference coordinate system and, e.g., along a center axis of the elastic mount 119.
  • the torsion mode of the elastic mount 119 can be resonantly excited.
  • the torsion could be imple- mented stepwise, i.e., non-resonant.
  • the light 180 is de- flected by a varying angle 181.
  • FIG. 1 also illustrates an actuator 172.
  • the actuator 172 is configured to excite movement of the spring element 111.
  • an eigenmode of the elastic mount 119 may be excited.
  • actuators 172 may be used in the various techniques described herein, e.g., to actuate tor- sion of the spring element 111.
  • piezoelectric actuators may be used.
  • bending piezoelectric actuators may be used.
  • An AC com- ponent of the magnetic field tuned to the resonance curve of the torsion mode of the elastic mount 119 facilitates resonant scanning of light at high repetition rates, e.g., in the range of 100 Flz - 2 kFIz.
  • a control unit 171 is configured to control operation of the actuator 172. Closed- loop control of the motion of the mirror 150 may be implemented by means of the control unit 171.
  • FIG. 2 illustrates aspects with respect to the scan unit 100.
  • FIG. 2 is a perspective view of an example structural implementation of the scan unit 100.
  • the scan unit 100 may be produced from silicon, e.g., using MEMS techniques.
  • the scan unit 100 includes a mirror 150.
  • the mirror 150 includes a reflective layer on a mirror front side (the mirror front side and the re- flective layer are obstructed from view in the perspective of FIG. 2).
  • the mirror 150 also includes a mirror back side 152.
  • the mirror back side 152 is arranged opposite to the mirror front side. As illustrated in FIG.
  • the mirror back side includes a frame structure including fins and cavities.
  • the mass moment of inertia of the mirror 150 can be tailored by appropriate geometrical implementation of the frame structure.
  • the eigenfrequency of the torsional eigenmode of the elastic mount 119 can be tuned.
  • the frame structure also provides rigidity against deformation of the reflective layer of the mirror front side due to acceleration during resonant scanning.
  • a total of four spring elements 111 -1 , 111 -2, 111 -3, 111 - 4 extend between the interface element 142 and the base 141.
  • an intermediate coupling 155 is provided which, however, is optional.
  • the spring elements 111 -1 , 111 -2, 111 -3, 111 -4 extend away from the mirror back side 152.
  • the interface 142 includes a limit stop 153 to avoid excessive deflection of the mirror 140 by engagement with a corresponding limit stop (not shown in FIG. 2).
  • the base 141 , the interface element 142, and the spring elements 111 -1 , 111-2, 111 -3, 111 -4 may be integrally formed. They may be produced from a single wafer.
  • FIG. 2 also illustrates a surface normal 151 A of the reflective layer of the mirror 150. From FIG. 2 it is apparent that the surface normal 151 A encloses an angle of approximately 45° with a center axis 119A of the elastic mount 119 formed by the spring elements 111 -1 , 111 -2, 111 -3, 111 -4. Thereby, periscope-style scanning of light 180 by torsion of the elastic mount 190 is facilitated if light impacts the reflec- tive layer parallel with the center axis 119A.
  • the angle between the surface normal 151 A and the spring elements 111 -1 , 111 - 2, 111-3, 111 -4 can be defined by laterally structuring the wafer defining the elastic mount 119.
  • FIG. 2 also illustrates aspects with respect to the dynamics of the elastic mount 119. Specifically, FIG. 2 illustrates aspects with respect to the torsion of the elastic mount 119.
  • FIG. 2 bottom, a sectional view along the axis A - A is illustrated. From the sectional view it is apparent that the spring elements 111 -1 - 111-4 are arranged having a fourfold rotational symmetry with respect to the center axis 119A, i.e., in particular at the edges of a fictitious square arranged in the drawing plane of the sectional view. Such an arrangement mitigates nonlinear effects.
  • Tor- sion 501 is illustrated in the sectional view.
  • the full lines illustrate the rest position of the spring elements 111 -1 - 111-4 and the dashed lines illustrate the actuated position of the spring elements 111-1 - 111-4, characterized by a tor- sional angle 502.
  • the torsional angle 502 of the spring elements 111 -1 - 111 -4 can be approximately equal to the scan angle by which the deflection angle 181 is changed vis-a-vis the rest position.
  • the spring elements 111 -1 , 111 -2 are arranged in a plane 988 in the rest position.
  • the spring elements 111 -3, 111 -4 are arranged in a further plane 989.
  • FIGs. 3 - 4 are further prospective views of the scan unit 100 according to the example of FIG. 2.
  • FIG. 3 illustrates the mirror front side 151 having a reflective layer 151 A deposited thereon.
  • the outer circumference 151 B of the reflective layer 151 A is illustrated in FIG. 4.
  • FIG. 4 illustrates aspects with respect to the frame structure 157 of the mirror back side 152.
  • the frame structure 157 includes fins 158 and cavities 159 formed in-between adjacent fins 158.
  • the frame structure 157 implements a web-like structure
  • the frame structure 147 in the example of FIG. 4, covers essentially the entire area of the mirror front side 151.
  • the frame structure 157 illustrated in FIG. 4 includes, both, radial fins 158 extending away from a center of the frame structure at the mirror back side 152, as well as circumferential fins 158 extending around the center of the frame struc- ture 157 and the mirror back side 152 at a fixed radius.
  • the elastic mount 119 is attached to the mirror 150 at the center of the mirror back side 152.
  • FIG. 5 is a flowchart of a method of producing a mirror according to various exam- pies. For example, the method according to FIG. 5 may be used to produce the mirror 150 according to any one of FIGs.
  • the method according to FIG. 5 can employ MEMS techniques.
  • the mirror may be fabricated from one or more wafers, e.g., a monocrystalline wafer or a polycrystalline wafer. According to ex- amples, more than a single wafer may be employed in connection with the produc- tion method of FIG. 5.
  • One or more wafers employed in connection with the pro- duction method of FIG. 5 may be made of silicon, SOI, or another semiconductor material, e.g., gallium arsenide, etc. Glass wafers, e.g., a Pyrex wafer, Borofloat wafer, or the like.
  • Typical silicon wafers used in the processes may be 500 pm thick and 200 mm or 300 mm in lateral diameter.
  • FIG. 5 is a two-step production method. Specifically, at block 1001 , a mirror front side is produced and, at block 1002, a mirror back side is produced.
  • the mirror front side may be defined by a first wafer and the mirror back side may be defined by a different, second wafer.
  • the mirror front side and the mirror back side are coupled with each other. Thereby, the mirror is defined.
  • the mirror front side may have a reflective layer, thereby being configured to deflect light.
  • the mirror back side may have a frame structure having fins and cavities, thereby providing structural rigidity to the mirror front side which is helpful when resonantly scanning the mirror in an oscillatory motion.
  • MEMS techniques may be employed: etching; dry etching; ion beam etching; reactive ion beam etching; DRIE; wet etch- ing; lithography; exposure of photoresist using a mask; lift off; material deposition; grinding; mounting to a glass wafer; and/or polishing.
  • release processes may be employed. Release processes help to reduce the amount of material that has to be removed by etch- ing. Rather than employing large-area etching of the material - which introduces heat and requires significant amounts of etching gas - trenches are etched. Trenches may be deeper than wide. For example, a width of a trench may not be larger than 20 % of a depth of the trench. Material inside a closed loop formed by a trench can then be released from the surrounding wafer material, because it is not connected in any manner by the surrounding wafer material. This corresponds to implementing a release of the wafer material.
  • the closed-loop formed by a trench may be aligned with a circumference of a cavity of the frame structure.
  • the frame structure may be produced using the release process.
  • various options are available to implement the release. This is due to the high flexibility obtained from the two-step production process.
  • the release is implemented using recesses in a glass wafer on which the wafer defining the mirror front side and/or the mirror back side is supported.
  • the release is implemented using wafer grinding. Grinding, as used herein, may include thinning and/or polishing.
  • a large count of mirror front sides and/or mirror back sides are produced by parallel processing of a wafer.
  • an array of mirror front side structures may be processed at the first wafer and/or an array of mirror back side structures may be processed at the second wafer.
  • large wafer diameters such as 150 millimeters or 300 millimeters, the production throughput can be increased.
  • individual mirror front sides and individual mirror back sides have to be isolated from the respective wa- fer. Such isolation, marking the transition from wafer-level processing to device- level processing, may, generally, occur prior to or after coupling the mirror front side and the mirror back side at block 1003.
  • wafer-level processing may generally correspond to any process step that is executed starting from a scenario in which multiple devices are ar- ranged on a wafer, e.g., in an array. There is a tendency that wafer-level pro- cessing exploits parallel processing of the multiple devices. Differently, device- level processing may correspond to processing individual devices individually, i.e., starting from a scenario in which multiple devices are not arranged on a wafer. There is a tendency that device-level processing relies on serial processing of the multiple devices.
  • the transition from wafer-level processing to device-level pro- cessing is typically marked by isolating a device from surrounding wafer material. Isolating the mirror on wafer level, after coupling the mirror front side and the mirror back side, has the advantage that the coupling of the mirror front side and the mirror back side can be efficiently implemented for large count of mirrors in parallel and, by exploiting positioning marks arranged at a large distance with respect to each other, highly precise alignment of the mirror front side of the mirror back side becomes possible.
  • different techniques may be employed for coupling at block 1003.
  • Direct wafer bonding may be used.
  • direct wafer bonding may be supported by using one or more glass wafers to which the wafers defining the mirror front side and/or defining the mirror back side are attached.
  • epoxy adhesive has the advantage of not having rely on high processed temperatures which, generally, helps to protect the integrity of, e.g., a reflective layer of the mirror.
  • the epoxy adhesive may be arranged on certain contact sur- faces of the mirror front side and the mirror back side.
  • One or more of the mirror front side and the mirror back side may be placed on a handling wafer for executing said coupling at block 1003.
  • the mirror mount is produced.
  • the mirror mount may be produced using a MEMS technique as explained above in connection with blocks 1001 and 1002.
  • the mirror mount may include one or more spring elements, e.g., as discussed in connection with mirror mount 119 and the preceding FIGs..
  • the mirror mount may be produced from a different wafer if compared to the one or more wafers used to produce the mirror front side and the mirror back side at blocks 1001 and 1002.
  • the mirror obtained from block 1003 and the mirror amount obtained from block 1004 are coupled with each other.
  • the cou- pling of the mirror and the mirror mount at block 1005 may be executed on device level, but, generally, it would also be possible to execute block 1005 on wafer level.
  • a center part of the mirror back side may include one or more notches or recesses to receive corresponding protrusions of the elastic mount.
  • FIGs. 6 - 10 illustrate aspects with respect to block 1002, i.e., the production of the mirror back side 152.
  • FIG. 6 illustrates etching of a respective wafer 701. Specifically, FIG. 6 illustrates etching (illustrated by the arrows) of the wafer 701.
  • trenches are etched, e.g., using a DRIE or another dry etching process.
  • the trenches 785 are aligned with the contours of the cavities 159 of the frame structure 157. For example, the trenches 785 may enclose the contours of the cavities 159, as a closed loop.
  • FIG. 6, left illustrates a schematic of the frame structure 157.
  • FIG. 6, left illustrates material 781 that is to remain, thereby forming the fins of the frame structure 157; and further illustrates material 782 to be removed.
  • FIG. 6, right by selectively etching the trenches along the contour of the cavities 159, not all material 782 is removed. Material 782 remains in the cavities which is not etched. This remaining material 782 is later on removed by a release process.
  • Such etching of the trenches helps to limit the material that has to be removed by etching. This, in turn, limits the required etching gas throughput, avoiding inhomo- geneous distribution of the etching gas along different lateral positions of the wafer 701. Further, the heat input is limited. This specifically applies for large-scale frame structures 157 having a low filling fraction, as typically encountered for mesoscopic mirrors. Also, the processing time is reduced.
  • FIG. 6, for sake of simplicity, does not illustrate a lithography mask used for later- ally confining the etching to the trenches 785.
  • FIG. 7 illustrates a next process step of producing the mirror back side 152 which follows the process step of FIG. 6.
  • the wafer 701 - after etching the trenches 785 - is attached to a handling wafer 709.
  • FIG. 8 illustrates the process step following the process step of FIG. 7.
  • Flere the side 705 of the wafer 701 - opposite to the side 704 from which etching is per- formed at the process step of FIG. 6 - is subject to grinding.
  • Flere large areas of the material are removed by using a grinding fluid and a grinding tool.
  • the thickness of the material 782 removed by grinding may be between 20 micrometers and 100 micrometers.
  • the depth of the trenches 785 may be larger, e.g., larger than 200 mi- crometers or around 400 micrometers.
  • the total amount of material 782 removed by etching may be less than 10% than the total amount of material 782 removed by grinding.
  • FIG. 9 illustrates a process step following the process step of FIG. 8.
  • the wafer 701 is separated from the handling wafer 709. Thereby, remaining material 782 and the wafer side 704 in between adjacent trenches 785 is released from the surrounding material. Only the material 781 remains, see FIG. 9, left.
  • the frame structure 157 has not yet been isolated from the surrounding wafer material rather, the material 781 of the frame structure 157 is embedded into a frame structure of the surrounding wafer material 781 via the extended radial fins 158-1 - 158-4. Flence, these fins 158-1 - 158-4 provide fixa- tion of the material 781 defining the frame structure 157 within the surrounding wafer material.
  • FIG. 10 illustrates the corresponding wafer 701 after the material 782 has been removed and the material 781 , defining the frame structure 157, remains. Isolation of the individual frame structures 157 has not yet taken place. The individual frame structures 157 are coupled with the surrounding wafer material via the extended fins 158-1 - 158-4.
  • FIGs. 11 - 14 illustrate aspects of producing a mirror back side, e.g., according to block 1002 of FIG. 5.
  • the process of producing the mirror front side may correspond to the process of producing the mirror back side.
  • FIG. 11 illustrates etching - e.g., dry etching such as DRIE etching - of a wafer 711. As illustrated in FIG. 11 , it is possible to remove all material 782 at the side 714 of the wafer 711 such that no release process is required subsequently. Again, a lithography mask for laterally confining said etching is not illustrated in FIG. 11 for sake of simplicity.
  • a reflective layer 770 is deposited on the side 714 of the wafer 711.
  • a gold or aluminum layer may be deposited. Evaporation or electron heating may be employed.
  • FIG. 13 illustrates the process step following the process step of FIG. 12.
  • the wafer 711 now coated with the reflective layer 770, is attached to a handling wafer 709.
  • the handling wafer 709 is attached to the side 714 of the wafer 711 which is also coated by the reflective layer 770.
  • FIG. 14 illustrates the process step following the process step of FIG. 13.
  • grinding is employed to remove the remaining material 782 at the side 715 of the wafer 711. Then, no material 782 remains, see FIG. 14, left.
  • the material 781 defining the mirror front side 151 is not laterally coupled with the surrounding wafer material 781 (and, hence, if the handling wafer 709 was removed, the mirror front side 151 would be released from the wafer 711 ). As will be explained hereinafter, this facilitates simple and reliable isolation of the mirror 150 on wafer level.
  • FIGs. 15 - 17 illustrate examples with respect to coupling the mirror front side 151 and the mirror back side 152 to obtain the mirror 150.
  • the wafer 711 defining the mirror front side 151 is attached to the handling wafer 709. Then, the wafer 711 , at side 715, is brought into contact with the wafer 701. For this, the fins 158 of the frame structure 157 are aligned with respect to the center of the mirror front side 151.
  • the wafer 701 and 711 may be coupled using an adhesive such as an epoxy adhesive or wafer bonding.
  • FIG. 16 illustrates the process step following the process step of FIG. 15.
  • FIG. 16 illustrates aspects with respect to isolating the mirror 150. Specifically, FIG. 16 illustrates cutting edges 790. Cutting is often referred to as wafer slicing or dicing. Illustrated in FIG. 16 as a scenario where the cutting edges 790 cut the fins 158-1
  • the mirror 150 is isolated from the sur- rounding wafer material 781 of the wafers 701 , 711.
  • the extended fins 158-1 - 158-4 are cut at a posi- tion offset from the outer circumference 151 B, these fins 158-1 - 158-4 extend beyond the outer circumference 151 B once the mirror 150 has been isolated (cf. FIG. 4).
  • the count of extended fins 158-1 - 158-4 may be set so as to provide structural stability prior to isolation of the mirrors 150 from the wafers 701 , 711. For example, as illustrated in connection with the example of FIG. 4, it may be sufficient to use for extended fins 158-1 - 158-4 in direction north, south, east, and west of the mirror 150.
  • FIGs. 11 - 17 the reflective layer 770 is deposited prior to coupling the mirror front side 151 and the mirror back side 152 (cf. FIGs. 12 and 15). Furthermore, the reflective layer 770 is deposited on the side 714 of the wafer 711 which is opposite to the side 715 which is grinded (cf. FIGs. 12 and 14).
  • FIGs. 18 - 21 illustrate another example.
  • FIG. 18 illustrates a process step which follows the process step of FIG. 12.
  • FIG. 18 illustrates aspects with respect to cou- pling the wafer 701 defining the mirror back side 152 and the wafer 711 defining the mirror front side 151.
  • FIG. 18 illustrates aspects with respect to cou- pling the wafer 701 defining the mirror back side 152 and the wafer 711 defining the mirror front side 151.
  • the mirror front side 151 and the mirror back side 152 are coupled prior to wafer thinning of the wafer 711 defin- ing the mirror front side 151. Furthermore, the mirror front side 151 and the mirror back side 152 are coupled prior to depositing the reflective layer 770.
  • direct wafer-wafer bonding may be used for said coupling.
  • FIG. 19 schematically illustrates the process step following the process step of FIG. 18.
  • a handling wafer 709 is attached to the wafer 701 , opposite to the side coupled to the wafer 711.
  • FIG. 20 illustrates the process step following the process step of FIG. 19.
  • wafer thinning is applied to the wafer 711 , by grinding the side 715 of the wafer 711.
  • the process step of FIG. 20 generally corresponds to the process step of FIG. 14; albeit, in the scenario of FIG. 20, grinding of the side 715 of the wafer 711 is performed prior to depositing the reflective layer 770 and after cou- pling the mirror front side 151 with the mirror back side 152.
  • FIG. 21 illustrates the process step following the process step of FIG. 20.
  • the reflective layer 770 is deposited on the side 715 of the wafers 711.
  • grinding in process step of FIG. 20, may be followed by polishing.
  • the reflective layer 770 is deposited after coupling the mirror front side 151 with the mirror back side 152. Also, grinding is executed after coupling.
  • the reflective layer 770 is deposited on the grinded side 715.
  • FIG. 22 illustrates the process step following the process step of FIG. 21.
  • FIG. 22 illustrates aspects with respect to cutting 790 the wafer 701 , 711 , for isolating the mirror 150.
  • the process of FIG. 22 generally corresponds to the process of FIG. 16.
  • a protective coating may be applied (not illustrated in FIG. 22).
  • FIG. 23 illustrates the isolated mirror 150 obtained from cutting at the process step of FIG. 22.
  • FIGs. 24 - 38 illustrate yet another MEMS process that may be used to produce the mirror 150.
  • a glass wafer 708 is provided.
  • recesses 708A are etched into the glass wafer. This may be achieved using wet etching, e.g., using hydrofluoric acid.
  • the particular shape of the recesses 708A is not decisive, because the recesses 708A are merely used for implementing a re- lease of material of the wafer 701 defining the mirror back side 152.
  • FIG. 26 illustrates the process step following the process step of FIG. 25.
  • the wafer 701 has been attached to the glass wafer 708. This can be achieved by silicon-on-glass (SOG) anodic wafer bonding.
  • SOG silicon-on-glass
  • FIG. 27 illustrates the process step following the process step of FIG. 26.
  • the pro- cess step of FIG. 27 essentially correspond to the process step of FIG. 6.
  • the trenches 785 are etched all the way to the recesses 708A. Thereby, the material 782 - defining the cavities 159 - is released, see FIG. 28. In FIGs. 27 and 28, again, for sake of simplicity a mask for etching the trenches 785 is not illustrated.
  • FIG. 29 illustrates producing the mirror front side 151 .
  • the respective wafer 711 is attached to a corresponding glass wafer 707, using SOG anodic wafer bond- ing or another process.
  • FIG. 30 illustrates the process step following the process step of FIG. 29.
  • Flere the side 715 of the wafer 711 is thinned, e.g., using grinding and/or polishing.
  • FIG. 31 illustrates a process step of coupling the wafer 711 defining the mirror front side 151 and the wafer 701 defining the mirror back side 152.
  • the process step of FIG. 31 follows the process steps of FIG. 28 and FIG. 30.
  • the wafer 701 and the wafer 711 or coupled using direct wafer bonding.
  • the wafer 711 is attached to the glass wafer 707 and the wafer 701 is attached to the glass wafer 708, thereby facilitating handling.
  • FIG. 32 illustrates the coupled wafers 701 , 711.
  • the wafer 711 is coupled via the thinned side 715.
  • FIG. 33 illustrates a process step following the process step of FIG. 32.
  • the glass wafer 707 is removed. This may be achieved using wet etching, e.g., using hydrofluoric acid.
  • a protective mask may be applied.
  • FIG. 34 illustrates a process step following the process step of FIG. 33.
  • trenches are etched to define the mirror front side 151.
  • the process step of FIG. 34 essentially correspond to the process step of FIG. 11.
  • DRIE etching may be used.
  • etching of the mirror front side 151 is implemented after coupling the wafer 111 and the wafer 101.
  • FIG. 35 illustrates a process step following the process step of FIG. 34.
  • the reflective layer 770 is deposited on the side 714 of the wafer 711.
  • the reflective layer 770 is deposited on the side 714 opposing the side 715 which has been thinned in the process step of FIG. 30 and via which the wafer 711 is coupled to the wafer 701.
  • the reflective layer 770 is deposited on the side 714 after coupling of the mirror front side 151 and the mirror back side 152.
  • FIG. 36 illustrates a process step following the process step of FIG. 35.
  • the wafer 711 at the side 714 - now covered with the reflective layer 770 - is attached to a handling wafer 709.
  • FIG. 37 illustrates a process step following the process step of FIG. 36.
  • the glass wafer 708 is removed, e.g., by wet etching. Further, cutting of the wafer 701 is implemented along the cutting lines 790.
  • the process step of FIG. 37 generally corresponds to the process step of FIG. 22 and the process step of FIG. 16.
  • FIG. 37 illustrates the mirror 150 obtained from the process step of FIG. 37 (in the scenario of FIG.’s 24 - 38, the cavities 159 of the frame structure 157 are illustrated in a simplified manner for sake of simplicity).
  • one or more ex- tended fins extend beyond the outer circumference of the mirror front side. How- ever, this is generally optional. In other examples, there may be no extended fins.
  • dedicated supporting structures may be provided to connect the mir- ror or parts of the mirror with the surrounding wafer material until isolating the mir- ror or the respective mirror parts from the surrounding wafer material by cutting the supporting structures. The supporting structures can be different from the ex- tended fins.
  • a possible materials and associated processes may include ceramics, e.g., from electrophoresis, epoxy-composite materials, glass, or graphite, etc.
  • Such materials can have an elasticity module that provides for significant rigidity / stiffness; thereby, dynamic deformation of the mirror - when actuate, or specifically when resonantly scanned - is sufficiently small. Thereby, larger scanning frequencies become possible.
  • such materials have a comparably low density, i.e., are lightweight. Thereby, higher resonance frequencies are supported.

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Abstract

A mirror (150) comprises a mirror front side (151) comprising a reflective layer. The mirror (150) also comprises a mirror back side (152). The mirror back side (152) comprises a frame structure (157) having fins (158) and cavities (159). The mirror front side (151) and the mirror back side (152) are not integrally formed.

Description

TWO-PART MIRROR
TECHNICAL FIELD
Various embodiments of the invention relate to techniques of producing a mirror, e.g., for a scan module configured for resonantly scanning light, e.g., for use by a LIDAR system. Various examples of the invention specifically relate to a two-part production process in which a mirror front side and a mirror back side are sepa- rately produced and subsequently coupled.
BACKGROUND
Mirrors for scanning light (scanning mirrors) are required in various use cases. One example use case is distance measurement using light (light detection and rang- ing; LIDAR). Pulsed or continuous-wave laser light is transmitted and, after reflec- tion at an object, detected. For providing a lateral resolution, the light may be scanned using a scanning mirror.
Techniques of producing scanning mirrors are known, e.g., from US 7,078,778 B2. Such techniques typically employ microelectromechanical systems (MEMS) ap- proaches. Here, the mirror is defined by a wafer such as a semiconductor wafer or specifically a silicon wafer. The wafer is processed using one or more of the fol- lowing techniques: lithography; dry etching; wet etching; lift off; etc..
Conventional techniques of producing a scanning mirror using MEMS techniques face certain restrictions and drawbacks. For example, in some scenarios, it may be desirable to produce a mirror having a comparably large diameter of the re- spective reflective layer. Specifically, this may facilitate implementation of large emitter apertures and/or detector apertures for the respective optical system. Then, when employing MEMS techniques, it is typically required to remove a sig- nificant amount of wafer material using etching. A typical MEMS technique relies on deep reactive ion beam etching (DRIE). An example DRIE process described by US 5,501 ,893 A. However, when applying such a DRIE process to remove large amounts of material - as typically encountered when producing large-diameter mirrors -, certain technical restrictions are encountered: for example, process gas/etching gas may not be easily supplied at the required high rates. This may result in concentration gradients across the wafer; thereby resulting in inhomoge- neous etching rates across the wafer. Inhomogeneous etching is highly undesira- ble, because structures positioned at different positions across the wafer are then processed differently. This makes it hard to implement a controlled process. A sec- ond drawback of conventional DRIE processes employed in connection with MEMS techniques is that heat input to the system increases for increasing volumes of material being removed. Then, when producing large-diameter mirrors, the cool- ing capacity - e.g., of back side cooling - may not be sufficient to limit the temper- ature increase. Furthermore, temperature gradients can be observed across the wafer, again leading to inhomogeneous processing. Conventional DRIE processes have limitations with respect to producing large mirrors being comparably light- weight.
SUMMARY
Therefore, a need exists for advanced techniques of producing mirrors using MEMS techniques. Specifically, a need exists for advanced techniques of produc- ing MEMS mirrors having a comparably large diameter of the respective reflective material. This need is met by the features of the independent claims. The features of the dependent claims define embodiments.
A method of producing a mirror includes producing a mirror front side and produc- ing a mirror back side. The mirror back side includes a frame structure. The frame structure includes fins and cavities. The method also includes coupling the mirror front side and the mirror back side to obtain the mirror.
MEMS techniques may be employed for producing the mirror. The mirror front side and the mirror back side may be defined by one or more wafers. For example, a glass MEMS technique may be employed.
For example, the frame structure may implement a web structure. For example, an area of the frame structure may cover not less than 50 % of the area of the mirror front side, optionally not less than 80 %, further optionally not less than 95 %. Thereby, structural rigidity can be provided to the mirror front side by the frame structure.
A mirror includes a mirror front side and a mirror back side. The mirror front side includes a reflective layer. The mirror back side includes a frame structure. The frame structure includes fins and cavities. The mirror front side and the mirror back side are not integrally formed.
The mirror front side and the mirror back side may thus be two parts that are cou- pled with each other.
The method may further include isolating the mirror from surrounding wafer mate- rial after said coupling. The mirror front side and the mirror back side may be made from at least one wafer including the surrounding wafer material. The method may further include isolating the mirror from surrounding wafer mate- rial, after said coupling. In other examples, the method may include isolating the mirror front side from surrounding wafer material and isolating the mirror back side from surround wafer material, prior to said coupling.
A mirror includes a mirror front side and a mirror back side. The mirror front side includes a reflective layer. The mirror back side includes a frame structure. The frame structure includes fins and cavities. At least one fin of the fins extends be- yond an outer circumference of the mirror front side.
Specifically, it would be possible that the at least one fin extends beyond an outer circumference of the reflective layer.
The mirror may be a mesoscopic mirror. The mirror may have a diameter not smaller than 4 millimeters, optionally not smaller than 6 millimeters, further option- ally not smaller than 8 millimeters.
A scan unit includes the mirror and an elastic mount. The scan unit may further include an actuator configured to resonantly scan the mirror by exciting an eigenmode of the elastic mount. For example, a torsional eigenmode of the elastic mount may be resonantly excited.
A LIDAR system includes such a scan unit. The LIDAR system may be configured to determine the distance to a target object, e.g., by implementing time of light measurements of laser light.
It is to be understood that the features mentioned above and those yet to be ex- plained below may be used not only in the respective combinations indicated, but also in other combinations or in isolation without departing from the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates a scan unit including a mirror and an elastic mount according to various examples.
FIG. 2 is a perspective view of a scan unit according to various examples.
FIG. 3 is a further perspective view of the scan unit according to FIG. 2. FIG. 4 is a further perspective view of the scan unit according to FIG. 2.
FIG. 5 is a flowchart of a method of producing a mirror and an elastic mount of a scan unit according to various examples. FIG. 6 illustrates a process step of producing a mirror back side according to vari- ous examples.
FIG. 7 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 6.
FIG. 8 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 7.
FIG. 9 schematically illustrates a process step of fabricating the mirror back side following the process step of FIG. 8.
FIG. 10 illustrates an array of mirror back sides on wafer level according to various examples. FIG. 11 schematically illustrates a process step of producing a mirror front side according to various examples. FIG. 12 schematically illustrates a process step of producing the mirror front side following the process step according to FIG. 11. FIG. 13 schematically illustrates a process step of producing the mirror front side following the process step according to FIG. 12.
FIG. 14 schematically illustrates a process step of producing the mirror front side following the process step according to FIG. 13.
FIG. 15 schematically illustrates a process step of producing a mirror by coupling a mirror front side and a mirror back side, wherein the process step according to FIG. 15 follows the process steps according to FIGs. 9 and 14. FIG. 16 schematically illustrates a process step of producing the mirror following the process step according to FIG. 15.
FIG. 17 schematically illustrates the mirror obtained from the process step accord- ing to FIG. 16.
FIG. 18 schematically illustrates a process step of producing a mirror, wherein the process step according to FIG. 18 follows the process steps according to FIG. 9 and FIG. 11. FIG. 19 schematically illustrates a process step of producing the, wherein the pro- cess step according to FIG. 19 follows the process step according according to FIG. 18.
FIG. 20 schematically illustrates a process step of producing the mirror following the process step of FIG. 19. FIG. 21 schematically illustrates a process step of producing the mirror following the process step according to FIG. 20.
FIG. 22 schematically illustrates a process step of producing the mirror following the process step according to FIG. 21.
FIG. 23 schematically illustrates a mirror obtained from the process step according to FIG. 22. FIG. 24 schematically illustrates a process step of producing a mirror back side according to various examples.
FIG. 25 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 24.
FIG. 26 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 25.
FIG. 27 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 26.
FIG. 28 schematically illustrates a process step of producing the mirror back side following the process step of FIG. 27. FIG. 29 schematically illustrates a process step of producing a mirror front side according to various examples.
FIG. 30 schematically illustrates a process step of producing the mirror front side following the process step of FIG. 29. FIG. 31 schematically illustrates a process step of producing a mirror following the process steps of FIGs.28 and 30.
FIG. 32 schematically illustrates a process step of producing the mirror following the process step of FIG. 31.
FIG. 33 schematically illustrates a process step of producing the mirror following the process step of FIG. 32. FIG. 34 schematically illustrates a process step of producing the mirror following the process step of FIG. 33.
FIG. 35 schematically illustrates a process step of producing the mirror following the process step of FIG. 34.
FIG. 36 schematically illustrates a process step of producing the mirror following the process step of FIG. 35.
FIG. 37 schematically illustrates a process step of producing the mirror following the process step of FIG. 36.
FIG. 38 schematically illustrates the mirror obtained from the process step of FIG. 37. DETAILED DESCRIPTION OF EMBODIMENTS
In the following, embodiments of the invention will be described in detail with ref- erence to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only. The drawings are to be regarded as being schematic representations and ele- ments illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect con- nection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firm- ware, software, or a combination thereof.
Hereinafter, techniques are described with respect to mirrors. The mirrors may be used to deflect light, e.g., laser light. The mirrors may be moved, e.g., by an elas- tical mount. Thereby, the light may be scanned. Hence, the techniques described herein may relate to scanning mirrors.
The mirrors described herein may find application in various use cases. Example use cases include, but are not limited to: LIDAR with lateral resolution; spectrome- tres; projectors; endoscopes; etc. The techniques described herein may facilitate production of mesoscopic mirrors, i.e., mirrors having a diameter of the respective reflective surface situated at a transition regime between micromirrors (typically having diameters in the submilli- meter range) and macroscopic mirrors (typically having diameters of the respective reflective layer in the centimeter regime, e.g., for multi-facetted polygon mirrors). For example, an area of a reflective surface of the mirror may be in the range of 100 mm2 to 300 mm2
The techniques described herein may facilitate production of large mirrors that are comparably light-weight. Specifically, the techniques described herein help to pro- duce mirrors having a web-like frame structure of low filling factor - hence, a light- weight structure providing structural rigidity. Such mesoscopic mirrors may have certain advantages: first, they may be built sufficiently lightweight - in particular if compared to macroscopic mirrors - to be resonantly actuated using an elastic mirror mount. This facilitates resonant scan- ning at comparably high frequencies, e.g., above 100 Hz up to kHz. Resonant scanning by elastic deformation is frictionless and, therefore, respective scanners typically have a higher mean time between failure if compared to scanners using rotational bearings, etc.. Second, mesoscopic mirrors may be small enough to be manufactured using MEMS processes on one or more wafers, or from other ap- propriate mass-production techniques. For example, such mesoscopic mirrors may be made from silicon using standard MEMS processes. Specifically if corn- pared to macroscopic mirrors this helps to facilitate automated production and to reduce production costs. Other examples include ceramics, e.g., manufactured from electrophoretic deposition. Third, mesoscopic mirrors may support compara- bly large emitter apertures and/or detector apertures of the corresponding optical system. This may help to implement a high measurement signal level, e.g., by collecting a large number of photons reflected from a target object when being applied in a LIDAR use case. This, particularly, applies if compared to micro- mirrors.
According to some examples, techniques of producing mirrors, specifically mesoscopic mirrors, are described. According to examples, a two-step production process is provided. Here, a mirror front side and a mirror back side can be indi- vidually produced, e.g., on different wafers or generally different proceses. Then, the mirror front side and the mirror back side are coupled to obtain the mirror. Such a two-step process may provide certain advantages vis-a-vis a one-step process with the mirror front side and the mirror back side are produced in-situ. For exam- pie, in such a two-step production process, the amount of material that is required to be removed by dry etching such as DRIE may be reduced. This may be achieved by combining the dry etching with release processes, rather than etching the ma- terial. Often, in-situ production processes do not offer the possibility to implement release processes. Specifically, the two-step production process helps to provide the mirror with a mirror back side having a frame structure. The frame structure includes fins - sometimes also referred to as ridges - and corresponding cavities between the fins. Specifically, the frame structure may be produced at a comparably low filling factor: the filling factor defines the area of the fins with respect to the overall area covered by the frame structure. At low filling factors, the fins occupy only a small area of the frame structure. The mirrors produced by the techniques described herein may have a filling factor of the frame structure which is not larger than 20%, optionally not larger than 5%, further optionally not larger than 2%. This may be at a diameter of the reflective layer of the mirror - which is typically fully or predomi- nantly covered by the frame structure - not smaller than 4 mm, optionally not smaller than 6 mm, further optionally not smaller than 8 mm. This provides for a large aperture of the corresponding optical system.
The frame structure provided by the mirror back surface supports the mirror front surface and the reflective material. Thereby, the mirror may be accelerated quickly, as is typically the case for resonant scanning where the mirror oscillates between stop positions and therefore exhibits periodic acceleration. Specifically, the frame structure can provide rigidity to the mirror front side, thereby reducing dynamic deformation of the reflective layer of the mirror front side when accelerating. This provides stability to the corresponding optical system; unwanted deflection or di- vergence of the light is avoided. At the same time, where the filling factor of the frame structure is limited, the mass moment of inertia of the mirror back side is limited. This facilitates high resonance of the eigenmodes of the mirror being res- onantly driven. Depending on the eigenmodes used for scanning, different move- ment patterns of the mirror can be implemented. Hence, the techniques described herein may facilitate one-dimensional or two-dimensional scanning. Scanning can relate to repetitive transmission of light at different emitter angles.
In various examples, laser light can be scanned. For example, coherent or inco- herent laser light may be used. Polarized or unpolarized laser light may be used. Pulsed laser light may be used. Laser pulses having a full width at half maximum in the range of femtoseconds or picoseconds or nanoseconds may be used. For example, a pulse duration in the range of 0.5 - 3 nanoseconds may be used. The laser light may have a wavelength in the range of 700 - 1800 nanometers, for example, 1550 nanometers or 950 nanometers. For sake of simplicity, hereinafter, references primarily made to laser light; the various techniques described herein could also be used for light originating from other light sources, e.g., wideband light sources or RGB light sources. RGB light sources can relate to light sources in the visible spectrum, wherein the color space is covered by superposition of different colors - e.g., red, green or blue.
In the various examples described herein, light may be scanned using a mirror mounted on an elastic mount. The elastic mount may comprise one or more spring elements. The one or more spring elements may provide a form-induced and/or material-induced elasticity and, therefore, may not be implemented flexurally rigid. Therefore, the one or more spring elements may also be referred to as support elements. The mirror may be coupled to a moving end of at least one of the one or more spring elements. Torsion and/or transversal movement of the one or more spring elements can trigger rotation and/or tilt, i.e., generally deflection, of the mir- ror. Thereby, different scan angles may be implemented. Resonant movement, i.e., oscillatory movement, of the one or more spring elements is possible.
It is possible that the one or more spring elements have a length in the range of 2 millimeters - 8 millimeters, e.g., in the range of 3 millimeters - 6 millimeters. The one or more spring elements may be of linear shape, in a rest position. The one or more spring elements can have a diameter of, e.g., 50 - 250 micrometers. The one or more spring elements may be made from silicon. Specifically, it would be possible to use torsional spring elements that can twist, but have a comparably high stiffness for flexure. In various examples, it would be possible that the one or more spring elements are produced by MEMS techniques. Hence, lithography and/or etching can be applied to a wafer to produce the one or more spring elements. For example, a DRIE pro- cess may be used. Silicon on insulator (SOI) wafers can be used to define stops for etching.
In the various examples described herein, the one or more spring elements can extend from the mirror back side. Hence, the one or more spring elements may enclose a significant angle with a reflective layer on the mirror front side, e.g., in the range of 30° - 90°, optionally 45°. In other examples, it would also be possible that the one or more spring elements are arranged in the plane defined by the reflective layer of the mirror front side.
Being applied in connection with an LIDAR technique, a respective scanner corn- prising a mirror may be used for, both, emitting laser light, as well as detecting reflected laser light. Hence, the detector aperture can be defined by the mirror. Such techniques are sometimes referred to as spatial filtering: by means of spatial filtering, a particularly large signal to noise ratio may be achieved, because light is selectively detected for the particular direction into which laser light has been emit- ted. Thereby, background noise from other areas is suppressed. By means of the large signal to noise ratio, long range measurements become possible.
FIG. 1 illustrates aspects with respect to a scan unit 100. The scan unit 100 corn- prises a base 141 . The base 141 defines a reference coordinate system. For ex- ample, the base 141 and a light source for light 180 (not illustrated in FIG. 1 ) could be arranged at fixed positions in the reference coordinate system. The scan unit 100 also includes a spring element 111. The spring element 111 can be deformed for resonant scanning of a mirror 150. The spring element 111 provides elasticity. The spring element 111 is coupled with the mirror 150 via an interface 142. The base 141 , the spring element 111 , and the interface 142 implement an elastic mount 119. By deformation of the spring element 111 , the mirror 150 is moved, thereby implementing different deflection angles 181 of the light 180.
For example, the elastic mount 119 can be configured to facilitate a rotation of the mirror 150 vis-a-vis the base 141 , respectively the reference coordinate system and, e.g., along a center axis of the elastic mount 119. This is achieved by torsion of the spring element 111. For example, the torsion mode of the elastic mount 119 can be resonantly excited. Alternatively or additionally, the torsion could be imple- mented stepwise, i.e., non-resonant. By means of the torsion, the light 180 is de- flected by a varying angle 181. FIG. 1 also illustrates an actuator 172. The actuator 172 is configured to excite movement of the spring element 111. For example, an eigenmode of the elastic mount 119 may be excited. As a general rule, different kinds and types of actuators 172 may be used in the various techniques described herein, e.g., to actuate tor- sion of the spring element 111. For example, piezoelectric actuators may be used. For example, bending piezoelectric actuators may be used. Alternatively or addi- tionally, it would be possible to use a magnetic actuator configured to create a magnetic field in the area of a magnet attached to the elastic mount. An AC com- ponent of the magnetic field tuned to the resonance curve of the torsion mode of the elastic mount 119 facilitates resonant scanning of light at high repetition rates, e.g., in the range of 100 Flz - 2 kFIz.
A control unit 171 is configured to control operation of the actuator 172. Closed- loop control of the motion of the mirror 150 may be implemented by means of the control unit 171. FIG. 2 illustrates aspects with respect to the scan unit 100. FIG. 2 is a perspective view of an example structural implementation of the scan unit 100. The scan unit 100 may be produced from silicon, e.g., using MEMS techniques. In the example of FIG. 2, the scan unit 100 includes a mirror 150. The mirror 150 includes a reflective layer on a mirror front side (the mirror front side and the re- flective layer are obstructed from view in the perspective of FIG. 2). The mirror 150 also includes a mirror back side 152. The mirror back side 152 is arranged opposite to the mirror front side. As illustrated in FIG. 2, the mirror back side includes a frame structure including fins and cavities. Thereby, the mass moment of inertia of the mirror 150 can be tailored by appropriate geometrical implementation of the frame structure. Thereby, the eigenfrequency of the torsional eigenmode of the elastic mount 119 can be tuned. The frame structure also provides rigidity against deformation of the reflective layer of the mirror front side due to acceleration during resonant scanning.
In the example of FIG. 2, a total of four spring elements 111 -1 , 111 -2, 111 -3, 111 - 4 extend between the interface element 142 and the base 141. In the example of FIG. 2, an intermediate coupling 155 is provided which, however, is optional. The spring elements 111 -1 , 111 -2, 111 -3, 111 -4 extend away from the mirror back side 152. The interface 142 includes a limit stop 153 to avoid excessive deflection of the mirror 140 by engagement with a corresponding limit stop (not shown in FIG. 2). The base 141 , the interface element 142, and the spring elements 111 -1 , 111-2, 111 -3, 111 -4 may be integrally formed. They may be produced from a single wafer.
FIG. 2 also illustrates a surface normal 151 A of the reflective layer of the mirror 150. From FIG. 2 it is apparent that the surface normal 151 A encloses an angle of approximately 45° with a center axis 119A of the elastic mount 119 formed by the spring elements 111 -1 , 111 -2, 111 -3, 111 -4. Thereby, periscope-style scanning of light 180 by torsion of the elastic mount 190 is facilitated if light impacts the reflec- tive layer parallel with the center axis 119A.
The angle between the surface normal 151 A and the spring elements 111 -1 , 111 - 2, 111-3, 111 -4 can be defined by laterally structuring the wafer defining the elastic mount 119.
FIG. 2 also illustrates aspects with respect to the dynamics of the elastic mount 119. Specifically, FIG. 2 illustrates aspects with respect to the torsion of the elastic mount 119. In FIG. 2, bottom, a sectional view along the axis A - A is illustrated. From the sectional view it is apparent that the spring elements 111 -1 - 111-4 are arranged having a fourfold rotational symmetry with respect to the center axis 119A, i.e., in particular at the edges of a fictitious square arranged in the drawing plane of the sectional view. Such an arrangement mitigates nonlinear effects. Tor- sion 501 is illustrated in the sectional view. Flere, the full lines illustrate the rest position of the spring elements 111 -1 - 111-4 and the dashed lines illustrate the actuated position of the spring elements 111-1 - 111-4, characterized by a tor- sional angle 502. Flere, when applying torsion to the elastic mount 119, the torsional angle 502 of the spring elements 111 -1 - 111 -4 can be approximately equal to the scan angle by which the deflection angle 181 is changed vis-a-vis the rest position.
From FIG. 2, it is apparent that the spring elements 111 -1 , 111 -2 are arranged in a plane 988 in the rest position. The spring elements 111 -3, 111 -4 are arranged in a further plane 989.
The total length 119B of the elastic mount 119 may be in the range of 2 millimeters - 10 millimeters, i.e., approximately equal to the diameter of the mirror 150. FIGs. 3 - 4 are further prospective views of the scan unit 100 according to the example of FIG. 2.
FIG. 3 illustrates the mirror front side 151 having a reflective layer 151 A deposited thereon.
The outer circumference 151 B of the reflective layer 151 A is illustrated in FIG. 4.
FIG. 4 illustrates aspects with respect to the frame structure 157 of the mirror back side 152. As illustrated in FIG. 4, the frame structure 157 includes fins 158 and cavities 159 formed in-between adjacent fins 158.
As illustrated in FIG. 4, the frame structure 157 implements a web-like structure The frame structure 147, in the example of FIG. 4, covers essentially the entire area of the mirror front side 151.
The specific design of the frame structure 157 illustrated in FIG. 4 is an example only. Flowever, as a general rule, it is possible that the frame structure 157, in accordance with the example of design of FIG. 4, includes, both, radial fins 158 extending away from a center of the frame structure at the mirror back side 152, as well as circumferential fins 158 extending around the center of the frame struc- ture 157 and the mirror back side 152 at a fixed radius. The elastic mount 119 is attached to the mirror 150 at the center of the mirror back side 152. Furthermore, from the multiple fins 158 of the frame structure 157, the fins 151 -1 , 158-2, 158-3, 158-4 extend beyond the outer circumference 151 B of the reflective layer 151 A of the mirror front side 151. This has certain advantages in connection with the method of production of the mirror 150. Specifically, by these extended fins 151 -1 , 158-2, 158-3, it is possible to conveniently isolate the mirror 150 from the surrounding wafer material in a MEMS technique for production. These and other techniques are discussed in connection with the following FIGs.. FIG. 5 is a flowchart of a method of producing a mirror according to various exam- pies. For example, the method according to FIG. 5 may be used to produce the mirror 150 according to any one of FIGs. 1 - 4. The method according to FIG. 5 can employ MEMS techniques. The mirror may be fabricated from one or more wafers, e.g., a monocrystalline wafer or a polycrystalline wafer. According to ex- amples, more than a single wafer may be employed in connection with the produc- tion method of FIG. 5. One or more wafers employed in connection with the pro- duction method of FIG. 5 may be made of silicon, SOI, or another semiconductor material, e.g., gallium arsenide, etc. Glass wafers, e.g., a Pyrex wafer, Borofloat wafer, or the like.
Typical silicon wafers used in the processes may be 500 pm thick and 200 mm or 300 mm in lateral diameter.
FIG. 5 is a two-step production method. Specifically, at block 1001 , a mirror front side is produced and, at block 1002, a mirror back side is produced. For example, the mirror front side may be defined by a first wafer and the mirror back side may be defined by a different, second wafer.
Next, at block 1003, the mirror front side and the mirror back side are coupled with each other. Thereby, the mirror is defined.
In detail, the mirror front side may have a reflective layer, thereby being configured to deflect light. On the other hand, the mirror back side may have a frame structure having fins and cavities, thereby providing structural rigidity to the mirror front side which is helpful when resonantly scanning the mirror in an oscillatory motion. In block 1001 , one or more of the following MEMS techniques may be employed: etching; dry etching; ion beam etching; reactive ion beam etching; DRIE; wet etch- ing; lithography; exposure of photoresist using a mask; lift off; material deposition; grinding; mounting to a glass wafer; and/or polishing.
Likewise, one or more of such MEMS techniques may be employed in block 1002.
In block 1001 and/or block 1002 release processes may be employed. Release processes help to reduce the amount of material that has to be removed by etch- ing. Rather than employing large-area etching of the material - which introduces heat and requires significant amounts of etching gas - trenches are etched. Trenches may be deeper than wide. For example, a width of a trench may not be larger than 20 % of a depth of the trench. Material inside a closed loop formed by a trench can then be released from the surrounding wafer material, because it is not connected in any manner by the surrounding wafer material. This corresponds to implementing a release of the wafer material. For example, the closed-loop formed by a trench may be aligned with a circumference of a cavity of the frame structure. Thus, the frame structure may be produced using the release process. As a general rule, various options are available to implement the release. This is due to the high flexibility obtained from the two-step production process. In one scenario, the release is implemented using recesses in a glass wafer on which the wafer defining the mirror front side and/or the mirror back side is supported. In another scenario, the release is implemented using wafer grinding. Grinding, as used herein, may include thinning and/or polishing.
Typically, at block 1001 and/or at block 1002, a large count of mirror front sides and/or mirror back sides are produced by parallel processing of a wafer. Hence, an array of mirror front side structures may be processed at the first wafer and/or an array of mirror back side structures may be processed at the second wafer. By implementing large wafer diameters such as 150 millimeters or 300 millimeters, the production throughput can be increased. Eventually, individual mirror front sides and individual mirror back sides have to be isolated from the respective wa- fer. Such isolation, marking the transition from wafer-level processing to device- level processing, may, generally, occur prior to or after coupling the mirror front side and the mirror back side at block 1003. Specifically, it would be possible that the coupling of the mirror front side and the mirror back side, at block 1003, is implemented on wafer level. Then, isolating the multiple mirrors can occur after executing block 1003. As a general rule, wafer-level processing may generally correspond to any process step that is executed starting from a scenario in which multiple devices are ar- ranged on a wafer, e.g., in an array. There is a tendency that wafer-level pro- cessing exploits parallel processing of the multiple devices. Differently, device- level processing may correspond to processing individual devices individually, i.e., starting from a scenario in which multiple devices are not arranged on a wafer. There is a tendency that device-level processing relies on serial processing of the multiple devices. The transition from wafer-level processing to device-level pro- cessing is typically marked by isolating a device from surrounding wafer material. Isolating the mirror on wafer level, after coupling the mirror front side and the mirror back side, has the advantage that the coupling of the mirror front side and the mirror back side can be efficiently implemented for large count of mirrors in parallel and, by exploiting positioning marks arranged at a large distance with respect to each other, highly precise alignment of the mirror front side of the mirror back side becomes possible.
Generally, different techniques may be employed for coupling at block 1003. Ex- amples include using an epoxy adhesive and silicon-silicon wafer bonding or, gen- erally, wafer bonding. Direct wafer bonding may be used. For example, direct wafer bonding may be supported by using one or more glass wafers to which the wafers defining the mirror front side and/or defining the mirror back side are attached. Using epoxy adhesive has the advantage of not having rely on high processed temperatures which, generally, helps to protect the integrity of, e.g., a reflective layer of the mirror. The epoxy adhesive may be arranged on certain contact sur- faces of the mirror front side and the mirror back side. One or more of the mirror front side and the mirror back side may be placed on a handling wafer for executing said coupling at block 1003.
To produce the scan unit, optionally, at block 1004, the mirror mount is produced. Again, the mirror mount may be produced using a MEMS technique as explained above in connection with blocks 1001 and 1002. The mirror mount may include one or more spring elements, e.g., as discussed in connection with mirror mount 119 and the preceding FIGs.. The mirror mount may be produced from a different wafer if compared to the one or more wafers used to produce the mirror front side and the mirror back side at blocks 1001 and 1002.
Finally, at optional block 1005, the mirror obtained from block 1003 and the mirror amount obtained from block 1004 are coupled with each other. Typically, the cou- pling of the mirror and the mirror mount at block 1005 may be executed on device level, but, generally, it would also be possible to execute block 1005 on wafer level. For example, it would be possible to couple the elastic mirror mount with the mirror using one or more interrelated indentation features provided on the respective con- tact surfaces of the elastic mount and the mirror back side. For example, a center part of the mirror back side may include one or more notches or recesses to receive corresponding protrusions of the elastic mount. With respect to the following FIGs., example implementations of the method of producing a mirror are described.
FIGs. 6 - 10 illustrate aspects with respect to block 1002, i.e., the production of the mirror back side 152.
FIG. 6 illustrates etching of a respective wafer 701. Specifically, FIG. 6 illustrates etching (illustrated by the arrows) of the wafer 701. In FIG. 6, trenches are etched, e.g., using a DRIE or another dry etching process. The trenches 785 are aligned with the contours of the cavities 159 of the frame structure 157. For example, the trenches 785 may enclose the contours of the cavities 159, as a closed loop.
Specifically, FIG. 6, left illustrates a schematic of the frame structure 157. FIG. 6, left illustrates material 781 that is to remain, thereby forming the fins of the frame structure 157; and further illustrates material 782 to be removed. As illustrated in FIG. 6, right, by selectively etching the trenches along the contour of the cavities 159, not all material 782 is removed. Material 782 remains in the cavities which is not etched. This remaining material 782 is later on removed by a release process. Such etching of the trenches helps to limit the material that has to be removed by etching. This, in turn, limits the required etching gas throughput, avoiding inhomo- geneous distribution of the etching gas along different lateral positions of the wafer 701. Further, the heat input is limited. This specifically applies for large-scale frame structures 157 having a low filling fraction, as typically encountered for mesoscopic mirrors. Also, the processing time is reduced.
FIG. 6, for sake of simplicity, does not illustrate a lithography mask used for later- ally confining the etching to the trenches 785. FIG. 7 illustrates a next process step of producing the mirror back side 152 which follows the process step of FIG. 6. Specifically, in FIG. 7, the wafer 701 - after etching the trenches 785 - is attached to a handling wafer 709. FIG. 8 illustrates the process step following the process step of FIG. 7. Flere, the side 705 of the wafer 701 - opposite to the side 704 from which etching is per- formed at the process step of FIG. 6 - is subject to grinding. Flere, large areas of the material are removed by using a grinding fluid and a grinding tool. This may also be referred to as wafer thinning. For example, the thickness of the material 782 removed by grinding may be between 20 micrometers and 100 micrometers. Differently, the depth of the trenches 785 may be larger, e.g., larger than 200 mi- crometers or around 400 micrometers. Flowever, because the trenches 785 have a laterally-confined width, the total amount of material 782 removed by etching may be less than 10% than the total amount of material 782 removed by grinding.
FIG. 9 illustrates a process step following the process step of FIG. 8. In FIG. 9, the wafer 701 is separated from the handling wafer 709. Thereby, remaining material 782 and the wafer side 704 in between adjacent trenches 785 is released from the surrounding material. Only the material 781 remains, see FIG. 9, left.
As illustrated in FIG. 9, the frame structure 157 has not yet been isolated from the surrounding wafer material rather, the material 781 of the frame structure 157 is embedded into a frame structure of the surrounding wafer material 781 via the extended radial fins 158-1 - 158-4. Flence, these fins 158-1 - 158-4 provide fixa- tion of the material 781 defining the frame structure 157 within the surrounding wafer material.
FIG. 10 illustrates the corresponding wafer 701 after the material 782 has been removed and the material 781 , defining the frame structure 157, remains. Isolation of the individual frame structures 157 has not yet taken place. The individual frame structures 157 are coupled with the surrounding wafer material via the extended fins 158-1 - 158-4.
FIGs. 11 - 14 illustrate aspects of producing a mirror back side, e.g., according to block 1002 of FIG. 5. Generally, the process of producing the mirror front side may correspond to the process of producing the mirror back side.
FIG. 11 illustrates etching - e.g., dry etching such as DRIE etching - of a wafer 711. As illustrated in FIG. 11 , it is possible to remove all material 782 at the side 714 of the wafer 711 such that no release process is required subsequently. Again, a lithography mask for laterally confining said etching is not illustrated in FIG. 11 for sake of simplicity.
Next, in FIG. 12, a reflective layer 770 is deposited on the side 714 of the wafer 711. For example, a gold or aluminum layer may be deposited. Evaporation or electron heating may be employed.
FIG. 13 illustrates the process step following the process step of FIG. 12. In the process step of FIG. 13, the wafer 711 , now coated with the reflective layer 770, is attached to a handling wafer 709. Specifically, the handling wafer 709 is attached to the side 714 of the wafer 711 which is also coated by the reflective layer 770.
FIG. 14 illustrates the process step following the process step of FIG. 13. In the process step of FIG. 14, grinding is employed to remove the remaining material 782 at the side 715 of the wafer 711. Then, no material 782 remains, see FIG. 14, left. As illustrated in FIG. 14, left, the material 781 defining the mirror front side 151 is not laterally coupled with the surrounding wafer material 781 (and, hence, if the handling wafer 709 was removed, the mirror front side 151 would be released from the wafer 711 ). As will be explained hereinafter, this facilitates simple and reliable isolation of the mirror 150 on wafer level. FIGs. 15 - 17 illustrate examples with respect to coupling the mirror front side 151 and the mirror back side 152 to obtain the mirror 150. The process step of FIG. 15 is the process step which follows the process step of FIG. 14. Specifically, in the process step of FIG. 15, the wafer 711 defining the mirror front side 151 is attached to the handling wafer 709. Then, the wafer 711 , at side 715, is brought into contact with the wafer 701. For this, the fins 158 of the frame structure 157 are aligned with respect to the center of the mirror front side 151.
As mentioned above, the wafer 701 and 711 may be coupled using an adhesive such as an epoxy adhesive or wafer bonding.
As will be appreciated from the above, the coupling, as illustrated by the process step of FIG. 15, may be implemented on wafer level. FIG. 16 illustrates the process step following the process step of FIG. 15. FIG. 16 illustrates aspects with respect to isolating the mirror 150. Specifically, FIG. 16 illustrates cutting edges 790. Cutting is often referred to as wafer slicing or dicing. Illustrated in FIG. 16 as a scenario where the cutting edges 790 cut the fins 158-1
- 158-4 at a position offset from the outer circumference 151 B of the mirror front side 151. Then, the mirror 150, as illustrated in FIG. 17, is isolated from the sur- rounding wafer material 781 of the wafers 701 , 711. As will be appreciated from the description above, because the extended fins 158-1 - 158-4 are cut at a posi- tion offset from the outer circumference 151 B, these fins 158-1 - 158-4 extend beyond the outer circumference 151 B once the mirror 150 has been isolated (cf. FIG. 4).
Generally, it is not required to implement all radial fins 158 as extended fins 158-1
- 158-4; rather, the count of extended fins 158-1 - 158-4 may be set so as to provide structural stability prior to isolation of the mirrors 150 from the wafers 701 , 711. For example, as illustrated in connection with the example of FIG. 4, it may be sufficient to use for extended fins 158-1 - 158-4 in direction north, south, east, and west of the mirror 150.
In the examples of FIGs. 11 - 17, the reflective layer 770 is deposited prior to coupling the mirror front side 151 and the mirror back side 152 (cf. FIGs. 12 and 15). Furthermore, the reflective layer 770 is deposited on the side 714 of the wafer 711 which is opposite to the side 715 which is grinded (cf. FIGs. 12 and 14). Other scenarios are conceivable, as explain in connection with the following FIGs. FIGs. 18 - 21 illustrate another example. FIG. 18 illustrates a process step which follows the process step of FIG. 12. FIG. 18 illustrates aspects with respect to cou- pling the wafer 701 defining the mirror back side 152 and the wafer 711 defining the mirror front side 151. In the example of FIG. 18, the mirror front side 151 and the mirror back side 152 are coupled prior to wafer thinning of the wafer 711 defin- ing the mirror front side 151. Furthermore, the mirror front side 151 and the mirror back side 152 are coupled prior to depositing the reflective layer 770.
Specifically, in the example of FIG. 18, direct wafer-wafer bonding may be used for said coupling.
FIG. 19 schematically illustrates the process step following the process step of FIG. 18. In FIG. 19, a handling wafer 709 is attached to the wafer 701 , opposite to the side coupled to the wafer 711. FIG. 20 illustrates the process step following the process step of FIG. 19. In FIG. 20, wafer thinning is applied to the wafer 711 , by grinding the side 715 of the wafer 711. As such, the process step of FIG. 20 generally corresponds to the process step of FIG. 14; albeit, in the scenario of FIG. 20, grinding of the side 715 of the wafer 711 is performed prior to depositing the reflective layer 770 and after cou- pling the mirror front side 151 with the mirror back side 152. FIG. 21 illustrates the process step following the process step of FIG. 20. In FIG. 21 , the reflective layer 770 is deposited on the side 715 of the wafers 711. To facilitate a plane reflective layer 770, grinding, in process step of FIG. 20, may be followed by polishing. As will be appreciated from FIG. 21 , the reflective layer 770 is deposited after coupling the mirror front side 151 with the mirror back side 152. Also, grinding is executed after coupling. The reflective layer 770 is deposited on the grinded side 715.
FIG. 22 illustrates the process step following the process step of FIG. 21. FIG. 22 illustrates aspects with respect to cutting 790 the wafer 701 , 711 , for isolating the mirror 150. As such, the process of FIG. 22 generally corresponds to the process of FIG. 16. To protect the reflective layer 770 in the process step of FIG. 22, a protective coating may be applied (not illustrated in FIG. 22). FIG. 23 illustrates the isolated mirror 150 obtained from cutting at the process step of FIG. 22.
FIGs. 24 - 38 illustrate yet another MEMS process that may be used to produce the mirror 150.
In FIG. 24, a glass wafer 708 is provided. Next, in the process step of FIG. 25, recesses 708A are etched into the glass wafer. This may be achieved using wet etching, e.g., using hydrofluoric acid. The particular shape of the recesses 708A is not decisive, because the recesses 708A are merely used for implementing a re- lease of material of the wafer 701 defining the mirror back side 152.
FIG. 26 illustrates the process step following the process step of FIG. 25. In FIG. 26, the wafer 701 has been attached to the glass wafer 708. This can be achieved by silicon-on-glass (SOG) anodic wafer bonding.
The wafer 701 is attached at the side of the glass wafer 708 into which the re- cesses 708A have been etched, to facilitate the release later on. FIG. 27 illustrates the process step following the process step of FIG. 26. The pro- cess step of FIG. 27 essentially correspond to the process step of FIG. 6. The trenches 785 are etched all the way to the recesses 708A. Thereby, the material 782 - defining the cavities 159 - is released, see FIG. 28. In FIGs. 27 and 28, again, for sake of simplicity a mask for etching the trenches 785 is not illustrated.
FIG. 29 illustrates producing the mirror front side 151 . Flere, the respective wafer 711 is attached to a corresponding glass wafer 707, using SOG anodic wafer bond- ing or another process. FIG. 30 illustrates the process step following the process step of FIG. 29. Flere, the side 715 of the wafer 711 is thinned, e.g., using grinding and/or polishing.
FIG. 31 illustrates a process step of coupling the wafer 711 defining the mirror front side 151 and the wafer 701 defining the mirror back side 152. The process step of FIG. 31 follows the process steps of FIG. 28 and FIG. 30. In FIG. 31 , the wafer 701 and the wafer 711 or coupled using direct wafer bonding. When coupling, the wafer 711 is attached to the glass wafer 707 and the wafer 701 is attached to the glass wafer 708, thereby facilitating handling.
FIG. 32 illustrates the coupled wafers 701 , 711. The wafer 711 is coupled via the thinned side 715. FIG. 33 illustrates a process step following the process step of FIG. 32. In FIG. 33, the glass wafer 707 is removed. This may be achieved using wet etching, e.g., using hydrofluoric acid. To avoid removal of the glass wafer 708, a protective mask may be applied.
FIG. 34 illustrates a process step following the process step of FIG. 33. In FIG. 34, trenches are etched to define the mirror front side 151. As such, the process step of FIG. 34 essentially correspond to the process step of FIG. 11. For example, DRIE etching may be used. In the example of FIG. 34 - different to the example of FIG. 11 - etching of the mirror front side 151 is implemented after coupling the wafer 111 and the wafer 101.
FIG. 35 illustrates a process step following the process step of FIG. 34. In the process step of FIG. 35, the reflective layer 770 is deposited on the side 714 of the wafer 711. Flence, the reflective layer 770 is deposited on the side 714 opposing the side 715 which has been thinned in the process step of FIG. 30 and via which the wafer 711 is coupled to the wafer 701. As will be appreciated, the reflective layer 770 is deposited on the side 714 after coupling of the mirror front side 151 and the mirror back side 152.
FIG. 36 illustrates a process step following the process step of FIG. 35. The wafer 711 , at the side 714 - now covered with the reflective layer 770 - is attached to a handling wafer 709.
FIG. 37 illustrates a process step following the process step of FIG. 36. Flere, the glass wafer 708 is removed, e.g., by wet etching. Further, cutting of the wafer 701 is implemented along the cutting lines 790. As such, the process step of FIG. 37 generally corresponds to the process step of FIG. 22 and the process step of FIG. 16.
Flowever, as will be appreciated from FIG. 37, left, it is not required to provide the extended fins 158-1 - 158-4. Specifically, the mirror back side 152 is at all times during the process of FIGs. 26 - 37 supported, either by the glass wafer 708 or by the handling wafer 709. Flence, there is no need for the extended fins 158-1 - 158- 4, or any other supporting structure. Therefore, the mask used for etching at FIG. 27 may differ from the mask used for etching at FIG. 16. FIG. 38 illustrates the mirror 150 obtained from the process step of FIG. 37 (in the scenario of FIG.’s 24 - 38, the cavities 159 of the frame structure 157 are illustrated in a simplified manner for sake of simplicity). Although the invention has been shown and described with respect to certain pre- ferred embodiments, equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present in- vention includes all such equivalents and modifications and is limited only by the scope of the appended claims.
For illustration, various examples have been described in which one or more ex- tended fins extend beyond the outer circumference of the mirror front side. How- ever, this is generally optional. In other examples, there may be no extended fins. For example, dedicated supporting structures may be provided to connect the mir- ror or parts of the mirror with the surrounding wafer material until isolating the mir- ror or the respective mirror parts from the surrounding wafer material by cutting the supporting structures. The supporting structures can be different from the ex- tended fins.
For further illustration, above, various examples have been described in which a mirror is manufactured from MEMS processing on Silicon. Respective structures may also be obtained using other kinds and types of processes. For example, a possible materials and associated processes may include ceramics, e.g., from electrophoresis, epoxy-composite materials, glass, or graphite, etc. Such materials can have an elasticity module that provides for significant rigidity / stiffness; thereby, dynamic deformation of the mirror - when actuate, or specifically when resonantly scanned - is sufficiently small. Thereby, larger scanning frequencies become possible. At the same time, such materials have a comparably low density, i.e., are lightweight. Thereby, higher resonance frequencies are supported.

Claims

Figure imgf000032_0001
1. A method of producing a mirror (150) for resonant scanning, comprising:
- producing a mirror front side (151 ),
- producing a mirror back side (152) comprising a frame structure (157) having fins (158) and cavities (159), and
- coupling the mirror front side (151 ) and the mirror back side (152) to ob- tain the mirror (150). 2. The method of claim 1 , wherein said producing the mirror back side (152) comprises:
- etching trenches (785) along a contour of the cavities (159) of the frame structure (157), said etching being from a first side (704) of a first wafer (701 ) de- fining the mirror front side (151 ), and
- implementing a release of wafer material from the cavities (159) of the frame structure (157) by means of the trenches (785).
3. The method of claim 2,
wherein the release is implemented using recesses in a glass wafer.
4. The method of claim 2,
wherein the release is implemented using wafer grinding from a second side (705) opposite to the first side (704). 5. The method of any one of the preceding claims,
wherein said coupling of the mirror front side (151 ) and the mirror back side (152) is implemented on wafer level.
6. The method of any one of the preceding claims,
wherein said coupling of the mirror front side (151 ) and the mirror back side (152) is implemented using direct wafer bonding of a first wafer defining the mirror front side (151 ) and a second wafer defining the mirror back side (152).
7. The method of claim 6,
wherein the first wafer is attached to a first glass wafer when coupling, wherein the second wafer is attached to a second glass wafer when cou- pling.
8. The method of any one of the preceding claims, further comprising:
- isolating the mirror (150) from surrounding wafer material by cutting at least one of the fins (158-1 , 158-2, 158-3, 158-4) of the frame structure (157) at a position offset from an outer circumference of the mirror front side (151 ).
9. The method of claim 8,
wherein, when isolating the mirror (150), the mirror front side (151 ) is not laterally coupled with a surrounding wafer material. 10. The method of any one of the preceding claims,
wherein the mirror front side (151 ) and the mirror back side (152) are cou- pled by aligning the fins (158) of the frame structure (157) with respect to a cen- ter of the mirror front side (151 ),
wherein, after said coupling, at least one fin (158-1 , 158-2, 158-3, 158-4) of the fins (158) of the frame structure (157) extends beyond an outer circumfer- ence of the mirror front side (151 ).
11. The method of any one of the preceding claims,
wherein the mirror front side (151 ) is coupled with the mirror back side (152) via a first side (714, 715) of a second wafer (711 ) defining the mirror front side (151 ),
wherein the method further comprises:
- depositing a reflective layer on a second side (714, 715) of the second wafer (711 ), the second side (714, 715) of the second wafer (711 ) being opposite to the first side (714, 715) of the second wafer (711 ),
wherein the reflective layer is deposited prior to or after said coupling of the mirror front side (151 ) and the mirror back side (152).
12. The method of claim 11 , further comprising:
- grinding the second side (714, 715) of the second wafer (711 ). 13. The method of claim 12,
wherein said grinding of the second side of the second wafer is executed after said coupling of the mirror front side (151 ) and the mirror back side (152).
14. The method of claim 13, further comprising:
- grinding the first side (714, 715) of the second wafer (711 ).
15. The method of any one of the preceding claims,
wherein the mirror front side (151 ) and the mirror back side (152) are cou- pled using at least one of an epoxy adhesive and a silicon-silicon wafer bonding.
16. The method of any one of the preceding claims, further comprising:
- producing an elastic mount, and
- coupling the mirror (150) and the elastic mount.
17. The method of claim 16,
wherein the elastic mount is coupled with the mirror (150) using one or more interrelated indentation features provided on respective contact surfaces of the elastic mount and the mirror back side (152).
18. A mirror (150), comprising:
- a mirror front side (151 ) comprising a reflective layer, and
- a mirror back side (152) comprising a frame structure (157) having fins (158) and cavities (159),
wherein the mirror front side (151 ) and the mirror back side (152) are not integrally formed.
19. A mirror (150), comprising:
- a mirror front side (151 ) comprising a reflective layer, and
- a mirror back side (152) comprising a frame structure (157) having fins
(158) and cavities,
wherein at least one fin (158-1 , 158-2, 158-3, 158-4) of the fins (158) ex- tends beyond an outer circumference (151 B) of the mirror front side (151 ). 20. The mirror (150) of claims 18 or 19,
wherein a diameter of the reflective layer of the mirror (150) is not smaller than 4 mm, optionally not smaller than 6 mm, further optionally not smaller than 8 mm. 21. The mirror (150) of any one of claims 18 - 20,
wherein a filling factor of the frame structure (157) is not larger than 20 %, optionally not larger than 5 %, further optionally not larger than 2 %.
22. The mirror (150) of any one of claims 18 - 21 ,
wherein an area of a reflective surface of the mirror (150) is in the range of 100 mm2 to 300 mm2.
23. The mirror (150) of any one of claims 18 - 22,
wherein the mirror (150) is made from at least one of: ceramics; epoxy; epoxy composite; glass; graphite; silicon; and/or a semiconductor material.
24. The mirror (150) of any one of claims 18 - 23,
wherein the mirror (150) is produced by a method of any one of claims 1 - 17.
25. A scanner (100), comprising:
- the mirror (150) of any one of claims 18 - 24,
- an elastic mount (119) coupled with the mirror (150), and
- an actuator (172),
wherein the actuator (172) is configured to resonantly scan the mirror
(150).
PCT/EP2018/083288 2017-12-11 2018-12-03 Two-part mirror WO2019115263A1 (en)

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