Abstract:
In one embodiment, a high-density spatial light modulator includes a substrate having a reflective surface and a reflective ribbon over the reflective surface. The ribbon may have one or more openings, such as rectangular slots. The openings allow light to pass through the ribbon and impinge on the reflective surface. Deflecting the ribbon towards the substrate thus allows for dynamically-controllable diffraction of incident light. The spatial light modulator pixel requires less space than a conventional light modulator, thus allowing for relatively large pixel count within a manufacturable device size. Other embodiments are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. provisional application No. 60/506,624, entitled “Spatial light modulator,” filed Sep. 26, 2003, by inventors Jahja I. Trisnadi and Clinton B. Carlisle. The disclosure of the aforementioned U.S. provisional application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to light modulators, and more particularly but not exclusively to micro electromechanical system (MEMS) based spatial light modulators.  
         [0004]     2. Description of the Background Art  
         [0005]     Spatial Light modulators fabricated using micro electromechanical system (MEMS) technology, in general, are well known. Examples of such light modulators include the Grating Light Valve™ (GLV™) light modulator available from Silicon Light Machines Corporation of Sunnyvale, Calif. Devices that are of the same type as GLV™ light modulators are hereinafter referred to as “ribbon light modulators”. Ribbon light modulators are described in the following disclosures, which are integral to this provisional application and can be found in the following patents: U.S. Pat. No. 5,311,360, entitled “Method and Apparatus for Modulating a Light Beam” to Bloom et al.; U.S. Pat. No. 5,841,579, entitled “Flat Diffraction Grating Light Valve” to Bloom et al.; U.S. Pat. No. 5,661,592, “Method of Making and an Apparatus for a Flat Diffraction Grating Light Valve” to Bornstein et al.; and U.S. Pat. No. 6,215,579, entitled “Method and Apparatus for Modulating an Incident Beam for Forming a Two-Dimensional Image” to Bloom et al. The aforementioned four U.S. patents are hereby incorporated by reference. Ribbon light modulators can be employed in various applications including optical networks, video, and printing.  
         [0006]      FIGS. 1 and 2  show schematic diagrams of conventional ribbon light modulator structures. In  FIG. 1 , each ribbon  102  with reflective layer  103  is electro-statically deflectable towards the substrate  104 , with reflective layer  106  thereon, to form a diffraction grating with adjustable diffraction strength. In  FIG. 2 , active deflectable ribbons  202  are interlaced with static un-deflectable ribbons  204  also to form an addressable diffraction grating with adjustable diffraction strength. In  FIG. 2 , the substrate  208  need not have a reflective surface layer. A ribbon  102  and a gap  108  (ribbon-gap) pair  110  in  FIG. 1  or an active ribbon  202  and a static ribbon  204  (ribbon-ribbon) pair  206  in  FIG. 2  constitutes a diffraction period. One or more periods can be addressed as a “pixel.” A pixel can be addressed to modulate incident light by diffraction. Thus, a pixel can be used to display or print a unit of an image, for example.  
         [0007]     While conventional spatial light modulators can be satisfactorily used in a wide variety of high resolution applications, there are emerging applications that may require even higher pixel resolution. Examples of these emerging applications include super high resolution display, maskless lithography, high resolution printing for printed circuit boards and flat panel displays, and a host of other applications. There are also existing applications, such as laser printers, that can be considerably improved by the use of high resolution spatial light modulators.  
       SUMMARY  
       [0008]     In one embodiment, a high-density spatial light modulator includes a substrate having a reflective surface and a reflective ribbon over the reflective surface. The ribbon may have one or more openings, such as rectangular slots. The openings allow light to pass through the ribbon and impinge on the reflective surface. Deflecting the ribbon towards the substrate thus allows for dynamically-controllable diffraction of incident light. The spatial light modulator pixel requires less space than a conventional light modulator, thus allowing for relatively large pixel count.  
         [0009]     These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  schematically shows a conventional ribbon light modulator structure.  
         [0011]      FIG. 2  schematically shows another conventional ribbon light modulator structure.  
         [0012]      FIG. 3  schematically shows a top view of a high-density ribbon light modulator in accordance with an embodiment of the present invention.  
         [0013]      FIG. 4  schematically shows a center cross-section of the ribbon light modulator of  FIG. 3 . 
     
    
       [0014]     The use of the same reference label in different drawings indicates the same or like components. Drawings are not necessarily to scale unless otherwise noted.  
       DETAILED DESCRIPTION  
       [0015]     In the present disclosure, numerous specific details are provided such as examples of apparatus, components, and methods to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.  
         [0016]     The present invention relates to a diffractive, spatial light modulator. While embodiments of the invention are described using linear ribbon light modulators as examples, it should be understood that the invention is not so limited and may also be employed in other types of spatial light modulators.  
         [0017]      FIGS. 3 and 4  show schematic diagrams of a high-density or high pixel count linear, diffractive, spatial ribbon light modulator  300  in accordance with an embodiment of the present invention.  FIG. 3  shows a top view of an embodiment of the light modulator  300 , while  FIG. 4  shows a cross-sectional view of the embodiment of the light modulator  300  as seen by cutting through the line illumination  302  denoted in  FIG. 3 . Light modulator  300  may have from a few to many thousands, or more, reflective ribbons, but only four ribbons  304  are shown for clarity of illustration. The ends of each ribbon  304  may be anchored using posts  306  or other means. The ribbons  304  and the substrate beneath them preferably have reflective top surfaces and are separated by an air gap. In the example of  FIG. 4 , the top surface of the substrate  402  is shown as having an overlying reflective layer  404 . Each ribbon  304  preferably also has such a reflective layer on its top surface (as illustrated in  FIGS. 1 and 2 ). Each ribbon has one or more openings in the form of rectangular slots  308 . The slots  308  allow portions of light illuminating the shaded region labeled “line illumination” to impinge on the reflective layer  404  of the substrate  402  underneath the ribbons  304 . In a preferred embodiment, the spatial light modulator  300  is configured such that a) the area-reflectivity product due to the reflective layer on the ribbons, and b) the area-reflectivity product due to the openings within and between the ribbons, are approximately equal. Light modulator  300  may be fabricated using a MEMS process technology similar to those employed in the fabrication of conventional ribbon light modulators.  
         [0018]     The middle portions (in between the anchored portions) of the reflective ribbons  304  may be deflected towards the substrate by electrostatic force, for example. Deflecting the ribbons  304  varies the distance between the top surfaces of the ribbons  304  and the reflective layer  404  of the substrate  402 , thereby varying the path length difference of light impinging on them. This allows incident light to be modulated by diffraction.  
         [0019]     In light modulator  300 , a diffraction period “         ” comprises a strip of ribbon surface and an opening. An addressable pixel “P” has “N” periods/pixel (i.e., P=N         ). In the example of  FIGS. 3 and 4 , each pixel has three periods. Note that the gaps  310  between ribbons have the same effect as the slots  308 . The ribbon width “W” is thus given by the equation W=P-(         /2). Light modulator  300  may be operated in zero-order or first-order mode. In zero-order mode, the 0 h  order components of the light are collected and modulation is obtained by diffracting the light away into first and higher orders. In the first-order mode, it is the modulated 1 st  order components of the light that are collected. However, since the period of light modulator  300  may be just a few wavelengths, the diffraction angle may get very large. Therefore, it is preferable to operate light modulator  300  in the zero-order mode.  
         [0020]     The distance “H” is the distance between the top reflective surfaces of the ribbons and the reflective layer of substrate. If H=(odd integer)(λ/4), where “λ” is the wavelength of the incident light, then the light modulator  300  is normally OFF. In other words, the un-deflected state is diffracting so light is discarded, corresponding to a dark pixel in the zero-order mode. If H=(even integer)(λ/4), then the light modulator  300  is normally ON. That is, the un-deflected state is specular, which corresponds to a bright pixel in the zero-order mode. Because a ribbon may snap down if the deflection exceeds H/3, the smallest even and odd integer multipliers are preferably four and five, respectively. A height margin δ may be added to the distance H to allow for uniformity calibration. In other words, a light modulator  300  that is normally OFF may be configured with H=M(λ/4)+δ, where M is an odd integer no less than five (i.e., M=5, or 7, or 9, or 11 etc.), while a light modulator  300  that is normally ON may be configured with H=M(λ/4)+δ, where M is an even integer no less than four (i.e., M=4, or 6, or 6, or  10  etc.).  
         [0021]     As a specific example,          =1 micrometer (μm) and N=3 periods/pixel result in P=3 μm (i.e., 1 μm×3). The slot width as well as the gap between ribbons are each equal to (         /2) or 0.5 μm (i.e., 1 μm/2). The ribbon width W is 2.5 μm (i.e., 3 μm−(1 μm/2)). For a light source having a wavelength λ=0.5 μm, a normally OFF light modulator may have a distance H of 0.625 μm (odd integer=5) or 0.875 μm (odd integer=7). For ten thousand (10,000) pixels, the die will be only about 30 mm (10,000×3 μm) long.  
         [0022]     For ribbon structures that are ˜λ, full vector diffraction analysis may be employed to optimize the device operation and account for polarization effects in the light field interaction with the structure. Also, the drive electronics for each pixel may be integrated in the same silicon as the ribbon structure to allow for very high pixel count and fine pixel pitch.  
         [0023]     A large pixel count, diffractive, spatial light modulator has been disclosed. The modulator may configured as a linear array, or alternatively, as a two-dimensional array. While specific embodiments have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.