Abstract:
An improved SLM that is capable of detecting when light incident on the SLM exceeds a predetermined threshold. A diode is fabricated around, or within the pixel array. Light incident on the array (and the diode) results in a current increase through the diode, which may detected and used to initiate a disable signal to control circuitry of the SLM.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates to spatial light modulators, and more particularly to detection of conditions in which the light incident on the spatial light modulator exceeds a predetermined intensity.  
         BACKGROUND OF THE INVENTION  
         [0002]    A Digital Micromirror Device™ (DMD™ ) is a type of microelectromechanical systems (MEMS) device. Invented in 1987 at Texas Instruments Incorporated, the DMD is a fast, reflective digital spatial light modulator. It can be combined with image processing, memory, a light source, and optics to form a digital light processing system capable of projecting large, bright, high-contrast color images.  
           [0003]    The DMD is fabricated using CMOS-like processes over a CMOS memory. It has an array of individually addressable mirror elements, each having an aluminum mirror that can reflect light in one of two directions depending on the state of an underlying memory cell. With the memory cell in a first state, the mirror rotates to +10 degrees. With the memory cell in a second state, the mirror rotates to −10 degrees. By combining the DMD with a suitable light source and projection optics, the mirror reflects incident light either into or out of the pupil of the projection lens. Thus, the first state of the mirror appears bright and the second state of the mirror appears dark. Gray scale is achieved by binary pulsewidth modulation of the incident light. Color is achieved by using color filters, either stationary or rotating, in combination with one, two, or three DMD chips.  
         SUMMARY OF THE INVENTION  
         [0004]    One aspect of the invention is an improved spatial light modulator (SLM) having an array of pixel elements and control circuitry. In one embodiment, the improvement comprises a diode placed around at least a portion of the perimeter of the pixel array, the diode operable to conduct current in response to light incident on the pixel array. A disable circuit is operable to receive the current from the diode and to deliver a disable signal to the control circuitry when the current exceeds a predetermined amplitude.  
           [0005]    An advantage of the invention is that it provides a cost effective means to detect overlight conditions, that is, conditions when light incident on the SLM exceeds a predetermined threshold. The collection diode may be fabricated using CMOS processes consistent with fabrication of the SLM. Existing voltages that are used to operate the SLM may be used to bias the diode.  
           [0006]    The diode may be used to detect the overlight conditions in a manner that is transparent to the user of the SLM. It may be fabricated in a manner such that it is integral to the SLM and does not affect its operation.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a top plan view of an SLM having a light collection diode surrounding its pixel array.  
         [0008]    [0008]FIG. 2 is a cross sectional view of the diode of FIG. 1.  
         [0009]    [0009]FIG. 3 illustrates a disable circuit, which senses current through the diode and provides a disable signal to control circuitry of the SLM.  
         [0010]    [0010]FIG. 4 illustrates an alternative embodiment, where the SLM array includes light collection diodes.  
         [0011]    [0011]FIG. 5 is a cross sectional view of a diode of the SLM array of FIG. 4.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The following discussion is related to use of a reverse-biased N-well diode as a light collector for a spatial light modulator (SLM). The didode forms a recombination region in the substrate of the SLM for photogenerated holes, which can be sensed as an increase in current through the diode. The sensed current may then be used to detect an “overlight” condition, that is, a condition in which the intensity of light incident on the SLM exceeds a predetermined intensity.  
         [0013]    The diode may be fabricated using standard CMOS fabrication processes. The biasing voltages for the diode may be the same as those used for operation of the DMD. Further, the bias voltage may be from a low current power supply so that the signal to noise ratio of the photogenerated current is substantial.  
         [0014]    [0014]FIG. 1 is a top view of an SLM  10  having a field diode  10  surrounding the pixel array  11 . FIG. 2 is a cross sectional view of the diode  11  of FIG. 1.  
         [0015]    For purposes of example herein SLM  10  is a DMD type SLM. As discussed in the Background, a DMD  10  is comprised of an array  11  of hundreds or thousands of micro-mirror elements, also referred to as “pixel elements”. Peripheral (active) circuitry  12  is used to load data to the pixel elements and control their operation. A control circuit  14  delivers control signals to the peripheral circuitry  12 .  
         [0016]    As illustrated in FIG. 2, the SLM substrate  21  is a p+ type material, in which the diode  13  is formed. A p− type epitaxial layer  22  is above the substrate  21 . An n− well is fabricated in layer  22 , and surrounds a smaller n+ region  23  at the surface of the DMD  10 . A voltage, V diode , provides a reverse bias to diode  13 . As indicated in FIG. 2, when light is incident on the surface of the SLM  10 , photogenerated electrons recombine in the field diode  13  or at another positive bias junction.  
         [0017]    [0017]FIG. 3 illustrates a disable circuit  13 , used to disable the SLM  10  when the incident light exceeds a predetermined intensity. As stated above, light incident on the SLM  10  is sensed as an increase in the diode current. A sense amp  31   a  senses this current and delivers it to a comparator  31   b.  A comparator  31   b  compares the diode current with a predetermined reference current. If the diode current exceeds the reference current, comparator  31   b  delivers a signal to control circuit  14 , which then disables the SLM  10 .  
         [0018]    When SLM  10  is a DMD, one possible DMD operating voltage that may be used for V diode  is the shield bias, which has no direct current. Another available voltage is the Vcc2 voltage.  
         [0019]    [0019]FIG. 4 illustrates an alternative embodiment of the invention. Rather than surrounding the SLM array with a field diode, SLM  40  has a modified array  41 , which includes positive biased n−well junctions.  
         [0020]    [0020]FIG. 5 is a cross sectional view of an n−well junction  51  in array  41 . An n− well  53  is formed in substrate  51  as described above in connection with FIG. 2. In a DMD type SLM  40 , the existing array n−well may be used to form the diode. Two smaller doped regions, one p+ type  52  and one n+ type  53 , are located within the n− well  53  at the surface of the SLM  40 . As illustrated, when light is incident on the SLM, photogenerated electrons recombine in the array  41  to set up a signal current that will be superimposed on the existing current.  
         [0021]    In both of the above-described embodiments, it may be desirable for the diode  13  or the diodes in array  41  to cover much of the exposure field of the SLM  10 . The diode(s) may thereby rely on a large surface area for collection efficiency. This helps ensure that the collection current is immune to surface variations in the doping profiles of the silicon. Thus, in the example of FIG. 1, diode  13  completely surrounds the array  11 . In the example of FIG. 4, the entire array  41  includes diodes. However, in other embodiments, diode  11  might surround only a portion of array  11 , or only a portion of array  41  might include diodes.  
         [0022]    Although the foregoing description is in terms of a DMD, the same concepts are applicable to other types of SLMs. For example, the embodiments of FIGS. 1 and 4 could be implemented to provide a disable signal for a liquid crystal device (LCD) array. It is anticipated that, consistent with FIGS. 1 and 4, the SLM will be comprised of an array such as arrays  11  and  41  and some sort of control circuitry  14  to be disabled.  
         [0023]    Other Embodiments  
         [0024]    Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.