Pixel and array architecture for piston-motion spatial light modulation

Abstract

An example apparatus and method for fabricating a plurality of independently addressable piston-motion micromirrors are disclosed. The apparatus includes a plurality of independently addressable piston-motion micromirrors. Each one of the independent addressable piston-motion micromirrors includes an independently addressable electrode, a suspension beam structure on the independently addressable electrode, a reflective surface coupled to the suspension beam structure, and a reflective surface coupled to the reflective surface body. A voltage is applied to the independently addressable electrode to create an electrostatic attraction of the reflective surface to provide a pixel-level phase shift.

Description

PIXEL AND ARRAY ARCHITECTURE FOR PISTON-MOTION SPATIAL LIGHT MODULATIONCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to United States Provisional Patent Application Serial No. 63 / 491 ,817, filed March 23, 2023, which is herein incorporated by reference in its entirety.REFERENCE TO GOVERNMENT FUNDING

[0002] The invention was made with government support under grant Number MH125556 awarded by the National Institutes of Health. The government has certain rights in the invention.FIELD OF THE INVENTION

[0003] The present disclosure describes a spatial light modulator that includes a plurality of configurable piston-motion micromirrors. The plurality of piston-motion micromirrors are independently addressable, and produce highspeed phase modulation, accommodating both a wide wavelength range for time-averaged polychromatic operation and CMOS-compatible voltage drive for scalability. The spatial light modulator may be used for holographic displays, biological interfacing, and the like.BACKGROUND

[0004] The ability to dynamically pattern or modulate light is widely employed in imaging and projection systems across a wide range of disciplines, including astronomy, biological microscopy, optical trapping, ophthalmoscopy, and augmented reality / virtual reality (AR / VR). While various approaches may be employed to achieve primitive forms of optical modulation such as focus tuning or beamsteering, sculpting light with enough control and precision to reproduce any optical wavefront, including ones scattered or emitted from natural objects or sources, requires the use of computer-generated holography (CGH).

[0005] Dynamic light sculpting under CGH is enabled by spatial light modulators (SLM), i.e., configurable diffractive surfaces capable of imparting arbitrary pixel-level phase and / or amplitude modulation to a coherent input beam of light in order to generate desired 3D illumination patterns via downstream interference. Current state-of-the-art SLMs are based on phase- only liquid crystal on silicon (LCoS), a technology that reorients birefringent crystals using applied electric fields, thereby creating local changes in refractive index that impart varying phase shifts to light at each SLM pixel. Given the maturity of this technology, current LCoS SLMs boast array formats on the order of megapixels. Yet liquid crystal backflow during settling fundamentally limits LCoS switching speeds to 100-300 Hz at room temperature and to a maximum of 500 Hz with cumbersome strategies including active heating / cooling, overdrive, and >2TT phase modulation.

[0006] Given this refresh rate barrier, the LCoS industry has turned to scaling up pixel count in an endeavor suffering from increasingly impractical device sizes, complexity, real-time computation, and cost. With such diminishing returns, improvements to the space-bandwidth product, i.e., information density, available for modulation with SLMs become marginal. And given that SLMs inherently produce monochromatic and speckled frames due to the coherence requirement of CGH, eschewing speed in favor of pixel count becomes especially disadvantageous. Indeed, higher-speed SLMs could be mobilized to produce full-color and speckle-free projections via time-averaged operation under allowable integration windows, i.e., by cycling rapidly across frames targeting different colors, depths, subfields, etc.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The teaching of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

[0008] FIGs. 1A-1C depict example augmented reality / virtual reality (AR / VR) projection display architectures using a high-speed spatial light modulator (SLM) of the present disclosure;

[0009] FIG. 2 depicts an example schematic of an optical neural interface employing the example high-speed SLM of the present disclosure;

[0010] FIG. 3 depicts an cross-sectional side view of an example of a pistonmotion micromirror pixel of the high-speed SLM of the present disclosure;

[0011] FIG. 4 depicts a flow chart of an example method for forming a plurality of independently addressable piston-motion micromirrors in the example high-speed SLM of the present disclosure;

[0012] FIG. 5 depicts an example structural stack-up (isometric and top- down views) of the example high-speed SLM of the present disclosure;

[0013] FIG. 6 depicts examples of beam extension via ingression into the reflective surface body;

[0014] FIG. 7 depicts a graph illustrating actuation curves (optical travel path vs. voltage relationships) for different amounts of beam extension of the present disclosure;

[0015] FIG. 8 depicts example layout schematics illustrating principle of driver integration with escape routing, of the present disclosure; and

[0016] FIG. 9 depicts example dummy pixel structures of the present disclosure.

[0017] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.DETAILED DESCRIPTION

[0018] The present disclosure provides examples of piston-motion micromirrors and methods for fabricating the same. As discussed above, the ability to dynamically pattern or modulate light is widely employed in imaging and projection systems across a wide range of disciplines, including astronomy, biological microscopy, optical trapping, ophthalmoscopy, and augmented reality / virtual reality (ARA / R). While various approaches may be employed to achieve primitive forms of optical modulation such as focus tuning or beamsteering, sculpting light with enough control and precision to reproduceany optical wavefront, including ones scattered or emitted from natural objects or sources, requires the use of computer-generated holography (CGH).

[0019] Dynamic light sculpting under CGH is enabled by spatial light modulators (SLM), i.e., configurable diffractive surfaces capable of imparting arbitrary pixel-level phase and / or amplitude modulation to a coherent input beam of light in order to generate desired 3D illumination patterns via downstream interference. Current state-of-the-art SLMs are based on phase- only liquid crystal on silicon (LCoS), a technology that reorients birefringent crystals using applied electric fields, thereby creating local changes in refractive index that impart varying phase shifts to light at each SLM pixel. Given the maturity of this technology, current LCoS SLMs boast array formats on the order of megapixels. Yet liquid crystal backflow during settling fundamentally limits LCoS switching speeds to 100-300 Hz at room temperature and to a maximum of 500 Hz with cumbersome strategies including active heating / cooling, overdrive, and >2TT phase modulation.

[0020] Given this refresh rate barrier, the LCoS industry has turned to scaling up pixel count in an endeavor suffering from increasingly impractical device sizes, complexity, real-time computation, and cost. With such diminishing returns, improvements to the space-bandwidth product, i.e., information density, available for modulation with SLMs become marginal. And given that SLMs inherently produce monochromatic and speckled frames due to the coherence requirement of CGH, eschewing speed in favor of pixel count becomes especially disadvantageous. Indeed, higher-speed SLMs could be mobilized to produce full-color and speckle-free projections via time-averaged operation under allowable integration windows, i.e., by cycling rapidly across frames targeting different colors, depths, subfields, etc.

[0021] Accordingly, various alternative SLM technologies have been explored for kilohertz to megahertz-rate array-scale phase modulation, including ferroelectric liquid crystals, silicon photonic phased arrays employing thermooptic or microelectromechanical system (MEMS)-based phase shifters, and tunable metasurfaces and resonant optical antennas. Yet array scaling and functional deployment challenges persist across these technologies due toassociated trade-offs, such as binary-only modulation in ferroelectric liquid crystals and digital micromirror devices (DMDs), high power consumption in thermo-optic phase shifters, large size and insertion loss in ME MS-based photonic MEMS shifters, and limited or co-dependent phase and amplitude modulation in tunable nanoantennas.

[0022] Advanced commercialization efforts for high-speed SLMs do however exist with piston-motion micromirror arrays, which employ electrostatic actuation to displace suspended micromirrors via open-loop voltage drive applied to fixed underlying electrodes, thereby imparting pixel-level phase shifts by altering the travel path of locally incident light. Existing products using such arrays include continuous deformable mirror (cDM) arrays and phase light modulators (PLMs), which adapted its surface micro-machined complementary metal-oxide- semiconductor (CMOS) platform to actuate piston-motion phase-shifting micromirrors instead of its hallmark binary amplitude DMDs. Overall, pistonmotion micromirrors offer promise as building blocks for high-speed SLMs because they possess megahertz-scale resonant frequencies that can surpass 10 kHz refresh rates under step settling, allow for random access patterning via quasi-static operation, and are agnostic to polarization, which is not the case for liquid crystals and metasurfaces.

[0023] Nevertheless, existing piston-motion micromirror array designs suffer from various limiting constraints that restrict their ability as SLMs to fully exploit fast speeds. Namely, currently available PLM devices offer poor pseudoanalog control in the form of only 16 phase levels and can only accommodate wavelengths ranging between 405 nm to 632 nm, which is considerably narrower than the 400-800 nm range required for full-color visual displays under time-averaged operation and markedly lower than the 800-1200 nm infrared range typically needed for multi-photon excitation in biological microscopy.

[0024] Similarly, cDM arrays exhibit inter-actuator coupling, which precludes fully independent pixel control of pixels, and require cumbersome high-voltage drivers that cannot be scaled to higher pixel counts. Therefore, the need for a configurable piston-motion micromirror array architecture that can serve to produce high-speed SLMs while accommodating both a wide wavelength rangefor time-averaged polychromatic operation and CMOS-compatible voltage drive for scalability remains unmet.

[0025] The most widely used AR / VR display technologies today include stereoscopic vision, which delivers distinct 2D images to each eye, and lightfield displays, which shape the origin and direction of light rays, typically through the use of multi-layer liquid crystal displays. Neither of these approaches is capable of generating optical wavefronts identical to ones reflected off physical 3D objects, nor are they able to generate images of sufficient quality and entirely free from eye strain. The only light-sculpting method that can recreate natural-looking 3D images is CGH, which surpasses stereoscopy and light-field techniques for multi-focal near eye displays since it does not suffer from vergence-accommodation conflicts and offers both parallax capabilities and the option of see-through real environment integration. In practice, however, CGH display implementation poses two main challenges: monochrome frames due to coherent light constraints, and a tradeoff between resolution and FoV that results in speckled images of poor quality. These limitations can be overcome by achieving high frame-to-frame refresh rates to exploit properties of human vision.

[0026] The human eye physiologically detects at rates of -1 kHz but persistence of vision limits perception by the human eye to -100 Hz. Therefore, cycling through multiple frames rapidly and alternating across light sources can introduce color to holographic experiences.

[0027] Similarly, shortcomings in image quality can be overcome with speed by applying established time multiplexing algorithms or by simply scaling a single frame’s FoV down to a subfield of sufficient resolution and rapidly sweeping across such subfields with lateral scanning tools to restore the full FoV.

[0028] A representative optical schematic of such a system, using a highspeed SLM 100 of the present disclosure is illustrated in FIGs. 1A-1C. For example, as illustrated in FIG. 1A, the system may include the SLM 100, a blue laser 102, a red laser 104, a green laser 106, a series of mirrors 108 and 110, a rapid XY scanner 112, and a lens 114. FIG. 1A illustrates an example, wherethe red laser 104 is projected to be displayed with various field of views FoV1 to FoV3. FIG. 1 B illustrates an example with the green laser 106 activated and FIG. 10 illustrates an example with the blue laser 102 activated.

[0029] Under the wide wavelength range (<1200 nm) and high-speed (~10 kHz) offered by the piston-motion micromirror-based SLM architecture in the SLM 100 of the present disclosure, high-speed CGH could enable near eye displays to create full-color, full-field, and artifact-free immersive experiences with unprecedented realism.

[0030] Recording and manipulating cells from biological tissue, such as the brain, at meaningful scales (i.e., thousands of simultaneously addressed cells) and within biologically relevant timescales (ex: ~1 millisecond in circuits of interconnected firing neurons) poses significant challenges in terms of scaling, precision, and safety that cannot be addressed with conventional electrodebased approaches. Accordingly, recent studies have turned to all-optical approaches for biological interfacing though the use of fluorescent membrane indicators of cellular activity and actuating proteins known as opsins that respond to light by triggering or inhibiting cell activity (such as neuron firing). Under such approaches, both recording and stimulation can be performed across different optical wavelength domains to avoid read / write crosstalk and without physically penetrating tissue. The precision offered by optical neural interfaces is further aided by auxiliary techniques, including multi-photon excitation and temporal focusing, which further confine illuminated target regions but require infrared-domain wavelengths. While LCoS-based CGH has already been leveraged in such all-optical interfaces, for instance to elicit realistic sensory precepts via stimulation of the primary visual cortex, the 10- 100x speeds (~10 kHz) and wide wavelength range (<1200 nm: visible and near-infrared) that the piston-motion micromirror array architecture of the present disclosure can produce could achieve unprecedented levels of spatiotemporal precision and scale to control and interrogate in vivo cell networks across multiple communication channels (i.e., optical wavelengths domains).

[0031] A representative optical schematic illustrating CGH-based biological interfacing with neurons is shown in FIG. 2. For example, FIG. 2 illustrates a laser 202, the SLM 100, a lens 204, and a subset of neurons 206.

[0032] A piston-motion micromirror pixel of the present disclosure includes multiple elements. For example, the piston-motion micromirror pixel of the present disclosure can include an independently addressable bottom electrode for voltage-based electrostatic drive, a planar reflective mirror body biased to a fixed global voltage, a suspension beam network for spring-like mirror deflection under electrostatic actuation, and a gap stop mechanism to prevent contact between suspended mirror and fixed electrode. Competing needs across each of these elements often hinders simplicity in the structural pixel stackup, increasing fabrication complexity, cost, reliability, and adaptability.

[0033] A cross-sectional view of an example pixel 302 of the SLM 100 of the present disclosure is illustrated in FIG. 3. For example, the pixel 302 may be a single independently addressable piston-motion micromirror out of a two dimensional array of a plurality of independently addressable piston-motion micromirrors of the SLM 100. For example, the pixel 302 may include a substrate 304, an independently addressable electrode 306 (also referred to as an electrode 306) fabricated on the substrate 304, a suspension beam structure 308 (also referred to as the suspension beam layer 308) formed on the independently addressable electrode 306, and a reflective surface body 310 coupled to the suspension beam structure 308, and a reflective surface 312 (also referred to herein as a reflective mirror, reflective surface, reflective mirror surface, and the like) coupled to the reflective body 310. In one embodiment, the combination of the reflective surface body 310 and the reflective surface 312 may also be referred to as a mirror body.

[0034] In one embodiment, the pixel 302 may also include an optional first sacrificial layer 330 and an optional second sacrificial layer 332. The optional sacrificial layers 330 and 332 may be oxide layers and are discussed in further details below.

[0035] In one embodiment, the pixel 302 may include vias 3181 to 318n(also referred to herein individually as a via 318 or collectively as vias 318) throughthe reflective surface body 310, the reflective surface 312, and a core plate 314 of the suspension beam structure 308. A voltage may be applied to the independently addressable electrode 306 to create an electrostatic attraction of the reflective surface 312.

[0036] The vias 318 may allow for improved sacrificial layer etching or faster settling times through reduced squeeze-film damping. The suspension beam structure 308 may be designed to allow for a spring like movement vertically, as shown by an arrow 320. As a result, a gap distance 322 may be changed to provide a pixel-level phase shift when a voltage is applied to the independently addressable electrode 306.

[0037] Both the fill factor, i.e., the percentage of active reflective mirror surface 312 across a given pixel region, and reflectance of the mirror surface 312 should be as close to 100% in order to maximize SLM efficiency. Since mirror planarity is also critical to optical performance, the topography of structures underneath this active region must not be propagated to the surface of the top-level reflective mirror 312, complicating conformal deposition of suspension beam structure 308 at high-fill factors.

[0038] Under a micromachined, conformally deposited fabrication approach (which offers the most flexibility in terms of adaptability to various substrates 304 and potential for compact integration and assembly), residual stress mismatches between the reflective surface 312 (which may be a metal layer comprising gold) and the reflective surface body 310 (which may comprise of polysilicon) may lead to undesirable mirror curling. Yet eliminating this curling by increasing the thickness of the reflective surface body 310 precludes the ability to incorporate the suspension beam structure 308 in the same layer as the increased beam thickness would result in high mirror rigidity requiring high- voltage drive.

[0039] The suspension beam structure 308 should be laid out in a way that is tileable across pixels 302, avoids buckling due to internal residual stresses, accommodates sufficient beam length for low-voltage drive, and incorporates a gap stop mechanism that does not unduly restrict pixel motion. Each mirror’s suspension beam structure 308 must also fit within the footprint of a single pixel302 or mirror without impinging upon the capacitive electrostatic interface area that exists between the fixed electrode 306 and overlapping suspended reflective surface body 310, which must be kept high in order to maximize electrostatic actuation for low-voltage drive.

[0040] The successful operation of a circular micromirror array consisting of 23,852 pixels for high-speed dwelling-capable focus tuning across a wide range of optical wavelengths (<1200 nm) under CMOS-compatible voltage drive was previously developed and demonstrated. In this prior work, reinforcement against mirror curling was achieved using a double-stack mirror body that employed part of the suspension beam layer, creating beam-only and mirror- only regions that preserved mirror planarity at the expense of fill factor (which was 70%), beam adjustability, and beam buckling tolerance. The clamping scheme of the two suspension beams supporting each mirror also introduced an unconstrained torsional degree of freedom in the micromirrors.

[0041] Under the new SLM architecture of the present disclosure, a novel reinforced and partially double-stacked mirror structure that is capable of overlapping with the suspension beam structure 308 via ingressions that allow for adjustments to beam length as shown in FIGs. 3 and 4 is adopted.

[0042] A planarization step is also incorporated prior to the deposition of the reflective metal layer in order to preserve mirror planarity while achieving fill factors in excess of 90%. The modified architecture also allocates four suspension beams (illustrated in FIG. 4) and gap stops to each mirror under a scheme that eliminates uncontrolled degrees of freedom as well as beam buckling for improved actuation stability. The architecture is thus able to satisfy high fill factor, low-voltage drive, stable actuation, and mirror planarity requirements within a similarly compact stack-up consisting of three structural layers.

[0043] In addition, the micromachinable architecture may be deposited over different types of substrates 304 for various embodiments of driver electronics integration, namely a though-silicon via array embodiment that would allow for backside flip-chip bonding of driver CMOS chips or a buried escape routing network embodiment that would allow for frontside flip-chip bonding of driverchips instead. Finally, the SLM architecture also offers adjustable non-reflective and / or diffractive pixels that may be tiled into the active region of the SLM array with minimal impact to optical performance for various reasons, including the alleviation of peripheral routing constraints.

[0044] FIG. 4 illustrates a flow chart for a method 400 forming a plurality of independently addressable piston-motion micromirrors, or pixels 302, in the example high-speed SLM 100 of the present disclosure. The steps of the method 400 may be performed by various wafer fabrication techniques using processes such as photolithography and etching (e.g., wet etch, dry etch, plasma etch, and the like).

[0045] FIG. 5 illustrates an isometric view and a top view for a process flow 500 to illustrate structurally various steps of the method 400. As shown in FIG.5 the architecture of the high-speed SLM 100 may include a micromachined set of three conformally deposited structure layers, which may be comprised of doped polycrystalline or monocrystalline silicon.

[0046] At step 402, the method 400 begins. At step 404, the method 400 forms an electrode layer on a substrate. For example, various different types of substrates (e.g., silicon) may be used. The electrode layer may be formed using any method to form any type of electrode.

[0047] As shown in step 502 of FIG. 5, an electrode layer 352 may be formed on the substrate 304. The electrode layer 352 may be the bottom most layer.

[0048] At step 406, the method 400 deposits a network of traces to form a plurality of independently addressable electrodes in the electrode layer. For example, the network of traces may be a conductive metal deposited on the electrode layer. The network traces may define the boundaries of each one of the plurality of independently addressable electrodes. In other words, each one of the plurality of independently addressable electrodes may be surrounded by traces of the network traces.

[0049] As shown in step 502 in FIG. 5, the electrode layer 352 may include a two-dimensional periodic array of independently addressable electrodes 306i to 306m(hereinafter also referred to individually as an electrode 306 or collectivelyas electrodes 306). The electrodes 306 are capable of subjecting the suspended mirror structures to an attractive electrostatic force under the application of voltage, leading to displacements that alter the optical travel distance of locally incident coherent light and, thereby, impart pixel-level phase shifts.

[0050] The electrodes 306 are separated from each other by a network of traces 352 fully surrounding each electrode 306 and providing global electrical biasing as well as mechanical anchors to the suspended network of micromirrors. This network of traces 352 also provides electromagnetic shielding for both the electrode plates.

[0051] At step 408, the method 400 inserts a suspension beam in each corner of the plurality of independently addressable electrodes to form a plurality of suspension beam structures and a core plate. For example each suspension beam may be inserted into the trace at a corner of a respective independently addressable electrode. Four suspension beams may be deployed around each corner of the respective independently addressable electrode. The four suspension beams may be coupled to the core plate. The suspension beams may be fabricated from silicon or polysilicon.

[0052] For example, step 504 of FIG. 5 illustrates the middle structural layer of the suspension beam structure 308. For example, the suspension beam structure 308 may include a plurality of suspension beams 360i to 360i (hereinafter also referred to individually as a suspension beam 360 or collectively as suspension beams 360). Each electrode 306 may include four suspension beams 360i to 3604. A core plate 314 may be coupled to each set of four suspension beams 360i to 3604 for each electrode 306. The suspension beams 360 and the core plate 314 may be fabricated from silicon or polysilicon.

[0053] Some of the suspension beams 360 may be coupled to other beams associated with different electrodes 306. For example, the suspension beam 3602 may be connected to suspension beam 360s that supports a different electrode 306. Suspension beam 360s may be connected to suspension beam 360e that supports a different electrode 306. Suspension beam 3604 may be connected to suspension beams 360?, 360s, and 360g that each support threedifferent electrodes. In other words, each electrode 306 may have their own set of four suspension beams 360, but some of the suspension beams 360 may be coupled other suspension beams 360 that support other electrodes 306.

[0054] In some embodiments, a first sacrificial layer (e.g., the first sacrificial layer 330 illustrated in FIG. 3) may be deposited and etched between the electrode layer 350 and the suspension beam structure 308. The first sacrificial layer may comprise silicon dioxide. The first sacrificial layer may set the gap distance 322 of the capacitive electrostatic interface between the fixed driving electrodes 306 and the globally biased suspended reflective surface 312 and the reflective surface body 310.

[0055] Since the additional optical travel distance at each pixel 302 corresponds to double the mirror displacement and since the actuation range is confined to one third of the gap distance 322 under parallel-plate voltage-driven electrostatic transduction, the gap between the electrode 306 and suspension beam structure 308, i.e., the thickness of the first sacrificial layer 330, should be greater than 1.8 microns (pm) in order to accommodate both the full spectrum of visible light as well the 800-1200 nm wavelength region commonly employed in biological microscopy and interfacing.

[0056] At step 410, the method 400 couples a reflective surface body to one or more of the plurality of suspension beam structures. For example, the reflective surface body may be formed from silicon or polysilicon.

[0057] Step 506 of FIG. 5 illustrates the reflective surface body 310 applied to the suspension beam structure 308 and the core plate 314.

[0058] At step 412, the method 400 applies a reflective surface to the reflective surface body. For example, the reflective surface may be a coating or a metal plate coupled to the reflective surface body. The reflective surface may be any metal material that has a high reflectivity, such as gold, aluminum, and the like.

[0059] Step 508 of FIG. 5 illustrates the reflective surface 312 coupled to the reflective surface body 310. As can be seen in FIG. 5, the reflective surface 312 may extend past the reflective surface body 310 over the suspension beams 360.

[0060] In one embodiment, a second sacrificial layer (e.g., the second sacrificial layer 332 illustrated in FIG. 3) may be deposited and etched between the core plate 314 of the suspension beam structure 308 and the reflective surface body 310. The second sacrificial layer may comprise silicon dioxide. As seen in FIG. 3, the second sacrificial layer 332 may extend past the core plate 314 and all the way up the suspension beams 360 on the outer perimeter of the suspension beam structure 308 located at the four corners of each pixel 302.

[0061] This second sacrificial layer 332, therefore, isolates the suspension beams 360 from the overlying non-reinforced reflective surface 312 periphery, and its thickness sets the mirror displacement at which the corners of the reflective surface 312 are mechanically stopped through contact with the anchor posts in the suspension beam structure 308. In order to avoid restrictions on the full actuation range of the mirrors while preserving the gap stop functionality that prevents electrode contact or shorting, the thickness of this second sacrificial layer 332 should be greater than the maximum achievable displacement under voltage drive and should therefore be greater than 600 nm for a maximum optical wavelength of 1200 nm.

[0062] A large opening may be employed in the second sacrificial layer to ensure contact between the core plate 314 in the suspension beam structure 308 and the larger overlying reflective surface 312.

[0063] Under this geometry, the reflective surface body 310 is able to extend to the edges of the pixel regions for a high fill factor (>90%) and maintain flatness via a planarization step performed between the deposition of the reflective surface body 310 and that of the reflective surface 312. The partial reinforcement of the reflective surface body 310 is sufficient to adequately withstand mirror curling and offers enough footprint to achieve low-voltage drive with the suspension beams. At step 414, the method 400 ends.

[0064] Although an example method 400 is illustrated and described above, other additional steps may be employed. For example, passivation of the conductive structural layers may also be employed in order to fully isolate them and prevent electrical shorting from debris or mirror pull-in. This may beachieved through patterned passivation steps performed after the deposition, doping, and annealing of each structural layer via conformal deposition methods such as atomic layer deposition. Passivation may also be achieved after the deposition of the reflective surface 312 and etching of the sacrificial layers 330 and 332 via thermal oxidation, conformal oxide deposition, or oxidation from the galvanic potential formed by the deposition of metal on polycrystalline silicon.

[0065] The architecture of the high-speed SLM 100 of the present disclosure also offers the ability to adjust the dynamic and steady-state transduction behavior of the mirrors under the same SLM array geometry, i.e., the same pixel-to-pixel pitch, via prescribed modifications to the suspension beam 360 and reinforcement via geometry. While each suspension beam 360 is clamped at one end to one of the four anchors at the corners of each pixel 302, the opposing guided end connected to the core plate 314 may be adjusted in terms of length and insertion angle as shown in FIG. 6.

[0066] This is achieved via ingressions of the suspension beams 360 into the reinforced double-stack region of the mirror body. In some embodiments, via openings can be removed from these ingression regions in order to isolate the ends of the suspension beams from the overlying mirror body.

[0067] Example 602 in FIG. 6 illustrates a “normal” suspension beam 360 having a length 610. The example 602 illustrates the reflective surface body 310 having the vias 318 supported by the cover plate 314 (hidden from view by the reflective surface body 310). The suspension beams 360 have no extension or a 0% extension.

[0068] As shown in example 604, an extension 370 may be ingressions of an end of the suspension beam 360 of varying lengths, which correspond to different extension ratios of the total suspension beam with respect to the base non-ingressing length, alter the spring constant, i.e., rigidity, of the suspension beams, allowing for adjustability to the pixel actuation curve and voltage range under the same array pitch and format. The extensions 370 are illustrated in FIG. 6 with dashed lines to indicate they would be hidden from view when looking at a top view down towards the reflective surface body 310. Forexample, the example 604 illustrates an example with a 25% extension. In other words, a length 612 of an extension 370 may be 25% of the length 610.

[0069] In addition, an angle 616 of the extensions 370 can be altered to minimize compressive internal stress, which may lead to beam buckling, or tensile stress in the suspension beam structure 308 or suspension beam layer. It has been found that a suspension insertion angle of 45° into the mirror body (as opposed to 90°) maximizes torque on the mirror body by orienting force vectors applied by each suspension beam away from the center of the mirror body. This allows the mirror body to rotate very slightly in order to alleviate any internal compressive or tensile stress in the suspension beam structure 308 or suspension beam layer. The alleviation of this stress also allows for improved uniformity in actuation behavior across pixels as local or global variations in stress can significantly impact the spring behavior of the suspension beams.

[0070] An example 606 illustrates the extension 370 having a 38% extension at the angle 616. In some embodiments, the extension 370 may be long enough to ingress below one of the vias 318 of the reflective surface body 310. In such instances, the via 318 may be removed where the extension 370 would be located, as illustrated in the example 606. Notably, vias 318 are missing where the extension 370 ingress into the reflective surface body 310.

[0071] FIG. 7 illustrates a graph 700 of actuation curves (e.g., optical travel path versus voltage relationships) for different amounts of beam extension. For example, higher extensions (or longer extension lengths) result in lower voltage ranges. The shaded spans correspond to range of possible deviations due to process variations.

[0072] Moreover, perforations may be selectively added or removed across the double stack region of the mirror body in order to adjust the squeeze film damping behavior governing the dynamic response and settling of the pixels 302. Near critical damping may therefore be achieved to minimize settling time by adjusting these perforations to the specific composition and pressure of the viscous fluid (ex: air, vacuum, inert gas, etc.) enveloping the SLM and permeating through the gap between the electrode layer 350 and suspension beam structure 308 or the suspension beam layer.

[0073] Since the described structural stack-up can be micromachined over a variety of substrates 304, the architecture of the high-speed SLM 100 of the present disclosure can accommodate different ways of achieving electronic driver integration, whereby active voltage drive channels are connected to each electrode 306 and global mirror biasing is provided in order to ensure independent and concurrent addressing of pixels 302 across the array in the high-speed SLM 100.

[0074] Namely, in one embodiment, the high-speed SLM 100 may be micromachined over a wafer substrate pre-processed to include arrays of front- to-back through-silicon vias that are aligned to the electrode plates of each pixel. Via openings across a base insulation layer may, therefore, be used to connect each electrode 306 to a separate through-silicon via, offering wafer backside access to each pixel. Under this embodiment, an application-specific integrated circuit (ASIC) driver consisting of an array of bumped metal pads, each providing independent voltage drive for a given mirror pixel 302, can be bonded to the wafer backside.

[0075] Alternatively, in another embodiment, a wafer with a top-side buried routing layer may be employed as shown in FIG. 8 in order to provide escape routing for each pixel 302 to the periphery of the SLM region. FIG. 8 illustrates an example a wafer 800 with an SLM region 804 and a flip-chip bonding region 806. Close-up views of the SLM region 804 and the flip-chip bonding region 806 are provided.

[0076] Escape traces 810 (which may consist of doped polycrystalline or monocrystalline silicon buried under an insulating layer of silicon nitride) are subsequently routed to a separate array of metal-coated landing pads 8O81 to 808o(hereinafter also referred to individually as a landing pad 808 or collectively as landing pads 808) that serves as a connection substrate for the front-side flip-chip bonding of one or several ASIC driver chips. Additional landing pads 808 can also be used to provide power supply and communication signals to the ASIC driver though traces connecting to wirebonds from the base substrate of the SLM plane. Biasing of the mirror body and underlying wafer bulk may also be provided through traces in the buried routing layer.

[0077] The same kind of escape routing may also be incorporated to the backside of the wafer 800 (with metal or doped silicon traces) with the through- silicon via embodiment in the event that the SLM array pitch and format are not matched to that of the ASIC’s bumped pad array format. Under both embodiments, separating the SLM region 804 from the flip-chip bonding region 806 across the substrate wafer plane allows for a partial sacrificial release process where only the SLM region 804 of the chip is immersed into the etchant responsible for the removal of the sacrificial material in the structural stack-up after bonding of the ASIC to the flip-chip bonding region 806. An extended buried trace routing area 812 separating the SLM from the ASIC serves as a buffer region that would ensure full immersion of the SLM region 804 and a fully dry ASIC region 804. This approach to driver integration and mirror release addresses situations in which the driver ASIC chip is not sufficiently coated or protected to withstand the impact of the etchant, which may be hydrofluoric acid (HF) in silicon dioxide-based sacrificial layers.

[0078] In embodiments of the architecture of the high-speed SLM 100 where escape routing (on either the frontside or backside of the SLM wafer 800) from the micromirror array is employed for driver integration, peripheral routing constraints may hamper efforts to scale up pixel count. Specifically, at sufficiently large pixel counts, the peripheral length of the micromirror array may not be large enough to accommodate all traces in the buried routing layer connecting to every pixel internal to the array. To alleviate this constraint, the SLM architecture of the present disclosure also includes an assortment of dummy pixel structures that may be placed along the internal or peripheral region of the micromirror array.

[0079] FIG. 9 illustrates an example of dummy pixel structures 910, 912, and 914. For example, the dummy pixel structure 910 may be a diffractive spacer dummy. The dummy pixel structure 912 may be a flat spacer dummy. The dummy pixel structure 914 may be a flat peripheral dummy. These dummy structures 910, 912, and 914 are square or rectangular pixels with a similar stackup to that of the active micromirror pixels 302, described above, but with afully filled suspension layer 922 devoid of suspension beams carve-outs in order to prevent actuation with maximal rigidity.

[0080] As illustrated in FIG. 9, internal dummy structures 910, 912, and 914 may be used to create spacer regions that increase the available peripheral track length for escape traces 810 through the buried routing layer under a given array format and pixel count. For example, FIG. 9 illustrates an example escape routing layout 904 for a 64x64 array 902. A close-up view of a dummy- filled spacer region 906 is also shown in FIG. 9. The close-up view of the dummy-filled spacer region 906 illustrates pixels 302i to 302p. In addition, the electrode layer 350, reflective surface body 310, and buried escape traces 810 can be seen in close-up view of the dummy-filled spacer region 906.

[0081] As noted above, the dummy structures 910, 912, and 914 can be used to increase the available peripheral track length for the escape traces 810. Different dummy structures 910, 912, and 914 can be used in different portions of the dummy-filled spacer region 906. For example, the dummy structure 910 may be used in a region labeled 1 in the dummy-filled spacer region 906, the dummy structure 912 may be used in a region labeled 2 in the dummy-filled spacer region 906, and the dummy structure 914 may be used in regions labeled 3 in the dummy-filled spacer region 906.

[0082] Since the overlying dummy structures 910, 912, and 914 remain rigidly suspended in the suspension and mirror body layers, the electrode layer 350 underneath these dummy structures 910, 912, and 914 may also be used as an additional escape layer for routing as shown in the close-up view of the dummy-filled spacer region 906, further increasing available regions for escape traces 810. For the escape traces 810 in the electrode layer 350, the rigid overlying dummy structure 910 serves as both physical protective covers and electromagnetic shielding. The dummy structures 910, 912, and 914 may be extended into the peripheral region of the array to form a surrounding wall, as shown in the 64x64 array 902 and in the close-up view of the dummy-filled spacer region 906, in order to offer continued cover to escape traces 810 in the electrode layer 350 and improve uniformity in pixel behavior by ensuringuniform etch and residual stress characteristics across all regions of the active internal pixel array.

[0083] In order to minimize the impact of their presence on the optical performance of SLM arrays, all internal and peripheral dummy pixel structures 910, 912, and 914 remain unmetallized such that optical reflection is minimized. For example, the dummy structure 910 may have a non-reflective surface 916 and body, the dummy structure 912 may have a non-reflective surface 918 and body, and the dummy structure 914 may have a non-reflective surface 920 and body. This considerably reduces the brightness of any static illumination profile that may result from reflection off non-active regions of the SLM and which may end up overlaid onto the intended dynamic illumination profile formed from the active phase masks produced by the SLM.

[0084] Furthermore, the static illumination profile may be deflected away from the target illumination region through the use of a diffractive surface profile on the dummy pixel structure 910. The diffractive profile may consist of a binary step surface formed by alternating between single-stack (suspension layer-only) regions and double-stack (suspension+mirror) body regions.

[0085] Across multiple dummy pixel structures 910, this binary step height surface forms a binary periodic phase mask with an adjustable period length that is smaller than the micromirror pixel pitch, producing diffraction spots that are outside of the SLM’s FoV or target illumination region. The step height and period of the binary phase surface may be selected and set during design and fabrication via specific combinations of etching and stacking across the three structural and two sacrificial layers in order to maximize diffraction efficiency at the desired operating wavelength. Alternatively, the step height of the binary phase surface may also be dynamically adjusted using low-rigidity dummy pixels with suspension beams (not shown) that can be actuated in unison under a single voltage drive channel, allowing for configurability to the diffractive surface with negligible impact to routing overhead.

[0086] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen orunanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

IN THE CLAIMSWhat is claimed is:

1. An apparatus, comprising: a plurality of independently addressable piston-motion micromirrors, wherein each one of the independent addressable piston-motion micromirrors, comprises: an independently addressable electrode; a suspension beam structure on the independently addressable electrode; a reflective surface body coupled to the suspension beam structure; and a reflective surface coupled to the reflective surface body, wherein applying a voltage to the independently addressable electrode creates an electrostatic attraction of the reflective surface to provide a pixel-level phase shift.

2. The apparatus of claim 1 , wherein the independently addressable electrode is separated from other independently addressable electrodes by a network of traces that surround the independently addressable electrode.

3. The apparatus of claim 2, wherein the suspension beam structure, comprises: four suspension beams, wherein each one of the four suspension beams is anchored at a corner of the network of traces that surround the independently addressable electrode.

4. The apparatus of claim 3, wherein each one of the four suspension beams extends at an angle towards the reflective surface body.

5. The apparatus of claim 4, wherein the angle comprises approximately 45 degrees and an amount of extension is between 25-40 percent of additionallength compared to a length of each one of the four suspension beams that does not extend at the angle towards the reflective surface body.

6. The apparatus of claim 1 , further comprising: a core plate located between the suspension beam structure and the reflective surface body to prevent the reflective surface body and the reflective surface from curling.

7. The apparatus of claim 1 , wherein a fill factor of the reflective surface is greater than 70 percent.

8. The apparatus of claim 1 , wherein each one of the independently addressable piston-motion micromirrors, further comprises: a first sacrificial layer located between the independently addressable electrode and the suspension beam structure to set a gap distance between the independently addressable electrode and the reflective surface.

9. The apparatus of claim 1 , wherein each one of the independently addressable piston-motion micromirrors, further comprises: a second sacrificial layer located between the suspension beam structure and the reflective surface.

10. The apparatus of claim 1 , further comprising: at least one dummy pixel structure to increase a peripheral length of the apparatus for escape routing from the plurality of independently addressable piston-motion micromirrors.11 . The apparatus of claim 10, wherein the at least one dummy pixel structure comprises: a fully filled suspension beam structure to prevent actuation; and a non-reflective surface and body coupled to the fully filled suspension beam structure.

12. A method for forming a plurality of independently addressable pistonmotion micromirrors, comprising: forming an electrode layer on a substrate; depositing a network of traces to form a plurality of independently addressable electrodes in the electrode layer; inserting a suspension beam in each corner of the plurality of independently addressable electrodes to form a plurality of suspension beam structures and a core plate; coupling a reflective surface body to one or more of the plurality of suspension beam structures; and applying a reflective surface to the reflective surface body.

13. The method of claim 12, further comprising: filling one or more of the plurality of suspension beam structures to prevent actuation; and coupling a non-reflective body to the one or more of the plurality of suspension beam structures that are filled.

14. The method of claim 12, further comprising: extending the suspension beam at an angle before the reflective body is coupled to the one or more of the plurality of suspension beam structures.

15. The method of claim 12, further comprising: forming a first sacrificial layer between the electrode layer and the suspension beam structures; and forming a second sacrificial layer between the plurality of suspension beam structures and the reflective surface body.

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