Patent ID: 12259546

DETAILED DESCRIPTION

FIG.1shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure or mask table MT configured to support a patterning device MA, a projection system PS and a substrate table WT configured to support a substrate W.

The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device10and a facetted pupil mirror device11. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device10and/or the faceted pupil mirror device11. For example, a micromirror array as described herein may be added to the illumination system IL in addition to the facetted field mirror device10and faceted pupil mirror device11as disclosed in U.S. Pat. No. 8,294,877 B2, which is hereby incorporated in its entirety by reference, or may be used to replace one or both of the faceted field mirror device10and the faceted pupil mirror device11as disclosed in U.S. Pat. No. 10,254,654 B2, which is hereby incorporated in its entirety by reference. In that case the illumination system IL, which now includes at least one micromirror array as described herein, is a programmable illuminator IL. Such a programmable illuminator IL may be used for conditioning a radiation beam used to illuminate the patterning device. For example, the programmable illuminator IL may be used to control or condition the EUV radiation beam B by providing it with a desired cross-sectional shape and/or a desired intensity distribution.

After being thus conditioned, the EUV radiation beam B illuminates the patterning device MA and interacts with it. As a result of this interaction, a patterned EUV radiation beam B′ is generated. The projection system PS is configured to project the patterned EUV radiation beam B′ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors13,14which are configured to project the patterned EUV radiation beam B′ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B′, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors13,14inFIG.1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B′, with a pattern previously formed on the substrate W.

A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

FIG.1ashows an inspection apparatus that is known from U.S. Pat. No. 9,946,167 B2, which is hereby incorporated in its entirety by reference.FIG.1acorresponds toFIG.3aof U.S. Pat. No. 9,946,167 B2. The inspection apparatus is a dark field metrology apparatus for measuring e.g. overlay and/or alignment.

In lithographic processes, it is desirable to frequently make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device and alignment, i.e. the position of alignment marks on the substrate. Various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target structure, e.g. a grating or mark(er), and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.

The dark field metrology apparatus shown inFIG.1amay be a stand-alone device/system or may be incorporated in the lithographic apparatus LA as an alignment system and/or as an overlay measurement system (not shown). An optical axis, which has several branches throughout the apparatus, is represented by a dotted line O. In this apparatus, light emitted by radiation source111(e.g., a xenon lamp) is directed onto a substrate W via a beam splitter115by an optical system comprising lenses112,114and objective lens116. These lenses are arranged in a double sequence of a 4F arrangement. Therefore, the angular distribution at which the radiation is incident on the substrate can be selected by defining a spatial intensity distribution in a plane that presents the spatial spectrum of the substrate plane, here referred to as a (conjugate) pupil plane. In particular, this can be done by inserting an aperture plate113of suitable form between lenses112and114, in a plane which is a back-projected image of the objective lens pupil plane. In the example illustrated, aperture plate113has different forms, labeled113N and113S, allowing different illumination modes to be selected. The illumination system in the present example forms an off-axis illumination mode. In the first illumination mode, aperture plate113N provides off-axis from a direction designated, for the sake of description only, as ‘north’. In a second illumination mode, aperture plate113S is used to provide similar illumination, but from an opposite direction, labeled ‘south’. Other modes of illumination are possible by using different apertures. The rest of the pupil plane is desirably dark, as any unnecessary light outside the desired illumination mode will interfere with the desired measurement signals.

A target structure (not shown), e.g. a grating or mark(er), on substrate W is placed normal to the optical axis O of objective lens116. A ray of illumination impinging on the target structure from an angle off the axis O gives rise to a zeroth diffraction order ray and two first diffraction order rays. Since the aperture in plate113has a finite width (necessary to admit a useful quantity of light) the incident rays will in fact occupy a range of angles, and the diffracted rays 0 and +1/−1 will be spread out somewhat. According to the point spread function of a small target, each order +1 and −1 will be further spread over a range of angles, not a single ideal ray. Note that the grating pitches and illumination angles can be designed or adjusted so that the first order rays entering the objective lens are closely aligned with the central optical axis.

At least the 0 and +1 orders diffracted by the target on substrate W are collected by objective lens116and directed back through beam splitter115. Both the first and second illumination modes are illustrated, by designating diametrically opposite apertures labeled as north (N) and south (S). When the incident ray is from the north side of the optical axis, that is when the first illumination mode is applied using aperture plate113N, the +1 diffracted rays, which are labeled +1(N), enter the objective lens116. In contrast, when the second illumination mode is applied using aperture plate113S the −1 diffracted rays (labeled −1(S) are the ones which enter the lens116.

A second beam splitter117divides the diffracted beams into two measurement branches. In a first measurement branch, optical system118forms a diffraction spectrum (pupil plane image) of the target on first sensor119(e.g. a CCD or CMOS sensor) using the zeroth and first order diffractive beams. Each diffraction order hits a different point on the sensor, so that image processing can compare and contrast orders. The pupil plane image captured by sensor119can be used for focusing the inspection apparatus and/or normalizing intensity measurements of the first order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

In the second measurement branch, an optical system including lenses120,122forms an image of the target on the substrate W on sensor123(e.g. a CCD or CMOS sensor). In the second measurement branch, an aperture plate referred to as field stop121is provided in a plane that is conjugate to the pupil-plane. This plane will be referred to as an ‘intermediate pupil plane’ when describing the invention. Field stop121functions to block the zeroth order diffracted beam so that the image of the target formed on sensor123is formed only from the −1 or +1 first order beam. The images captured by sensors119and123are output to image processor and controller PU, the function of which will depend on the particular type of measurements being performed. Note that the term ‘image’ is used here in a broad sense. An image of the grating lines as such will not be formed, if only one of the −1 and +1 orders is present.

The illumination system of the inspection apparatus comprises an illuminator110. As shown inFIG.1a, this illuminator110comprises lens112and aperture plate113. More details of the inspection apparatus can be found in U.S. Pat. No. 9,946,167 B2.

FIG.1bshows a programmable illuminator140for use in the inspection apparatus ofFIG.1a. This programmable illuminator140can be used in the inspection apparatus ofFIG.1ainstead of the illuminator110. The programmable illuminator140comprises a micromirror array133according to the present invention as well as a low NA relay 4F system135comprising a pair of lenses. Radiation or light from a radiation source130(not part of the programmable illuminator140), e.g. a broad band radiation source or white light source, may be directed via an optional fiber131and an optional collimating lens system132to the micromirror array133. A processing unit PU can control the micromirror array133in such a way that the micromirrors134, or more precise the mirrors in the micromirrors134, in the micromirror array133are tilted individually. By tuning the tilt angle of each individual mirror independently, the spatial distribution of the light that is output by the low NA relay system135can be controlled and various illumination modes can be made as desired without having to use aperture plates. If the programmable illuminator140is used in the inspection apparatus ofFIG.1ait interfaces with lenses114, meaning that the light that is output by the low NA relay system135is received by the lenses114ofFIG.1a.

In order to control the spectral distribution of the light that is output by the low NA relay system135at least part of the mirrors may comprise a grating on top of the mirror surfaces (not shown). The grating may be the same for all mirrors or, alternatively, different gratings, e.g. gratings having different pitches, may be used. By appropriate control of the micromirror array133the light that is output by the low NA relay system135comprises a single wavelength or a single (narrow) range of wavelengths. It is however also possible to control the micromirror array133in such a way that the light that is output by the low NA relay system135comprises a number of different wavelengths or a number of different (narrow) ranges of wavelengths. The gratings may be lithographically patterned on the mirror surfaces. Each mirror with grating diffracts light of different wavelengths in different directions according to the associated grating equation. A portion of the diffracted light is captured by the low NA relay system135and an image is formed. By tuning the angle of each mirror independently, the light distribution at the output can be controlled both spatially and spectrally as (a) certain diffraction order(s) will be captured by the low NA relay system135and (an)other diffraction order(s) will not be captured. Such a spatial and spectral light distribution can be used advantageously for example for illuminating and measuring an overlay target structure on a substrate or for measuring the position of an alignment mark on a substrate. In this text, the terms target structure, target, mark, marker and grating are, where the context allows, all synonyms of each other.

The spectral bandwidth of the diffracting beam which can be captured by the low NA relay system135is dλ=P·NA where P is the pitch of the grating and NA is the numerical aperture of the low NA relay system135. With P=500 nm and NA=0.02 the spectral bandwidth is 10 nm, meaning that a diffraction order of the grating comprises a range or band of wavelengths of 10 nm.

The spatial resolution of the low NA relay system135is ˜λ/NA. With λ=850 nm and NA=0.02 the spatial resolution is 42.5 micrometer. If the size of the mirrors Is greater than 42.5 micrometer, each mirror can be resolved. A reasonable size of a mirror is 100×100 micrometer.

By rotating/tilting the mirrors around their individual axis, a different central wavelength band can be directed into the low NA relay system135. The rotating range of each mirror required for operation over the visible wavelength range should be ΔV/2P, where Δλ=400 nm for an operating wavelength range of 450 nm-850 nm. This means that each mirror must be able to rotate by 0.4 radians.

The MEMS system inFIG.2is a micromirror with a mirror (not shown) and four electrostatic actuators21for displacing the mirror. In other embodiments (not shown) the micromirror may have a different number of electrostatic actuators21for displacing the mirror. In all these embodiments the micromirror has one or more electrostatic actuators21for displacing the mirror. A number of micromirrors as shown inFIG.2can be arranged in an array to form a micromirror array.

FIG.2shows a schematic diagram of a part of a micromirror array20for displacing a mirror (not shown) in the array. The part of the array20comprises electrostatic actuators21being two pairs of comb actuators. Each of the four comb actuators21comprises a fixed part22afixed to the substrate and a moving part22bmovable relative to the substrate. The movable part22bis shaped as a trapezium and is flexibly anchored to the substrate at each corner of the trapezium by anchors23. The anchors23provides a flexible connection between the substrate and the movable part22bof the comb actuator21. The movable part22bof each comb actuator is connected to a post24, which supports the mirror. By applying a voltage to the comb actuators21, the movable part22bof the actuators21moves relative to the substrate and exerts a force on the post24, which deflects and thereby displaces the mirror. The magnitude of the displacement is a function of the applied voltage. By selectively applying a voltage to the pairs of comb actuators, the mirror can be tipped and/or tilted. The micromirror array20has four-fold rotational symmetry about the axis25.

FIG.3shows a schematic diagram of a part of a micromirror array30for displacing a mirror (not shown) in the array. The micromirror array30may be the micromirror array20ofFIG.2. The micromirror array30has four-fold rotational symmetry about the axis39. The part of the array30comprises a post31for supporting the mirror connected to four pairs of spring elements32,35,36,37. The post31typically connects to the center of the back of the mirror (i.e. a portion of the mirror opposite to the reflective surface). Referring for example to the pair of spring elements32, it comprises an upper spring element33aand a lower spring element33b. Here the terms “upper” and “lower” refer different distances above a plane of the substrate. The spring elements33a,33bare strips of flexible material, typically strips of silicon. The upper spring element33aof each pair of spring elements32is connected to an electrostatic actuator (not shown) such as a comb actuator. The spring elements33a,33bcan transmit forces from the electrostatic actuators to the post31in order to deflect and/or translate the post31relative to the substrate, and thereby displace the mirror relative to the substrate. Flexible connectors34are thin silicon strands extending from the post31in a directly parallel to the plane of the substrate, and are mounted on the post31for transferring heat from the mirror to a heat sink (not shown), while not significantly impeding motion of the post31. The flexible connectors34may be formed in the same silicon layer as the upper spring elements33a.

Suppose that the electrostatic actuators corresponding to the pairs of spring elements35,37act to translate the lower end of the post39in the direction indicated by the arrow38, which is transverse to the axis39. This causes the other pair of spring elements32,36to be deformed. That is, the bottom spring element of each of the pairs of spring elements32,36is displaced in the direction38relative to the top spring element of the pair of spring elements, and this tends to rotate the bottom spring element about its elongation direction relative to the substrate. This rotates the post31about that elongation direction. Thus, the post31, and hence the mirror it supports, is tilted relative to the top surface of the substrate.

FIG.4ashows a schematic diagram of a top view of a part of a micromirror array40(which may be one of the micromirror arrays20,30). The part of the array40comprises a post41for supporting a mirror (not shown) and two pairs of comb actuators42connected to the post41via spring elements43. Each comb actuator42may comprise a part shaped as a trapezium and flexibly anchored to the substrate at some or all corners of the trapezium by anchors44. The part of the array40shown inFIG.4aalso comprises a heat diffuser45comprising a heat sink (not shown) and flexible connectors46connecting the post41to the heat sink.

FIG.4aalso illustrates in more detail the construction of each of the comb actuators42. As in known comb actuator designs, the fixed part of each comb actuator comprises two parallel conductive portions42a,42bsurrounded by a portion of the moving part43cof the comb actuator42. From each of the conductive portions42a,42bextend, in a respective opposite directions towards the moving part42cof the comb actuator, a plurality of conductive elements (“fixed teeth”). The fixed teeth of each of the conductive portions42a,42bare interleaved with a corresponding plurality of “moving teeth” of approximately the same length mounted on the moving part42cof the comb actuator, and extending towards the corresponding conductive portion42a,42b. Forces are developed between the moving teeth and the corresponding fixed teeth according to the differing voltages of the conductive portions42a,42band the moving part42c. Typically, these forces are relatively constant for a range of movement of the moving part42crelative to the conductive portions42a,42bwhich is approximately the length of the teeth.

When, for example, the conductive portion42band the moving part42cof each comb actuator are placed at zero volts, and the conductive portion42ais set at a non-zero voltage (e.g. positive 100 volts), an attractive force is generated between the fixed teeth of conductive portion42aand the moving teeth of the moving part42cof the comb actuator. This draws the moving part42cto the left inFIG.4a, by a distance approximately equal to the length of the teeth. Conversely, if the conductive portion42aand the moving part42care both grounded, and a non-zero voltage is applied to the conductive portion42b, the moving part42cof the comb actuator is urged to the right.

FIG.4bshows a similar part of the micromirror array, but including four protrusions47from the mirror and connected to the post41. The protrusions47are part of a sensing element for sensing displacement of the mirror. The sensing element further comprises an electrode (not shown) underneath each protrusion47, which is arranged to sense a capacitance between the protrusion47and that electrode. The sensing element may be a part of a displacement control feedback system, in which the measured capacitance is used to determine a mirror position. The feedback system may then adjust the voltage applied to the comb actuators42based on the determined mirror position and based on a target mirror position. The protrusions may also have the additional advantage of providing a larger bonding area between the mirror and the post41.

FIG.5shows a cross sectional diagram of a part of a mirror50in a micromirror array which may be the micromirror array20,30and/or40described above. The mirror50is typically rectangular (which in this document is used to include square) with each side being in the range 0.5 mm to 2.5 mm. For example, it may be a square having a surface area of 1 mm2and a through thickness of 100 μm. The mirror50is connected to one end of a post51, which supports the mirror. The post51may be a cuboid having 20 μm×20 μm end surfaces and a 150 μm through thickness (i.e. height). The lower end of the post51is connected to a first end of a first spring element52(bottom spring element). The opposite end of the first spring element52is connected (via a vertically extending post) to the first end of a second spring element53(top spring element). A second end of the second spring element53is connected to the moving part54aof a comb actuator55. One corner of the moving part54aof the comb actuator is connected to the substrate57by a post591which is mounted on the substrate57by a connection592(note that the post591and the connection592are at a different distance in the direction into the diagram from the first spring element52and the second spring element53; that is, the plane containing the post51, the first spring element52and the second spring element53does not contain the post591or the connection592). Thus, the mirror50is supported by the post51which in turn is supported by the first spring element52, which in turn is supported by the second spring element53, which in turn is supported by the moving part54aof the comb actuator, which in turn is supported by the substrate57using the post591and the connection592.

The connection between the moving part54aand the substrate57provided by the post591and the connection592is resilient, and flexes as the moving part54aof the comb actuator moves laterally (i.e. in the left-right direction inFIG.5) relative to the substrate57.

Thus the post51is supported from the substrate57by a plurality of members, including multiple resilient members, which permit the post51to move relative to the substrate57. Specifically, the post51is laterally translatable relative to the substrate57, and additionally the post51is deflectable relative to the substrate57, thus changing the relative orientation of the surface of the mirror and the top surface of the substrate57.

By applying a voltage to the comb actuator55, the moving part54acan be pulled towards the fixed part54bof the comb actuator55. The two spring elements52and53couple the post51to the comb actuator55so as to transfer forces from the comb actuator to the post51in order to displace the mirror50. The first spring element52may be a 1 μm thick strip of polysilicon and the second spring element53may a 1 μm thick strip of single crystalline silicon. The combs of the comb actuator55may be 30 μm thick polysilicon.

A flexible connector56, being a part of a heat diffuser, is connected between the post51and the substrate57. The flexible connector56is connected to the substrate57by a post593and connection594, which do not lie in the plane containing the post51, the first spring element52and the spring element53. One or more (and typically all) of flexible connector56, the post593and connection594are arranged to flex as the post51moves relative to the substrate57.

The flexible connector56of the heat diffuser and the second spring element53may be formed in the same silicon layer. The heat diffuser may also be arranged to electrically connect the post51to the substrate57. Only a half of the cross section of the mirror50and the underlying MEMS components is shown inFIG.5, with the dashed line58indicating mirror symmetry.

Embodiments of the micromirror array can provide tip and tilt displacement range of +/−120 mrad and a mirror accuracy of 100 prad. Embodiments of the micromirror array can be operated at high light intensities as required for EUV, and may work at 40 to 60 kW/m2 of absorbed thermal power density (which implies an incident light power density on the surface of the mirror which is even larger). This is orders of magnitude higher than the absorbed thermal power density of micromirror arrays used in some other applications. This is possible because the comb actuators21are operative to provide, even at a relatively low actuator voltage (e.g. under about 100 volts), such a strong force that they are able to deform the flexible connector56even though the flexible connector56is thick enough to provide high heat conductivity to the substrate. Due to the high thermal conductivity, the micromirror array may in use have a temperature of under about 100 degrees Celsius.

The distance by which the outer edge of the mirror can move may be in the range 50 μm-120 μm, such as about 80 μm. Typically, known micromirror arrays permit a smaller range of motion than this, such as only a few microns. The greater spacing is achieved in this embodiment because the actuation force is applied (laterally) to the pillar, rather than, for example, by an electrostatic actuator having a first conductive plate mounted on the mirror and a second conductive plate mounted on the substrate. This would typically limit the range of movement of the mirror to be the range of relative movement of the plates such that the electrostatic actuator is able to operate effectively. Typically, this distance is only a few microns. Furthermore, using a comb actuator means that the actuator is efficient even if the moving and fixed parts of the actuator move relatively by an amount roughly equal to the length of the teeth.

Also described herein are methods of forming a micromirror array. A method according to an embodiment comprises providing a plurality of wafers of silicon, forming elements of the micromirror array in the wafers and then bonding the wafers to each other.

FIG.6shows a schematic diagram of five silicon wafers that are bonded together to form a micromirror array. The five wafers comprise a wafer600for forming the mirror601, a wafer602for forming the upper spring element603and optionally for forming the flexible connectors604of the heat diffuser, a wafer605for forming the combs606of the comb actuators and for forming the lower spring element607, a wafer608being an interposer wafer for providing electrical connections to the micromirror array and also for forming a substrate609which supports the micromirror array, and a wafer610for providing electric connections to the interposer wafer608.

A method of forming the micromirror array may comprise the following steps:a. Providing a first wafer602(“upper spring wafer”), which may be a SOI wafer with 1 μm highly doped silicon film. The upper spring603pattern is formed in the wafer, and optionally also the flexible connectors604of the heat diffuser.b. Providing a second wafer605(“comb wafer”) and forming the combs606in the second wafer605. The comb wafer may comprise two SOI wafers bonded together, one SOI wafer having a thin (e.g. 1 um) highly doped silicon film and the other SOI wafer having a thicker (e.g. 30 um) layer of highly doped silicon. Alternatively the comb wafer605may comprise one SOI wafer with a thin (e.g. 1 um) highly doped silicon film and, deposited thereon, a highly doped silicon layer (e.g. 30 um thick). The wafer605is patterned to form the combs606of the comb actuators.c. Bonding the first wafer600to the second wafer605so as to to connect the upper spring element603to the comb actuator. The step of bonding may comprise fusion bonding with cavities. After bonding, a handle wafer of the second wafer605can be removed, followed by via patterning, metal fill and patterning or CMP, and lithography and etching to form the lower spring elements607.d. Providing a third wafer608(“interposer wafer”), which may be an SOI wafer with a 100 um silicon film. The third wafer608is patterned to form cavity holes.e. Bonding the third wafer608to the first and second wafers602and605so as to connect the comb actuators to the substrate609. The step of bonding may comprise fusion bonding with cavities. After bonding, a handle wafer of the third wafer608may be removed, followed by via etching through the silicon and oxide of the third wafer608, oxide liner deposition, then further via etching through silicon and oxide into the second wafer602, TSV Cu fill and CMP, and redistribution layer (RDL) pattern formation on the non-bonded side of the interposer wafer608.f. Forming connections to the heat sink and sensing elements. The step of forming connections to the heat sink and to the sensing elements may comprise removing a handle wafer of the first wafer602, followed by via hole patterning and etching though the highly doped silicon film of the first wafer602, metal fill and patterning, depositing bonding metal and patterning, and removing the oxide membrane on top of the highly doped silicon film of the first wafer602.g. Providing a fourth wafer600(“mirror wafer”). The mirror wafer600may be an SOI wafer with 250 um silicon film. Providing the mirror wafer600may comprise, depositing bonding material and patterning, forming protrusions being capacitor top-plates of the sensing elements, depositing a hard mask and patterning, providing a resist mask and etching silicon (e.g. 100 um etch and over-etch), removing the resist mask and etching the silicon further (e.g. 150 um), and removing the hard mask.h. Bonding the mirror wafer600to the stack of wafers comprising the first wafer602, the second wafer605and the third wafer608, so as to form the post connecting the mirror to the substrate609. The bonding may comprise eutectic bonding.i. Forming bump pads on the third wafer608and etching to release the post supporting the mirror601.j. Providing a fifth wafer610(“electronics wafer”). The step of providing the fifth wafer610may comprise providing HV, analog, and digital CMOS components in the electronics wafer, forming TSVs (e.g. 5000 to 10000 connections), and forming bump balls for connecting to the interposer wafer608.k. Attaching the electronics wafer610to the interposer wafer608using the solder bumps on the respective wafers608and610.l. Removing a handle wafer of the mirror wafer600to release the mirror601, followed by dicing (e.g. laser dicing) to complete the micromirror array.

FIG.7shows a schematic diagram of an alternative method of forming a micromirror array comprising only three wafers that are bonded together. The three wafers comprise a mirror wafer700for forming the mirror701, a middle wafer702for forming spring elements703, the combs704of the comb actuators, and the substrate705which supports the micromirror array, and an electronics wafer706for providing electric connections707to the micromirror array.

The method of forming the micromirror array may comprise the following steps:a. Providing a first wafer702(“middle wafer”). The step of providing may comprise providing an SOI wafer with a 1 um highly doped silicon film, depositing oxide, etching of anchor trenches, filling trenches with polysilicon followed by CMP, patterning oxide to provide a masking layer for the lower spring element703, and epitaxially growing a 30 um thick silicon layer on the patterned oxide followed by CMP. The step of providing the middle wafer may further comprise, silicon dry reactive ion etching (DRIE) using the oxide as stopping layer, filling the etched silicon with oxide, epitaxially growing a 1 um silicon layer (poly and single crystalline) for the upper spring elements703, and etching the silicon layer to form the upper spring elements703. The step of providing the middle wafer may further comprise flipping the wafer, via etching, isolation layer deposition, filling vias with metal and patterning (e.g. to form 5000 to 10000 connections).b. Providing a second wafer700(“mirror wafer”). The mirror wafer700may be an SOI wafer with 250 um silicon film. Providing the mirror wafer700may comprise, depositing bonding material and patterning, forming protrusions being capacitor top-plates of the sensing elements, depositing a hard mask and patterning, providing a resist mask and etching silicon (e.g. 100 um etch and over-etch), removing the resist mask and etching the silicon further (e.g. 150 um), and removing the hard mask.c. Bonding the first and second wafers together. The bonding may comprise eutectic bonding.d. Releasing the spring elements703and the combs704. The step of releasing may comprise forming bump bonding pads (e.g. 10 to 20 per mirror), etching vias through a handle wafer of the first wafer to form space for the movement of the lower spring elements703to form a path for subsequent vapor HF etch, etching oxide using vapor HF etch to release the spring element703and the combs704.e. Providing a third wafer706(“electronics wafer”). The step of providing the electronics wafer706may comprise providing HV, analog, and digital CMOS components in the electronics wafer, forming TSVs (e.g. 5000 to 10000 connections), and forming bump balls for connecting to the middle wafer702.f. Attaching the electronics wafer706to the middle wafer702using the solder bumps on the respective wafers.g. Removing a handle wafer of the mirror wafer700to release the mirror701, followed by dicing (e.g. laser dicing) to complete the micromirror array.

Embodiments are provided according to the following clauses:

1. A micromirror array comprising:

a substrate;a plurality of mirrors for reflecting incident light;for each mirror of the plurality of mirrors, a respective post supporting the mirror;

andfor each mirror of the plurality of mirrors, one or more electrostatic actuators connected to the substrate for applying force to the post to displace the post relative to the substrate, thereby displacing the mirror.
2. A micromirror array according to clause 1, wherein the one or more electrostatic actuators comprise at least a pair of comb actuators, wherein each comb actuator comprises a static part fixed to the substrate and a moving part which is movable relative to the substrate and connected to the post, at least one plurality of elongate conductive elements extending from the static part and being interleaved with a plurality of elongate conductive elements extending from the moving part.
3. A micromirror array according to clause 2, wherein the one or more electrostatic actuators include two pairs of comb actuators connected to the post and arranged to enable tip and tilt displacement control of the mirror, wherein each comb actuator comprises a static part fixed to the substrate and a moving part which is movable relative to the substrate and connected to the post.
4. A micromirror array according to clause 2 or 3, wherein the moving part of each comb actuator is shaped as a trapezium and anchored to the substrate at some or all corners of the trapezium.
5. A micromirror array according to any one of the preceding clauses, wherein each electrostatic actuator is connected to the post by one or more spring elements.
6. A micromirror array according to any one of the preceding clauses and comprising, for each mirror of the plurality of mirrors, a sensing element for sensing displacement of the mirror.
7. A micromirror array according to clause 6, wherein the sensing element comprises a protrusion from the mirror and an electrode connected to the substrate and arranged to sense a capacitance between the protrusion and the electrode.
8. A micromirror array according to clause 6, wherein the sensing element comprises a piezoresistor coupled to the post.
9. A micromirror array according to any one of the preceding clauses, and comprising, for each mirror of the plurality of mirrors, a heat diffuser for diffusing heat from the mirror to the substrate.
10. A micromirror array according to clause 9, wherein the heat diffuser comprises a heat sink and one or more flexible connectors connecting the heat sink to the post.
11. A micromirror array according to any one of the preceding clauses, wherein each mirror of the plurality of mirrors is for reflecting light having a wavelength of substantially 13.5 nm.
12. A programmable illuminator comprising a micromirror array according to any one of clauses 1 to 11 for conditioning a radiation beam.
13. A programmable illuminator according to clause 12 and comprising a displacement control feedback system configured to determine for each mirror of the plurality of mirrors a position of the mirror and to adjust a voltage applied to the one or more electrostatic actuators based on the determined position and based on a predefined target position of the mirror.
14. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, comprising a programmable illuminator according to clauses 12 or 13 for conditioning a radiation beam used to illuminate the patterning device and/or for conditioning a radiation beam used to measure a target structure on the substrate.
15. An inspection apparatus, comprising a programmable illuminator according to clauses 12 or 13 for conditioning a radiation beam used to measure a target structure on a substrate.
16. A method of forming a micromirror array comprising: providing a substrate;forming a plurality of mirrors for reflecting incident light and, for each mirror of the plurality of mirrors, a respective post supporting the mirror;forming, for each mirror of the plurality of mirrors, one or more electrostatic actuators connected to the substrate for applying force to the post to displace the post relative to the substrate, thereby displacing the mirror.
17. A method according to clause 16, wherein the step of forming the one or more electrostatic actuators comprises forming at least a pair of comb actuators, wherein each comb actuator comprises a static part fixed to the substrate and a moving part which is movable relative to the substrate and connected to the post, at least one plurality of elongate conductive elements extending from the static part and being interleaved with a plurality of elongate conductive elements extending from the moving part.
18. A method according to clause 16, wherein the step of forming the one or more electrostatic actuators includes forming two pairs of comb actuators arranged to enable tip and tilt displacement control of the mirror, wherein each comb actuator comprises a static part fixed to the substrate and a moving part which is movable relative to the substrate and connected to the post.
19. A method according to clause 17 or 18, wherein each comb actuator is shaped as a trapezium and anchored to the substrate at some or all corners of the trapezium.
20. A method according to any one of clauses 16 to 19, wherein the step of forming the one or more electrostatic actuators comprises forming one or more spring elements connecting the one or more electrostatic actuators to the post.
21. A method according to any one of clauses 16 to 20 and comprising, for each mirror of the plurality of mirrors, forming a sensing element for sensing a displacement of the mirror.
22. A method according to clause 21, wherein the step of forming the sensing element comprises forming a protrusion from the mirror and an electrode connected to the substrate and arranged to sense a capacitance between the protrusion and the electrode.
23. A method according to clause 21, wherein the step of forming the sensing element comprises forming a piezoresistor coupled to the post.
24. A method according to any one of clauses 16 to 23 and comprising, for each mirror of the plurality of mirrors, forming a heat diffuser for diffusing heat from the mirror to the substrate.
25. A method according to clause 24, wherein the step of forming the heat diffuser comprises forming a heat sink and one or more flexible connectors connecting the heat sink to the post.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. 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 the invention as described without departing from the scope of the claims set out below.