Micro-mirror array

Abstract

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

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to EP application 19192311.9, filed on Aug. 19, 2019, and EP application 19199718.8, filed on Sep. 26, 2019, which are hereby incorporated by reference in their entirety. Field of the Invention

[0003] The present invention relates to a micromirror array, a programmable illuminator including such a micromirror array, a lithographic apparatus including such a programmable illuminator, an inspection apparatus including such a programmable illuminator, and a method for forming such a micromirror array. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus can project a pattern provided on a patterning device onto a layer of radiation-sensitive material (resist) provided on a substrate. The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to endow an incident radiation beam with a patterned cross-section that corresponds to a pattern to be created in a target portion of the substrate; the term “light valve” can also be used herein. Generally, the pattern will correspond to a particular functional layer in a device (such as an integrated circuit or other device) to be created in the target portion. Examples of such patterning devices include:

[0005] - a mask (or reticle). The concept of a mask is well known in lithography and includes mask types such as binary, alternating phase-shift, and attenuated phase-shift masks, as well as various hybrid mask types. Depending on the pattern on the mask, the 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. The mask can be supported by a support structure such as a mask table or a mask clamp. The support structure ensures that the mask can be held at a desired position in the incident radiation beam and, if desired, the mask can be moved relative to the beam;

[0006] - Programmable mirror arrays. An example of such a device is a matrix-addressable surface with a viscoelastic control layer and a reflective surface. The basic principle behind such a device is that (for example) the addressed regions of the reflective surface reflect incident light as diffracted light, while the unaddressed regions reflect incident light as non-diffracted light. Using an appropriate filter, the non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light; in this way, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. Alternative embodiments of the programmable mirror array employ a matrix arrangement of micromirrors, such that each micromirror in the matrix can be individually tilted about an axis, for example by applying an appropriate local electric field, or by employing electrostatic or piezoelectric actuation means. Again, the mirrors are matrix-addressable such that the addressed mirrors will reflect an incident radiation beam in a different direction than the unaddressed mirrors; in this way, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. Appropriate electronic means can be used to perform the required matrix addressing. In both of the above cases, the patterning means can include one or more programmable mirror arrays. For example, more information about the mirror arrays mentioned herein can be gleaned from U.S. Patents US 5,296,891 and US 5,523,193 and PCT patent applications WO 98 / 38597 and WO 98 / 33096, which are hereby incorporated by reference. Such programmable mirror arrays can be supported by a support structure (such as a frame or a stage), which can be fixed or movable as required; and

[0007] - Programmable LCD arrays. An example of such a construction is given in U.S. Patent US 5,229,872, which is hereby incorporated by reference. Such programmable LCD arrays can be supported by a support structure (such as a frame or a stage), which can be fixed or movable as required.

[0008] For simplicity, the remainder of this document may in some places specifically refer itself to examples involving masks and mask tables; however, the general principles discussed in such cases should be seen in the broader context of the patterning device as set forth above.

[0009] In order to project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum feature size that can be formed on the substrate. Compared to a lithographic apparatus using radiation with a wavelength of, for example, 193 nm, a lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 - 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0010] In addition to the wavelength (λ) of the radiation and the numerical aperture (NA) of the projection lens, the shape of the illumination source (or more generally, the angular intensity distribution) is one of the most important parameters for achieving high resolution in lithography.

[0011] In the illumination system of a lithographic apparatus, a micromechanical mirror array including an array of hundreds or thousands of micromirrors (hereinafter generally simply referred to as "mirrors") can be used to control the cross-sectional shape and intensity distribution of light. Each micromirror reflects a light spot, and changing the angle of the micromirror changes the position of the spot, thereby changing the shape of the radiation beam.

[0012] Microelectromechanical systems (MEMS) technology can be used to fabricate and control the mirrors. For example, electrostatic or piezoelectric MEMS systems can be used to turn the mirrors.

[0013] Currently, there are micromechanical mirror arrays for shaping light having a wavelength in the deep ultraviolet spectrum (DUV), such as λ = 193 nm. However, these micromechanical mirror arrays cannot be effectively used for the shorter wavelengths required for light in the extreme ultraviolet spectrum (EUV), such as λ = 13.5 nm. New micromechanical mirror array technologies are needed for EUV radiation. Moreover, new advantageous applications for such new micromechanical mirror array technologies are needed for EUV and / or non-EUV radiation, such as visible light or DUV radiation. SUMMARY OF THE INVENTION

[0014] According to a first aspect of the invention, there is provided a micromechanical mirror array which can be used, for example, in the illumination system of a lithographic apparatus or an inspection apparatus to adjust a radiation beam. The micromechanical mirror array includes 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 and connected to the substrate, and there is one or more struts which connect the piezoelectric actuator or each piezoelectric actuator to the mirror. The (plural) struts can be operated to support the mirror from the (plural) piezoelectric actuators. Applying a voltage to the piezoelectric actuator can cause the actuator to move the strut 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 micromechanical mirror array includes four piezoelectric actuators for each mirror, arranged to achieve tip and tilt displacement control of the mirror.

[0015] For each mirror in the array, the micromirror array may further include a heat spreader for spreading the 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 temperature rise will degrade the device performance. Typically, the micromirror array is used to operate in an environment where the gas pressure is much less than one atmosphere, and in practice is typically substantially in a vacuum, so heat convection is essentially zero. Instead, the heat spreader allows the heat to be conducted away, such as being conducted to the substrate. Typically, the heat spreader includes a flexible element that is connected between the mirror and the substrate and is arranged to bend when the mirror moves. Note that there is a trade-off between the increased flexibility of the heat spreader and the increased ability of the heat spreader to conduct heat away from the mirror. Using (a) piezoelectric actuator(s) allows an increased force to be applied to the flexible element, and thus allows the heat spreader to be selected to provide improved thermal conductivity.

[0016] In one example, the heat spreader may include a heat sink and a thermally conductive post connecting the heat sink to the mirror. The heat sink may include a flexible membrane that allows the post to pivot when the mirror is displaced. The flexible membrane may be a patterned silicon layer, which has the advantage of being readily available in a CMOS manufacturing process without the need for additional masks or process steps. The flexible membrane may include grooves that pass through the flexible membrane and extend from the 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 not to impede the movement of the mirror. The piezoelectric actuator of the preferred embodiment is selected to provide a greater level of force than the electrostatic actuators used in some conventional systems, and is sufficient to deform the flexible member, even though it has a sufficient cross-sectional area (e.g., as measured at the intersection of the flexible member with a cylindrical surface having an axis coincident with the axis of the post) to allow greater heat spreading than that provided for a conventional mirror array. This allows the present micromirror array to be used in applications where conventional mirror arrays are not suitable.

[0017] The heat sink may include a layer of metal (such as aluminum), which has a relatively high thermal conductivity compared to, for example, silicon. The thermally conductive post may also be conductive and grounded to prevent charge from accumulating on the mirror, which could otherwise impede the displacement control of the mirror.

[0018] A piezoelectric actuator may include a flexible material strip connected at one end to a substrate, where struts are located at opposite ends of the flexible material strip, and may include a piezoelectric material layer disposed on the flexible material strip. The piezoelectric material layer and the strip may thus form a cantilever that is anchored to the substrate at one end (fixed relative to the substrate) and connected to a mirror via the struts at the opposite (movable) end. By applying a voltage to the piezoelectric material layer, the layer may expand or contract, and thus apply stress to the strip and cause it to bend. Each piezoelectric actuator may also include hinges connected to the ends of the strip and the struts. The hinges have a smaller cross-section in the elongation direction of the strip (i.e., the end-view cross-section of the strip). For example, the hinges may be formed of the same material as the flexible material strip (typically silicon), but are patterned to have a smaller cross-section to increase their flexibility, and thus serve as hinges between the strip and the struts. The reduced cross-sectional area may also reduce the thermal conductivity of the hinges compared to the flexible material strip, and thus this is advantageous in preventing heating of the piezoelectric actuator. The struts may include a thermal isolation layer (such as an oxide) to reduce or prevent heat transfer from the mirror to the piezoelectric actuator. The struts may also be configured to electrically isolate the mirror from the piezoelectric actuator. This may prevent charge accumulated on the mirror from affecting the piezoelectric actuator.

[0019] For each mirror in the array, the micro-mirror array may further include sensing elements for sensing the displacement of the mirror. The sensing elements may allow for precise determination of the mirror position (e.g., tilt and inclination angles), which may be important for providing feedback to the piezoelectric actuators. The sensing elements may be connected to the piezoelectric actuators. For example, the sensing elements may include piezoresistors arranged such that displacement of the mirror deflects (i.e., deforms) the piezoresistors. The piezoresistors may have one (fixed) end connected to the substrate and another (movable) end connected to one of the mirror, the strut, and the piezoelectric actuator. The voltage output from the piezoresistors may be proportional to the displacement of the mirror.

[0020] Each mirror in the array is preferably adapted to reflect light having a wavelength in a range of about 13 nm, such as a narrow range centered substantially at 13.5 nm. This enables the micro-mirrors to be used with a lithographic apparatus operating in the extreme ultraviolet (EUV) spectrum.

[0021] According to a second aspect of the invention, there is provided a programmable illuminator comprising a micro-mirror array for modulating a radiation beam according to the first aspect of the invention.

[0022] The programmable illuminator may also include a displacement control feedback system configured to determine the position of each mirror in the micromirror array and to adjust the voltage applied to the associated piezoelectric actuator based on the determined position and a predetermined target position of the mirror. The performance of the piezoelectric actuator may change over time such that the initial calibration of the displacement to the applied voltage is no longer valid, and the displacement control feedback system can be used to adapt the applied voltage based on the measured mirror position. The feedback system may include or utilize sensing elements of the micromirror array to determine the mirror position.

[0023] According to a third aspect of the invention, there is provided a lithographic apparatus configured to project a pattern from a patterning device onto a substrate. The lithographic apparatus includes a programmable illuminator according to the second aspect of the invention, which is configured to condition the radiation beam used to illuminate the patterning device and / or to condition the radiation beam used to measure a target structure on the substrate. The micromirror array in the programmable illuminator can be used in the illumination system of the lithographic apparatus to, for example, control or adjust the cross-sectional shape and / or intensity distribution of the light or radiation beam used to illuminate the patterning device. Alternatively or additionally, the micromirror array in the programmable illuminator can be used in the alignment system and / or overlay measurement system of the lithographic apparatus to control or adjust the spectral and / or spatial distribution of the light or radiation beam used to measure the position of alignment marks or target structures on the substrate and / or to perform overlay measurements of marks or target structures on the substrate, respectively.

[0024] According to a fourth aspect of the invention, there is provided an inspection apparatus including a programmable illuminator according to the second aspect of the invention, which is configured to condition the radiation beam used to measure a target structure on the substrate. For example, the micromirror array in the programmable illuminator can be used to control or adjust the spectral and / or spatial distribution of the light or radiation beam used by the inspection apparatus to measure a target structure, such as a mark, on the substrate in order to determine the position of the target structure for alignment purposes and / or to perform overlay measurements.

[0025] According to a fifth aspect of the invention, there is provided a method of forming a micromirror array. The method can be used to form a micromirror array according to the first aspect of the invention. The method of forming a micromirror array includes 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 includes forming one or more struts for connecting the mirror to the at least one piezoelectric actuator.

[0026] The method may include: forming a heat diffuser for diffusing heat from a mirror by forming a heat sink and a heat conducting post connected to the mirror, wherein the bonding step connects the heat conducting post to the heat sink. The step of forming the heat sink may include forming a flexible film that allows the heat conducting post to pivot when the mirror is displaced. The flexible film may be formed by patterning a silicon layer. The step of patterning the silicon layer may include forming a groove through the silicon layer that extends from the outer edge of the heat diffuser towards the heat conducting post. The groove may be a curved groove.

[0027] The step of forming the piezoelectric actuator may include: forming a flexible material strip connected to a substrate at one end and a piezoelectric material layer disposed on the flexible material strip.

[0028] The step of forming the strut may include: providing a thermal isolation layer in the strut to reduce or prevent heat transfer from the mirror to the piezoelectric actuator.

[0029] The method may further include: for each mirror in the array, forming at least one sensing element connected to at least one piezoelectric actuator, the at least one sensing element being configured to sense the displacement of the mirror. The step of forming the sensing element may include forming a piezoresistor that is arranged such that displacement of the mirror deflects the piezoresistor.

[0030] The step of forming at least one piezoelectric actuator for each mirror may include: forming four piezoelectric actuators, and the step of forming at least one strut then includes: forming four struts connected to the mirror, wherein the bonding step connects each of the four struts to a corresponding one of the four piezoelectric actuators. Description of the Drawings

[0031] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0032] Figure 1 A lithography system including a lithography apparatus having a (programmable) illuminator and a radiation source is depicted;

[0033] Figure 1a A known inspection apparatus is depicted;

[0034] Figure 1b Depicted is used in Figure 1a The programmable illuminator in the inspection apparatus.

[0035] Figure 2 A portion of a micromirror array including a mirror and four piezoelectric actuators is depicted;

[0036] Figure 3 A schematic cross-section of a portion of a micromirror array according to an embodiment is depicted;

[0037] Figure 4 Depicts a portion of a micromirror array according to another embodiment, the micromirror array having a plurality of different sensing elements for sensing the displacement of the mirrors;

[0038] Figure 5 Depicts a portion of a sensing element having a piezoresistor;

[0039] Figure 6 Depicts a circuit diagram of a Wheatstone bridge;

[0040] Figure 7a Depicts the configuration of the resistors in the sensing element;

[0041] Figure 7b Depicts another configuration of the resistors in the sensing element;

[0042] Figure 8a Depicts the first step in a method of forming a micromirror array;

[0043] Figure 8b Depicts the second step in a method of forming a micromirror array;

[0044] Figure 8c Depicts the third step in a method of forming a micromirror array;

[0045] Figure 8d Depicts the fourth step in a method of forming a micromirror array;

[0046] Figure 8e Depicts the fifth step in a method of forming a micromirror array;

[0047] Figure 8f Depicts the sixth step in a method of forming a micromirror array;

[0048] Figure 8g Depicts the seventh step in a method of forming a micromirror array;

[0049] Figure 8h Depicts the eighth step in a method of forming a micromirror array;

[0050] Figure 8i Depicts the ninth step in a method of forming a micromirror array; and

[0051] Figure 8j Describes the tenth step in a method of forming a micromirror array. Detailed Description

[0052] Figure 1A lithography system is shown that includes a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes 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.

[0053] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Additionally, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and / or the faceted pupil mirror device 11. For example, in addition to the faceted field mirror device 10 and the faceted pupil mirror device 11 disclosed in US 8,294,877 B2, which is incorporated herein by reference in its entirety, a micromirror array as described herein may be added to the illumination system IL, or may be used to replace one or both of the faceted field mirror device 10 and the faceted pupil mirror device 11 disclosed in US 10,254,654 B2, which is incorporated herein by reference in its entirety. In this case, the illumination system IL is a programmable illuminator IL that now includes at least one micromirror array as described herein. Such a programmable illuminator IL can be used to condition the radiation beam used to illuminate the patterning device. For example, the programmable illuminator IL can be used to control or condition the EUV radiation beam B by providing a desired cross-sectional shape and / or a desired intensity distribution to the EUV radiation beam B.

[0054] After being conditioned as such, the EUV radiation beam B illuminates the patterning device MA and interacts therewith. 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 this purpose, the projection system PS may include a plurality of mirrors 13, 14 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' so as to form an image having features that are smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although in Figure 1 the projection system PS is shown as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0055] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B’ with the pattern previously formed on the substrate W.

[0056] A relatively high vacuum, i.e., a small amount of gas (e.g., hydrogen gas) at a pressure far below atmospheric pressure, may be provided in the radiation source SO, the illumination system IL, and / or the projection system PS.

[0057] 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 capable of generating EUV radiation.

[0058] Figure 1a An inspection apparatus known from US 9,946,167 B2 is shown, which US 9,946,167 B2 is incorporated herein by reference in its entirety. Figure 1a corresponding to US 9,946,167 B2 Figure 3 a. The inspection apparatus is a dark-field metrology apparatus for measuring, for example, overlay and / or alignment.

[0059] In a lithographic process, it is desirable to frequently measure the created structures, for example for process control and verification. Various tools for performing such measurements are known, including scanning electron microscopes typically used to measure critical dimension (CD), and dedicated tools for measuring overlay, the alignment accuracy of two layers in a device, and alignment (i.e., the position of alignment marks on a substrate). Various forms of scatterometers have been developed for the lithography field. These devices direct a radiation beam onto a target structure (e.g., a grating or a mark), and measure one or more properties of the scattered radiation (e.g., the intensity at a single reflection angle as a function of wavelength; the intensity at one or more wavelengths as a function of reflection angle; or the polarization as a function of reflection angle) to obtain a “spectrum” from which an interesting property of the target can be determined. The determination of the interesting property can be performed by various techniques: for example, by iterative methods such as rigorous coupled-wave analysis or finite element methods to reconstruct the target structure; library search; and principal component analysis.

[0060] Figure 1aThe dark-field metrology apparatus shown can be a stand-alone device / system or can be incorporated into a lithographic apparatus LA as an alignment system and / or as a overlay measurement system (not shown). The optical axis having several branches throughout the apparatus is indicated by the dashed line O. In this apparatus, light emitted by a radiation source 111 (e.g., a xenon lamp) is guided via a beam splitter 115 to a substrate W through an optical system including lenses 112, 114 and an objective 116. These lenses are arranged in a double sequence in a 4F arrangement. Thus, the angular distribution of the radiation incident on the substrate can be selected by defining a spatial intensity distribution in a plane (here called the (conjugate) pupil plane) that presents the spatial spectrum of the substrate plane. In particular, this can be achieved by inserting an aperture plate 113 of a suitable form between lenses 112 and 114 in a plane that is the back-projected image of the objective pupil plane. In the example shown, the aperture plate 113 has different forms, labeled 113N and 113S, allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 113N provides off-axis from a specified direction, which for descriptive purposes is designated "north". In a second illumination mode, the aperture plate 113S is used to provide a similar illumination, but from the opposite direction, labeled "south". Other illumination modes are possible by using different apertures. The remainder of the pupil plane is desirably dark because any unwanted light outside the desired illumination mode will interfere with the desired measurement signal.

[0061] A target structure (not shown) (e.g., a grating or a mark) on the substrate W is placed perpendicular to the optical axis O of the objective 116. The illumination light rays impinging on the target structure from an angle off the axis O produce a zero-order diffracted ray and two first-order diffracted rays. Since the apertures in the plate 113 have a finite width (necessary to allow a useful amount of light), the incident light rays will actually occupy a certain angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. According to the point spread function of a small target, each order +1 and -1 will be further spread out over an angular range rather than being a single ideal ray. Note that the grating pitch and the illumination angle can be designed or adjusted such that the first-order rays entering the objective are closely aligned with the central optical axis.

[0062] At least the 0th and +1st orders diffracted by the target on the substrate W are collected by the objective 116 and guided back through the beam splitter 115. The first illumination mode and the second illumination mode are illustrated by specifying diametrically opposite apertures labeled north (N) and south (S). When the incident light rays come from the north side of the optical axis, i.e., when the aperture plate 113N is used to apply the first illumination mode, the +1 diffracted ray labeled +1(N) enters the objective 116. Conversely, when the aperture plate 113S is used to apply the second illumination mode, the -1 diffracted ray (labeled -1(S)) is the ray entering the lens 116.

[0063] The second beam splitter 117 splits the diffracted beam into two measurement branches. In the first measurement branch, the optical system 118 uses the zero-order diffracted beam and the first-order diffracted beam to form a diffracted spectrum (pupil plane image) of the target on the first sensor 119 (such as a CCD or CMOS sensor). Each diffracted order hits a different point on the sensor, enabling image processing to compare and contrast the orders. The pupil plane image captured by the sensor 119 can be used to focus the inspection device and / or normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0064] In the second measurement branch, an optical system including lenses 120, 122 forms an image of the target on the substrate W on the sensor 123 (such as a CCD or CMOS sensor). In the second measurement branch, an aperture plate called the field stop 121 is provided in a plane conjugate to the pupil plane. When describing the present invention, this plane will be referred to as the "intermediate pupil plane". The field stop 121 is used to block the zero-order diffracted beam, such that the image of the target formed on the sensor 123 is formed only by the -1 or +1 first-order beams. The images captured by the sensors 119 and 123 are output to an image processor and controller PU, whose functions will depend on the specific type of measurement being performed. Note that the term "image" as used herein is in a broad sense. If only one of the -1 and +1 orders is present, an image of that grating line will not be formed.

[0065] The illumination system of the inspection device includes an illuminator 110. As Figure 1a shown, this illumination system 110 includes a lens 112 and an aperture plate 113. More details of the inspection device can be found in US9,946,167B2.

[0066] Figure 1b Shown is a programmable illuminator 140 used in the Figure 1a inspection device. This programmable illuminator 140 can be used in place of the illuminator 110 in the Figure 1aIn the inspection device. The programmable illuminator 140 includes a micromirror array 133 according to the present invention and a low-NA relay 4F system 135 including a pair of lenses. Radiation or light from a radiation source 130 (not part of the programmable illuminator 140) (such as a broadband radiation source or a white light source) can be guided to the micromirror array 133 via an optional optical fiber 131 and an optional collimating lens system 132. The processing unit PU can control the micromirror array 133 in such a way that the micromirrors 134 (more precisely, the mirrors in the micromirrors 134, or the mirrors in the micromirror array 133) are individually tilted. By independently tuning the tilt angles of each individual mirror, the spatial distribution of the light output by the low-NA relay system 135 can be controlled, and various illumination patterns can be performed as needed, without having to use an aperture plate. If the programmable illuminator 140 is used in Figure 1a the inspection device, then it cooperates with the lens 114, which means that the light output by the low-NA relay system 135 is Figure 1a received by the lens 114 of

[0067] To control the spectral distribution of the light output by the low-NA relay system 135, at least some of the mirrors can include gratings on top of the mirror surface (not shown). The gratings can be the same for all the mirrors, or alternatively, different gratings can be used, such as gratings with different pitches. By appropriate control of the micromirror array 133, the light output by the low-NA relay system 135 includes a single wavelength or a single (narrow) wavelength range. However, the micromirror array 133 can also be controlled in such a way that the light output by the low-NA relay system 135 includes multiple different wavelengths or multiple different (narrow) wavelength ranges. The gratings can be lithographically patterned on the mirror surface. Each mirror with a grating diffracts light of different wavelengths in different directions according to the associated grating equation. The diffracted light is partially captured by the low-NA relay system 135 and forms an image. By independently tuning the angle of each mirror, the light distribution at the output can be controlled both spatially and spectrally, because certain diffraction orders will be captured by the low-NA relay system 135 while other diffraction orders will not be captured. This spatial and spectral light distribution can be advantageously used, for example, for illuminating and measuring the overlay target structures on a substrate or for measuring the positions of alignment marks on a substrate. In this context, the terms "target structure", "target", "mark", "flag", and "grating" are all synonyms of each other where the context allows.

[0068] The spectral bandwidth of the diffracted beam that can be captured by the low-NA relay system 135 is dλ = P·NA, where P is the grating pitch and NA is the numerical aperture of the low-NA relay system 135. For P = 500 nm and NA = 0.02, the spectral bandwidth is 10 nm, which means that the diffractive orders of the grating cover a wavelength range or band of 10 nm.

[0069] The spatial resolution of the low-NA relay system 135 is ~λ / NA. For λ = 850 nm and NA = 0.02, the spatial resolution is 42.5 μm. If the size of the mirror is larger than 42.5 μm, each mirror can be resolved. A reasonable size for the mirror is 100×100 μm.

[0070] By rotating / tilting the mirrors about their respective axes, different central wavelength bands can be directed into the low-NA relay system 135. The rotation range for each mirror required for operation over the visible wavelength range should be Δλ / 2P, where for a working wavelength range of 450 nm - 850 nm, Δλ = 400 nm. This means that each mirror must be able to rotate 0.4 radians.

[0071] Figure 2 The MEMS system shown is a micromirror having a mirror 20 and four piezoelectric actuators 21 for displacing the mirror 20. In other embodiments (not shown), the micromirror can have a different number of piezoelectric actuators 21 for displacing the mirror 20. In all of these embodiments, the micromirror has at least one piezoelectric actuator 21 for displacing the mirror 20. As Figure 2 The multiple micromirrors shown can be arranged in an array to form a micromirror array.

[0072] Figure 2 An MEMS system with a mirror 20 is shown, and the mirror 20 can be part of a micromirror array according to an embodiment. The MEMS system has a four-fold rotational symmetry about an axis. Specifically, four piezoelectric actuators 21 are symmetrically arranged below the mirror 20 to effect tipping and tilting displacements of the mirror 20. The mirror 20 is typically rectangular (used herein to include square) with each side in the range of 0.5 mm to 2.5 mm. For example, it can be 1 mm 2A square of surface area. In other embodiments, the mirror can be of other shapes, such as hexagonal. Each piezoelectric actuator 21 has a flexible strip 22 of material, which is fixed at one end to a underlying substrate (not shown) and is connected to a strut 24 via a hinge 23. The flexible strip 22 of material has an elongation direction which is substantially parallel to the plane of the front surface of the reflector 20 when at rest. The flexible strip 22 of material has a piezoelectric material layer (e.g., PZT) thereon, and a voltage can be applied to the piezoelectric material layer to activate the piezoelectric actuator 21. When the piezoelectric actuator 21 is activated, the strip 22 bends and acts as a cantilever to displace the mirror 20 via the strut 24. The magnitude of the displacement is a function of the applied voltage (and is also a function of other parameters, such as the geometry of the piezoelectric actuator). The hinge 23 is formed by narrowing the strip 22 at one end such that the cross-sectional area of the hinge 23 is less than the cross-sectional area of the strip 22 along the elongation direction of the strip 22. Thus, the rigidity of the hinge 23 is less than that of the strip 22, which allows it to act as a hinge. A hinge typically needs to bend in a plane transverse to the axis direction, which is itself transverse to the elongation direction of the strip 22.

[0073] The mirror 20 is also connected to a heat spreader, which includes a heat conducting post 25 connected to the center of the back surface of the mirror 20 and a heat sink 26 connected to the other end of the post 25. In the stationary state, the length direction of the post 25 is an axis of four-fold rotational symmetry of the MEMS system. The heat conducting post 25 is arranged to transfer heat from the mirror 20 to the heat sink 26, which spreads the heat over a relatively large surface area. The heat sink 26 includes a flexible membrane of a patterned silicon layer that is circular in shape. Optionally, there can be multiple layers; if so, one or more (e.g., all) layers can be silicon, and one or more layers can be a material other than silicon. The flexible membrane has a bending groove 27 formed therein, and the groove 27 increases the flexibility of the flexible membrane. In use, when the mirror 20 is displaced, the heat conducting post 25 pivots and elastically deforms the flexible membrane.

[0074] Each piezoelectric actuator 21 is associated with a sensing element 28, and the piezoelectric actuator 21 is a piezoresistor that is fixed at one end to the strut 24 and at the other end to the underlying substrate. When the strut 24 is displaced, the piezoresistor is compressed / deformed, which changes the electrical properties of the piezoresistor, whereby the displacement can be determined. For example, the piezoresistor can be connected in a Wheatstone bridge, which is configured such that the output voltage of the bridge is a function of the displacement of the mirror 20. The output of the piezoresistor is temperature sensitive, and temperature compensation can be used to improve the accuracy of the displacement measurement.

[0075] Figure 3FIG. shows a schematic cross-section of a portion of a micromirror array 300 according to an embodiment. The mirror 301 is connected to the piezoelectric actuator 302 via a strut 303. The strut 303 includes silicon, germanium, aluminum, and oxide layers with a combined thickness of 151 μm. The oxide layer 304 has a thickness of 1 μm and provides thermal and electrical insulation to protect the piezoelectric actuator 302.

[0076] The piezoelectric actuator 302 includes a flexible material strip 305, which is a 5-μm-thick silicon strip, connected to the substrate 306 at opposite ends of the strut 303. The piezoelectric actuator 302 also includes a piezoelectric material layer 307, which is a PZT layer with a thickness in the range of 500 nm to 2 μm. The piezoelectric material layer 307 has top and bottom electrodes 308 made of platinum (Pt) and LaNiO3 (LNO) for applying a voltage to the piezoelectric material layer 307. The piezoelectric material layer is bonded to the flexible material strip 305 by a nitride (SiN) layer 309 with a thickness of 100 nm. The stack of SiN / Pt / LNO / PZT / Pt / LNO is covered by silicon oxide and aluminum oxide layers 310. Applying a voltage to the electrodes 308 causes the piezoelectric material layer 307 to contract or expand, but because the layer is constrained at the interface with the flexible material strip 305, the combined system of the piezoelectric material layer 307 and the flexible material strip 305 bends. It is this bending motion of the piezoelectric actuator 302 that displaces the strut 303 and thereby also displaces the mirror 301.

[0077] A sensing element 311 including a piezoresistor is embedded in the flexible material strip 305 and covered by an oxide. The sensing element 311 is arranged to sense the deflection of the piezoelectric actuator 302.

[0078] The piezoelectric actuator 302 is electrically connected to the substrate 306 through a through-silicon via (TSV) 312. An application-specific integrated circuit (ASIC) 313 can be used to apply a voltage to the piezoelectric actuator 302 and also to derive an output voltage from the sensing element 310.

[0079] The mirror 301 is connected to the heat sink 314 through the heat conducting posts 315. The heat sink 314 and the heat conducting posts 315 together form a heat diffuser for diffusing the heat from the mirror 301. The post 315 includes silicon, germanium and aluminum layers. The heat sink 314 includes a flexible element, specifically a flexible silicon film in this example. The flexible element allows the post 315 to move when the mirror 301 is displaced. The heat sink 314 is electrically connected to the substrate 306 and can be grounded to prevent charge accumulation on the mirror 301. The heat sink 314 and the flexible material strip 305 are in the same plane and can be formed from the same silicon wafer. The spacing between the lower surface of the mirror 301 and the upper surface of the heat sink 314 and / or the flexible material strip 305 can be in the range of 50 μm - 120 μm, such as about 80 μm. Typically, known micro - mirror arrays have a smaller spacing than this, such as only a few microns. In this embodiment, a larger spacing is achieved because the (piezoelectric) actuation force is generated on the flexible material strip 305 rather than, for example, by an electrostatic actuator mounted on the mirror itself, which would typically limit the movement range of the mirror to the relative movement range of the components of the electrostatic actuator.

[0080] The spacing between the lower surface of the heat sink 314 and / or the flexible material strip 305 and the upper surface of the substrate can be in the range of 50 μm - 120 μm, such as about 80 μm.

[0081] Each mirror can be provided with one or more control units. The one or more control units can be operated to identify the address corresponding to the mirror in the received control signal, and after identifying the address, generate a control voltage for one or more piezoelectric actuators of the mirror based on the control information additionally included in the control signal. The control unit can be implemented as an ASIC 313. The ASIC 313 receives the control signal using the vias 312 and controls the corresponding piezoelectric actuator 302 based on it; in this case, for a given piezoelectric actuator 302, the address in the control signal can specify not only the mirror but also the ASIC 313. Using the control unit, an external control system can individually control all the mirrors in the mirror array by transmitting the same control signal to the control units of all the mirrors, such that each control unit identifies the control signal addressed to it and accordingly controls the corresponding (multiple) piezoelectric actuators. It is possible to position the ASIC 313, for example, within a structure supported above the substrate 306 and close to the flexible material strip 305 (e.g., substantially coplanar with the flexible material strip 305), because this embodiment can be formed in multiple layers by a MEMS process, as described below.

[0082] Figure 4A schematic top view of a MEMS system 40 for controlling a mirror (not shown) in a micromirror array according to an embodiment is shown. The system 40 includes four piezoelectric actuators 41 connected to the mirror through respective struts 42. Each piezoelectric actuator 41 includes a strip of flexible material 43 and a hinge 44, where the strut 42 is connected to the hinge 44 at one end of the strip 43, and where the other end of the strip 43 (i.e., in a substantially fixed positional relationship) is connected to a substrate (not shown). The system 40 also includes a heat spreader 45 for spreading heat from the mirror during use. The heat spreader 45 includes a circular silicon layer fixed to the substrate along the outer edge of the silicon layer and a heat conducting post connected to the center of the silicon layer.

[0083] The system 40 includes Figure 4 any one or more of the five different types of sensing elements 46a to 46e shown, the sensing elements being for sensing the displacement of the mirror. Each of the sensing elements 46a to 46e includes a piezoresistor arranged such that the displacement of the mirror causes stress in the piezoresistor. Preferably, each of the actuators is provided with only one of the five types of sensing elements 46a to 46e, and the same type of sensing element is used in each of the four actuators.

[0084] The first type of sensing element 46a includes a curved beam located in an annular space between the heat spreader 45 and the piezoelectric actuator 41, the curved beam being fixed to the substrate at one end and to the strut 42 at the opposite end. One or more piezoresistors may be formed in the beam.

[0085] The second type of sensing element 46b includes two folded beams, each of the two folded beams including a respective piezoresistor, each piezoresistor being located in an annular space between the heat spreader 45 and the piezoelectric actuator 41. One of the folded beams is connected to the substrate and the strut 42, while the other folded beam is connected to two different points of the substrate to provide a reference value.

[0086] The third type of sensing element 46c includes a straight beam including a piezoresistor connected to the strut 42 at one end and to the substrate at the other end.

[0087] The fourth type of sensing element 46d includes a curved beam including a piezoresistor, the beam being located outside the piezoelectric actuator 41. The beam is connected to the substrate at one end and to the strut 42 at the opposite end.

[0088] The fifth type of sensing element 46e includes four piezoresistors fixed to the heat spreader 45. The fifth sensing element 46e is for sensing the deformation of the heat spreader caused by the displacement of the mirror.

[0089] Figure 5 Schematic top view showing a part of the sensing element 50, which can be the Figure 4 first sensing element 46a in, but other sensing elements have a similar structure. The sensing element 50 includes a flexure beam 51 and a folded piezoresistor 52 in an annular space 53 between a piezoelectric actuator 54 and a thermal diffuser 55. A temperature sensor 56 can be provided to measure the temperature of the piezoresistor 52. The temperature sensor can be implemented as a bipolar transistor or a diode, for example, because for such devices, the current is a function of temperature.

[0090] Figure 6 Schematic circuit diagram showing a Wheatstone bridge with a supply voltage Vs, resistors R1, R2, R3, and R4, and an output voltage V 0 . The circuit can be part of a sensing element for sensing the displacement of a mirror in a micromirror array. One or more of the resistors R1 to R4 can be piezoresistors of the sensing element.

[0091] Figure 7a Schematic view showing a specific configuration of the resistors R1 to R4 of the Wheatstone bridge in a sensing element 70 including a flexure beam 71. R1 is a piezoresistor extending along the elongation direction of the beam 71 towards the moving end of the beam. R2 to R4 are located at the ends of the sensing element 70 fixed to the substrate. When the beam 71 deflects / is stressed due to the displacement of the mirror, the resistance of R1 changes, while the resistances of R2 to R4 remain substantially constant.

[0092] Figure 7b Schematic view showing an alternative configuration of the resistors R1 to R4 of the Wheatstone bridge in a sensing element 70 including a flexure beam 71. In this configuration, two piezoresistors (R1 and R4) extend along the elongation direction of the beam 71, while R2 and R3 are located at the ends of the sensing element 70 fixed to the substrate. When the beam 71 deflects / is stressed due to the displacement of the mirror, the resistances of R1 and R4 change, while the resistances of R2 and R3 remain substantially constant.

[0093] As described above, one or more of the piezoresistors R1 to R4 can be provided with a temperature sensor. In this case, the temperature value output by the (multiple) temperature sensors can be used (using a circuit 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 the resistance and deflection / stress in the resistors R1 to R4.

[0094] Embodiments of the micromirror array can provide a tilt and a tilt displacement range of + / - 120 mrad and a mirror accuracy of 100 μrad. Embodiments of the micromirror array can operate at the high light intensities required for EUV and can operate at an absorption heat power density of 40 to 60 kW / m 2 This is several orders of magnitude higher than the absorption heat power density of micromirror arrays used in some other applications. This is possible because the piezoelectric actuators 41 can be operated to provide strong forces even at relatively low actuator voltages (e.g., below about 100 V), enabling them to deform the flexible element (flexible member 314), even though the flexible element is thick enough to provide high thermal conductivity to the substrate. Due to the high thermal conductivity, the micromirror array can have a temperature below about 100 degrees Celsius during use.

[0095] A method of forming a micromirror array is also described herein. Figures 8a - 8j Some steps of an embodiment of this method are shown.

[0096] As Figure 8a shown, the method includes: providing a first silicon wafer 800 for forming piezoelectric actuators and sensing elements. The first wafer 800 may be referred to as an "actuator wafer". The actuator wafer may be a silicon-on-insulator (SOI) wafer having a 4-μm silicon film 801. Low-voltage active devices such as sensing elements can be formed in the wafer 800 using a complementary metal oxide semiconductor (CMOS) front-end-of-line (FEOL) process. Then a CMOS back-end-of-line (BEOL) process can be used to form metal interconnect layers for connecting the low-voltage devices to other circuit devices. Then chemical mechanical polishing / planarization (CMP) can be used to form a smooth surface having a planar oxide layer 802. A Cu damascene process can be used to form a Cu bonding matrix 803 having Cu pads 804 for subsequent Cu-Ox hybrid bonding to another wafer.

[0097] As Figure 8b shown, the method further includes: providing a second silicon wafer 805, which will be the substrate on which the micromirror array is to be fixed. The second wafer 805 may be referred to as an "insert wafer". The insert wafer 805 may be an SOI wafer having a 100-μm silicon film, as Figure 8b shown. A high-voltage (HV) CMOS process (FEOL and BEOL) can be used to form HV drivers in the wafer. A TSV process can be used to form electrical connections 806 through the silicon film 807 of the second wafer 805. Planarization (e.g., CMP or wet etching) and Cu pad formation can be performed after the TSV process. Cu pads 808 are arranged to connect to the Cu bonding matrix 803 of the first wafer 800.

[0098] Figure 8c Shows how to bond the first wafer 800 and the second wafer 805 using, for example, a Cu / oxide hybrid bond 809.

[0099] As Figure 8d shown, the "handle wafer" used to process the first wafer 800 can be removed to expose the first wafer 800. Then, selective box removal can be used to leave a thin layer (e.g., 5 μm) of silicon 810 of the first wafer 800 bonded to the second wafer 805. Al can be deposited and patterned on the first wafer 800 for subsequent connection to the mirror. The first wafer 800 can be patterned to form a piezoelectric actuator 811. Al 2 O 3 and / or TiN can be deposited on the first wafer to prevent EUV radiation and plasma.

[0100] As Figure 8e shown, the method further includes: providing a third silicon wafer 812 for forming a mirror. The third wafer 812 can be referred to as the "mirror wafer" 812. The mirror wafer 812 can be an SOI wafer with a 250-μm silicon film. The method can include performing a cavity etch on the mirror wafer 812 to allow for a thermal insulator (e.g., 1 μm to 2 μm), followed by Ge deposition for subsequent bonding to the first wafer 800. Using a hard mask (e.g., nitride) and a resist mask, the mirror wafer 812 is etched to form struts 813 ("beam connectors") for connection to the piezoelectric actuator and to form thermal conduction columns 814 ("center columns") for connection to the heat sink in order to form a heat spreader. A mirror release trench 815 is etched around the periphery of the mirror.

[0101] As Figure 8f shown, the third wafer 812 is bonded to the first wafer 800 in order to connect the mirror to the piezoelectric actuator. The bonding step can include an aligned Ge / Al eutectic bond. The Al / Ge bonding layer 816 is both thermally and electrically conductive, which can allow for effective heat transfer from the mirror through the thermal conduction columns to the heat sink. Some box-shaped oxides can be left on the first wafer 800 to reduce thermal and electrical conduction at some bonding locations (such as at the struts connected to the piezoelectric actuator).

[0102] In Figure 8gIn this case, the stack of joined wafers (wafer 1 "actuator wafer" 800, wafer 2 "insert wafer" 805, and wafer 3 "mirror wafer" 812) (which may be collectively referred to as "device wafers") is inverted such that the operating wafer 817 of the mirror wafer 812 becomes the support wafer. The operating wafer of the second wafer 805 can be removed from the second wafer 805, and box removal can be used to expose the TSVs 806 in the second wafer 805. This can be followed by dielectric deposition, patterning, and bump formation.

[0103] In Figure 8h this case, lithography and silicon etching are used to form cavities 818 in the second wafer 805 below the piezoelectric actuator and the heat sink. Note that in use, the exterior of the flexible membrane 314 (e.g., the radially outward portion of the groove) contacts the wall 819. The wall 819 is located between the cavity 818a registered with the heat sink and the cavity 818b registered with the piezoelectric actuator. The wall 819 is capable of conducting heat from the flexible membrane 314 to the substrate. The method then includes etching the dielectric layer to expose the piezoelectric actuator (i.e., to release the flexible material strip), the sensing element, and the scribe lines.

[0104] As Figure 8i shown, the method further includes: providing a fourth silicon wafer 820 for sealing the second wafer 805. The fourth wafer 820 may be referred to as the "support wafer" 820 and includes TSVs 821 for connection to the second wafer 805. Bump bonding can be used to bond the fourth wafer 820 to the second wafer 805. An Al redistribution layer (RDL) and connection pads can be formed on the back side of the fourth wafer 820.

[0105] In Figure 8j this case, the mirror 822 is released by removing the front-side operating wafer 817. The controller chip can be glued and wire-bonded to the back side of the fourth wafer 820.

[0106] Although specific reference may be made herein to the use of a lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guiding and detecting patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0107] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in a manner different from that described. The above description is intended to be illustrative and not restrictive. Thus, it will be apparent to those skilled in the art that the described invention may be modified without departing from the scope of the claims set forth below.

Claims

1. A micro - mirror array, comprising: a substrate; a plurality of mirrors for reflecting incident light; at least one piezoelectric actuator, for each of the plurality of mirrors, the at least one piezoelectric actuator is configured to displace the mirror and is connected to the substrate; and one or more struts connecting the mirror to the at least one piezoelectric actuator, and for each of the plurality of mirrors, the micro - mirror array includes a heat spreader for dissipating heat from the mirror, the heat spreader includes a heat sink and a heat - conducting post connecting the heat sink to the mirror.

2. The micro - mirror array according to claim 1, wherein the heat sink includes a flexible membrane, and when the mirror is displaced, the flexible membrane allows the heat - conducting post to pivot.

3. The micro - mirror array according to claim 2, wherein the flexible membrane includes a patterned silicon layer.

4. The micro - mirror array according to claim 3, wherein the flexible membrane includes grooves passing through the silicon layer and extending from the outer edge of the heat spreader towards the heat - conducting post.

5. The micro - mirror array according to claim 4, wherein the grooves are curved grooves.

6. The micro - mirror array according to any one of claims 1 to 5, wherein the heat - conducting post is conductive and grounded.

7. The micro - mirror array according to any one of the foregoing claims, wherein the at least one piezoelectric actuator comprises: a flexible material strip, the flexible material strip is connected to the substrate at one end, and one of the one or more struts is located at the opposite end of the flexible material strip; and a piezoelectric material layer disposed on the flexible material strip.

8. The micro - mirror array according to claim 7, wherein the at least one piezoelectric actuator further includes a hinge connected to the end of the flexible material strip and the strut, and wherein the hinge has a smaller cross - section than the flexible material strip in the elongation direction of the strip.

9. The micro - mirror array according to any one of the foregoing claims, wherein the one or more struts include a heat - insulating layer to reduce or prevent heat transfer from the mirror to the at least one piezoelectric actuator.

10. The micro - mirror array according to any one of the foregoing claims, wherein the one or more struts are configured to electrically isolate the mirror from the at least one piezoelectric actuator.

11. The micro - mirror array according to any one of the foregoing claims, for each of the plurality of mirrors, the micro - mirror array includes at least one sensing element connected to the at least one piezoelectric actuator, the at least one sensing element is configured to sense the displacement of the mirror.

12. The micro - mirror array according to claim 11, wherein the sensing element includes a piezoresistor, and the piezoresistor is arranged such that the displacement of the mirror deflects the piezoresistor.

13. The micro - mirror array according to claim 12, wherein the sensing element further includes a temperature sensor for measuring the temperature of the piezoresistor.

14. The micromirror array according to any one of the preceding claims, wherein each of the plurality of mirrors is configured to reflect light having a wavelength of substantially 13.5 nm.

15. The micromirror array according to any one of the preceding claims, wherein the at least one piezoelectric actuator comprises four piezoelectric actuators arranged to effect tilting and tipping displacement control of the mirror.

16. A programmable illuminator comprising the micromirror array according to any one of claims 1 to 15, the micromirror array being configured to condition a radiation beam.

17. The programmable illuminator according to claim 16, the programmable illuminator comprising a displacement control feedback system configured to determine the position of each of the plurality of mirrors and to adjust the voltage applied to the one or more piezoelectric actuators based on the determined position and a predetermined target position of the mirror.

18. A lithographic apparatus for projecting a pattern from a patterning device onto a substrate, comprising the programmable illuminator according to claim 16 or 17, the programmable illuminator being configured to condition the radiation beam used to illuminate the patterning device and / or to condition the radiation beam used to measure a target structure on the substrate.

19. An inspection apparatus comprising the programmable illuminator according to claim 16 or 17, the programmable illuminator being configured to condition the radiation beam used to measure a target structure on a substrate.

20. A method of forming a micromirror array, comprising: providing a substrate; forming a plurality of mirrors for reflecting incident light; for each of the plurality of mirrors, forming at least one piezoelectric actuator configured to displace the mirror and connected to the substrate; and forming one or more struts connecting the mirror to the at least one piezoelectric actuator, and the method comprising: for each of the plurality of mirrors, forming a heat spreader for dissipating heat from the mirror by forming a heat sink and a heat conducting post connected to the mirror.

21. The method according to claim 20, wherein the step of forming the heat sink comprises: forming a flexible membrane that allows the heat conducting post to pivot when the mirror is displaced.

22. The method according to claim 21, wherein the step of forming the flexible membrane comprises: patterning a silicon layer.

23. The method according to claim 21 or 22, wherein the step of forming the flexible membrane comprises: forming a groove extending from an outer edge of the heat spreader towards the heat conducting post.

24. The method according to claim 23, wherein the groove is a curved groove.

25. The method according to any one of claims 20 to 24, wherein the step of forming at least one piezoelectric actuator comprises: forming a curved strip comprising a silicon layer and a piezoelectric material layer; and Form a hinge that is connected to one end of the bent strip and the strut, wherein the hinge is thinner than the strip.

26. The method according to any one of claims 20 to 25, wherein the step of forming one or more struts comprises: Providing a thermal insulation layer in the strut or each strut to reduce or prevent heat transfer from the mirror to the at least one piezoelectric actuator.

27. The method according to any one of claims 20 to 26, comprises: For each of the plurality of mirrors, forming at least one sensing element for sensing the displacement of the mirror.

28. The method according to claim 27, wherein the step of forming the at least one sensing element comprises: Forming a piezoresistor that is arranged such that the displacement of the mirror deflects the piezoresistor.

29. The method according to claim 28, wherein the step of forming the at least one sensing element comprises: Forming a temperature sensor to measure the temperature of the piezoresistor.

30. The method according to any one of claims 20 to 29, wherein the step of forming at least one piezoelectric actuator comprises: Forming four piezoelectric actuators that are arranged to achieve control of the tipping and tilting displacement of the mirror.

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