NL2008318A - Lithographic apparatus, substrate table and device manufacturing method. - Google Patents

Description

LITHOGRAPHIC APPARATUS, SUBSTRATE TABLE AND DEVICEMANUFACTURING METHOD

FIELD

[0001] The present invention relates to a lithographic apparatus, a substrate table, and amethod for manufacturing a device.

BACKGROUND

[0002] A lithographic apparatus is a machine that applies a desired pattern onto asubstrate, usually onto a target portion of the substrate. A lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). In that instance, apatterning device, which is alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of the IC. This pattern can betransferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate(e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate willcontain a network of adjacent target portions that are successively patterned.

[0003] Lithography is widely recognized as one of the key steps in the manufacture ofICs and other devices and/or structures. However, as the dimensions of features madeusing lithography become smaller, lithography is becoming a more critical factor for enablingminiature IC or other devices and/or structures to be manufactured.

[0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleighcriterion for resolution as shown in equation (1):

(1) where λ is the wavelength of the radiation used, NA is the numerical aperture of theprojection system used to print the pattern, k\ is a process dependent adjustment factor, alsocalled the Rayleigh constant, and CD is the feature size (or critical dimension) of the printedfeature. It follows from equation (1) that reduction of the minimum printable size of featurescan be obtained in three ways: by shortening the exposure wavelength λ, by increasing thenumerical aperture NA or by decreasing the value of kv

[0005] In order to shorten the exposure wavelength and, thus, reduce the minimumprintable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source.EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm,for example within the range of 13-14 nm, or example within the range of 5-10 nm such as6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by anelectron storage ring.

[0006] EUV radiation may be produced using a plasma. A radiation system forproducing EUV radiation may include a laser for exciting a fuel to provide the plasma, and asource collector module for containing the plasma. The plasma may be created, forexample, by directing a laser beam at a fuel, such as particles of a suitable material (e.g.tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasmaemits output radiation, e.g., EUV radiation, which is collected using a radiation collector. Theradiation collector may be a mirrored normal incidence radiation collector, which receives theradiation and focuses the radiation into a beam. The source collector module may includean enclosing structure or chamber arranged to provide a vacuum environment to support theplasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

[0007] It is desirable to ensure that a pattern projected onto a substrate is aligned withpatterns already located on the substrate. This may be done by using an alignmentapparatus to align the mask with a substrate table which holds the substrate, therebyallowing alignment of the mask relative to the substrate to be determined. In order to alignthe mask with the substrate table the substrate table may be provided with a sensor.

[0008] It is desirable to provide a lithographic apparatus with a substrate table having asensor that is novel and inventive compared with known sensors.

SUMMARY OF THE INVENTION

[0009] According to an aspect of the invention, there is provided substrate table with asensor, the sensor comprising a block of material provided with a layer of material opaque toradiation, the layer of material having at least one window configured to allow thetransmission of the radiation, the sensor further comprising a wavelength conversionmaterial located at the window, and a waveguide positioned to receive radiation emitted bythe wavelength conversion material, the waveguide being embedded in the block of materialand configured to guide radiation emitted by the wavelength conversion material through theblock of material and towards a detector.

[00010] The block of material may be a semiconductor chip or a dielectric block.

[00011] The waveguide may have a height and/or width of less than 30 microns.

[00012] The detector may be provided in the substrate table.

[00013] The detector may be provided in the semiconductor chip, or be provided next tothe semiconductor chip.

[00014] The detector may be one of a plurality of detectors.

[00015] The window may be one of a plurality of windows.

[00016] A set of windows may extend in a first direction, and at least some of the windowsmay have different positions in a second direction, the second direction being transverse tothe first direction.

[00017] At least some of the windows may be provided at different heights.

[00018] Each window may be provided with a separate piece of wavelength conversionmaterial.

[00019] The waveguide may be one of a plurality of waveguides.

[00020] Each window may be associated with a different waveguide.

[00021] At least some of the waveguides may extend to different depths within thesemiconductor chip.

[00022] At least some of the waveguides may be spaced laterally apart from each other.

[00023] A dopant configured to amplify the radiation emitted by the wavelengthconversion material may be provided in the waveguide, and an optical pump may bearranged to directly pump radiation into the waveguide, the pump radiation having awavelength configured to excite the dopant.

[00024] The wavelength conversion material may be configured to emit radiation having awavelength in the wavelength range 500-2000 nm when EUV radiation is incident upon thewavelength conversion material.

[00025] The layer of opaque material may be opaque to EUV radiation. The layer ofopaque material may be opaque to radiation in the wavelength range 500-2000 nm.

[00026] Different windows may be provided with wavelength conversion materialconfigured to emit radiation at different wavelengths. Alternatively, different waveguidesmay be provided with filters (e.g. Bragg gratings) which are configured to pass radiation ofdifferent wavelengths. A wavelength multiplexer may be located at an opposite end of thewaveguides from the windows, the wavelength multiplexer being arranged to multiplex theradiation of different wavelengths (e.g. for transmission of a multiplexed signal via an opticalfiber).

[00027] According to an aspect of the invention there is provided a lithographic apparatuscomprisingan illumination system configured to condition a radiation beam, a supportconstructed to support a patterning device, the patterning device being capable of impartingthe radiation beam with a pattern in its cross-section to form a patterned radiation beam, asubstrate table according to any preceding aspect of the invention, the substrate table beingconstructed to hold a substrate, and a projection system configured to project the patternedradiation beam onto a target portion of the substrate.

[00028] According to an aspect of the invention there is provided a device manufacturingmethod comprising using the lithographic apparatus of clause 16 to manufacture devices,wherein the method includes using the sensor to measure an optical property of an EUV

radiation beam, and using the sensor to measure alignment of the substrate table and amask.

[00029] According to an aspect of the invention there is provided a lithographic apparatuscomprising an illumination system configured to condition a radiation beam, a supportconstructed to support a patterning device, the patterning device being capable of impartingthe radiation beam with a pattern in its cross-section to form a patterned radiation beam, asubstrate table with a sensor, the sensor comprising a block of material provided with a layerof material opaque to radiation, the layer of material having at least one window configuredto allow the transmission of the radiation, a wavelength conversion material located at thewindow, and a waveguide positioned to receive radiation emitted by the wavelengthconversion material, the waveguide being embedded in the block of material and beingconfigured to guide radiation emitted by the wavelength conversion material through the block of material and towards a detector, the substrate table being constructed tohold a substrate, and a projection system configured to project the patterned radiation beamonto a target portion of the substrate.

[00030] According to an aspect of the invention there is provided a device manufacturingmethod comprising using a lithographic apparatus comprising an illumination systemconfigured to condition a radiation beam; a support constructed to support a patterningdevice, the patterning device being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate table with a sensor, thesensor comprising a block of material provided with a layer of material opaque to radiation,the layer of material having at least one window configured to allow the transmission of theradiation, a wavelength conversion material located at the window, and a waveguidepositioned to receive radiation emitted by the wavelength conversion material, thewaveguide being embedded in the block of material and being configured to guide radiationemitted by the wavelength conversion material through the block of material and towards adetector, the substrate table being constructed to hold a substrate; and a projection systemconfigured to project the patterned radiation beam onto a target portion of the substrate,using the sensor to measure an optical property of an EUV radiation beam, and using thesensor to measure alignment of the substrate table and the patterning device.

[00031] According to an aspect of the invention there is provided a device manufacturingmethod comprising patterning an EUV beam of radiation with a pattering device, projecting apatterned beam of radiation onto a substrate supported by a substrate table with a projectionsystem, measuring an optical property of the EUV radiation beam with a sensor in thesubstrate table, the sensor comprising a block of material provided with a layer of materialopaque to radiation, the layer of material having at least one window configured to allow thetransmission of the radiation, a wavelength conversion material located at the window, and a waveguide positioned to receive radiation emitted by the wavelength conversion material,the waveguide being embedded in the block of material and being configured to guideradiation emitted by the wavelength conversion material through the block of material andtowards a detector, and measuring alignment of the substrate table and the patterningdevice with the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[00032] Embodiments of the invention will now be described, by way of example only,with reference to the accompanying schematic drawings in which corresponding referencesymbols indicate corresponding parts, and in which:

[00033] Figure 1 depicts a lithographic apparatus according to an embodiment of theinvention;

[00034] Figure 2 is a more detailed view of the apparatus of Figure 1;

[00035] Figure 3 is a schematic view of the substrate table and a sensor of thelithographic apparatus according to an embodiment of the invention;

[00036] Figure 4 is a schematic view of the sensor of Figure 3;

[00037] Figure 5 is a schematic view of an embodiment of the sensor;

[00038] Figure 6 is a schematic view of part of the sensor according to an embodiment ofthe invention; and

[00039] Figure 7 is a schematic view of part of the sensor according to an alternativeembodiment of the invention.

DETAILED DESCRIPTION

[00040] Figure 1 schematically depicts a lithographic apparatus 100 including a sourcecollector module SO according to one embodiment of the invention. The apparatuscomprises: an illumination system (illuminator) IL configured to condition a radiation beam B(e.g. EUV radiation); a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask or a reticle) MA and connected to a first positioner PMconfigured to accurately position the patterning device; a substrate table (e.g. a wafer table)WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to asecond positioner PW configured to accurately position the substrate; and a projectionsystem (e.g. a reflective projection system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

[00041] The illumination system may include various types of optical components, suchas refractive, reflective, magnetic, electromagnetic, electrostatic or other types of opticalcomponents, or any combination thereof, for directing, shaping, or controlling radiation.

[00042] The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of the lithographic apparatus,and other conditions, such as for example whether or not the patterning device is held in avacuum environment. The support structure can use mechanical, vacuum, electrostatic orother clamping techniques to hold the patterning device. The support structure may be aframe or a table, for example, which may be fixed or movable as required. The supportstructure may ensure that the patterning device is at a desired position, for example withrespect to the projection system.

[00043] The term “patterning device” should be broadly interpreted as referring to anydevice that can be used to impart a radiation beam with a pattern in its cross-section such asto create a pattern in a target portion of the substrate. The pattern imparted to the radiationbeam may correspond to a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

[00044] The patterning device may be transmissive or reflective. Examples of patterningdevices include masks, programmable mirror arrays, and programmable LCD panels.Masks are well known in lithography, and include mask types such as binary, alternatingphase-shift, and attenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of small mirrors, each ofwhich can be individually tilted so as to reflect an incoming radiation beam in differentdirections. The tilted mirrors impart a pattern in a radiation beam that is reflected by themirror matrix.

[00045] The projection system, like the illumination system, may include various types ofoptical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic orother types of optical components, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use of a vacuum. It may bedesired to use a vacuum for EUV radiation since other gases may absorb too muchradiation. A vacuum environment may therefore be provided to the whole beam path withthe aid of a vacuum wall and vacuum pumps.

[00046] As here depicted, the apparatus is of a reflective type (e.g. employing a reflectivemask).

[00047] The lithographic apparatus may be of a type having two (dual stage) or moresubstrate tables (and/or two or more mask tables). In such “multiple stage” machines theadditional tables may be used in parallel, or preparatory steps may be carried out on one ormore tables while one or more other tables are being used for exposure.

[00048] Referring to Figure 1, the illuminator IL receives an extreme ultra violet (EUV)radiation beam from the source collector module SO. Methods to produce EUV light include,but are not necessarily limited to, converting a material into a plasma state that has at leastone element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range.In one such method, often termed laser produced plasma ("LPP") the required plasma canbe produced by irradiating a fuel, such as a droplet, stream or cluster of material having therequired line-emitting element, with a laser beam. The source collector module SO may bepart of an EUV radiation system including a laser, not shown in Figure 1, for providing thelaser beam exciting the fuel. The resulting plasma emits output radiation, e.g. EUVradiation, which is collected using a radiation collector, disposed in the source collectormodule. The laser and the source collector module may be separate entities, for examplewhen a C02 laser is used to provide the laser beam for fuel excitation.

[00049] In such cases, the laser is not considered to form part of the lithographicapparatus and the radiation beam is passed from the laser to the source collector modulewith the aid of a beam delivery system comprising, for example, suitable directing mirrorsand/or a beam expander. In other cases, the source may be an integral part of the sourcecollector module, for example when the source is a discharge produced plasma EUVgenerator, often termed as a DPP source.

[00050] The illuminator IL may comprise an adjuster for adjusting the angular intensitydistribution of the radiation beam. Generally, at least the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprisevarious other components, such as facetted field and pupil mirror devices. The illuminatormay be used to condition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

[00051] The radiation beam B is incident on the patterning device (e.g. mask) MA, whichis held on the support structure (e.g. mask table) MT, and is patterned by the patterningdevice. After being reflected from the patterning device (e.g. mask) MA, the radiation beamB passes through the projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g.an interferometric device, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and another position sensor PS1 can beused to accurately position the patterning device (e.g. mask) MA with respect to the path ofthe radiation beam B. Patterning device (e.g. mask) MA and substrate W may be alignedusing mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[00052] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WTare kept essentially stationary, while an entire pattern imparted to the radiation beam isprojected onto a target portion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a different target portion C can beexposed.

2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WTare scanned synchronously while a pattern imparted to the radiation beam is projected ontoa target portion C (i.e. a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the support structure (e.g. mask table) MT may be determinedby the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (e.g. mask table) MT is kept essentiallystationary holding a programmable patterning device, and the substrate table WT is movedor scanned while a pattern imparted to the radiation beam is projected onto a target portionC. In this mode, generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of the substrate table WT orin between successive radiation pulses during a scan. This mode of operation can bereadily applied to maskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

[00053] Combinations and/or variations on the above described modes of use or entirelydifferent modes of use may also be employed.

[00054] Figure 2 shows the apparatus 100 in more detail, including the source collectormodule SO, the illumination system IL, and the projection system PS. The source collectormodule SO is constructed and arranged such that a vacuum environment can be maintainedin an enclosing structure 220 of the source collector module SO. In an embodiment, an EUVemitting plasma 210 may be produced by irradiating a fuel, such as a droplet, stream orcluster of material having the required line-emitting element provided by a fuel source 200,with a laser beam 205 emitted from a light source LA, such as a laser. In an embodiment,an EUV radiation emitting plasma 210 may be formed by a discharge produced plasmasource. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor orSn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range ofthe electromagnetic spectrum. The very hot plasma 210 is created by, for example, anelectrical discharge causing an at least partially ionized plasma. Partial pressures of, forexample, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required forefficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) isprovided to produce EUV radiation.

[00055] The source collector module SO may include a radiation collector CO, which maybe a so called normal incidence radiation collector. Radiation that is reflected by theradiation collector CO is focused in a virtual source point IF. The virtual source point IF iscommonly referred to as the intermediate focus, and the source collector module is arrangedsuch that the intermediate focus IF is located at or near an opening 221 in the enclosingstructure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[00056] Subsequently the radiation traverses the illumination system IL, which mayinclude a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged toprovide a desired angular distribution of the radiation beam 21, at the patterning device MA,as well as a desired uniformity of radiation intensity at the patterning device MA. Uponreflection of the beam of radiation 21 at the patterning device MA, held by the supportstructure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by theprojection system PS via reflective elements 28, 30 onto a substrate W held by the substratetable WT.

[00057] More elements than shown may generally be present in illumination optics unit ILand projection system PS. The grating spectral filter may optionally be present, dependingupon the type of lithographic apparatus. Further, there may be more mirrors present thanthose shown in the Figures, for example there may be 1- 6 additional reflective elementspresent in the projection system PS than shown in Figure 2.

[00058] The substrate table WT is provided with an alignment sensor AS according to anembodiment of the invention.

[00059] Figure 3 schematically shows a substrate table WT that corresponds with thesubstrate table shown in Figures 1 and 2. An alignment sensor AS is provided in thesubstrate table WT. The alignment sensor AS includes a grating 1, which is representedschematically by a series of lines. Although only three lines are shown in Figure 3, thegrating 1 may comprise any suitable number of lines. The alignment sensor AS is formed ina block of material, which in this embodiment is a semiconductor chip 2 (e.g. formed fromsilicon or some other suitable semiconductor). Part of the semiconductor chip 2 is shown inan enlarged view beneath the substrate table WT in Figure 3.

[00060] As may be seen from the enlarged view, the alignment sensor AS comprises awavelength conversion material 4 and a waveguide 5. The waveguide is embedded in thesemiconductor chip 2 and is configured to receive and guide radiation emitted by thewavelength conversion material 4. The waveguide 5 comprises a core 6 and cladding 7, thecladding surrounding the core. The core 6 has a higher refractive index from the cladding 7.This difference in refractive index is sufficiently large that radiation emitted by thewavelength conversion material 4 is substantially confined by the waveguide, so that theradiation is guided along the waveguide 5. The wavelength conversion material may for example be a material that emits infrared or visible radiation when EUV radiation is incidentupon it. The wavelength conversion material may for example emit radiation in the range500-2000 nm.

[00061] Part of the alignment grating 1 is shown in the enlarged view. The alignmentgrating comprises a layer of material 8 that is opaque to EUV radiation, the layer of materialbeing provided with a window 9. The opaque material 8 may for example be aluminium, ormay be any other suitable material. The opaque material 8 blocks EUV radiation, whereasEUV radiation may pass through the window 9. The opaque material 8 and the window 9thus form part of the alignment grating.

[00062] Figure 3 also schematically shows the semiconductor chip 2 in cross-sectionalong line A. As may be seen, the waveguide 5 has a rectangular cross-section (although itmay have any other suitable cross-sectional shape). The height hc of the core 6 (and thewidth) may be of the order of the wavelength of radiation that is emitted by the wavelengthconversion material 4, or may be greater than this. In this context the wavelength of theradiation is the wavelength of the radiation when it is in the core 6, which will be significantlyshorter than the wavelength of the radiation in air. The thickness (and the width) of the core6 may be less than the wavelength of radiation that is emitted by the conversion material.However, if it is significantly less, then this may give rise to substantial losses of radiation asthe radiation travels along the waveguide. In general, the smaller the core 6, the moreradiation will be lost from the waveguide as the radiation propagates, particularly at cornersor curves of the waveguide. The smaller the core 6, the less radiation is inside the core andmore radiation is outside the core. The radiation that is outside the core 6 will be inside thecladding 7. However, if the cladding is not sufficiently thick, then radiation will leak from thecladding into the semiconductor chip 2, thereby reducing the amount of radiation that isavailable for detection by a detector. Conversely, if the core is bigger, then more radiationwill be retained in the core, and more radiation will be available for detection by the detector.In addition, a bigger core may collect more radiation from the wavelength conversionmaterial 4.

[00063] The thickness td of the cladding 7 may be of the order of the wavelength ofradiation that is emitted by the wavelength conversion material 4, or may be greater thanthis. In this context, the wavelength of the radiation is the wavelength of the radiation whenit is in the cladding 7, which will be significantly shorter than the wavelength of the radiationin air. Although the cladding 7 may have a thickness that corresponds with the wavelengthof radiation that is emitted by the wavelength conversion material 4, the cladding may bethicker than this. Providing thicker cladding may reduce the amount of radiation that is lostwhen the radiation travels along the waveguide 6, since less radiation will leak from thecladding into the semiconductor chip 2.

[00064] The height hw of the waveguide 5 may be three or more times the wavelength ofthe radiation that is emitted by the wavelength conversion material 4. The width of thewaveguide 5 may similarly be three or more times the wavelength of the radiation that isemitted by the wavelength conversion material 4.

[00065] The wavelength conversion material 4 may, for example, emit infrared or visibleradiation. The height of the core may therefore, for example, be less than 10 microns, lessthan 5 microns, less than 2 microns, or less than 1 micron. The width of the core may, forexample, be less than 10 microns, less than 5 microns, less than 2 microns, or less than 1micron. The thickness of the cladding may, for example, be less than 10 microns, less than5 microns, less than 2 microns, or less than 1 micron. The height and/or width of thewaveguide may thus be less than 30 microns, less than 15 microns, less than 6 microns, orless than 3 microns

[00066] The opaque material 8 may, for example, be Al, Cu, TiN, or any other suitablematerial (e.g. metal). The opaque material may, for example, be provided as a layer that isaround 100 nm thick, or as a layer with some other suitable thickness.

[00067] The waveguide core 6 may be formed from Si, SiC, Si3N4, InGaAsP, or any othersuitable material (e.g. any suitable semiconductor such as semiconductor formed from lll-Vgroup and/or ll-VI group elements and/or group IV elements). The waveguide cladding 7may be formed from Si02, Si3N4, SiC, InGaAsP or any other suitable material (e.g. anysuitable semiconductor such as semiconductor formed from lll-V group and/or ll-VI groupelements, and/or group IV elements). The waveguide cladding 7 may consist of the samematerial as the semiconductor chip 2. Where InGaAsP is used, this may be provided in anInP block or a GaAs block (e.g. the semiconductor chip 2 may be formed from InP or GaAs).Where Si, SiC or Si3N4 is used, this may be provided in a Si or Si02 block (e.g. thesemiconductor chip 2 may be formed from Si). Alternatively, the semiconductor chip may beformed from CdTe. The semiconductor chip 2 may be formed from any suitablesemiconductor material (e.g. semiconductor formed from lll-V group and/or ll-VI groupelements, or from group IV elements such as Ge). The semiconductor material used to formthe semiconductor chip 2 may have a lattice that is matched to the lattice of materials usedto form the cladding 7 and core 6. The semiconductor material used to form thesemiconductor chip 2 may have a lattice with dimensions that are sufficiently close to thelattice dimensions of the cladding 7 and core 6 that stresses arising at material interfaces donot cause the waveguide 5 to be formed incorrectly.

[00068] Where InGaAsP is used, its optical properties (including refractive index) andmechanical properties may be tuned by changing the relative concentrations of theconstituent materials. For ln(x)Ga(1-x)As(y)P(1-y) the values of x and y may be variedbetween 0 and 1 to change the relative concentrations and tune the optical and mechanical properties. Similarly, optical properties of other materials may be tuned by changing therelative concentrations of constituent materials.

[00069] As mentioned above, the refractive index nc of the core 6 is greater than therefractive index nd of the cladding 7, i.e. nc > nd.

[00070] The wavelength of radiation generated by the wavelength conversion material 4may be taken into account when choosing which material to use to form the waveguide core6 and cladding 7 (e.g. materials that are substantially transmissive at that wavelength maybe chosen).

[00071] The wavelength conversion material 4 may, for example, be a scintillationmaterial such as YAG:Ce, or any other suitable scintillation material. The scintillationmaterial may for example be P43. P43 is a phosphorus formed by using Td to activate aGd202S host material.

[00072] The wavelength conversion material 4 may be a semiconductor. EUV radiationphotons have an energy of around 90eV, and thus provide enough energy to exciteelectrons in a semiconductor material. If the semiconductor material has a direct bandgap,then photons will be emitted when these electrons relax to the ground state. Suitable directbandgap semiconductors are semiconductors from the lll-V group and the ll-VI group. Itmay be desirable that the wavelength conversion material emits radiation having awavelength in the range 500-2000 nm, since relatively low loss waveguides may beconstructed to guide radiation in this wavelength range. This wavelength range correspondswith a photon energy in the range 0.6 - 2.4eV. A semiconductor material that has abandgap that falls within this range may be used as the wavelength conversion material.Semiconductor materials that have bandgaps in this range include GaAs (bandgap around1.4 eV), AIGaAs (bandgap around 1.42 - 2.16 eV), InGaAs (bandgap around 0.36 - 1.43eV), and InGaAsP (bandgap around 0.9 - 1.3 eV). Other semiconductor materials may beused. Properties of the wavelength conversion material 4 may be tuned by changing therelative concentrations of constituent materials.

[00073] Fabrication of the semiconductor chip 2, including the waveguide 5, may be viaconventional lithographic techniques or other techniques known in the art. Patterns may beexposed onto resist, processed and etched, in order to form the waveguides and thealignment grating, and may be used to form the wavelength conversion material. Thelithographic techniques may, for example, use Si and materials that may be grown on Si.However, the lattice dimensions of materials such as GaAs and AIGaAs are different from Si,and it may therefore not be possible to grow these materials onto Si (or related materials).Therefore, the wavelength conversion material 4 may be provided by gluing the materialonto the semiconductor chip 2 rather than growing it. For example, once the waveguide 5has been formed, the wavelength conversion material 4 (e.g. InGaAs or AIGaAs) may be glued onto the top of the waveguide, the wavelength conversion material being provided asa layer that extends across the semiconductor chip (and potentially across a plurality ofsemiconductor chips). In an alternative method, the wavelength conversion material may bebonded to the semiconductor chip 2 using Van der Waals bonding. Following gluing orbonding of the wavelength conversion material 4 onto the waveguide, lithographic patterningand etching may be used to remove unwanted wavelength conversion material, such thatthe wavelength conversion material remains only in a desired location or locations (e.g. asshown in Figure 3). Additional semiconductor chip material and cladding material may thenbe provided on the semiconductor chip using conventional lithographic techniques.Following this, the alignment grating 1 may be formed using conventional lithographictechniques.

[00074] The semiconductor chip 2 may be one of a plurality of semiconductor chips thatare all formed together on a semiconductor wafer (also referred to as a substrate).

[00075] The cladding 7 may be formed from a single material or may be formed from acombination of materials. Different materials may be used to form the cladding 7 on differentsides of the core 6. For example, the materials used to form the cladding 7 above the core 6may be different from the materials used to form the cladding beneath the core and/or maybe different from the materials used to form the cladding on the left and/or right sides of thecore.

[00076] The core 6 may be formed from different materials. For example, alternatinglayers of semiconductor may be used to form the core 6. In an embodiment, alternatinglayers of InGaAsP and InP may be used to form the core 6. This may be done to provide thecore 6 with a desired refractive index.

[00077] Figure 3 shows only one window 9 and one waveguide 5. Figure 4 is a cross-sectional view of the alignment sensor AS, which is less enlarged than Figure 3 and whichtherefore can show more windows and waveguides. Five waveguides 15a-e are all locatedbeneath one another in the semiconductor chip 2 (i.e. they are substantially verticallyaligned). The waveguides 15a-e are all provided at different depths such that they areseparated in the z-direction. This allows the waveguides to pass to one end of thesemiconductor chip 2 without intersecting with one another. Each of the waveguides 15a-emay have the structure shown in Figure 3 (i.e. a core surrounded by cladding). However,this structure is omitted from Figure 4 for simplicity of illustration. The waveguides 15a-e areconfigured to guide radiation in the x-direction. A discontinuity 14 is shown in Figure 4, thediscontinuity indicating that the waveguides 15a-e may be significantly longer than could beshown in Figure 4. The waveguides 15a-e may for example extend for more than 1 cm, mayextend for more than 2 cm, or may extend for more than 3 cm.

[00078] The waveguides 15a-e initially extend downwardly from the wavelengthconversion material 4, then extend diagonally before finally extending horizontally. Thedownwardly extending portions of the waveguides 15a-e have different lengths, and thediagonally extending portions of the waveguides also have different lengths. This allows thehorizontally extending portions of the waveguides 15a-e to be provided at different depths.

[00079] Some radiation may be lost as the radiation passes from the downwardlyextending portion of a waveguide to the diagonally extending portion. Similarly, someradiation may be lost as the radiation passes from the diagonally extending portion of awaveguide to the horizontally extending portion. This will not affect the performance of thealignment sensor, provided that sufficient radiation is received at the detector to allow theradiation to be detected with a desired signal to noise ratio. It may be possible to use thealignment sensor effectively even if significant loss of radiation occurs in the waveguide (e.g.up to 80% loss of radiation).

[00080] If loss of radiation in the waveguide is likely to have a significant effect upon theperformance of the alignment sensor, then the waveguide may be configured such that theloss of radiation is less than a desired amount. This may be achieved, for example, byensuring that angles subtended between different portions of the waveguide do not exceed apredetermined value. It may be achieved by providing the waveguides with curves ratherthan corners.

[00081] If the amount of radiation emitted from the wavelength conversion material issufficiently high, then the diagonal portion of the waveguides may be omitted. Thewaveguide may, for example, instead include a 90° corner. Some radiation may be lost atthis corner, but sufficient radiation may be retained to allow the alignment sensor to be usedeffectively.

[00082] A detector 16a-e is provided at the end of each waveguide 15a-e, each detectorbeing configured to detect radiation that has travelled along that waveguide. The detectorsmay, for example, be located in the semiconductor chip 2. Alternatively, the detectors maybe located next to the semiconductor chip 2 (as shown), for example, being held in asubstrate 17. The detectors 16a-e may, for example, be photodiodes. The detectors 16a-emay, for example, be provided in a two-dimensional array. The detectors 16a-e may, forexample, be a CCD array. Output signals from the detectors 16a-e are passed toprocessing electronics (not illustrated).

[00083] As mentioned above, an opaque material 8 is provided on top of thesemiconductor chip 2. The opaque material is provided with a series of windows 9. Thewindows 9 are periodically separated and therefore form a grating 1 in the opaque material8. A piece of wavelength conversion material 4 is provided beneath each window 9. Eachpiece of wavelength conversion material 4 is separated by semiconductor material of the semiconductor chip 2. The semiconductor chip 2 may be formed from a material that isopaque to EUV radiation and is also opaque to radiation emitted by the wavelengthconversion material 4. The semiconductor chip material may thereby prevent cross-talkoccurring between pieces of wavelength conversion material. Additionally or alternatively,metal or some other material that is opaque to radiation emitted by the wavelengthconversion material may be provided between waveguides in order to prevent cross-talkoccurring between them. Metal may, for example, be provided at an outer boundary of thecladding 7. The metal may, for example, have a thickness of less than 0.5 microns.

[00084] During alignment, EUV radiation is incident upon the alignment sensor AS. EUVradiation that is incident upon the opaque material 8 is blocked. However, EUV radiationthat falls on the windows 9 passes through the windows and is incident upon the wavelengthconversion material 4. The wavelength conversion material 4 emits radiation at a longerwavelength than EUV radiation (e.g. emitting visible or infrared radiation). This radiation isguided along the waveguides 15a-e and travels to the detectors 16a-e. Radiation receivedby the detectors may be used to obtain alignment between a mask MA and the substratetable WT (see Figure 2).

[00085] An embodiment of the invention is shown schematically in Figure 5. Figure 5schematically shows part of an alignment sensor viewed from above in partial cross-section.Detectors are omitted from Figure 5. However, detectors may be provided, for example, inthe manner described above. The alignment sensor comprises a semiconductor chip 2 ontowhich an opaque material 8 has been provided. The opaque material 8 is provided with aseries of windows 9, the windows having a periodic separation such that they form a grating1. A series of waveguides 25a-e extend in the x-direction. Each waveguide 25a-e connectswith a different window 9, and is configured to guide radiation in the x-direction. Althoughnot shown in Figure 5, wavelength conversion material is located at each window 9, thewavelength conversion material being configured to receive EUV radiation and emit radiationwith a longer wavelength (e.g. visible or infrared radiation).

[00086] As may be seen from Figure 5, each waveguide 25a-e is separated in the y-direction, thereby allowing radiation to be guided by the waveguides without cross-talkoccurring between them. Since the waveguides are separated in the y-direction there is noneed for them to be separated vertically (i.e. in the z-direction), although they may also beseparated in this direction. The waveguides 25a-e may, for example, all pass along thesemiconductor chip 2 at the same depth (although they may have different depths).Detectors (not shown) may, for example, be located at opposite ends of the waveguides25a-e from the windows 9.

[00087] In embodiments of the invention (not illustrated), some waveguides may beseparated in the z-direction and separated in the y-direction. The waveguides may have any suitable form. Although the portions of the waveguides that extend to the detectors extendin the x-direction in the Figures, the waveguides may extend in the y-direction, or in anyother direction (e.g. substantially transverse to an optical axis of the lithographic apparatus).Although the portions of the waveguides that extend to the detectors are parallel to oneanother in the figures, the waveguides may be non-parallel. Some waveguides may extendin opposite directions, substantially opposite directions, or different directions. Where this isthe case, detectors may accordingly be provided at different locations to receive radiationthat has been guided by the waveguides. Detectors may, for example, be located ondifferent sides of the semiconductor chip 2.

[00088] A separate waveguide may be provided for each window. Alternatively, awaveguide may be configured to receive radiation from more than one window (followingwavelength conversion by wavelength conversion material). A single waveguide may beprovided for all windows. Similarly, a single piece of wavelength conversion material may beprovided for all windows.

[00089] The alignment sensor AS may, for example, be used when it is desired to align apatterning device MA with a substrate table WT (see Figures 1 and 2). This may beachieved by directing EUV radiation through a grating provided on the patterning device MAsuch that an image of the grating is formed in the vicinity of the substrate table WT (referredto here as the mask grating image). The substrate table WT may be positioned such thatthe alignment grating AS overlaps with the mask grating image. The position of thesubstrate table WT relative to the mask grating image is measured, thereby allowingalignment between the patterning device MA and the substrate table to be achieved.

[00090] EUV radiation of the mask grating image that falls on the opaque material 8 of thealignment grating 1 is not transmitted and is therefore not detected by the detectors 16a-e.EUV radiation that falls upon the windows 9 (i.e., the transmissive portions of the alignmentgrating 1) is incident upon wavelength conversion material 4. This EUV radiation thuscauses radiation to be emitted by the wavelength conversion material 4, the radiationtravelling along the waveguides 15a-e, 25a-e to the detectors. The amount of radiation thatis detected by the detectors depends upon the alignment of the mask grating image relativeto the alignment sensor grating 1. If the grating image and the alignment sensor grating 1both have the same period, then a maximum signal will be seen when bright portions of themask grating image are aligned with windows 9 of the alignment grating. A minimum signalwill be seen when bright portions of the mask grating image are aligned with opaque material8 of the alignment grating 1. An intermediate signal will be seen when bright portions of themask grating image partially overlap with windows 9 of the alignment sensor grating 1.

[00091] In one alignment method, the substrate table WT is moved transverse to the z-direction (e.g. is moved in the x-direction) such that the alignment grating moves through the mask grating image. The intensity of radiation detected by the detectors is monitored duringthis movement, and the variation of intensity is used to determine the position of thealignment grating 1 relative to the mask grating image. This allows the position of thesubstrate table WT to be determined relative to the position of the patterning device, therebyallowing the patterning device and the substrate table to be aligned. A coarse alignmentmay already have been performed in order to ensure that the position of the grating 1 iswithin the capture range needed to achieve correct determination of the relative positions ofthe substrate table WT and the patterning device MA. This coarse alignment may, forexample, be performed using another alignment grating having a longer period, or may beperformed using some other alignment system.

[00092] In an embodiment, the output signals from the detectors 16a-e may all be addedtogether so that the processor (not shown) processes a single radiation intensity value.Alternatively, a single detector may be used to detect the radiation delivered by thewaveguides 15a-e, 25a-e instead of a plurality of detectors. Either of these approaches mayallow alignment of the lithographic apparatus to be performed in the same way as is done inconventional lithographic apparatus (where a single photodiode is located directly beneath asubstrate table alignment grating).

[00093] In an embodiment, the output signals from the detectors 16a-e may be processedindividually by the processor. Where this is done, the processor is provided with moreinformation than if only a single radiation intensity value is used. This may allow theprocessor to measure alignment in a different manner to the conventional manner. Forexample, the different intensities detected from each waveguide may be used to determinethe relative positions of the substrate table WT and the patterning device MA while movingthe substrate table through a smaller scanning movement than in prior art systems.

[00094] In an embodiment in which the output signals from the detectors 16a-e areprocessed individually by the processor, alignment may be measured without using ascanning movement of the substrate table WT (or of the patterning device MA). The alignedposition of the substrate table WT (relative to the patterning device MA) may be determinedby comparing the output signals from the detectors 16a-e to see which detector has thehighest output signal. The detector that has the highest output signal may be considered toindicate a central point of the pattern projected from the patterning device MA, and thisinformation may be used to achieve alignment of the substrate table WT and patterningdevice.

[00095] An embodiment of the invention which may be used when determining alignmentwithout using a scanning movement of substrate table WT (or of the patterning device MA) isshown schematically in Figure 6. Figure 6 shows part of an alignment sensor viewed fromabove. A layer of material 108 which is opaque to EUV radiation is provided, the layer of material having two sets of windows 109, 110. Wavelength conversion material 4 isprovided in each window. A waveguide (not shown) passes from each window to a detector(also not shown) in the same manner as described above in relation to other embodiments.The waveguide may have properties as described above in relation to other embodiments.

[00096] The windows 109, 110 are staggered in the x-direction. That is, each of thewindows of a given set of windows 109, 110 is offset in the x-direction relative to adjacentwindows. The x-direction offsets may be equal to one another or may be different. The x-direction offsets may be the same for each set of windows 109,110 or may be different.

[00097] Also shown in Figure 6 are images of two alignment grating lines 40a, 40b. Theimages are formed by EUV radiation which has passed through openings of a grating in apatterning device MA (see Figure 2), the openings being imaged onto the layer of material108 by the projection system PS. As may be seen, the images 40a, 40b partially overlapwith the sets of windows 109,110.

[00098] Referring to the first image 40a, it may be seen that the image does not overlapwith first and second windows 109a,b of the set of windows. The image partially overlapswith third and fourth windows 109c,d of the set of windows 109a and is fully overlaid overfifth and sixth windows 109e,f of the set of windows. Detectors which detect radiationemitted by wavelength conversion materials 4 located in the windows therefore outputsignals which may be used to determine the position in the x-direction of the image 40a.Little or no radiation will be received from the first two windows 109a,b of the set of windows.A small amount of radiation will be received from the third window 109c and a larger amountof radiation will be received from the fourth window 109d. A still larger amount of radiationwill be received from the fifth and sixth windows 109e,f. A processer of the lithographicapparatus may thus determine the position of a left hand edge of the image 40a, and maythereby determine the position of the image.

[00099] Referring to the second image 40b, it may be seen that the image overlaps in adifferent manner with the set of windows 110. Again, the intensity of radiation detected bydetectors connected to the windows allows the position of the image 40b to be determined.

[000100] The sets of windows 109, 110 allow the positions of the images 40a, 40b of linesof a patterning device MA alignment mark to be determined without scanning movement ofthe substrate table WT. This allows alignment of the substrate table WT relative to thepatterning device MA to be determined without performing a scanning movement of thesubstrate table.

[000101] Although only two sets of windows 109, 110 are shown in Figure 6, any suitablenumber of sets of windows may be used. Similarly, although each set of windows 109, 110comprises six windows, a set of windows may comprise any suitable number of windows.Although each window is referred to as having a separate waveguide and detector, more than one window may be associated with a given waveguide. A detector may receiveradiation from more than one window.

[000102] The differences in x-direction positions of the windows 109, 110 may determinethe accuracy with which the alignment of the substrate table WT relative to the patterningdevice MA may be determined. Staggering the windows with a smaller separation mayprovide a higher accuracy of alignment measurement, and staggering with a larger spacingmay provide a lower accuracy of alignment measurement.

[000103] Sets of windows which extend in the y-direction may be provided in order to allowalignment of the substrate table WT relative to the patterning device MA in the y-direction.

[000104] Figure 7 shows schematically viewed from above part of an alignment sensor ASaccording to a further alternative embodiment of the invention. Three alignment gratings201 a-c are formed in a layer of material 208 which is opaque to EUV radiation. The gratings201 a-c comprise windows 209, each window being provided with wavelength conversionmaterial 4.

[000105] The right hand side of Figure 7 schematically shows in cross-section some of thewindows of the first grating 201a, some of the windows of the second grating 201b and someof the windows of the third grating 201c. In each instance, wavelength conversion material 4is provided in the windows and waveguides 215 pass through semiconductor material 2 todetectors (not shown).

[000106] In the first grating 201 a the windows are provided at or close to the surface of thesemiconductor material. In the second grating 201b, the windows are provided in a recessin the semiconductor material. Because the windows are provided in a recess, they have alower position in the z-direction than the windows of the first grating 201a. In the thirdgrating 201c the windows are provided above the semiconductor material, the layer of EUVopaque material 208 having been provided as a thicker layer in this location in order to allowthe windows to be raised relative to the semiconductor material. The windows of the thirdgrating 201c are higher in the z-direction than the first set of windows 201a. The windows ofthe gratings 201 a-c are provided at different heights.

[000107] As may be seen, in each case the wavelength conversion material 4 is located ator adjacent to an upper surface of the windows.

[000108] The embodiment shown in Figure 7 allows the position of the alignment sensorAS relative to an image focal plane of the projection system PS to be determined. In Figure7 images 240a, 240b of alignment mark lines provided in a patterning device MA are shown.The images are shown as having sharp edges. However, in practice, the sharpness of theedges of the image will depend upon the position in the z-direction relative to the image focalplane of the projection system. Taking as an example the case in which the upper surface ofthe semiconductor material 2 lies in the image focal plane of the projection system, the alignment grating line images 240a, 240b will have sharp edges in the vicinity of the firstalignment grating 201a because the first alignment grating lies in or close to the image focalplane of the projection system. The second alignment grating 201b will be beneath theimage focal plane of the projection system. As a result, the alignment grating images 240a,240b will have diffuse edges, and the intensity of radiation incident upon the windows of thesecond alignment grating 201b will be reduced. Similarly, the third alignment grating 201cwill lie above the image focal plane, and as a result the edges of the alignment gratingimages 240a, 240b will be diffuse and the intensity of radiation incident upon the windows ofthe third alignment grating will be reduced.

[000109] A processor connected to detectors (not shown) which receive radiation emittedby the wavelength conversion material 4 will determine, based upon the intensity of radiationreceived at the detectors, that the image focal plane corresponds with the position in the z-direction of the first alignment grating 201 a (or that it is closer to this plane than to the planesof the second or third alignment gratings 201b, 201c). The alignment sensor AS thus allowsthe position of the substrate table WT relative to the image focal plane of the projectionsystem PS to be determined without performing a scanning movement of the substrate tablein the z-direction.

[000110] Although alignment gratings at three different z-direction levels are shown inFigure 3, any suitable number of alignment gratings may be provided. The alignmentgratings may be provided at any suitable combination of heights. Alignment gratings withdifferent heights may be arranged in height order, i.e. with the lowest alignment grating atone end of a set of alignment gratings and with the highest alignment grating at an oppositeend of the set. The alignment gratings may alternatively be not in height order, for examplewith heights in a random or pseudo-random order. The accuracy with which the z-directionposition of the substrate table WT may be determined may depend upon the z-directionseparation between alignment gratings.

[000111] Although three alignment gratings 201 a-c are shown in Figure 7, any number ofalignment gratings may be provided. For a given z-direction separation of alignmentgratings, increasing the number of alignment gratings will increase the capture range overwhich the position of the focal plane of the projection system PS may be determined.

[000112] The z-direction separation of the alignment gratings will determine the accuracywith which the position of the focal plane of the projection system may be determined.Reducing the z-direction separation will increase the accuracy, and increasing the z-directionseparation will reduce the accuracy.

[000113] In addition to obtaining information about the position of the substrate table WT inthe z-direction, the alignment sensor AS shown in Figure 7 may also be used to determine x-direction alignment of the substrate table WT relative to the patterning device MA.

[000114] Features described further above in relation to the embodiments illustrated inFigures 3-5 may also be applied to the embodiment illustrated in Figures 6 and 7. Indeed,features of any embodiments of the invention may be combined with one another.

[000115] The wavelength conversion material 4 may have an upper surface which issubstantially co-planar with an upper surface of the EUV opaque material 8. Thewavelength conversion material 4 may be provided at any suitable height relative to the EUVopaque material 8.

[000116] In an embodiment in which alignment is measured without using a scanningmovement of the substrate table, the period of the alignment sensor grating may be differentfrom the period of the mask grating image. Where this is the case, each window of thealignment senor grating will have a slightly different overlap with the mask grating image.This allows more information to be gathered regarding the relative position of the alignmentsensor and the mask grating image than would be the case if alignment sensor grating hadthe same period as the mask grating image.

[000117] In embodiments in which scanning movement of the substrate table WT is notneeded in order to align the substrate table with the patterning device MA, alignment may beperformed more quickly, thereby allowing the throughput of the lithographic apparatus to beincreased.

[000118] In embodiments of the invention, the waveguides may be embedded insemiconductor material (e.g. in a semiconductor chip). The waveguides may be consideredto be integrally formed within the semiconductor material (e.g. semiconductor chip). Thewavelength conversion material and alignment grating may also be considered to beintegrally formed with the semiconductor material. In this context, the term ‘integrallyformed’ may be considered to mean that the waveguides, the wavelength conversionmaterial, alignment grating and semiconductor material (e.g. semiconductor chip) form asolid block. This construction may be advantageous because the alignment sensor ofembodiments of the invention is robust, compared with, for example, an alignment sensorcomprising separately provided alignment grating, wavelength conversion material, opticalfiber, and detector, which are connected together. The alignment sensor may be morestable than an alignment sensor comprising separately provided components, since thecomponents are less likely to move relative to one another. The alignment sensor may alsohave a longer life than an alignment sensor comprising separately provided components,since the separately provided components are more likely to become detached from oneanother.

[000119] In embodiments in which a separate waveguide is provided for each window ofthe diffraction grating, the diffraction grating may have any desired length. If a singlewaveguide were to be used to collect all radiation transmitted by the diffraction grating, then this would limit the length of the diffraction grating, since properties of the waveguide mayrestrict the cross-sectional area of the waveguide.

[000120] Because separate waveguides and separate detectors are used for each windowof the diffraction grating, a large amount of information is available to the processor. As aresult, the alignment sensor may be used to measure properties other than the relativepositions of the patterning device MA and substrate table WT (i.e. the sensor is not merelyan alignment sensor). For example, properties of the EUV radiation beam may be measuredby the sensor. These properties may include spatial variation of the intensity of the EUVradiation beam, aberration of the radiation beam, the position of the focal plane of theradiation beam, etc. The properties may be measured by using the patterning device MA toposition an appropriate aperture or pattern in the EUV radiation beam, then measuringradiation that is received by different detectors of the sensor, e.g. as a function of theposition of the alignment sensor and/or the patterning device MA.

[000121] In a conventional alignment sensor, a detector is located immediately beneaththe alignment grating. The detector detects radiation that is transmitted by the alignmentgrating and transmits an electrical signal. The detector may be a significant source of heat,and this heat may introduce errors into alignment measurements. The alignment sensormay, for example, be located at an edge of the substrate table WT, and heat from thedetector may cause distortion of the edge of the substrate table. If an interferometer is usedto measure the position of the substrate table WT, then this distortion of the edge of thesubstrate table may introduce an error into the measured position of the substrate table.

[000122] Embodiments of the invention may avoid this disadvantage because thedetectors are located away from the alignment grating (and may be located away from anedge of the substrate table WT). The detectors may, for example, be located sufficiently farfrom the alignment grating that heat emitted by the detectors does not reach the alignmentgrating, or sufficiently far that heat that reaches the detectors does not have a significanteffect upon alignment measurements. For example, the detectors may be locatedsufficiently far from the alignment grating that heat emitted by the detectors does not causemechanical distortion of the substrate table WT between the alignment grating and asubstrate held on the substrate table WT. In addition, detectors may, for example, belocated sufficiently far from the edge of the substrate table WT that heat emitted by thedetectors does not reach an adjacent edge of the substrate table, or sufficiently far that heatdoes not cause significant distortion between the alignment mark and an adjacent edge ofthe substrate table.

[000123] The detectors may, for example, be provided at a location that is adjacent to acooling apparatus, e.g. an active cooling apparatus such as an apparatus that uses watercirculation.

[000124] In an embodiment, detectors are not provided at the end of the waveguides.Instead, additional waveguides may be provided, the additional waveguides being arrangedto receive radiation that has travelled along the waveguides, and to guide that additionalradiation. Detectors may be provided at far ends of the additional waveguides. Theadditional waveguides may for example be optical fibers, or may be formed in asemiconductor material. The additional waveguides may, for example, be used to carryradiation to a location in the substrate table WT that is located away from the alignmentsensor AS (e.g. located away from the semiconductor chip). This may reduce the likelihoodof unwanted thermal effects affecting alignment measurements. The additional waveguidesmay, for example, be used to carry radiation to a location that is located away from thesubstrate table WT (e.g. optical fibers that pass as a cable from the substrate table).

[000125] Wavelength conversion materials 4 that emit different wavelengths of radiationmay be used. Referring to Figure 4 for example, the wavelength conversion materialassociated with each waveguide 15a-e may emit a different wavelength. Where this is thecase, radiation that travels to ends of the waveguides 15a-e may be multiplexed into a singleadditional waveguide (e.g. an optical fiber). The multiplexed radiation may travel along theadditional waveguide and then be de-multiplexed at an opposite end of the additionalwaveguide and detected. The de-multiplexer may, for example, comprise one or moregratings that are configured to direct the radiation towards different detectors in awavelength-dependent manner.

[000126] In an embodiment, instead of different wavelength conversion materials 4emitting different wavelengths of radiation, wavelength conversion materials may emit broad¬band radiation. Filters may be provided in the waveguides, each waveguide being providedwith a different filter, the different filters being configured to select a different wavelength ofradiation for each waveguide. The filters may for example be Bragg gratings. Wavelengthbased multiplexing may be used with this embodiment in the manner explained above.

[000127] In an embodiment, time division multiplexing may be used instead of wavelengthbased multiplexing. This may be achieved by applying different delays to pulses of radiationemitted by different wavelength conversion materials 4. The lithographic apparatusgenerates pulsed EUV radiation, and as a result the radiation emitted by the wavelengthconversion materials will naturally be pulsed, all of the pulses being generated at the sametime. The pulses may be separated in time by providing the waveguides with differentlengths, or by providing photonic crystals arranged to delay the propagation of the radiationemitted by the wavelength conversion materials 4. Pulses of radiation emitted by thewavelength conversion materials 4 may thus be arranged to arrive at a detector in series,thereby allowing the detector to separately detect the pulses.

[000128] In illustrated embodiments of the invention, an alignment grating that extends inthe x-direction is shown. An alignment grating that extends in the y-direction may beprovided. A pair of alignment gratings that extend in perpendicular directions may beprovided.

[000129] In an embodiment, the alignment grating may comprise a two-dimensional arrayof rectangles (for example comprising a set of squares in a chess-board type configuration).Where this is the case, the same alignment grating may be used to obtain alignment in twoperpendicular directions (e.g. the x-direction and the y-direction). Different waveguides mayextend from each window of the alignment grating. At least some of the waveguides mayextend to different depths in order to allow the waveguides to move away from the alignmentgrating without crossing one another. The detectors may, for example, be provided in anarray. The detectors may for example comprise a CCD array.

[000130] In an embodiment, radiation that is detected may be converted into a digitalsignal by the processor. A radiation emitter (e.g. an LED) on the substrate table WT may bearranged to emit modulated radiation that represents the digital signal. A detector (e.g. acamera) located away from the substrate table may be used to detect the digital signalemitted by the radiation emitter. The signal detected by the detector may then travel to acontroller, processor or other electronics that forms part of the lithographic apparatus.

[000131 ] A waveguide may include a dopant that may be used to amplify the radiation thatis travelling along the waveguide. The dopant may, for example, be YAG:Ce, or any othersuitable material. Optical pumping of the dopant may, for example, be provided using LEDsor other optical sources that have a wavelength that is sufficiently short to excite the dopantto an excited state (the wavelength is shorter than the wavelength of radiation that is emittedfrom the wavelength conversion material and that travels along the waveguide). Dopantmay be provided in the waveguide if the loss of radiation is expected to be so high that anundesirably low signal to noise ratio will be seen at the detector.

[000132] The semiconductor chip 2 may include other components that may be usefulduring operation of the lithographic apparatus. For example, the semiconductor chip 2 mayinclude alignment gratings (or other alignment marks), which may be used to measure thealignment of substrates W relative to the substrate table WT. The alignment gratings may,for example, be formed using aluminium or some other suitable masking material. Thealignment gratings may be unconnected to optical sensors, waveguides or othercomponents.

[000133] A layer of zirconium (not shown) may be provided over the windows 9, 109, 209(and optionally also over the opaque material 8). The zirconium transmits the majority ofEUV radiation that is incident upon it but blocks other wavelengths (e.g. infrared, visible,DUV) that may be emitted by the source collector module SO. The zirconium may thus help to reduce or eliminate the influence of non-EUV radiation on measurements made using thesensor AS. The zirconium layer may, for example, be around 100 nm thick, or may haveany other suitable thickness. The zirconium layer may be covered with a protective layer toprevent oxidation. The protective layer may, for example, be TiN (e.g. around 10 nm thick),or may be any other suitable material. A different material may be used instead of zirconiumto block non-EUV radiation while allowing EUV radiation to be transmitted.

[000134] In illustrated embodiments of the invention, the windows 9 are open space.However, the windows 9 may comprise material that allows the transmission of at leastsome EUV radiation (i.e. such that EUV radiation travels through the material to thewavelength conversion material 4). The windows 9 may, for example, be formed from Si02,or may be formed from any other suitable material.

[000135] The wavelength conversion material may be provided in the windows 9, 109, 209instead of being provided beneath the windows. The wavelength conversion material maybe provided in part in the windows 9, 109, 209 and in part below the windows. Thewavelength conversion material 4 may be provided above the windows, or in part above thewindows. Where this is the case, some of the radiation emitted by the wavelengthconversion material may be blocked by the opaque material 8. In general, the wavelengthconversion material may be said to be located at the windows, meaning that the wavelengthconversion material may be located on top, in and/or beneath the window.

[000136] As mentioned further above, the waveguides may be considered to be embeddedin the semiconductor material (e.g. semiconductor chip 2). The waveguides may beconsidered to be provided in conduits formed in the semiconductor material (e.g.semiconductor chip 2). The waveguides and the semiconductor chip 2 may be consideredto form a solid block (i.e. no air being present at boundaries between the waveguides andthe semiconductor chip).

[000137] Although the windows 9, 109, 209 form a grating in illustrated embodiments, thewindows may be arranged to form any desired pattern. The term ‘window’ is not intended toimply that the window is entirely surrounded by opaque material 8. Opaque material may beomitted from one or more sides of the windows 9,109, 209.

[000138] Although the waveguides 5, 15 have been shown beneath the wavelengthconversion material 4 in embodiments of the invention, in other embodiments, one or morewaveguides may be located in some other position. For example, a waveguide may belocated to one side of a piece of waveguide conversion material.

[000139] It is not essential that the waveguide is in contact with the wavelength conversionmaterial. For example, there may be a gap between the wavelength conversion materialand the waveguide. The gap may be filled with a material that provides a degree ofrefractive index matching between the wavelength conversion material and the waveguide.

[000140] The term “semiconductor chip” may be interpreted as meaning a piece ofsemiconductor (e.g. a block of semiconductor). Semiconductor waveguides of embodimentsof the invention are provided within the piece of semiconductor.

[000141] The semiconductor chip is an example of a block of material within which anembedded waveguide may be provided. In an alternative example, the block of materialmay be a dielectric, for example glass, quartz, or some other suitable dielectric. Anembedded waveguide may be formed in the block of dielectric by locally changing therefractive index of the dielectric along the desired path of the waveguide. This may be donein a glass block for example by using laser pulses to locally change the refractive index.

[000142] The term ‘waveguide’ may be considered to mean a structure that guidesradiation. The waveguide may comprise a central portion formed from material with a higherrefractive index than adjacent material, e.g. such that total internal reflection occurs at theboundary between the central portion and the adjacent material. The adjacent material may,for example, be cladding that surrounds the central portion (which may be a core of thewaveguide). Alternatively, the waveguide may comprise a central portion that is surroundedby metal. The metal may act to confine the radiation emitted by the wavelength conversionmaterial such that the radiation is guided along the waveguide (the central portion acting asa guide). The central portion surrounded by metal may for example be air or may besemiconductor (e.g. Si02).

[000143] In an embodiment, the waveguide may comprise photonic crystals embedded inthe semiconductor chip (or block of other material), the photonic crystals acting aswaveguides.

[000144] Cartesian coordinates have been used when describing embodiments of theinvention since this is a convenient way of expressing information. The Cartesiancoordinates should not be taken to imply that the invention or components of the inventionmust have a particular orientation.

[000145] Where a material is described as being “opaque” this does not necessarily meanthat absolutely no radiation passes through it. A small amount of radiation may pass throughthe material, but this is significantly less than the amount of radiation that passes through amaterial designed to transmit the radiation (e.g. waveguide core material).

[000146] Although described embodiments of the invention are directed towardsmeasurement of EUV radiation, embodiments of the invention may be used to measureradiation at other wavelengths. For example embodiments of the invention may be used tomeasure DUV radiation (e.g. at wavelengths used by DUV lithographic apparatus).

[000147] Although specific reference may be made in this text to the use of lithographicapparatus in the manufacture of ICs, it should be understood that the lithographic apparatusdescribed herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-paneldisplays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan willappreciate that, in the context of such alternative applications, any use of the terms “wafer”or “die” herein may be considered as synonymous with the more general terms “substrate”or “target portion", respectively. The substrate referred to herein may be processed, beforeor after exposure, in for example a track (a tool that typically applies a layer of resist to asubstrate and develops the exposed resist), a metrology tool and/or an inspection tool.Where applicable, the disclosure herein may be applied to such and other substrateprocessing tools. Further, the substrate may be processed more than once, for example inorder to create a multi-layer 1C, so that the term substrate used herein may also refer to asubstrate that already contains multiple processed layers.

[000148] The term “lens”, where the context allows, may refer to any one or combination ofvarious types of optical components, including refractive, reflective, magnetic,electromagnetic and electrostatic optical components.

[000149] The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 5-20 nm, for example within the range of13-14 nm, or example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.

[000150] While specific embodiments of the invention have been described above, it will beappreciated that the invention may be practiced otherwise than as described. For example,the invention may take the form of a computer program containing one or more sequencesof machine-readable instructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having such a computerprogram stored therein. The descriptions above are intended to be illustrative, not limiting.Thus it will be apparent to one skilled in the art that modifications may be made to theinvention as described without departing from the scope of the clauses set out below. Otheraspects of the invention are set out as in the following numbered clauses: 1. A substrate table with a sensor, the sensor comprising: a block of material provided with a layer of material opaque to radiation, the layer ofmaterial having at least one window configured to allow the transmission of the radiation;a wavelength conversion material located at the window; anda waveguide positioned to receive radiation emitted by the wavelength conversionmaterial, the waveguide being embedded in the block of material and being configured toguide radiation emitted by the wavelength conversion material through the block of materialand towards a detector.

2. The substrate table of clause 1, wherein the block of material is a semiconductor chipor a dielectric block.

3. The substrate table of any preceding clause, wherein the detector is provided in thesubstrate table.

4. The substrate table of clause 3, wherein the detector is provided in thesemiconductor chip, or is provided next to the semiconductor chip.

5. The substrate table of clause 3 or clause 4, wherein the detector is one of a pluralityof detectors.

6. The substrate table of any preceding clause, wherein the window is one of a pluralityof windows.

7. The substrate table of clause 6, wherein a set of windows extends in a first direction,and wherein at least some of the windows have different positions in a second direction, thesecond direction being transverse to the first direction.

8. The substrate table of clause 6 or clause 7, wherein at least some of the windows areprovided at different heights.

9. The substrate table of any of clauses 6 to 8, wherein each window is provided with aseparate piece of wavelength conversion material.

10. The substrate table of any preceding clause, wherein the waveguide is one of aplurality of waveguides.

11. The substrate table of clause 6 and clause 10, wherein each window is associatedwith a different waveguide.

12. The substrate table of clause 9 or clause 10, wherein at least some of thewaveguides extend to different depths within the semiconductor chip.

13. The substrate table of any preceding clause, wherein a dopant configured to amplifythe radiation emitted by the wavelength conversion material is provided in the waveguide,and wherein an optical pump is arranged to directly pump radiation into the waveguide, thepump radiation having a wavelength configured to excite the dopant.

14. The substrate table of any preceding clause, wherein the wavelength conversionmaterial is configured to emit radiation having a wavelength in the wavelength range 500-2000 nm when EUV radiation is incident upon the wavelength conversion material.

15. The substrate table of any preceding clause, wherein the layer of opaque material isopaque to EUV radiation and is opaque to radiation in the wavelength range 500-2000 nm.

16. A lithographic apparatus comprising: an illumination system configured to condition a radiation beam;a support constructed to support a patterning device, the patterning device beingcapable of imparting the radiation beam with a pattern in its cross-section to form apatterned radiation beam; a substrate table according to any of clauses 1 to 15, the substrate table beingconstructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a targetportion of the substrate.

17. A device manufacturing method comprising using the lithographic apparatus ofclause 16 to manufacture devices, wherein the method includes using the sensor tomeasure an optical property of an EUV radiation beam, and using the sensor to measurealignment of the substrate table and a patterning device.

18. A lithographic apparatus comprising: an illumination system configured to condition a radiation beam;a support constructed to support a patterning device, the patterning device beingcapable of imparting the radiation beam with a pattern in its cross-section to form apatterned radiation beam; a substrate table with a sensor, the sensor comprising a block of material provided with a layer of material opaque to radiation, thelayer of material having at least one window configured to allow the transmission of theradiation, a wavelength conversion material located at the window, anda waveguide positioned to receive radiation emitted by the wavelengthconversion material, the waveguide being embedded in the block of material andbeing configured to guide radiation emitted by the wavelength conversion material throughthe block of material and towards a detector, the substrate table being constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a targetportion of the substrate.

19. A device manufacturing method comprising: using a lithographic apparatus comprising an illumination system configured tocondition a radiation beam; a support constructed to support a patterning device, thepatterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table with a sensor, the sensorcomprising a block of material provided with a layer of material opaque to radiation, the layerof material having at least one window configured to allow the transmission of the radiation,a wavelength conversion material located at the window, and a waveguide positioned toreceive radiation emitted by the wavelength conversion material, the waveguide beingembedded in the block of material and being configured to guide radiation emitted by thewavelength conversion material through the block of material and towards a detector, thesubstrate table being constructed to hold a substrate; and a projection system configured toproject the patterned radiation beam onto a target portion of the substrate; using the sensor to measure an optical property of an EUV radiation beam; and using the sensor to measure alignment of the substrate table and the patterning device.

20. A device manufacturing method comprising: patterning an EUV beam of radiation with a pattering device; projecting a patterned beam of radiation onto a substrate supported by a substratetable with a projection system; measuring an optical property of the EUV radiation beam with a sensor in thesubstrate table, the sensor comprising a block of material provided with a layer of materialopaque to radiation, the layer of material having at least one window configured to allow thetransmission of the radiation, a wavelength conversion material located at the window, and awaveguide positioned to receive radiation emitted by the wavelength conversion material,the waveguide being embedded in the block of material and being configured to guideradiation emitted by the wavelength conversion material through the block of material andtowards a detector; and measuring alignment of the substrate table and the patterning device with the sensor.

Claims (1)

1. Een lithografieinrichting omvattende: een belichtinginrichting ingericht voor het leveren van een stralingsbundel; een drager geconstrueerd voor het dragen van een patroneerinrichting, welke patroneerinrichting in staat is een patroon aan te brengen in een doorsnede van de stralingsbundel ter vorming van een gepatroneerde stralingsbundel; een substraattafel geconstrueerd om een substraat te dragen; en een projectieinrichting ingericht voor het projecteren van de gepatroneerde stralingsbundelop een doelgebied van het substraat, met het kenmerk, dat de substraattafel is ingerichtvoor het positioneren van het doelgebied van het substraat in een brandpuntsvlak van deprojectieinrichting.A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being able to apply a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.