A micromirror array comprises a substrate, a plurality of minors for reflecting incident light and, for each mirror (20) of the plurality of minors, at least one piezoelectric actuator (21) for displacing the minor, wherein the at least one piezoelectric actuator is connected to the substrate. The micromirror array further comprises one or more pillars (24) connecting the minor to the at least one piezoelectric actuator. Also disclosed is a method of forming such a micromirror array. The micromirror array may be used in a programmable illuminator. The programmable illuminator may be used in a lithographic apparatus and/or in an inspection apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 19192311.9 which was filed on Aug. 19, 2019 and of EP application 19199718.8 which was filed on Sep. 26, 2019 which are incorporated herein in its entirety by reference.

FIELD

The present invention relates to a micromirror array, a programmable illuminator comprising such a micromirror array, a lithographic apparatus comprising such a programmable illuminator, an inspection apparatus comprising such a programmable illuminator and a method for forming such a micromirror array.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device onto a layer of radiation-sensitive material (resist) provided on a substrate. The term “patterning device” as employed in this text should be broadly interpreted as referring to a device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. Examples of such patterning devices include:A mask (or reticle). The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask may be supported by a support structure such as a mask table or mask clamp. This support structure ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such a device is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis, for example by applying a suitable localized electric field, or by employing electrostatic or piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning means can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. Such a programmable mirror array may be supported by a support structure such as a frame or table, for example, which may be fixed or movable as required; andA programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. Such a programmable LCD array may be supported by a support structure such as a frame or table, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and a mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.

To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation (here referred to often as simply “light”, though the wavelength may not be in the visible range). The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

Besides the wavelength (λ) of the radiation and the Numerical Aperture (NA) of the projection lens, the shape, or more generally the angular intensity distribution, of the illumination source is one of the most important parameters in enabling high resolution in lithography.

A micromirror array, comprising an array of hundreds or thousands of micromirrors (often referred to below simply as “mirrors”), can be used in the illumination system of a lithographic apparatus to control the cross-sectional shape and intensity distribution of the light. Each micromirror reflects a spot of light and changing the angles of the micromirrors changes the positions of the spots and thus changes the shape of the radiation beam.

Microelectromechanical systems (MEMS) technology may be used to manufacture and control the mirrors. For example, an electrostatic or piezoelectric MEMS system may be used to angle the mirrors.

Currently micromirror arrays exist for shaping light having a wavelength in the deep ultraviolet spectrum (DUV), e.g. λ=193 nm. However, these micromirror arrays cannot be effectively used at shorter wavelengths as required for light in the extreme ultraviolet spectrum (EUV), e.g. λ=13.5 nm. New micromirror array technology is required for use with EUV radiation. Also, advantageous new applications for this new micromirror array technology are desired, for use with EUV and/or non-EUV radiation, e.g. visible light or DUV radiation.

SUMMARY

According to a first aspect of the present invention there is provided a micromirror array, which for example may be used in the illumination system of a lithographic apparatus or an inspection apparatus to condition a radiation beam. The micromirror array comprises a substrate and a plurality of mirrors for reflecting incident light. For each mirror, there is at least one piezoelectric actuator for displacing the mirror connected to the substrate, and one or more pillars connecting the or each piezoelectric actuator to the mirror. The pillar(s) may be operative to support the mirror from the piezoelectric actuator(s). Applying a voltage to the piezoelectric actuator can cause the actuator to move the pillar and thereby displace the mirror, in order to change the angle of the mirror and thereby change the shape of the radiation beam. Preferably, the micromirror array includes four piezoelectric actuators for each mirror, arranged so as to enable tip and tilt displacement control of the mirror.

The micromirror array may also comprise, for each mirror in the array, a heat diffuser for diffusing heat from the mirror. In use, the micromirror array will absorb some energy from the incident light, which increases the temperature of the device. This increase in temperature can decrease device performance. Typically, the micromirror array is intended to operate in environment with a gas pressure far less than one atmosphere, in fact typically substantially in a vacuum, so heat convection is substantially zero. Instead, the heat diffuser allows heat to be conducted away, such as to the substrate. Typically, the heat diffuser comprises a flexible element connected between the mirror and the substrate and arranged to flex as the mirror is moved. Note that there is a trade-off between increased flexibility of the heat diffuser, and increased ability for the heat diffuser to conduct heat away from the mirror. Using piezoelectric actuator(s) allows increased force to be applied to the flexible element, in turn allowing the heat diffuser to be selected to provide improved thermal conductivity.

In one example, the heat diffuser may comprise a heat sink and a thermally conductive post connecting the heat sink to the mirror. The heat sink may comprise a flexible membrane, which allows the post to pivot when the mirror is displaced. The flexible membrane can be a patterned silicon layer, which has the advantage of being readily available in a CMOS manufacturing process, without requiring further masks or process steps. The flexible membrane can comprise grooves through the flexible membrane and extending from an outer edge of the heat sink towards the thermally conductive post. The grooves, which may be curved grooves, increase the flexibility of the membrane so as to not impede motion of the mirror. The piezoelectric actuators of preferred embodiments are selected to provide a level of force which is greater than the electrostatic actuators used in some conventional systems, and which is sufficient to deform the flexible member even though it has sufficient cross-sectional area (for example, as measured at the intersection of the flexible member with a circular-cylindrical surface with an axis coinciding with an axis of the post) to permit greater heat diffusion than that provided for conventional mirror arrays. This allows the present micromirror array to be used in applications for which the conventional mirror arrays would be unsuitable.

The heat sink may comprise a layer of metal, such as aluminum, which has a relatively high thermal conductivity compared to e.g. silicon. The thermally conductive post may also be electrically conductive and connected to ground, so as to prevent charge build up on the mirror, which may otherwise impede displacement control of the mirror.

The piezoelectric actuators may comprise a strip of flexible material connected at one end to the substrate, with the pillar being located at the opposite end of the strip of flexible material, and a layer of piezoelectric material provided on the strip of flexible material. The strip and the layer of piezoelectric material may thereby form a cantilever, anchored to the substrate at one (fixed relative to the substrate) end and connected to the mirror via the pillar at the opposite (moving) end. By applying a voltage to the layer of piezoelectric material, the layer can expand or contract and thereby stress the strip and cause it to bend. Each piezoelectric actuator may also comprise a hinge connected to an end of the strip and to the pillar. The hinge has a smaller cross section than the strip in the elongation direction of the strip (i.e. the cross section looking end-on at the strip). For example, the hinge may be formed from the same material as the strip of flexible material (typically silicon), but be patterned to have a smaller cross section to increase its flexibility and thereby cause it to act as a hinge between the strip and the pillar. The reduced cross-sectional area can also decrease the thermal conductivity of the hinge compared to the strip of flexible material, which may therefore be advantageous in preventing heating of the piezoelectric actuator. The pillar may comprise a thermally isolating layer (e.g. oxide) to reduce or prevent heat transfer from the mirror to the piezoelectric actuator. The pillar may also be configured to electrically isolate the mirror from the piezoelectric actuator. This may prevent charge build up on the mirror from affecting the piezoelectric actuator.

The micromirror array may further comprise, for each mirror in the array, a sensing element for sensing displacement of the mirror. The sensing element can allow accurate determination of the mirror position (e.g. tip and tilt angles), which may be important for providing feedback to the piezoelectric actuators. The sensing element may be connected to the piezoelectric actuator. For example, the sensing element may comprise a piezoresistor arranged so that displacement of the mirror causes the piezoresistor to deflect (that is, be deformed). The piezoresistor may have one (fixed) end connected to the substrate and another (moving) end connected to one of the mirror, the pillar, and the piezoelectric actuator. The voltage output from the piezoresistor may be proportional to the displacement of the mirror.

Each mirror in the array is preferably suitable for reflecting light having a wavelength in the range of about 13 nm, such as a narrow range centered substantially on 13.5 nm. This enables the micromirror to be used with a lithographic apparatus operating in the extreme ultraviolet (EUV) spectrum.

According to a second aspect of the present invention there is provided a programmable illuminator that comprises a micromirror array according to the first aspect of the present invention for conditioning a radiation beam.

The programmable illuminator may further comprise a displacement control feedback system configured to determine for each mirror in the micromirror array a position of the mirror and to adjust a voltage applied to the associated piezoelectric actuators based on the determined position and based on a predefined target position of the mirror. The performance of the piezoelectric actuators may change over time, so that the initial calibration of displacement to applied voltage is no longer valid, and the displacement control feedback system can be used to adapt applied voltage based on the measured mirror position. The feedback system may comprise or make use of the sensing element of the micromirror array to determine the mirror position.

According to a third aspect of the present invention there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus comprises a programmable illuminator according to the second aspect of the present invention 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. The micromirror array in the programmable illuminator may be used in an illumination system of a lithographic apparatus for example to control or condition a cross-sectional shape and/or intensity distribution of the light or radiation beam that is used to illuminate the patterning device. Alternatively or in addition, the micromirror array in the programmable illuminator may be used in an alignment system and/or overlay measurement system, respectively, of the lithographic apparatus to control or condition a spectral and/or spatial distribution of the light or radiation beam that is used to measure a position of an alignment mark(er) or target structure on the substrate and/or to perform an overlay measurement of a mark(er) or target structure on the substrate, respectively.

According to a fourth aspect of the present invention there is provided an inspection apparatus that comprises a programmable illuminator according to the second aspect of the present invention for conditioning a radiation beam used to measure a target structure on a substrate. For example, the micromirror array in the programmable illuminator may be used to control or condition a spectral and/or spatial distribution of the light or radiation beam that is used by the inspection apparatus to measure a target structure, e.g. a mark(er), on the substrate in order to determine the position of that target structure for alignment purposes and/or in order to perform an overlay measurement.

According to a fifth aspect of the present invention there is provided a method of forming a micromirror array. The method may be used to form a micromirror array according to the first aspect of the present invention. The method of forming a micromirror array comprises: providing a substrate, forming a plurality of mirrors for reflecting incident light and for each mirror in the array, forming at least one piezoelectric actuator for displacing the mirror and connected to the substrate. The method further comprises forming one or more pillars for connecting the mirror to the at least one piezoelectric actuator.

The method may comprise forming a heat diffuser for diffusing heat from the mirror by forming a heat sink and a thermally conductive post connected to the mirror, wherein said step of bonding causes the thermally conductive post to connect to the heat sink. The step of forming the heat sink may comprise forming a flexible membrane, which allows the thermally conductive post to pivot when the mirror is displaced. The flexible membrane can be formed by patterning a silicon layer. The step of patterning the silicon layer may comprise forming grooves through the silicon layer that extend from an outer edge of the heat diffuser towards the thermally conductive post. The grooves may be curved grooves.

The step of forming a piezoelectric actuator may comprise forming a strip of flexible material connected at one end to the substrate and a layer of piezoelectric material provided on the strip of flexible material.

The step of forming the pillar may comprise providing a thermally isolating layer in the pillar to reduce or prevent heat transfer from the mirror to the piezoelectric actuator.

The method may further comprise, for each mirror in the array, forming at least one sensing element connected to the at least one piezoelectric actuator for sensing displacement of the mirror. The step of forming the sensing element may comprise forming a piezoresistor arranged so that displacement of the mirror causes the piezoresistor to deflect.

The step of forming at least one piezoelectric actuator for each mirror may include forming four piezoelectric actuators, and said step of forming at least one pillar then includes forming four pillars connected to the mirror, wherein said step of bonding causes each of the four pillars to connect to a respective piezoelectric actuator of the four piezoelectric actuators.

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 plate1135is 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 Δλ/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 shown inFIG. 2is a micromirror with a mirror20and four piezoelectric actuators21for displacing the mirror20. In other embodiments (not shown) the micromirror may have a different number of piezoelectric actuators21for displacing the mirror20. In all these embodiments the micromirror has at least one piezoelectric actuator21for displacing the mirror20. A number of micromirrors as shown inFIG. 2can be arranged in an array to form a micromirror array.

FIG. 2shows a MEMS system with a mirror20which may be a part of a micromirror array according to an embodiment. The MEMS system has a four-fold rotational symmetry about an axis. In particular, four piezoelectric actuators21are arranged symmetrically under the mirror20to enable tip and tilt displacement of the mirror20. The mirror20is 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 square with a 1 mm2surface area. In other embodiments the mirror may be another shape, such as hexagonal. Each piezoelectric actuator21has a curved strip of flexible material22, fixed to the underlying substrate (no shown) at one end and connected via a hinge23to a pillar24. The strip of flexible material22has an elongation direction which at rest lies substantially parallel to the plane of the front surface of the mirror20. The strip of flexible material22has a layer of piezoelectric material (e.g. PZT) on it, to which a voltage can be applied in order to activate the piezoelectric actuator21. When activating the piezoelectric actuator21, the strip22bends, acting as a cantilever, to displace the mirror20via the pillar24. The magnitude of the displacement is a function of the applied voltage (as well as being a function of other parameters, such as the geometry of the piezoelectric actuator). The hinge23is formed by narrowing the strip22at one end, so that the cross sectional area of the hinge23is smaller than that of the strip22along the elongation direction of the strip22. The hinge23is therefore less stiff than the strip22, which allows it to act as a hinge. The hinge is typically required to flex in a plane transverse to an axis direction, which itself is transverse to the elongation direction of the strip22.

The mirror20is also connected to a heat diffuser, comprising a thermally conductive post25connected to the center of the back of the mirror20, and a heat sink26connected to the other end of the post25. In a rest condition, the length direction of the post25is the axis of four-fold rotational symmetry of the MEMS system. The thermally conductive post25is arranged to transfer heat from the mirror20to the heat sink26, which diffuses the heat over a relatively large surface area. The heat sink26comprises a flexible membrane being a circular, patterned silicon layer. It may alternatively have multiple layers; if so, one or more (e.g. all) of the layers may be silicon, and one or more layers may be of material of other than silicon. The flexible membrane has curved grooves27formed in it, which increase the flexibility of the flexible membrane. In use, when the mirror20is displaced, the thermally conductive post25pivots and elastically deforms the flexible membrane.

Each piezoelectric actuator21is associated with a sensing element28, being a piezoresistor fixed to the pillar24at one end and to the underlying substrate at the other. As the pillar24is displaced, the piezoresistor is stressed/deformed, which changes the electric properties of the piezoresistor, from which the displacement can be determined. For example, the piezoresistor may be connected in a Wheatstone bridge, configured so that the output voltage of the bridge is a function of the displacement of the mirror20. The output from the piezoresistor is temperature sensitive, and temperature compensation may be used to increase the accuracy of displacement measurements.

FIG. 3shows a schematic diagram of a cross section of a part of a micromirror array300according to an embodiment. A mirror301is connected to a piezoelectric actuator302via a pillar303. The pillar303comprises layers of silicon, germanium, aluminum and oxide having a combined thickness of 151 μm. The oxide layer304has a thickness of 1 μm and provides both thermal and electrical insulation to protect the piezoelectric actuator302.

The piezoelectric actuator302comprises a strip of flexible material305, being a 5 μm thick strip of silicon, connected to a substrate306at the opposite end from the pillar303. The piezoelectric actuator302also comprises a layer of piezoelectric material307being a layer of PZT having a thickness in the range of 500 nm to 2 μm. The layer of piezoelectric material307has top and bottom electrodes308made of platinum (Pt) and LaNiO3 (LNO) for applying a voltage to the layer of piezoelectric material307. The layer of piezoelectric material is bonded to the strip of flexible material305by a layer of nitride (SiN)309having a thickness of 100 nm. The stack of SiN/Pt/LNO/PZT/Pt/LNO is covered by a layer of silicon oxide and aluminum oxide310. Applying a voltage to the electrodes308causes the layer of piezoelectric material307to contract or expand, but because the layer is constrained at the interface to the strip of flexible material305, the combined system of the layer of piezoelectric material307and the strip of flexible material305bends. It is this bending motion of the piezoelectric actuator302which displaces the pillar303and thereby also the mirror301.

A sensing element311comprising a piezoresistor is embedded in the strip of flexible material305and covered by oxide. The sensing element311is arranged to sense the deflection of the piezoelectric actuator302.

The piezoelectric actuator302is electrically connected to the substrate306by a through silicon via (TSV)312. Application specific integrated circuits (ASICs)313can be used to apply the voltage to the piezoelectric actuator302and also to derive the output voltage from the sensing element310.

The mirror301is connected to a heat sink314by a thermally conductive post315. The heat sink314and the thermally conductive post315together form a heat diffuser for dissipating heat from the mirror301. The post315comprises layers of silicon, germanium and aluminum. The heat sink314comprises a flexible element, specifically in this example a flexible membrane of silicon, which allows the post315to move as the mirror301is displaced. The heat sink314is electrically connected to the substrate306, and may be grounded to prevent charge build up on the mirror301. The heat sink314and the strip of flexible material305are in the same plane and may be formed from the same silicon wafer. The spacing between the lower surface of the mirror301and the upper surface of the heat sink314and/or the strip of flexible material305may be in the range 50 μm-120 μm, such as about 80 μm. Typically, known micromirror arrays have a smaller spacing than this, such as only a few microns. The greater spacing is achieved in this embodiment because the (piezoelectric) actuation force is generated on the strip of flexible material305, rather than, for example, by an electrostatic actuator mounted on the mirror itself, which would typically limit the range of movement of the mirror to the the range of relative movement of components of the electrostatic actuator.

The spacing between the lower surface of the heat sink314and/or the strip of flexible material305, and the upper surface of the substrate may be in the range 50 μm-120 μm, such as about 80 μm.

Each mirror may be provided with one or more control units which are operative to recognize, in a received control signal, an address corresponding to the mirror, and, upon recognizing the address, to generate control voltages for one or more piezoelectric actuators of the mirror based on control information additionally contained in the control signal. The control units may be implemented as the ASICs313which receive the control signals using the vias312and control the corresponding piezoelectric actuator302based on it; in this case, the address in the control signal may specify not only the mirror but also the ASIC313for a given piezoelectric actuator302. Using the control units, an external control system is able to individually control all the mirrors of the mirror array, by transmitting identical control signals to the control units of all of the mirrors, such that each control unit recognizes control signals addressed to it, and controls the corresponding piezoelectric actuator(s) accordingly. Positioning the ASICs313within the structure, e.g. supported above the substrate306and proximate to (e.g. substantially in plane with) the strip of flexible material305, is achievable because the embodiment may be formed in multiple layers by a MEMS process, as described below.

FIG. 4shows a schematic diagram of a top view of a MEMS system40for controlling a mirror (not shown) in a micromirror array according to an embodiment. The system40comprises four piezoelectric actuators41connected to the mirror by respective pillars42. Each piezoelectric actuator41comprises a strip of flexible material43and a hinge44, wherein the pillar42is connected to the hinge44at one end of the strip43and wherein the other end of the strip43is connected to (that is, in a substantially fixed positional relationship to) the substrate (not shown). The system40also comprises a heat diffuser45for diffusing heat from the mirror when in use. The heat diffuser45comprises a circular silicon layer fixed to the substrate along an outer edge of the silicon layer and a thermally conductive post connected to the center of the silicon layer.

The system40comprises any one or more of five different types of sensing elements46ato46eillustrated inFIG. 4for sensing a displacement of the mirror. Each sensing element46ato46ecomprises a piezoresistor arranged so that displacement of the mirror induces stress in the piezoresistor. Preferably, each of the actuators is provided with only one of these five types of sensing element46ato46e, and the same type of sensing element is used in each of the four actuators.

A first type of sensing element46acomprises a curved beam located in the annular space between the heat diffuser45and the piezoelectric actuator41and fixed to the substrate at one end and to the pillar42at the opposite end. One or more piezoresistors may be formed in the beam.

A second type of sensing element46bcomprises two folded beams comprising respective piezoresistors, each located in the annular space between the heat diffuser45and the piezoelectric actuator41. One of the folded beams is connected to the substrate and to the pillar42, whereas the other folded beam is connected to two different points of the substrate to provide a reference value.

A third type of sensing element46ccomprises a straight beam comprising a piezoresistor connected to the pillar42at one end and to the substrate at the other end.

A fourth type of sensing element46dcomprises a curved beam comprising a piezoresistor, the beam being located outside the piezoelectric actuator41. The beam is connected to the substrate at one end and to the pillar42at the opposite end.

A fifth type of sensing element46ecomprises four piezoresistors fixed to the heat diffuser45. The fifth sensing element46eis for sensing deformation of the heat diffuser caused by displacement of the mirror.

FIG. 5shows a schematic diagram of a top view of a part of a sensing element50, which may be the first sensing element46ainFIG. 4but the other sensing elements have a similar construction. The sensing element50comprises a curved beam51and a folded piezoresistor52in the annular space53between the piezoelectric actuator54and the heat diffuser55. A temperature sensor56may be provided to measure the temperature of the piezoresistor52. The temperature sensor may for example be implemented as a bipolar transistor or a diode, since for such devices the current is a function of temperature.

FIG. 6shows a circuit diagram of a Wheatstone bridge having a supply voltage Vs, resistors R1, R2, R3and R4, and output voltage Vo. The circuit may be a part of the sensing element for sensing displacement of the mirror in a micromirror array. One or more of the resistors R1to R4may be piezoresistors of the sensing element.

FIG. 7ashows a schematic diagram of a particular configuration of resistors R1to R4of the Wheatstone bridge in a sensing element70comprising a curved beam71. R1is a piezoresistor extending along the elongation direction of the beam71towards the moving end of the beam. R2to R4are located at the end of the sensing element70that is fixed to the substrate. As the beam71is deflected/stressed due to displacement of the mirror, the resistance of R1changes while the resistances of R2to R4remain substantially constant.

FIG. 7bshows a schematic diagram of an alternative configuration of the resistors R1to R4of the Wheatstone bridge in a sensing element70comprising a curved beam71. In this configuration, two piezoresistors (R1and R4) extend along the elongation direction of the beam71, while R2and R3are located at the end of the sensing element that is fixed to the substrate. As the beam71is deflected/stresses due to displacement of the mirror, the resistance of R1and R4changes while the resistances of R2and R3remain substantially constant.

As mentioned above, one or more of the piezoresistors R1to R4may be provided with a temperature sensor. In this case, the temperature value output by the temperature sensor(s) may be employed (using circuitry which is not shown) to modify the operation of the sensing element to correct for temperature variations in the piezoresistors, i.e. to correct for temperature variations in the relationship between resistance and deflection/stress in the resistors R1to R4.

Embodiments of the micromirror array can provide tip and tilt displacement range of +/−120mrad and a mirror accuracy of 100 μrad. Embodiments of the micromirror array can be operated at high light intensities as required for EUV, and may work at 40 to 60 kW/m2of 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 piezoelectric actuators41are operative to provide, even at a relatively low actuator voltage (e.g. under about 100V), such a strong force that they are able to deform the flexible element (flexible member314) even though the flexible element is 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.

Methods of forming a micromirror array are also described herein.FIGS. 8ato 8jillustrate some of the steps of an embodiment of such a method.

As illustrated inFIG. 8a, the method comprises providing a first silicon wafer800for forming the piezoactuators and the sensing elements. The first wafer800may be referred to as the “actuator wafer”. The actuator wafer may be a silicon on insulator (SOI) wafer with a 4 μm silicon film801. Low voltage active devices such as the sensing elements can be formed in the wafer800using a Complementary Metal Oxide Semiconductor (CMOS) Front End of Line (FEOL) process. A CMOS Back End of Line (BEOL) process can then be used to form metal interconnect layers for connecting the low voltage devices to other circuitry. Chemical Mechanical Polishing/Planarization (CMP) can then be used to form a smooth surface with a planar oxide layer802. A Cu damascene process can be used for forming a Cu bonding matrix803with CU pads804for subsequent Cu-Ox hybrid bonding to another wafer.

As illustrated inFIG. 8b, the method further comprises providing a second silicon wafer805which will become the substrate on which the micromirror array is fixed. The second wafer805may be referred to as the “interposer wafer”. The interposer wafer805may be an SOI wafer with a 100 μm silicon film as seen inFIG. 8b. A high voltage (HV) CMOS process (both FEOL and BEOL) may be used to form a HV driver in the wafer. A TSV process can be used to form electrical connections806through the silicon film807of the second wafer805. The TSV process can be followed by planarization (e.g. CMP or wet etch) and Cu pad formation. The Cu pads808are arranged to connect to the Cu bonding matrix803of the first wafer800.

FIG. 8cillustrates how the first and second wafers800and805are bonded, e.g. using a Cu/oxide hybrid bond809.

As shown inFIG. 8d, the “handle wafer” used for handling the first wafer800may be removed to expose the first wafer800. Selective box removal can then be used to leave a thin layer (e.g. 5 μm) of silicon810of the first wafer800bonded to the second wafer805. Al can be deposited and patterned on the first wafer800for subsequent connection to a mirror. The first wafer800can be patterned to form the piezoelectric actuators811. Al2O3and/or TiN can be deposited on the first wafer for protection against EUV radiation and against plasma.

As illustrated inFIG. 8e, the method further comprises providing a third silicon wafer812for forming the mirror. The third wafer812may be referred to as the “mirror wafer”812. The mirror wafer812may be an SOI wafer with a 250 μm silicon film. The method may comprise performing a cavity etch on the mirror wafer812to allow for a thermal barrier (e.g. 1 μm to 2 μm), followed by Ge deposition for subsequent bonding to the first wafer800. Using a hardmask (e.g. nitride) and a resist mask the mirror wafer812is etched to form pillars813(“beam connectors”) for connecting to the piezoelectric actuators and to form a thermally conductive post814(“center post”) for connecting to a heat sink so as to form a heat diffuser. A mirror release trench815is etched around the periphery of the mirror.

As shown inFIG. 8f, the third wafer812is bonded to the first wafer800in order to connect the mirror to the piezoelectric actuators. The step of bonding may comprise aligned Ge/Al eutectic bonding. The Al/Ge bonding layer816is both thermally and electrically conducting, which can allow efficient heat transfer from the mirror through the thermally conductive post to the heat sink. Some of the box oxide may have been left on the first wafer800to reduce thermal and electrical conduction at some bonding locations such as at the pillars connected to the piezoelectric actuators.

InFIG. 8g, the stack of bonded wafers (wafers 1 “actuator wafer”800, 2 “interposer wafer”805and 3 “mirror wafer”812), which may collectively be referred to as the “device wafer”, is turned upside down, so that the handle wafer817of the mirror wafer812becomes the supporting wafer. The handle wafer of the second wafer805can be removed from the second wafer805and box removal may be used to reveal the TSVs806in the second wafer805. This can be followed by dielectric deposition, patterning and bump formation.

InFIG. 8h, lithography and silicon etching is used to form cavities818in the second wafer805underneath the piezoelectric actuators and the heat sink. Note that in use an outer portion of the flexible membrane314(e.g. a portion radially outward of the grooves) is in contact with a wall819. The wall819is between a cavity818ain register with the heat sink and a cavity818bin register with the piezoelectric actuators. The wall819is able conduct heat from the flexible membrane314to the substrate. The method then comprises etching a dielectric layer to reveal the piezoelectric actuators (i.e. to release the strip of flexible material), the sensing elements and dicing scribes.

As illustrated inFIG. 8i, the method further comprises providing a fourth silicon wafer820for sealing the second wafer805. The fourth wafer820may be referred to as the “support wafer”820and comprises TSVs821for connecting to the second wafer805. Bump bonding may be used to bond the fourth wafer820to the second wafer805. An Al redistribution layer (RDL) and connection pads can be formed on the back of the fourth wafer820.

InFIG. 8j, the mirror822is released by removing the front side handle wafer817. A controller chip can be glued and wirebonded to the back of the fourth wafer820.