MEMS-based phase space light modulation architecture

Abstract

The invention discloses a phase space light modulation architecture based on MEMS. Described examples include an apparatus having a drive electrode 506 on a substrate. The apparatus has a stage 504 suspended over the drive electrode and conductively coupled to a stage electrode 512, wherein the stage is configured to move in a direction perpendicular to the substrate surface in response to a voltage difference applied between the drive electrode and the stage electrode. The device is also provided with a lens column 508 on the platform. The apparatus has a mirror 510 coupled to the platform by a mirror column 508, wherein the mirror is rectangular.

Description

[0001] This application is a divisional application of Chinese patent application 202080069718.X, filed on October 15, 2020, entitled “MEMS-based Phase Spatial Light Modulation Architecture”. Technical Field

[0002] The present disclosure relates generally to light modulation, and more particularly to examples of phase spatial light modulation. Background Art

[0003] Holographic projection uses interference between portions of a coherent light source reflected from a phase spatial light modulator to project an image to a specified distance or "far field" or Fraunhofer region. Phase spatial light modulation is a form of light modulation in which the reflection phase of each pixel determines the phase of the reflected light relative to other light reflected from other pixels of the phase spatial light modulator (PSLM). Thus, if the PSLM shifts one pixel by a quarter wavelength, the light reflected from the first pixel is a quarter wavelength out of phase with the light reflected from the second, unshifted pixel. The light from the first and second pixels then interferes in a controllable way that directs the combined light.

[0004] A PSLM may have thousands or millions of controllable pixels. A PSLM can project an image based on the interference of reflected coherent light directed by pixels of different phases. In contrast, a conventional amplitude spatial light modulator (SLM) reflects or does not reflect selected pixels to create a projected image. The image projected by this PSLM has several uses. It can scan a scene for use in a light detection and ranging (LIDAR) system that changes the image by direction to help detect objects in the scene. In other uses, the user views the image directly, such as using a head-up display projected onto the windshield of a car.

[0005] On the LIDAR side, a PSLM can scan a holographic projection across the field of view. An amplitude-only SLM can do this using an imaging configuration by turning on a single micromirror at a time, but with very low efficiency (1 / (number of pixels)). Another way to scan the beam is to use an amplitude-only SLM to create holograms, each corresponding to a different beam position. However, this can only be about 12% efficient due to attenuation and the generation of unused diffraction orders. However, using a PSLM to create a virtual phase ramp, the efficiency can be much higher - in some cases close to 100%. Instead of attenuating unwanted light like an SLM, a PSLM can manipulate the light wavefront to direct the light where it is needed. By manipulating the light wavefront, a PSLM device can create an image at a distant plane (without a projection lens) or at some intermediate plane. In the case of an image at a distant plane, a PSLM device exploits the Fourier transform relationship at the Fraunhofer far-field plane. PSLM devices can also generate digital holograms by introducing phase curvature to focus the light at various intermediate distances. Summary of the invention

[0006] According to one example, a device includes a drive electrode on a substrate. The device has a platform suspended above the drive electrode and conductively coupled to the platform electrode, wherein the platform is configured to move in a direction perpendicular to the surface of the substrate in response to a voltage difference applied between the drive electrode and the platform electrode. The device also has a mirror post on the platform. The device has a reflector coupled to the platform through the mirror post, wherein the reflector is rectangular. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a schematic diagram showing the process of projecting a holographic image.

[0008] Figure 2 is a schematic diagram showing the output of holographic projection using a phase spatial light modulator.

[0009] Figure 3 is a schematic diagram of an example holographic image.

[0010] Figure 4 is a schematic diagram of three example pixels of an example PSLM.

[0011] Figure 5 is a side view of an example cell.

[0012] Figure 6 is a perspective image on the example cells.

[0013] Figure 7 is a schematic diagram of an example projection system.

[0014] Figure 8 is a process flow chart for manufacturing an example pixel in an example PSLM.

[0015] Figure 9A-9H (collectively referred to as "Figure 9") is a diagram Figure 8 Schematic diagram of the steps of the process flow. DETAILED DESCRIPTION

[0016] In the drawings, corresponding numerals and symbols generally refer to corresponding parts unless otherwise indicated. The drawings are not necessarily drawn to scale.

[0017] In this specification, the term "coupled" may include connections made with intermediate elements, and additional elements and various connections may exist between any elements that are "coupled."

[0018] In this specification, the term "phase spatial light modulator" (PSLM) is used. A phase spatial light modulator (PSLM) describes a device consisting of an array of units, each unit imparting a different optical phase delay according to an electrical signal applied to each unit. PSLM devices can be composed of liquid crystal devices (LCDs), liquid crystal on silicon devices (LCOS), or microelectromechanical systems (MEMS) devices. MEMS PSLMs can be composed of an array of small mirrors that are actuated to displace in a direction perpendicular to the plane of the array. The function of a PSLM is to change the shape of the light wavefront / wavefront incident on the device. A PSLM can impose a linear phase delay on a wavefront that has the effect of manipulating a light beam in different directions. A PSLM can also impart a curved wavefront that can focus the wavefront like a lens. The main advantage of a PSLM is that it can be quickly reconfigured to steer or focus a light beam to a desired direction or to a desired plane.

[0019] The optical function of the PSLM in a LIDAR system is different from that of a digital micromirror device (DMD). When a DMD is used in a receiver in a LIDAR system, light from the external scene is imaged onto the DMD array, and various pixels are turned on or off depending on whether the pixel is imaging laser light or background light. In the case of the PSLM, the light falls on the device without the help of imaging optics. Specific areas or points of interest within the scene can be selected by applying a spatial wavefront pattern on the PSLM so that the area of ​​interest is directed toward the detector. As a result, light that is not in the area of ​​interest is directed to an area away from the detector. In this way, the PSLM can perform a similar function to an imaging DMD, namely directing laser light to the detector while directing background light away from the detector.

[0020] The PSLM can be the same or similar device used to steer the LIDAR laser beam in a LIDAR system. A linear phase function can be displayed on the PSLM so that it directs the laser in a desired direction. Changing the phase front of each beam direction causes the beam to scan in a specific pattern required to obtain a range or reflectivity image of a scene. In a similar manner, different linear phase functions can be displayed on the PSLM to direct the received light in different directions towards the detector. Additionally, by displaying a curved phase function on the PSLM, the beam can be focused on the detector.

[0021] In an example arrangement, the shape factor mismatch problem of a holographic projection image using a phase spatial light modulator is solved by providing a phase spatial light modulator having a picture element whose shape factor matches the desired shape factor. In one example, a device includes a drive electrode on a substrate. The device has a platform suspended above the drive electrode and conductively coupled to the platform electrode, wherein the platform is configured to move in a direction perpendicular to the surface of the substrate in response to a voltage difference applied between the drive electrode and the platform electrode. The device also has a mirror column on the platform. In one example, the device has a reflector coupled to the platform by the mirror column, wherein the reflector is rectangular.

[0022] Figure 1 is a schematic diagram showing a process 100 for projecting a holographic image 108. The algorithm for generating a PSLM image exploits the Fourier transform relationship between the PSLM phase image and the far-field image. In a simple example, an input image 102 is processed using an inverse fast Fourier transform (IFFT) to produce a frequency domain image 104. The frequency domain image 104 is processed using one of several processes for mapping the frequency domain image 104 to produce a phase pattern 106 for input to a spatial light modulator. The mapping is done using one of several algorithms. Gerchberg and Saxton, "A practical algorithm for the determination of the phase from image and diffraction plane pictures," Optik 35, 237-246 (1972), describes an example algorithm, which is hereby incorporated by reference in its entirety. The mapping produces a phase pattern 106, which includes a phase assignment for each pixel of the phase spatial light modulator. A coherent light source, such as a laser (not shown), is reflected from the phase spatial light modulator having the pattern 106 to produce an image 108 in the far field or at a specified depth.

[0023] Figure 2is a schematic diagram showing the output of a holographic projection using a phase spatial light modulator 208. In this example, the projection creates an image in the far field. Coherent light is reflected from a PSLM 208 having PxP pixels. The light is reflected to the far field 206 where an image 202 is produced. Because the image is an interference pattern rather than a collection of pixels, the size of the image 202 on each side is sin -1 (λ / P), where λ is the wavelength of the light used to create the interference pattern and P is the x and y dimensions of the pixel of PSLM 208. In addition, interference may produce artifact or shadow images 204A, 204B, 204C, and 204D. The shape factor of the image produced by interference is proportional to the shape factor of the pixel of PSLM 208, rather than the shape factor of the arrangement of the pixels on PSLM 208.

[0024] Figure 3 is a schematic diagram of an example holographic image 302. PSLM 308 includes a P y The y dimension and P x In this example, the coherent light reflected from the PSLM 308 produces an image 302 in the far field 306. The resulting image 302 has a sin -1 (λ / P x ) and sin -1 (λ / P y ). Thus, the x dimension of the pixels of PSLM 208 determines the x dimension of the image, and the y dimension of the pixels of PSLM 208 determines the y dimension of image 302. Additionally, interference may produce artifact or shadow images 304A, 304B, 304C, and 304D.

[0025] Figure 4 is a schematic diagram of three example pixels 402, 422 and 442 of an example PSLM. Figure 4 Pixel mirrors 410, 430, 450 are shown in outline to show the structure underneath the mirrors. Pixel 402 includes two platform posts 414 that support platform 404 and conductively connect platform 404 to two platform electrodes 412. Platform 404 supports mirror 410 via posts 408. Drive electrodes 406 move platform 404 vertically (into the page) as described below. Mirror 410 (and therefore pixel 402) has a rectangular form factor of 2:1. Therefore, if the width of mirror 410 is W, the height of mirror 410 (the distance between platform posts 414) is 2W. In one example, W is 0.5μ.

[0026] Pixel 422 includes two platform posts 434 that support platform 424 and conductively connect platform 424 to two platform electrodes 432. Platform 424 supports mirror 430 via posts 428. Drive electrodes 426 move platform 424 vertically (into the page) as described below. Mirror 430 (and therefore pixel 422) has a rectangular form factor of 3:1. Thus, if the width of mirror 430 is W, the height of mirror 430 (the distance between platform posts 434) is 3W.

[0027] Pixel 442 includes two platform posts 454 that support platform 444 and conductively connect platform 444 to two platform electrodes 452. Platform 444 supports reflector 450 via posts 448. Figure 4 As indicated, a larger platform 444 allows three posts 448 for greater stability. Whether rectangular (including square) or other form factors, multiple posts 448 provide additional stability for the reflector 450 and help maintain vertical motion of the reflector 450. The drive electrode 446 moves the platform 444 vertically (into the page) as described below. The reflector 450 (and therefore the pixel 442) has a rectangular form factor of 4:1. Therefore, if the width of the reflector 450 is W, the height of the reflector 450 (the distance between the platform posts 454) is 4W.

[0028] Figure 5 5 is a side view of an example pixel 502. The platform 504 is connected to two platform electrodes 512 via platform posts 514. Posts 508 support a reflector 510 above the platform 504. Figure 5As shown, when a voltage is applied to the drive electrode 506 and a reference voltage (e.g., ground) is applied to the platform electrode 512, an electrostatic force pulls the platform 504 downward, thereby pulling the mirror 510 downward. The amount of movement is determined by the applied voltage. In other examples, the pixel 502 uses two or more drive electrodes 506, which can be individually addressed by a drive circuit (not shown). The electrostatic force applied is proportional to the area of ​​the drive electrode 506 and the platform 504. Therefore, using multiple electrodes, the amount of force can be precisely controlled by selecting the drive electrode 506 or a combination of drive electrodes 506, thereby precisely controlling the vertical position of the mirror 510, while applying the same voltage to each selected one of the drive electrodes 506. The phase shift provided by the pixel 502 is determined by the vertical positioning of the mirror 510. For example, if the pixel is lowered by a quarter wavelength (1 / 4λ), the light reflected from the pixel will travel an additional half wavelength relative to the pixel that is not lowered (1 / 4λ down to the mirror, 1 / 4λ back). In another example, if a pixel is lowered by one-eighth wavelength (1 / 8λ), light reflected from that pixel will travel an additional quarter wavelength relative to a pixel that is not lowered (1 / 8λ down to the mirror, 1 / 8λ back). Using coherent light, these phase differences can be exploited to create holographic projections.

[0029] Figure 6 602. is a perspective view on an example pixel 602. In this example, the length of the pixel 602 is eight times the width, which provides an 8:1 form factor. Two flexible hinges 616 and two platform posts 614 support the platform 604. The platform post 614 is conductively coupled to two platform electrodes 612. A mirror post 608 supports a reflector 610 above the platform 604. The drive electrode 606 is below the platform 604. Circuitry (not shown) in the substrate 618 applies signals to the platform electrodes 612 and the drive electrode 606 to position the reflector 610 so as not to increase or increase the path length of the light reflected by the reflector 610 by a selected distance that increases the path length by a selected portion of the wavelength of the light. As described above, the drive electrode 606 can be a single electrode or multiple electrodes and can be driven by multiple voltages, by a single voltage applied to one or more drive electrodes 606, or a combination thereof. In this example, the hinge 616 is in the same layer as the platform 604 (as further explained below) and is attached to the opposite side of the platform 604. Additionally, the hinge 616 is attached to the platform 604 at a point that provides a distance between the attachment point and the platform posts 614. The attachment of the hinge 616 on the side of the platform parallel to the line between the platform posts 614 reduces the torsional forces on the platform 604 when balanced by the hinge 616 on the opposite side. The distance from the posts 614 allows for greater hinge flexibility and less stress on the hinge 616 for a given deflection of the platform 604.

[0030] Figure 7 is a schematic diagram of an example projection system 700. Lens 704 collimates light from light source 702 onto PSLM 706. Light source 702 is a coherent light source, such as a laser or laser diode. To provide a full color gamut, the light source may include several laser sources with different colors. The voltage applied to the pixel of the PSLM is adjusted for different wavelengths of different colors, and the time of projection of the colors is staggered to provide the desired color. Fresnel lens 708 is optional. PSLM 706 reflects light through Fresnel lens 708 and outputs light thus after phase shifting.

[0031] Figure 8 is a schematic diagram of a process flow 800 for manufacturing an example pixel in an example PSLM. Figure 9A-9H (collectively referred to as "FIG. 9") illustrates the steps of process flow 800 in a series of cross-sectional views. Step 802 forms a platform electrode and at least one drive electrode by depositing and patterning a first conductive layer on a substrate. Fig.9A As shown, in this step, the process deposits and patterns a conductive layer, such as aluminum, titanium-aluminum, titanium or titanium nitride. For example, chemical vapor deposition (CVD) or physical vapor deposition (PVD) forms a conductive layer with a thickness of to The conductive layer is patterned by photolithography and etching to form drive electrodes 906 and platform electrodes 912 on substrate 918. Substrate 918 is an insulating layer on a semiconductor integrated circuit (not shown) that is coupled to drive electrodes 906 and platform electrodes 912 and provides control signals thereto. In one example, substrate 918 is silicon dioxide. Step 804 is to form a first sacrificial layer on the substrate and on the platform electrodes and (one or more) drive electrodes. Thus, a first sacrificial layer 954 is formed on substrate 918, (one or more) drive electrodes 906, and platform electrodes 912, as shown in FIG. Fig. 9B Examples of materials for the first sacrificial layer 954 include silicon dioxide, silicon nitride, photoresist, polyimide, germanium, germanium oxide, polysilicon, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), and spin-on glass (SOG). The specific method used to form the first sacrificial layer 954 depends on the material used. In one example, the first sacrificial layer 954 is spin-coated onto Thickness of SOG.

[0032] Step 806 is to form (one or more) post openings in the first sacrificial layer 954 to expose the platform electrode, and to form the posts, hinges, and platforms by depositing and patterning a second conductive layer on the first sacrificial layer and in the (one or more) post openings. Fig. 9C As shown, this step uses CVD or PVD to deposit a conductive layer (e.g., aluminum, titanium-aluminum, titanium, or titanium nitride) onto to The conductive layer is patterned and etched using photolithography and etching. This step forms two platform posts 914 on the sacrificial layer 954 that contact the platform electrode 912, the hinge 916, and the platform 904. Step 808 is to form a second sacrificial layer on the posts, hinges, platforms, and the first sacrificial layer. Fig.9D As shown, the second sacrificial layer 956 is formed on the pillar 914, the hinge 916, the platform 904 and the exposed portion of the first sacrificial layer 954, for example, with a thickness of Examples of the material of the second sacrificial layer 956 include silicon dioxide, silicon nitride, photoresist, polyimide, germanium, germanium oxide, polysilicon, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), and spin-on glass (SOG). Fig.9E Is from Fig.9D A view of the platform 904 and hinge 916 at the top of the page.

[0033] Step 810 is to form a mirror post opening in the second sacrificial layer. Fig.9F As shown, photolithography and etching form mirror post openings 907 in the second sacrificial layer 956. Step 812 is to deposit a third conductive layer onto the arrive The thickness of the reflector and the mirror column are formed, and the third conductive layer is patterned and etched on the second sacrificial layer and in the (one or more) mirror column openings. Figure 9G As shown, the reflector 910 is on the second sacrificial layer 956 and the reflector post 908 is on the platform 904 in the opening 907. Step 814 is to remove the first sacrificial layer and the second sacrificial layer by isotropic etching, leaving Figure 9H The structure shown.

[0034] Modifications are possible in the described examples, and other examples are possible within the scope of the claims.

Claims

1. A device, wherein include: a drive electrode on the substrate; a platform suspended above the drive electrode and conductively coupled to the platform electrode, wherein the platform is configured to move in a direction perpendicular to a surface of the substrate in response to a voltage difference applied between the drive electrode and the platform electrode; a mirror column on said platform; and A reflector is coupled to the platform through the mirror column, wherein the reflector is rectangular.

2. The device according to claim 1, further comprising: include: two platform columns; as well as at least two flexible hinges; Wherein the platform electrode is coupled to at least one of the two platform posts, and the platform is suspended from the two platform posts by at least two flexible hinges located on opposite sides of the platform.

3. The device of claim 2, wherein the opposing sides are parallel to a line between the two platform posts.

4. The device of claim 2, wherein the at least two flexible hinges and the platform are adjacent.

5. The apparatus of claim 1, wherein the reflector is longer in a first dimension than in a second dimension, wherein the first dimension and the second dimension are perpendicular. The device according to claim 1 , wherein the driving electrode is two or more electrodes.

7. The device of claim 6, wherein the two or more electrodes are individually addressable.

8. The apparatus of claim 1, wherein the mirror column is a first mirror column, and further include: A second mirror column is on the platform and supports the reflector.

9. A projection system, wherein include: light source; A phase spatial light modulator comprising: a drive electrode on the substrate; a platform suspended above the drive electrode, the drive electrode conductively coupled to the platform electrode, wherein the platform is configured to move perpendicular to the surface of the substrate by a voltage difference between the drive electrode and the platform electrode; a mirror column on said platform; and a reflector coupled to the platform via the mirror post, wherein the reflector is rectangular; and Wherein light from the light source is adapted to be reflected from the phase spatial light modulator.

10. The projection system according to claim 9, further comprising: include: two platform columns; as well as at least two flexible hinges; Wherein the platform electrode is coupled to at least one of the two platform posts, and the platform is suspended from the two platform posts by the at least two flexible hinges located at opposite sides of the platform.

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