GB2513170A

GB2513170A - A detection system and a method of making a detection system

Info

Publication number: GB2513170A
Application number: GB1307052.9A
Authority: GB
Inventors: Marijan Macek; Janez Trontelj; Aleksander Sesek
Original assignee: Univerza v Ljubljani
Current assignee: Univerza v Ljubljani
Priority date: 2013-04-18
Filing date: 2013-04-18
Publication date: 2014-10-22
Anticipated expiration: 2033-04-18
Also published as: GB201307052D0; GB2513170B

Abstract

A bolometric detection system for detecting electromagnetic radiation with frequencies from 300GHz to 1THz is disclosed with a narrow or wide band antenna 6 directly connected to a suspended metal film thermistor 5 fabricated on a thin (3 - 5Î¼m) self-supporting dielectric membrane 3. The thermistor 5 is suspended above the membrane 3, which is fabricated with a planarised process to minimise the thermistor dimensions scattering. The detection system includes a reflecting cavity 9 which is a Î»/4 THz mirror. The depth dR of the THz mirror 9 can be controlled solely by the thickness of the silicon wafer 7. The thickness can alternatively be controlled by providing a separate silicon wafer 7 with a metallised groove 9 that is bonded by a low temperature bonding process to the detecting silicon wafer 1. The system is optimised for a thermistor resistance of about 800Î©, which is slightly more than twice the impedance of free space (Z0 = 377Î©).

Description

A DETECTION SYSTEM AND A METHOD OF MAKING A DETECTION SYSTEM

The present invention relates to a detection system and a method of making the same, the detection system for detecting electromagnetic radiation having frequencies in the range of about 300GHz to 1THz, particularly at the lower end of this range.

Detection of electromagnetic waves having wavelengths within the range from 100pm to 1mm (i.e. between far infrared (FIR) waves and microwaves) is a great challenge. Detection is done typically via some kind of small antenna resistively or capacitively coupled to a sensing detector. This thermally sensitive detector has a low thermal conductance and is typically fabricated as a micro-bridge structure by known micromachining techniques. In this way a single detector or detector array can be fabricated.

The response of a thermistor or bolometer detector is proportional to its resistivity.

Therefore the resistivity of the thermistor or bolometer should be as high as possible.

However the resistivity of the detector should be roughly equal to the impedance of free space (Z0 3770). Therefore it is desirable for the detecting system to be designed such that the antenna of the system and the efficient, high resistive, thermistor are coupled efficiently.

Typically the antenna is placed over the dielectric layer on top of a silicon substrate. In some systems, the resistively coupled detector is limited to low ohmic values of about 1000, resulting in more pure properties being detected. A better response however is obtained by a capacitively coupled detector.

In order to address the difficulties with detecting electromagnetic radiation, complex systems have been designed in which double SOI substrates, sometimes called also DSOI substrates (comprising bulk silicon/silica/thin silicon (of a few micrometers)/silica/thin silicon layer) are used to reduce interference between the antenna, the underlying silicon and the resonant cavity. Alternative solutions are directed to collection of electromagnetic waves by absorption. However all the above systems are directed to detection of electromagnetic radiation in the higher frequencies in the vicinity of FIR radiation, with wavelengths of about 100pm.

There remains a need for an improved detection system for detecting electromagnetic radiation having frequencies ranging from 300GHz to ITHz (300pm -1mm).

In accordance with the present invention, from a first broad aspect, there is provided a detection system for detecting electromagnetic radiation having a frequency in the range of about 300GHz to ITHz, the detection system comprising a thermistor, an antenna, a dielectric layer, and a cavity substrate, wherein the antenna is directly connected to the thermistor, the thermistor is suspended above at least a portion of the dielectric layer such that there is a spacing between the thermistor and the dielectric layer, and the cavity substrate comprises a reflecting cavity, the reflecting cavity having a cavity depth (dR) of about one quarter of the wavelength of the electromagnetic radiation to be detected (A14).

Thus there is provided a detection system in which an antenna is in direct, physical contact with a thermistor that is suspended in a micro-bridge structure to provide a detection system with improved sensitivity for detecting electromagnetic radiation having frequencies in the range of about 300GHz to 1THz, particularly those around the 300GHz end of this range. Furthermore as the antenna and thermistor are directly connected, the antenna can be used to bias the thermistor and also can provide an output signal connection for the detection system, thus providing a sensitive yet compact system with a reduced number of components.

The gap or spacing between the suspended thermistor and the dielectric layer is required because the response of the thermistor is inversely proportional to the thermal conductance of the system. It may be that more than 50% of the heat conductivity is directed into the dielectric layer and substrate under the thermistor. The spacing therefore should be suitable sized and may be at least about 5pm.

The detection system comprises a cavity substrate in which a reflecting cavity is provided for reflecting any electromagnetic radiation that passes the antenna and reaches the cavity back to the antenna. Thus the reflecting cavity is configured to have a depth (dR) corresponding to about one quarter of the wavelength of the electromagnetic radiation to be detected (A14). The cavity substrate comprises a suitable material for forming such a cavity and may comprise silicon. The silicon may be in the form of a <100> orientated monocrystalline silicon wafer. To form the cavity in such a wafer, potassium hydroxide etching can be used to provide an accurately dimensioned and relatively planar reflecting cavity. For measuring electromagnetic radiation having a wavelength in the range of about 300pm to 1mm (i.e. a frequency range of about 3000Hz to ITHz), the reflecting cavity has a depth (dR) in the range of about 75 to 250pm.

To enhance the efficiency of the reflecting cavity, the reflecting cavity may further comprise a reflective layer substantially covering the inner surface of the reflecting cavity forming a THz mirror, the reflective layer comprising a reflecting metal film layer. Thus the electromagnetic radiation that passes the antenna is reflected back to the antenna by the 1Hz mirror thus further improving the antenna gain.

The detection system may be formed by depositing one or more layers and components on a suitable surface, which may comprise a support substrate. The support substrate supports the dielectric layer. The support substrate may comprise silicon and may be a clOO> orientated monocrystalline silicon wafer. The support substrate co-operates with the cavity substrate to define the detection system and may be bonded to the cavity substrate by at least one of low temperature polymer bonding, low temperature soldering, and conductive bumps electrically interconnecting the two wafers.

The detection system comprises a dielectric layer, which may be a single layer of a particular material or may comprise a plurality of layers of one or more materials. The dielectric layer may comprise a first layer comprising a low stress silicon oxynitride film membrane. A second layer of the dielectric layer may comprise a silicon nitride layer. A third layer may comprise a silicon dioxide layer. A fourth layer may be provided intermediate the first and second layers. The fourth layer may comprise a phosphor-silicate glass layer. Thus a thin dielectric membrane of a low stress material is provided to minimise buckling or breaking of the released membrane. The intermediate (fourth) layer minimises interlayer stress between the first and second layers. The layers may be formed by any suitable process. The low stress dielectric membrane may be formed by plasma enhanced chemical vapour deposition (PECVD). This layer may have a thickness of about I to 10pm, or may be about 3 to 5pm thick. The silicon nitride layer may be formed by low pressure chemical vapour deposition (LPCVD). This layer may have a thickness of about 100 to lSOnm. The silicon dioxide layer may be formed by thermally growing the layer on, for example, the support substrate. This layer may have a thickness of about 3Onm. The phosphor-silicate glass layer may be formed separately and laid over the silicon nitride layer or may be formed thereon by a suitable deposition process. The lateral size of the finished membrane 5 depends on the wavelength of radiation and is roughly A/2 of the incoming THz radiation 20, i.e. in the order of about 1mm for a single pixel. The combination of phosphor-silicate glass and low stress SiON prevents buckling or breaking of the released membrane 3. The detection system comprises an antenna, which is formed of a material suitable for receiving electromagnetic radiation having a frequency in the range of about 300GHz to ITHz.

The antenna may comprise at least one layer of aluminium. The antenna may comprise multiple layers of aluminium having intermediate dielectric layers between each pair of aluminium layers, with the aluminium in one layer being connected with the aluminium in a second layer only through vias or through holes in the dielectric layer.

The detection system comprises a thermistor, which may comprise a metal film thermistor and the film may be a thin film. The thermistor may comprise a titanium or a bismuth thin film. The film may be configured to have a sheet resistivity of about 300/sq.

Thus the film has a relatively low thermal conductivity and is suited for use in the detection system. The thermistor may have a high temperature coefficient of resistance to obtain a high response and may comprise a protective oxide to passivate the thermistor.

The thermistor should have a resistance that is roughly about the same as the impedance of free space. The thermistor may have a resistance of about 7500. With the sheet resistivity being about 300Isq, the ratio of the length (L) to the width (W) of the thermistor (Nsq = L/W) may be about 25. The thermistor may have a length of about 12pm and a width of about 0.5pm. A thermistor of about this size is advantageous because the heat conduction to the substrate is proportional to the area of the thermistor so larger thermistors are not desirable and photolithography limitations make small thermistors difficult to fabricate. Furthermore the 1/f noise that is not usually attributable to larger metal thin film resistors may become problematic if the thermistor were to be smaller.

The detection system may require connection to an external source or system and external connectors may therefore be provided. At least one electrical connector may be provided as part of the detection system. Therefore the detection system may further comprise at least one electrical connector, the electrical connector directly connecting the antenna to the thermistor. The connector may be a separately provided component or the electrical connector may be integrally formed with the antenna. The antenna and the electrical connector may comprise at least one aluminium layer.

Providing an integral antenna and connector (or connectors) is advantageous as it simplifies the design and fabrication of the detection system and is a reliable connection and further enables the antenna to be used as an electrical connection and also to bias the thermistor to which it is directly attached.

The above detection system is a sensitive detector for electromagnetic waves having a frequency in the range of about 300GHz to 11Hz. Furthermore the design of the detection system is such that it can be reliably fabricated with a high level of accuracy and planarity. Thus In accordance with the present invention, from a further broad aspect, there is provided a method of providing a detecting portion of a detection system for detecting electromagnetic radiation having frequencies in the range of about 3000Hz to 11Hz, the method comprising the steps of removing a portion of a supporting substrate, thereby providing a groove in a first surface of the supporting substrate, providing a first layer over the supporting substrate and the groove substantially covering the exposed surfaces thereof, the first layer comprising dielectric material, filling the covered groove with a removable material such that the removable material substantially fills the groove and comprises a flanged portion extending beyond the rim of the groove, providing a thermistor on the removable material such that the thermistor is spaced apart from the dielectric material by the removable material and is centred substantially above the groove, the thermistor having planar dimensions less than the planar dimensions of the surface of the removable material on to which thermistor is provided, providing a metal layer over the removable material and the thermistor, the metal layer having at least one planar dimension greater than the planar dimension of the surface of the removable material on to which thermistor is provided thereby extending beyond the outer periphery of the removable material, removing the removable material, thereby forming a bridge structure comprising the thermistor suspended above the dielectric material from the metal layer, and exposing the first layer by removing a portion of the supporting substrate from a surface opposite the first surface of the supporting substrate.

The method advantageously enables a thermistor micro-bridge structure to be fabricated over a dielectric layer on a supporting substrate, the thermistor in direct contact only with the antenna and being spaced from the dielectric layer. The method of fabrication is simpler and more reliable than traditional micromachining techniques yet provides a system with a high degree of planarity and accurate dimensions.

The supporting substrate is formed from material suitable for use in detecting electromagnetic radiation in the range of about 300GHz to ITHz. The supporting substrate may comprise a <100> orientated monocrystalline silicon wafer and the step of removing a portion of the supporting substrate to provide the groove comprises covering the surface of the silicon wafer with a mask, which may be provided by thermally growing silicon dioxide on the surface and depositing a silicon nitride layer over the silicon dioxide. The step further comprises etching a central portion of the mask to provide an unmasked central portion, which may comprise plasma etching. The step further comprises etching the unmasked, central portion of the substrate to provide a groove having a depth (d0) of at least about 5pm, which may comprise anisotropic etching with a potassium hydroxide etchant.

The method may further comprise the step of covering the unmasked, etched surface with a protective layer, preferably comprising the step of thermally growing silicon dioxide on the unmasked, etched surface and depositing a silicon nitride layer over the silicon dioxide.

The method may further comprise the step of covering the protective layer and the mask with a stress reduction layer. This layer may comprise phosphor silicate glass.

The method may further comprise the step of planarising the covered surface of the groove prior to the step of providing a first layer over the supporting substrate.

The step of providing a first layer over the supporting substrate and the groove substantially covering the exposed surfaces thereof, may comprise depositing a silicon oxynitride membrane over the supporting substrate and the groove, the membrane preferably having a thickness of about 3 to 5pm.

The step of filling the covered groove with a removable material may comprises filling the covered groove with at least one of a negative tone photoresist, and a positive tone photoresist such that the groove is completely filled and the material further extends beyond the rim of the groove, and curing the removable material by at least one of flooding the material with UV radiation, and sequentially baking the material at a temperature of up to about 160t.

This method of fabrication is advantageous because photolithography of sub-micrometre scale for a thermistor layer having the required the topography is extremely difficult. Thus the present method advantageously provides an improved method for fabricating a detection system having an etched cavity in the substrate silicon that is sufficiently planarised. Using planarisation techniques with negative and positive tone photoresists, the whole structure is almost planar before the deposition of the thermistor thin film layer. The method provides a surface on which the thermistor can be formed and by which it will initially be supported until the supporting structures of the finished system are put in place, after which the removable material can be removed to leave the gap or spacing underneath the suspended thermistor.

The step of providing the thermistor may comprise deposition of a thin metal film on the removable material. The metal may be titanium or may be bismuth. The thin metal film may have a sheet resistivity of about SOQIsq.

The step of providing a metal layer over the removable material and the thermistor may comprise deposition of at least one metal film thereon. The film may comprise aluminium. This advantageously enables both the antenna and any desired electrical connections to be integrally formed over the thermistor in a single step.

Once the metal layer is provided and connected to the thermistor, the removable material can be removed and the thermistor will be suspended by the metal layer. The step of removing the removable material may comprise performing oxygen plasma stripping to release the photoresist, and removing the material from the cavity, thus providing a gap between the thermistor and the dielectric membrane.

The above method steps provide a detecting portion of a detection system but it may be necessary to remove a portion of the substrate on a side opposite to that in which the groove was formed, in order to expose the thermistor and antenna to any incident radiation. The step of exposing the first layer may comprise etching the substrate at the surface opposite the first surface of the supporting substrate by anisotropic etching in potassium hydroxide or deep reactive ion etching.

Thus a detecting portion for detecting electromagnetic radiation is provided.

However the radiation may not all be detected as it passes the thermistor and antenna.

Therefore a reflecting cavity portion may be provided to the detection system to reflect the transmitted radiation back to the antenna. Therefore from a further broad aspect, the present invention provides a method of providing a reflecting cavity portion of a detection system for detecting electromagnetic radiation in the range of about 300GHz to ITHz, the method comprising the steps of removing a portion of a cavity substrate, thereby providing a reflecting cavity in a first surface of the cavity substrate, the reflecting cavity having a depth (dR) of about one quarter of the wavelength of the electromagnetic radiation to be detected (A14), and providing a reflective layer in the reflecting cavity so as to substantially cover the exposed surface of the cavity to form a THz mirror. To provide the complete detection system, from a further broad aspect of the present invention there is provided a method of providing a detection system for detecting electromagnetic radiation in the range of about 300GHz to ITHz, the method comprising the step of bonding the above-described detecting portion to the above described reflecting cavity portion such that the thermistor is substantially centrally aligned with the reflecting cavity, wherein the step of exposing the first layer of the supporting substrate by removing a portion thereof from a surface opposite the first surface of the supporting substrate is performed after bonding of the portions together. This is advantageous when fabricating an array of such detection systems because the top wafer having the reflecting cavities acts as a protection for the thermistor micro -bridge structures and as a support for the fragile sensor wafers during subsequent back-side etching and chipping.

The bonding step may comprise low temperature polymer bonding. The low temperature polymer may comprise a negative tone polymer, which may be SU8 negative tone polymer. The bonding step may further comprise applying a force to the portions to push them together with the polymer therebetween and curing the polymer.

The curing may be achieved with a temperature between about 150 and 200C.

The bonding step may comprise low temperature soldering.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a detection system in accordance with the present invention; Figure 2 schematically illustrates fabrication of a groove in a supporting substrate of a detection system in accordance with the present invention; Figure 3 schematically illustrates fabrication of a dielectric layer in the groove and on the supporting substrate of figure 2; Figure 4 schematically illustrates filling of the groove of figure 3 with a removable material and a thermistor provided on the surface of the material; Figure 5 schematically illustrates an integrally formed antenna and connectors supporting the thermistor of figure 4, after the removable material has been removed from the groove; and Figure 6 schematically illustrates a reflecting cavity substrate bonded to the supporting substrate of figure 5.

Figure 1 schematically illustrates a detection system in accordance with the present invention. The detection system is a THz detector, either in a form of a single pixel or an array. The system comprises a mono-crystalline silicon wafer I with orientation <100>, which is the sensor wafer 1. The main components of the detector are a membrane 3, an antenna (which is integrally formed with electrical connectors 6 -the antenna is not shown in the figures, but it is completely centred on the membrane 3 and covered by a reflecting cavity 9), a sensing micro-bridge structure made of thin metal film thermistor 5 and the reflecting cavity 9, which is a THz mirror, in an upper silicon wafer 7. A gap 4 is provided between the thermistor 5 and the membrane 3. Masking layers 2 are present between the membrane 3 and the silicon wafer 1, which are exposed to the incident THz radiation 20 where the silicon wafer I has been etched away, as discussed in detail below.

The membrane 3 comprises a dielectric material with thickness of few pm (about 3 to 5pm) and is a low stress PECVD silicon oxynitride (SiON) film. This film 3 is deposited on the top of a LPCVD silicon nitride (SIN) masking layer 2, with an intermediate layer of phosphor-silicate glass (PSG, not shown) between the membrane 3 and the layer 2, to reduce any possible interlayer stress.

The antenna may be wide band or narrow band, depending on the system application. The antenna absorbs the incoming THz radiation 20 and is made from the same metal 6 as used for electrical connection of the thermistor 5, which is aluminium.

The thermistor 5 is arranged as a sensing micro-bridge structure and is a thin metal film thermistor. The thermistor material should have a relatively high sheet resistivity (therefore low thermal conductivity) and a high temperature coefficient of resistance TC to obtain a high response. A protective oxide (not shown) is also provided to passivate the thermistorS. The thermistorS can be titanium Ti or bismuth Bi. The system 100 is targeted to resistor values of round 7500. Assuming a typical sheet resistivity of the thermistor 5 of about SOOIsq, the ratio of the length L (see figure 4) and the width W (see figure 8) of the thermistor 5, i.e. Nsq=UW, should be about 25. Therefore the length L of the thermistor S is about 12pm and the width W is about SOOnm. Larger thermistors are not recommended since the heat conduction to the substrate is proportional to the area of the thermistor. Smaller thermistors are not practical, due to the photolithography limitations and at such small dimensions the I/f noise may be problematic (this noise not usually being attributed to metal thin film thermistors with larger dimensions). -10-

Since the response of the thermistor 5 is inversely proportional to the thermal conductance, the micro-bridge structure is suspended from the under laying surface (membrane 3) with a gap 4 that is greater than 5pm. A sacrificial photoresist layer 30 (see figure 4) is used to temporarily suspend the thermistor 5 during the course of fabrication as discussed in detail below. It should be noted that photolithography in the sub-micrometre scale of a thermistor layer with the required topography including a height of 5pm is almost impossible. To overcame the problems with the field of depth of the optical aligners, a planarised process for forming a plurality of pixels with cavities 4 etched in to the substrate silicon I has thus been devised. Using planarisation techniques with negative and positive tone photoresists, the whole structure is almost planar before the deposition of the thermistor thin film layer S occurs.

The supporting legs 6 for supporting the thermistor 5 are made of metal as discussed above and also provide the function of electrical wiring connectors for the integrally formed antenna and thus connect the antenna to the thermistor 5. The same layer 6 also provides an input and output function of the sensor array. Whilst the illustrated arrangement uses a single metal for wiring, there is no limitation against using a double metal for wiring, with an intermediate dielectric layer, a via mask and a passivation film as is known in CMOS processing.

The reflecting cavity or THz mirror 9 is made on the separate silicon wafer as discussed in detail below. The 1Hz mirror 9 in the <100> orientated silicon wafer 7 is made by anisotropically etching with potassium hydroxide (KOH), using a LPCVD fabricated SiN mask 2. The depth dR of the cavity of the THz mirror 9 typically equals A14 of the targeted electromagnetic (EM) radiation 20 to be detected. The surface of the THz mirror 9 is covered with a reflecting metal film 10, which reflects all the EM radiation 20 that initially passes the antenna back to the antenna, thus increasing the gain.

The silicon wafer? with the THz mirror 9 is connected to the bottom sensor silicon wafer I by a low temperature wafer bonding process. One suitable process is polymer bonding using an SU8 polymer intermediate layer 8. This is a low temperature bonding process that is suitable for the described system. The top silicon wafer 7 acts not only as reflector of the EM radiation 20, but at the same time it also supports the fragile chip which is composed mainly of the thin membrane 3 surrounded by thick silicon rims. The top wafer 7 also protects the suspended micro-bridge thermistors 5 during sawing of a multi or single pixel system wafer into multiple chips and during bonding. However, by using appropriate conductive bumps, i.e. indium bumps, the connectors and the electronics as well can be fabricated on the top wafer 7 along with the THz mirrors 9.

Furthermore, an appropriate low temperature solder process should enable even vacuum tight encapsulation, improving the sensitivity of the thermistor 5 two or three-fold.

However in this case, the membrane 3 has to be thicker, to resist mechanical stress due to the pressure difference between the vacuum in the THz mirror 9 and the surrounding atmosphere.

A 1Hz detection system 100 realised by the above-mentioned invention is of high quality as was shown by simulations. Radiation efficiency of a dipole centred for 300GHz applications is between 86 and 90% in the frequency range 270 to 330GHz.

The designed antenna has a radiation pattern with good directivity (11.3dBi), taking into account also efficiency and losses due to the antenna connections). The antenna and the resistively coupled thermistor are in resonance at 2940Hz while the real value of the impedance is about 8000, which is very suitable for the proposed thermistor application.

A process of fabricating the system of figure 1 will now be described. The process is performed on each of the two silicon wafer substrates, namely sensor wafer 1, which has the sensor array(s) and cavity wafer 7, which has the THz mirror or mirrors 9.

The cavity wafer 7 can, if desired, have the necessary wiring and read out electronics for the arrays fabricated on it, but that will not be described in this arrangement.

The process for the sensor wafer 1 starts with formation of the recessed gap 4 on the front side of the sensor wafer 1. The starting material is <100> silicon as discussed above. Since the whole structure is supported by the dielectric membrane 3, the resistivity of the material is less important. The only role of the silicon wafer I is to support the thin membrane 3. A combination of a LPCVD deposited SiN layer (having a thickness of about 100-l5Onm, over a thin, thermally grown Si02 layer (having a thickness of about 3Onm)) is used as an etching mask 2a as shown in figure 2. After masking and plasma etching of the SiN layer, anisotropic etching using KOH is performed. The depth dG of the etched groove 4 is 5pm or greater, since more than 50% of the heat conductivity is directed into the substrate underneath the bridge 5. After the etching is finished, a thin layer of SiN/Si02 2b is deposited over the etched silicon wafer 1, in the same way as already mentioned for the mask 2a.

Next the thin (3 -5pm thick) dielectric membrane 3 is fabricated as shown in figure 3. The membrane 5 is deposited by PECVD onto the SiN (although the SiN is -12-covered beforehand with a thin layer of PSG glass (not shown) to eliminate interlayer stress). The lateral size of the finished membrane 5 depends however on the wavelength and is roughly A/2 of the incoming THz radiation 20, i.e. in the order of about 1 mm for a single pixel. Therefore the low stress SiON in combination with underlaying phosphor-silicate glass is used as a material to prevent buckling or breaking of the released membrane 3.

Before deposition of the thermistor thin film 5, thorough planarisation of the recessed groove 4 is required. The quality of the pattern transfer during lithography is limited by photoresist thickness, in the case of contact printing, or by the depth of field, in the case of projection printing. Planarisation techniques in combination with negative and positive tone photoresists are used to ensure the flatness of the structure as shown in figure 4. The polymer layer also has to be highly cross-linked. The negative resist is over-flooded by UV radiation and the positive resist is sequentially baked up to 160°C.

Higher temperatures are not recommended since excessive temperature curing of a photosensitive polymer can prevent subsequent stripping with oxygen plasma.

As shown in figure 5, the next step is the deposition of metal layer 6, and fabrication of the receiving antenna, support legs for the thermistor and of the interconnections to the bonding pads to provide input and output communication with the finished system.

Metallisation can be provided as a single layer (preferably aluminium since there is no need for passivation) or can be multilayer (typically double layer) combining an appropriate via mask, an intermediate dielectric and passivation, all steps known from CMOS processing. Where double layered metallisation is used, the first metal layer, the interlayer dielectric and the passivation are deposited prior to planarisation of the recessed gap 4. The second metal layer only connects the thermistor 5 with the first metal through the via openings.

After the finished metal masking the micro-bridge thermistorS is suspended over the groove or cavity 4 by removing the photoresist 30 by oxygen plasma release (as shown in figure 5). As the thermistor 5 is smaller than the photoresist 30, and the photoresist 30 overlapped the rim of the cavity 4 (providing a flanged portion 32), when the photoresist 30, 32 is released, the thermistor 5 is suspended only by the metal 6 and does not physically contact the dielectric membrane 3. The only material directly connected to the thermistor 5 is the metal 6, which forms the supporting/connecting -13-portions and the antenna.

The bottom, sensing wafer 1 of figure 5 is thus ready for bonding with the cavity wafer 7 as shown in figure 6. However the 1Hz mirror 9 is first formed in the cavity wafer 7. The cavity wafer 7 is formed from a separate silicon wafer. Using anisotropic etching with a SiN layer as an etch mask, a cavity is etched into the wafer 7 up to the desired depth dR, which is typically about A14 of the incoming EM radiation 20. The silicon surface of the cavity is metallised with a metal layer 10 to form the THz mirror 9, which reflects the radiation 20 that initially passes the antenna back to the antenna, thus improve the antenna gain.

The wafers 1, 7 are then bonded together as shown in figure 6. Wafer bonding in the simplest form is performed using low temperature polymer bonding with, for instance, an SU8 negative tone polymer as an adhesive 8. The exact temperature and force for bonding depends on the geometry, but typically the thickness of the polymer layer has to be at least few times thicker than the non-planarity of the bottom wafer 1. Curing temperatures for the SU8 polymer are somewhere between about 150 and 20000. The top wafer 7 with THz mirrors 9 in a multipixel array acts also as a protection for the thermistor micro-bridge 5 and as support for the fragile sensor wafer I during subsequent back-side etching and chipping (described below).

As previously mentioned, bonding of two wafers 1, 7 can be done also by low temperature soldering which provides vacuum tight encapsulation with improved thermistor properties or conductive bumps can electrically interconnect the sensor wafer 1 with the cavity wafer 7 with necessary read out electronics.

After the wafers 1, 7 are bonded together, the silicon from the pixel element of the sensor wafer I has to be etched away so that the EM radiation 20 can reach the sensor system. Back side anisotropic etching using a SiN mask (as discussed previously) is usually done using KOH to etch the silicon. However for pixels having smaller dimensions of separation it is recommended to switch to the so called deep reactive-ion etching (DRIE) technique to reduce the periodicity of the pixels. Returning to figure 1, a schematic cross-section of the finished pixel of the sensor array is shown after the backside etching (and any separation into individual pixels).

With a system according to the present invention any problems with the underlying silicon are eliminated, since the whole structure (including the antenna and the resistively coupled thermistor) is realised over the thin dielectric layer composed of -14-materials with relatively low (compared to silicon) index of refraction, like SiON and P30 glass on the top of the cavity wafer, which are optimised for particular wavelengths. -15-

Claims

CLAIMS1. A detection system for detecting electromagnetic radiation having frequencies in the range of about 3000Hz to ITHz, the detection system comprising: a thermistor, -an antenna, a dielectric layer, and a cavity substrate, wherein: the antenna is directly connected to the thermistor, the thermistor is suspended above at least a portion of the dielectric layer such that there is a gap between the thermistor and the dielectric layer, and the cavity substrate comprises a reflecting cavity, the reflecting cavity having a cavity depth (dp) of about one quarter of the wavelength of the electromagnetic radiation to be detected (A14).
2. A detection system as claimed in claim 1, wherein the gap between the thermistor and the dielectric layer is at least about 5pm.
3. A detection system as claimed in claim 1 or 2, wherein the cavity substrate comprises silicon, preferably a <100> orientated monocrystalline silicon wafer, and the cavity comprises a potassium hydroxide etched cavity having a depth dR in the range of about 75 to 250pm.
4. A detection system as claimed in any one of claims 1, 2 or 3, wherein the reflecting cavity of the cavity substrate further comprises a reflective layer substantially covering the inner surface of the reflecting cavity, the reflective layer comprising a reflecting metal film layer.
5. A detection system as claimed in any one of the preceding claims, wherein the dielectric layer comprises a first layer comprising a low stress silicon oxynitride film membrane, a second layer comprising a silicon nitride layer, a third layer comprising a silicon dioxide layer, and a fourth layer intermediate the first and -16-second layers, the fourth layer comprising a phosphor-silicate glass layer.
6. A detection system as claimed in claim 5, wherein the membrane has a thickness of about Ito 10pm, preferably about 3 to 5pm.
7. A detection system as claimed in claim 5 or 6, wherein the dielectric layer has a lateral size of about one half of the wavelength of the electromagnetic radiation to be detected (A/2).
8. A detection system as claimed in any one of the preceding claims, wherein the antenna comprises at least one layer of aluminium.
9. A detection system as claimed in any one of the preceding claims, wherein the thermistor comprises a thin metal film thermistor, preferably comprising a titanium or a bismuth thin film, the film preferably having a sheet resistivity of about 300/sq.
l0.A detection system as claimed in any one of the preceding claims, wherein the thermistor has a resistance of about 7500.
l1.A detection system as claimed in any one of the preceding claims, wherein the thermistor is configured such that the ratio of the thermistor width to the thermistor length is about 25.
12.A detection system as claimed in any one of the preceding claims, wherein the thermistor has a length of about 12pm and a width of about 0.Spm.
13.A detection system as claimed in any one of the preceding claims, further comprising a support substrate for supporting the dielectric layer, the support substrate preferably comprising silicon, preferably a <100> orientated rnonocrystalline silicon wafer.
14. A detection system as claimed in claim 13, wherein the support substrate is -17-bonded to the cavity substrate by at least one of low temperature polymer bonding, low temperature soldering, or conductive bumps electrically interconnecting the two wafers.
15. A detection system as claimed in any preceding claim, further comprising at least one electrical connector, the electrical connector directly connecting the antenna to the thermistor.
16.A detection system as claimed in claim 15, wherein the electrical connector is integrally formed with the antenna, preferably the antenna and the electrical connector comprising at least one aluminium layer.
17.A method of providing a detecting portion of a detection system for detecting electromagnetic radiation having frequencies in the range of about 300GHz to 11Hz, the method comprising the steps of: removing a portion of a supporting substrate, thereby providing a groove in a first surface of the supporting substrate, providing a first layer over the supporting substrate and the groove substantially covering the exposed surfaces thereof, the first layer comprising dielectric material, filling the covered groove with a removable material such that the removable material substantially fills the groove and comprises a flanged portion extending beyond the rim of the groove, providing a thermistor on the removable material such that the thermistor is spaced apart from the dielectric material by the removable material and is centred substantially above the groove, the thermistor having planar dimensions less than the planar dimensions of the surface of the removable material on to which thermistor is provided, providing a metal layer over the removable material and the thermistor, the metal layer having dimensions in at least one direction greater than the dimensions of the surface of the removable material on to which thermistor is provided thereby extending beyond the outer periphery of the removable material, -18-removing the removable material, thereby forming a bridge structure comprising the thermistor suspended above the dielectric material from the metal layer, and exposing the first layer by removing a portion of the supporting substrate from a surface opposite the first surface of the supporting substrate.
18.A method of providing a detecting portion of a detection system as claimed in claim 17, wherein supporting substrate comprises a <100> orientated nionocrystalline silicon wafer and the step of removing a portion of the supporting substrate to provide the groove comprises: covering the surface of the silicon wafer with a mask, the mask preferably provided by thermally growing silicon dioxide on the surface and depositing a silicon nitride layer over the silicon dioxide, etching a central portion of the mask to provide an unmasked central portion, the etching step preferably comprising plasma etching, and etching the unmasked, central portion of the substrate to provide a groove having a depth d0 of at least about 5pm, the etGhing step preferably comprising anisotropic etching with a potassium hydroxide etchant.
19.A method of providing a detecting portion of a detection system as claimed in claim 18, further comprising the step of covering the unmasked, etched surface with a protective layer, preferably comprising the step of thermally growing silicon dioxide on the surface and depositing a silicon nitride layer over the silicon dioxide.
20.A method of providing a detecting portion of a detection system as claimed in claim 19, further comprising the step of covering the protective layer and the mask with a stress reduction layer, preferably comprising phosphor silicate glass.
21.A method of providing a detecting portion of a detection system as claimed in claim 20, further comprising the step of planarising the covered surface of the groove prior to the step of providing a first layer over the supporting substrate.
22. A method of providing a detecting portion of a detection system as claimed in any -19-one of claims 17 to 21, wherein the step of providing a first layer over the supporting substrate and the groove substantially covering the exposed surfaces thereof, comprises depositing a silicon oxynitride membrane over the supporting substrate and the groove, the membrane preferably having a thickness of about 3 to 5pm.
23. A method of providing a detecting portion of a detection system as claimed in any one of claims 17 to 22, wherein the step of filling the covered groove with a removable material comprises: filling the covered groove with at least one of a negative tone photoresist, and a positive tone photoresist such that the groove is completely filled and the material further extends beyond the rim of the groove, and curing the removable material by at least one of flooding the material with UV radiation, and sequentially baking the material at a temperature of up to about 160CC.
24. A method of providing a detecting portion of a detection system as claimed in any one of claims 17 to 23, wherein the step of providing a thermistor on the removable material comprises deposition of a thin metal film, preferably comprising titanium or bismuth, on the removable material, preferably such that the film preferably has a sheet resistivity of about 300/sq.
25. A method of providing a detecting portion of a detection system as claimed in any one of claims 17 to 24, wherein the step of providing a metal layer over the removable material and the thermistor comprises deposition of at least one metal film, preferably comprising aluminium, to provide an antenna and electrical connectors for the system.
26.A method of providing a detecting portion of a detection system as claimed in claim 25, wherein the step of removing the removable material comprises: performing oxygen plasma stripping to release the photoresist, and removing the material from the cavity, thus providing a gap between the thermistor and the dielectric material.-20 -
27. A method of providing a detecting portion of a detection system as claimed in any one of claims 17 to 26, wherein the step of exposing the first layer comprises etching the supporting substrate at the surface opposite the first surface of the supporting substrate by at least one of potassium hydroxide etching and deep reactive ion etching.
28.A method of providing a reflecting cavity portion of a detection system for detecting electromagnetic radiation having frequencies in the range of about 300GHz to 11Hz, the method comprising the steps of: removing a portion of a cavity substrate, thereby providing a reflecting cavity in a first surface of the cavity substrate, the reflecting cavity having a depth (dR) of about one quarter of the wavelength of the electromagnetic radiation to be detected (A14), and providing a reflective layer in the reflecting cavity so as to substantially cover the exposed surface of the cavity.
29.A method of providing a reflecting cavity portion as claimed in claim 28, wherein the cavity substrate comprises a <100> orientated silicon wafer and the step of removing a portion of a cavity substrate comprises: providing a silicon nitride mask covering the outer periphery of the surface of the cavity substrate, and anisotropically etching the cavity substrate to provide the reflecting cavity having the required depth (dR).
30. A method of providing a reflecting cavity portion as claimed in claim 29, further comprising the step of covering the inner surface of the cavity with a layer of reflecting metal.
31.A method of providing a detection system for detecting electromagnetic radiation having frequencies in the range of about 3000Hz to ITHz, the method comprising the step of: bonding the supporting substrate of any one of claims 17 to 27 to the cavity -21 -substrate of any one of claims 28 to 30, such that the thermistor is substantially centrally aligned with the reflecting cavity, wherein the step of exposing the first layer of the supporting substrate by removing a portion thereof from a surface opposite the first surface of the supporting substrate is performed after bonding of the substrates together.
32. A method of providing a detection system as claim in claim 31, wherein the bonding step comprises low temperature polymer bonding.
33.A method of providing a detection system as claim in claim 32, wherein the low temperature polymer comprises a negative tone polymer, preferably SU8 negative tone polymer, and the bonding step further comprises applying a force to the substrates to push them together with the polymer therebetween and curing the polymer, preferably at a temperature between about 150 to 2OOC.
34.A method of providing a deteGtion system as Glaim in claim 31, wherein the bonding step comprises low temperature soldering.