Abstract:
Devices and methods for forming a spatial light modulator with high contrast are described. Light absorbing materials are used within a chamber that houses a spatial light modulator. The light absorbing materials absorb reflected light that is not intended for forming a part of a display image. The light absorbing material can form an aperture layer, wherein light to form the display image is transmitted through an opening in the aperture layer. An array of spatial light modulators can be within the housing and dummy spatial light modulators may be formed to enable easy alignment of the array with the opening in the aperture layer.

Description:
BACKGROUND 
   The present disclosure relates to the packaging of spatial light modulators. 
   In manufacturing spatial light modulators, multiple spatial light modulators are commonly fabricated on a semiconductor wafer. The spatial light modulators are then sealed in micro chambers, and subsequently separated into individual dies. The micro chambers typically include transparent windows through which the spatial light modulators to receive and output optical signals. To ensure the optical performance of the spatial light modulators, it is important to prevent unwanted scattered light in the micro chambers from exiting the transparent window. 
   SUMMARY 
   In one general aspect, an encapsulated spatial light modulator (SLM) is described. The SLM includes a spatial light modulator on a substrate within a chamber, an encapsulation cover in part defining the chamber, a spacer wall between the substrate and the encapsulation cover, wherein the spacer wall has an inner surface adjacent to the spatial light modulator; and a first light absorbing material on the inner surface of the spacer wall, the light absorbing material configured to absorb light in the chamber. 
   In another general aspect, methods for operating an array of tiltable mirrors encapsulated in a chamber on a substrate are described. A method can include the following steps. At least one of the tiltable mirrors is tilted to an on position. A first packet of incident light is reflected off of the tiltable mirror to produce a first reflected light in the on position. The first reflected light is transmitted out of the chamber, wherein the chamber comprises an encapsulation cover and a spacer wall between the substrate and the encapsulation cover. The tiltable mirror is tilted to an off position. A second packet of incident light is reflected off of the tiltable mirror to produce a second reflected light in the off position. The second reflected light is absorbed by a first light absorbing material on a surface of the spacer wall in the chamber. 
   In another yet general aspect, methods of fabricating an encapsulation device for a plurality of spatial light modulators are described. The methods can include the following steps. A plurality of openings are formed in an encapsulation cover. An aperture layer is formed on the encapsulation cover, the aperture layer comprising a plurality of openings. Spacer walls are formed on the encapsulation cover. A layer of a first light absorbing material is formed on the spacer walls and the aperture layer, thereby producing an encapsulation device, wherein the first light absorbing material is configured to absorb light in the chambers. 
   Implementations of the system may include one or more of the following features. The encapsulation cover can be transparent to visible, UV, or IR light. The first light absorbing material can include a zirconium compound, such as zirconium oxide or zirconium nitride. The device can include an aperture layer on a surface of the encapsulation cover, wherein the aperture layer has an opening over the spatial light modulator. The aperture layer can include a metal oxide or carbide, such as a chromium compound. The aperture layer can be inside the chamber. The SLM can include a second light absorbing material on a surface of the aperture layer, wherein the second light absorbing material is configured to absorb light in the chamber. The second light absorbing material can include a chromium compound or a zirconium compound. The SLM can include a third light absorbing material on a surface of the substrate, wherein the third light absorbing material is configured to absorb unwanted light in the chamber. The third light absorbing material can include a zirconium compound. The third light absorbing material can be on a portion of the surface of the substrate not covered by the spatial light modulator. The spacer wall can include a metallic material. The spacer wall can be sealed to the encapsulation cover or the substrate with an adhesive. The spacer wall can be bonded to the encapsulation cover or the substrate. The spacer wall can define a cavity height between the substrate and the encapsulation cover, and the cavity height can be between about 0.2 and 2.0 microns, such as between 0.5 and 1 micron. The spatial light modulator can include a tiltable mirror configured to tilt to an on position and an off position. The tiltable mirror can be configured to reflect light out of the chamber when the tiltable mirror is at the on position. The encapsulation cover can be substantially parallel to a surface of the substrate and the mirror and reflect light in an on direction when the mirror is at the on position and the on direction is substantially perpendicular to the encapsulation cover. The tiltable mirror can be configured to reflect light toward the first light absorbing material when the tiltable mirror is at the off position, wherein reflected light is absorbed by the first light absorbing material. The SLM can also include electric contacts on the substrate, where the one or more electric contacts are configured to send electric signals to or receive electric signals from the spatial light modulator. The electric contacts can be positioned outside of the chamber. The SLM can include an aperture layer on a surface of the encapsulation cover, and the SLM can include an array of tiltable mirrors where the array is characterized by a first lateral dimension and a second lateral dimension substantially orthogonal to the first dimension, and the aperture layer comprises an opening above the array of tiltable mirrors. The first lateral dimension of the array of tiltable mirrors can be wider than a corresponding dimension of the opening in the aperture layer. 
   The spacer wall can be formed by forming a conductive layer on the encapsulation cover, forming a mask layer on the conductive layer, wherein the mask layer comprises a plurality of openings and electroplating the spacer walls on the conductive layer and in the openings of the mask layer. The step of forming a layer of a first light absorbing material can include the following steps: coating a photo resist layer on the spacer walls, the aperture layer, and on a surface of the encapsulation cover that corresponds to the openings in the aperture layer; irradiating a portion of the photo resist layer that is in the openings of the aperture layer; removing the photo resist layer on the spacer walls and the aperture layer; subsequently depositing the first light absorbing material on the spacer walls and the aperture layer, and on the photo resist layer; and removing the photo resist layer on the surface of the encapsulation cover and the first light absorbing material thereon. The spacer walls of the encapsulation device can subsequently be connected to a surface of a substrate having a plurality of spatial light modulators to form a plurality of chambers on the substrate with each chamber including at least one spatial light modulator. The spacer walls can be sealed to the surface of the substrate by an adhesive or bonded to the surface by plasma bonding. A portion of the substrate and a portion of the encapsulation cover can be cut to form two or more dies each containing at least one chamber encapsulating one of the spatial light modulators. 
   Various implementations of the methods and devices described herein may include one or more of the following advantages. The disclosed spatial light modulators can have improved optical performance. Unwanted light may be absorbed in a micro chamber that encapsulates the spatial light modulator. The optical noise in the output optical signal can therefore be reduced. The contrast between an “on” state and an “off” state of the spatial light modulator may also be increased. The specification also discloses manufacturing processes for encapsulation devices that include light absorbing components that can absorb the unwanted light in the chambers. Furthermore, a plurality of spatial light modulators on a substrate can be encapsulated in a common process. The manufacturing efficiency is thus improved. 
   Although the invention has been particularly shown and described with reference to multiple embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles, devices and methods described herein. 
       FIG. 1A  is a schematic cross-sectional view of a spatial light modulator encapsulated in a chamber. 
       FIG. 1B  is a schematic top view of the spatial light modulator encapsulated in the chamber shown in  FIG. 1A . 
       FIG. 2A  is a schematic of an enlarged top view of the spatial light modulator including an array of pixel cells each including a micro mirror. 
       FIG. 2B  is a cross-sectional view of an exemplary micro mirror in the spatial light modulator of  FIG. 2A . 
       FIGS. 3A and 3B  illustrate directions of incident light and reflected light when a micro mirror plate in a pixel cell of a spatial light modulator is tilted to an “on” and an “off” direction respectively. 
       FIG. 4  is a schematic diagram showing incident light and reflected light in the chamber when a micro mirror plate in a pixel cell of a spatial light modulator is tilted to an “off” direction. 
       FIG. 5  is a flowchart showing the steps of fabricating an encapsulation device and encapsulating a spatial light modulator on a substrate using the encapsulation device. 
       FIG. 6  is a top view of an encapsulation cover assembly. 
       FIGS. 7A-7I  are cross-sectional views along A-A in  FIG. 6 , showing the steps of fabricating an encapsulation device and encapsulating a spatial light modulator on a substrate using the encapsulation device. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1A and 1B , a packaged spatial light modulator  100  includes a spatial light modulator  110  formed or mounted onto a substrate  120 . The spatial light modulator  110  can be mounted on the substrate  120  by wire bonding or flip-chip bonding. The spatial light modulator  110  can also be formed in one or more layers on a wafer. The substrate  120  can include an electric circuit  127  that electrically connects the spatial light modulator  110  to electric contacts  125  (outside of a chamber  135 ). The electric contacts  125  allow the spatial light modulator  110  to receive external electric signals or to output electric signals. The electric circuits  127  can, for example, include conductor-metal-oxide semiconductor (CMOS) transistors. 
   The spatial light modulator  110  is encapsulated by an encapsulation device  130  in a chamber  135 . The encapsulation device  130  can include an encapsulation cover  140  that can be made of a material that is transparent to visible, UV, or IR light. An opaque aperture layer  145  can be formed on the lower surface of an encapsulation cover  120 . The aperture layer  145  can be made of an opaque material, such as a metal oxide or carbide, for example, chromium oxide. The lower surface of the aperture layer  145  can be coated with a layer  152  of a light absorbing material. An aperture  148  in the opaque aperture layer  145  above the spatial light modulator  110  defines a transparent window for optical communications between the spatial light modulator  110  and outside of the chamber  135 . The aperture  148  is defined by aperture boundary  148 A. 
   The encapsulation device  130  can also include spacer walls  150  that are connected to the aperture layer  145  of the encapsulation cover  140  and to the substrate  120 . The spacer walls  150  include internal surfaces  150 B facing the spatial light modulator  100 . For example, the spacer walls  150  can be sealed to the substrate  120  by a polymer adhesive or bonded to the substrate  120  by plasma in the areas  150 A (the contact areas between the spacer walls  150  and the encapsulation cover  140  or the substrate  120 ). The spacer walls  150  can be made of an inorganic material, such as glass. The height of the spacer walls  140  can be about 0.2 to 2.0 microns, or 0.5 to 1 micron. The encapsulation cover  140  can optionally include antireflective coatings on the upper or the lower surfaces. 
   The surfaces of the spacer walls  150  inside the chamber  135  are also coated by a layer  152  of a light absorbing material. Optionally, an outside surface of the spacer walls  150  can also be coated by a layer of light absorbing material. In some embodiments, the light absorbing material absorbs light more efficiently than the aperture layer  145 . The upper surfaces of the substrate  120  that is outside of the spatial light modulator  110  and inside the chamber  135  are also coated with a layer  122  of a light absorbing material, as shown in  FIG. 1A . The light absorbing materials on the layer  122 , the layer  152 , and the aperture layer  145  can include for example a zirconium compound such as zirconium oxide or zirconium nitride. 
   Referring to  FIG. 2A , the spatial light modulator  110  can include a plurality of pixel cells  210 ,  220  that can be distributed in an array that is characterized by two lateral dimensions “W” and “H” (only a few pixel cells are shown for the sake of simplicity). Some pixel cells  210  are under the aperture  148  defined by the aperture boundary  148 A. The pixel cells  210  are thus under the window defined by the aperture  148  and can easily receive or output optical signals from or to the outside of the chamber  135 . 
   In some embodiments, some other pixel cells  220  in the spatial light modulator  110  are positioned under the aperture layer  145 . The pixel cells  220  are not used for optical communications or light modulations during device operation. The pixel cells  220  can be referred as dummy pixel cells. One purpose for the dummy pixel cells is to overcome possible registration error between the aperture  148  and the spatial light modulator  110 . When an encapsulation device  130  is bonded to the substrate  120 , small alignment errors may occur in the relative lateral positions between the spatial light modulator  110  and the aperture  148 . If the active area of the spatial light modulator  110  is made exactly same size as that of the aperture  148 , a small lateral misalignment between the spatial light modulator  110  and the aperture  148  can produce an inactive area inside the aperture  148 , that is, certain areas under the aperture  148  may not include pixel cells for optical communications such as spatial light modulations. The array of the pixel cells  210 ,  220  in the spatial light modulator  110  is therefore made larger than the aperture  148  to ensure the pixel cells  210 ,  220  fill the area within the aperture boundary  148  despite potential alignment errors. In other words, at least one of the lateral dimensions “W” and “H” of the array of pixel cells  210  and  220  is wider than the corresponding width of the opening  148 . 
   Referring to  FIG. 2B , a pixel cell  210  or  220  can include a tiltable micro mirror  200 . The tiltable micro mirror  200  can include a mirror plate  202  that includes a flat reflective upper layer  203   a , a middle layer  203   b  that provides the mechanical strength for the mirror plate, and a bottom layer  203   c . The upper layer  203   a  can be formed of a reflective material such as aluminum, silver, or gold. The layer thickness can be in the range of between about 200 and 1000 angstroms, such as about 600 angstroms. The middle layer  203   b  can be made of a silicon based material, for example, amorphous silicon, typically about 2000 to 5000 angstroms in thickness. The bottom layer  203   c  can be made of an electrically conductive material that allows the electric potential of the bottom layer  203   c  to be controlled relative to the step electrodes  221   a  or  221   b . The bottom layer  203   c  can be made of titanium and have a thickness in the range of about 200 to 1000 angstroms. 
   A hinge  206  is connected with the bottom layer  203   c  (the connections are out of plane of view and are thus not shown in  FIG. 2B ). The hinge  206  is supported by a hinge post  205  that is rigidly connected to the substrate  120 . The mirror plate  202  can include two hinges  206  connected to the bottom layer  203   c . The two hinges  206  define an axis about which the mirror plate  202  can be tilted. The hinges  206  can extend into cavities in the lower portion of mirror plate  202 . For ease of manufacturing, the hinge  206  can be fabricated as part of the bottom layer  203   c.    
   Step electrodes  221   a  and  221   b , landing tips  222   a  and  222   b , and a support frame  208  can also be fabricated over the substrate  120 . The heights of the step electrodes  221   a  and  221   b  can be in the range from between about 0.2 microns and 3 microns. The step electrode  221   a  is electrically connected to an electrode  281  whose voltage Vd can be externally controlled. Similarly, the step electrode  221   b  is electrically connected with an electrode  282  whose voltage Va can also be externally controlled. The electric potential of the bottom layer  203   c  of the mirror plate  202  can be controlled by an electrode  283  at potential Vb. 
   Bipolar electric pulses can individually be applied to the electrodes  281 ,  282 , and  283 . Electrostatic forces can be produced on the mirror plate  202  when electric potential differences are created between the bottom layer  203   c  on the mirror plate  202  and the step electrodes  221   a  or  221   b . An imbalance between the electrostatic forces on the two sides of the mirror plate  202  causes the mirror plate  202  to tilt from one orientation to another. 
   The landing tips  222   a  and  222   b  can have a same height as that of a second step in the step electrodes  221   a  and  221   b  for manufacturing simplicity. The landing tips  222   a  and  222   b  provide a gentle mechanical stop for the mirror plate  202  after each tilt movement. The landing tips  222   a  and  222   b  can also stop the mirror plate  202  at a precise angle. Additionally, the landing tips  222   a  and  222   b  can store elastic strain energy when they are deformed by electrostatic forces and convert the elastic strain energy to kinetic energy to push away the mirror plate  202  when the electrostatic forces are removed. The push-back on the mirror plate  202  can help separate the mirror plate  202  and the landing tips  222   a  and  222   b . Alternatively, the micro mirror  200  can be formed without landing tips  222   a  and  222   b.    
   Details about the structures and operations of micro mirrors are disclosed for example in commonly assigned U.S. Pat. No. 7,167,298, titled “High contrast spatial light modulator and method” and U.S. patent application Ser. No. 11/564,040, entitled “Simplified manufacturing process for micro mirrors”, filed Nov. 28, 2006, the content of which are incorporated herein by reference. 
   Referring to  FIGS. 3A and 3B , the un-tilted position for the mirror plate  202  is typically the horizontal direction parallel to the upper surface of the substrate  120 . The mirror plate  202  can be tilted by a tilt angle θ on  from the un-tilted position to an “on” position. The flat reflective upper layer of the mirror plate  202  can reflect the incident light  351  to produce the light  352  along the “on” direction. Since the incident angle (i.e., the angle between the incident light  330  and the mirror normal direction) and the reflection angle (i.e. the angle between the reflected light  340  and the mirror normal direction) are the same, the incident light  330  and the reflected light  340  form an angle 2θ on  that is twice as large as the tilt angle θ on  of the mirror plate  202 . The “on” direction is typically configured to be perpendicular to the substrate  120 . 
   The mirror plate  202  can be symmetrically tilted in an opposite direction to an “off” position. The mirror plate  202  can reflect the incident light  351  to form reflected light  353  traveling in the “off” direction. Because the incident angle for the incident light  330  is 3θ on , the reflection angle should also be 3θ on . Thus the angle between the light  352  and the light  353  is 4θ on , four times as large as the tilt angle θ on  of the mirror plate  202 . Typically, the tiltable micro mirror  200  is designed to produce the light  353  that travels substantially in the lateral direction. 
   Referring to  FIG. 4 , the light  353  reflected by the mirror plate  202  can travel in the “off” direction inside the chamber  135  ( FIG. 4  illustrates only a single mirror plate for clarity; all of the mirror plates of the spatial light modulator would similarly be positioned in the chamber  135 ). The light  353  can impinge on the layer  152  of light absorbing material coated on the internal surfaces of the spacer walls  150  and be absorbed by the light absorbing material in the layer  152 . Other unwanted light in the chamber  135  can include light scattered by the surfaces and objects in the chamber  135 . The unwanted light can also be absorbed by the layer  122  on the surface of the substrate  120  and the aperture layer  145  on the lower surface of the encapsulation cover  140 . When the mirror plate  202  is tilted to an “off” direction, it is desirable that no light can travel outside of the chamber  135  through the aperture  148 . An important measure for the performance of the spatial light modulator  110  is the ratio of the output light intensities when the mirror plate is tilted to the “on” and the “off” directions. The effective absorption of light  353  and other unwanted light in the chamber  135  in the disclosed system can significantly reduce the unwanted light exiting the aperture  148  when the mirror pale is tilted to an “off” position. The contrast and the performance of the spatial light modulator  110  can thus be improved. 
     FIG. 5  is a flowchart showing the steps of fabricating an encapsulation device  130  and encapsulating a spatial light modulator  110  on a substrate  120  using the encapsulation device  130 . Referring to  FIGS. 6 and 7A , an encapsulation cover  140  having a plurality of openings  315  is first provided (step  510 ). As described above, the encapsulation cover  140  is made of a transparent material. Each opening  315  between chambers  135  to be defined by the intact portions of the cover  140 . The openings  315  are provided for accessing the electric contacts  125  on the substrate  120  after the spatial light modulators  110  are encapsulated in chambers  135 . 
   An opaque aperture layer  145  is next formed and patterned on a surface of the encapsulation cover  120  ( FIG. 7B , step  520 ). The patterned aperture layer  145  defines a plurality of apertures  148  each associated with an opening  315  (and a chamber  135  to be formed). A plurality of spacer walls  150  are next formed on the patterned aperture layer  145  ( FIG. 7C , step  530 ). The spacer walls can be adjacent to the openings and surrounding the apertures  148 . Examples of the materials for the spacer walls  150  can include a metal such as nickel, and copper. The spacer walls  150  can be formed by first forming a conductive layer on the encapsulation cover  120 . A mask layer can then be formed on the conductive layer. The mask layer can have openings in the area where the spacer walls are to be built. The spacer walls are then formed in the openings by electrochemical plating. The spacer walls  150  can be formed by successive formation of a plurality of layers. Details about forming spacer walls using electrochemical plating are disclosed in commonly assigned pending U.S. Ser. No. 11/680,600, entitled “Fabricating tall micro structure”, filed Feb. 28, 2007, this disclosure of which is incorporated herein by reference. 
   A negative photo resist is next spin-coated on the spacer walls  150  and the aperture layer  145 , and the portion of the encapsulation cover  120  in the apertures  148  ( FIG. 7D , step  540 ). A photo resist layer  710  is formed on the surfaces of the spacer walls  150  and the aperture layer  145 . A portion  710 A of the photo resist layer is formed within the apertures  148 . Photon irradiation is next applied from the side of the encapsulation cover  120  that is opposite to the photo resist layer  710  ( FIG. 7E , step  550 ). Since the aperture layer  145  is opaque and the encapsulation cover  120  is transparent, only the portion  710 A of the photo resist layer  710  in the aperture  148  is exposed to the photon irradiation. The photo resist layer  710 A is subsequently cured by baking. The photo resist layer  710  is then removed by a developer while a cured photo resist layer  715  remains on the portion of the encapsulation cover  120  that is within the apertures  148  ( FIG. 7F , step  560 ). 
   A layer of light absorbing material is next deposited on the surfaces of the spacer walls  150  and the aperture layer  145 , and the cured photo resist layer  715  ( FIG. 7G , step  570 ). The light absorbing material can include a zirconium compound such as zirconium oxide and zirconium nitride. The light absorbing material can alternatively include amorphous carbon. The light absorbing material can be anisotropically deposited using chemical vapor deposition (CVD). An encapsulation device  130  is finally formed by lifting off the cured photo resist layer  715  and the portion of the light absorbing material  152  on the cured photo resist layer  715  ( FIG. 7H , step  580 ). 
   The encapsulation device  130  can then be used to encapsulate a plurality of spatial light modulators  110  on substrate  120  ( FIG. 7I , step  590 ). The surfaces of the spacer walls  150  are sealed to the upper surface of the substrate  120  with a polymer adhesive, such as epoxy or bonded to the upper surface of the substrate  120  by plasma bonding. A plurality of chambers  135  are thereby formed, each encapsulating one or more spatial light modulators  110 . One or more electric contacts  125  are positioned on the substrate  120  in the opening  315  next to each chamber  135 . The substrate  120  and the encapsulation cover  140  can then by diced to form individual dies each containing an encapsulated spatial light modulator  110  (step  600 ). 
   The above disclosed methods and devices may include one or more of the following advantages. The disclosed spatial light modulators can have improved optical performances. Unwanted light may be absorbed in a micro chamber that encapsulates the spatial light modulator. The optical noise in the output optical signal can therefore be reduced. The contrast between an “on” state and an “off” state of the spatial light modulator may also be increased. The specification also discloses manufacturing processes for encapsulation devices that include light absorbing components that can absorb the unwanted light in the chambers. Furthermore, a plurality of spatial light modulators on a substrate can be encapsulated in a common process. The manufacturing efficiency is thus improved. 
   It is understood that the disclosed systems and methods are compatible with other light absorbing materials and other processes for introducing the light-absorbing materials in the chambers. The encapsulation cover and the spacer walls can be made of different materials and formed by different processes. The spacer walls can be connected to the encapsulation cover and the substrate by different sealing or bonding techniques. The spatial light modulators compatible with the disclosed system and methods can include many optical devices other than tiltable micro mirrors. The tiltable mirrors can be tilted to more positions than the disclosed on and off position. The tiltable mirrors may not include mechanical stops for stopping the tilt movement of the mirror plates. The positions of the tiltable mirrors may be defined by balances between electrostatic forces and elastic forces. The relative positions, form factors, dimensions, and shapes of the chambers, the spatial light modulators, and the electric contact can also vary without deviating from the present application.