Abstract:
One embodiment relates to a hybrid micro electromechanical systems (MEMS) based spatial light modulator (SLM) capable of operating in both analog and digital modes. The hybrid SLM includes a substrate having an upper surface, a number of movable ribbons disposed a predetermined distance above the upper surface of the substrate, the ribbons having light reflective surfaces formed on their upper side facing away from the upper surface of the substrate, and a number of standoffs having a predetermined height positioned between a lower surface of the movable ribbons and the upper surface of the substrate. The standoffs are configured to limit the ribbon deflection of movable ribbons toward the upper surface of the substrate when the SLM is operated in digital mode with snap-down voltages applied between the ribbon and drive electronics in the substrate. Other embodiments are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims the benefit of U.S. Provisional Patent Application No. 60/655,680, entitled “Hybrid Analog/Digital Grating Light Valve,” filed Feb. 22, 2005, by inventor David T. Amm. 

   TECHNICAL FIELD 
   The present invention is directed generally to spatial light modulators, and more particularly, but not exclusively to, micro electromechanical systems (MEMS) based spatial light modulators. 
   BACKGROUND OF THE INVENTION 
   Spatial light modulators (SLMs) are devices or arrays of one or more devices that can control or modulate an incident beam of light in a spatial pattern that corresponds to an electrical input to the devices. The incident light beam can be modulated in intensity, phase, polarization or direction. Some modulation can be accomplished through the use of micro electromechanical systems (MEMS) in which electrical signals move micromechanical structures to modulate light incident thereon. 
   One type of MEMs based SLM is a ribbon light modulator, such as a Grating Light Valve (GLV™) commercially available from Silicon Light Machines, Inc., of Sunnyvale, Calif. Referring to  FIG. 1 , a ribbon light modulator generally includes a number of ribbons  102  each having a light reflective surface supported over a reflective surface of a substrate  104 . One or more of the ribbons are deflectable toward the substrate to form an addressable diffraction grating with adjustable diffraction strength. The ribbons  102  may be electrostatically deflected towards the substrate  104  by integrated drive electronics formed in or on the surface of the substrate. Light reflected from the movable ribbons adds as vectors of magnitude and phase with that reflected from stationary ribbons or a reflective surface beneath the ribbons, thereby modulating light reflected from the SLM. 
   SUMMARY 
   One embodiment of the invention relates to a hybrid micro electromechanical systems (MEMS) based spatial light modulator (SLM) capable of operating in both analog and digital modes. The hybrid SLM includes a substrate having an upper surface, a number of movable ribbons disposed a predetermined distance above the upper surface of the substrate, the ribbons having light reflective surfaces formed on their upper side facing away from the upper surface of the substrate, and a number of standoffs having a predetermined height positioned between a lower surface of the movable ribbons and the upper surface of the substrate. The standoffs are configured to limit the ribbon deflection of movable ribbons toward the upper surface of the substrate when the SLM is operated in digital mode with snap-down voltages applied between the ribbon and drive electronics in the substrate. 
   Another embodiment relates to a method of operating a micro electromechanical systems (MEMS) based spatial light modulator (SLM). Determinations are made (a) of a corrected level is determined for an intensity of reflected light by a pixel of the MEMS-based SLM when the pixel is in an ON state and (b) of an analog voltage to apply between movable ribbons of the pixel and a substrate thereunder such that the movable ribbons are deflected from an undeflected state by an analog distance so as to reduce the intensity of the reflected light by the pixel to the corrected level. The pixel is controllably set in the ON state by applying the analog voltage between the movable ribbons and the substrate. The pixel is controllably set in an OFF state by applying a snap-down voltage between the movable ribbons and the substrate thereunder. Applying the snap-down voltage deflects a top surface of the movable ribbons to a predetermined height above a top surface of the substrate. 
   A micro electromechanical systems (MEMS) based spatial light modulator (SLM) including at least the following. Circuitry is configured to controllably set the pixel in the ON state by applying an analog voltage between the movable ribbons and the substrate such that the movable ribbons are deflected from an undeflected state so as to reduce the intensity of the reflected light by the pixel to a corrected level. In addition, circuitry is configured to controllably set the pixel in an OFF state by applying a snap-down voltage between the movable ribbons and the substrate thereunder. Applying the snap-down voltage deflects a top surface of the movable ribbons to a predetermined height above a top surface of the substrate. 
   Other embodiments are also disclosed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and various other features and advantages of the present invention will be apparent upon reading of the following detailed description in conjunction with the accompanying drawings and the appended claims provided below, where: 
       FIG. 1  is a perspective view of a conventional ribbon type spatial light modulator (SLM); 
       FIG. 2  is a schematic cross sectional view of ribbons for a diffractive SLM according an embodiment of the present invention, operating in 0th order; 
       FIG. 3  is a schematic cross sectional view of the device of  FIG. 2 , with the ribbons deflected to provide 50% attenuation; 
       FIG. 4  is a schematic cross sectional view of the device of  FIG. 2  in the OFF state; 
       FIG. 5  is a graph of projected intensity-voltage characteristic for the device of  FIG. 2 ; 
       FIG. 6  is a flow chart of a method of operating a pixel of a MEMS based SLM in accordance with an embodiment of the invention; and 
       FIG. 7  is a schematic diagram of an apparatus in accordance with an embodiment of the invention 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 
   Conventional MEMS-Based Spatial Light Modulators 
   Ribbon light modulators can be employed in various applications including displays, optical networks, and printing. Generally, MEMS-based SLMs are either analog or digital modulators. 
   Digital modulators operate in a “contact” or snap-down mode in which the electrostatic attraction causes the ribbon to snap down and contact the substrate. The ribbon typically snaps down to the substrate if the deflection exceeds one third the distance between the ribbon and the substrate surface. 
   Analog modulators operate in the non-contact mode at deflections less than the snap-down voltage, and thus has the capability of continuous (or “analog”) intensity modulation. However, analog modulators generally are made using a thick sacrificial layer to provide the necessary distance between the ribbon and the substrate surface. This requirement of a thick sacrificial layer reduces ribbon damping and power handling characteristics of the analog modulator. This results in slower switching speeds. 
   In contrast, digital modulators operating in contact mode generally have a much thinner sacrificial layer. Thus, the ribbon damping is significantly higher, and faster switching and damping speeds are achieved. Unfortunately, because the digital modulator is operated in a “digital” or ON/OFF mode, and intensity attenuation must be performed using pulse-width-modulation techniques—this does not work well with moving media, such as in printing applications. 
   Hybrid MEMS-Based Spatial Light Modulators 
   As discussed above, analog MEMS-based SLMs are generally disadvantageous in their reduced damping speeds, while digital MEMS-based SLMs are generally disadvantageous in their need to use PWM for intensity attenuation. Accordingly, there is a need for a device or modulator which has the benefits of both the analog and digital modulators described above while minimizing their detrimental characteristics. 
   It is further desirable that the modulator have analog capability up to about 50% attenuation for uniformity correction. It is also desirable that the device is capable of operating at a higher voltage to “snap-down” the ribbon into contact to produce a digital “OFF” state. 
   The present disclosure is directed to a hybrid MEMS-based SLM capable of operating in both analog and digital modes. The hybrid SLM of the present disclosure may be used in numerous applications including, for example, displays, optical networks, maskless lithography, and printing applications, such as high power thermal printing. The hybrid SLM of the present disclosure is a device which has the benefits of both the analog and digital modulators described above while minimizing their detrimental aspects. 
   If the geometry and dimensions disclosed herein are used, a hybrid modulator may be configured to have analog capability up to about 50% attenuation—this is the type of attenuation that is required for uniformity correction. Moreover, a higher voltage may be applied to “snap-down” the ribbon into contact to produce a digital “OFF” state. Such a hybrid modulator meets the requirements of certain applications, such as high-power thermal printing. 
   The ON state (0th order) must be continuously variable in order to correct variation due to laser profiles, illumination optics, and MEMs and electronics. Once this correction is established, the circuitry may be configured so as to be able to toggle between this corrected state, and an OFF state—for example, in a digital printing application. 
   An exemplary embodiment of how this device may be designed and operated is now described in detail with reference to  FIGS. 2 through 5 . The example used is particularly suited to a high power thermal printing where the laser wavelength is approximately 820 nanometers (˜820 nm), and thus a ribbon deflection of approximately 200 nanometers (˜200 nm) is required in order to extinguish the 0th (zero-th) order light. 
     FIG. 2  shows a schematic cross sectional view of ribbons for an FLV style SLM, operating in 0th order. By FLV style, it is meant a ribbon type SLM having a number of moving ribbons  108  interlaced with a number of static or reference ribbons  106 . 
   In this example, the sacrificial layer  202 , and therefore the separation between a lower surface  204  or underside of the ribbon  108  and an upper surface  206  of the substrate  104 , is 0.3 micrometers (μm). The ribbons  108  further include standoffs  208  on the underside of the ribbon  108  which extend about 1 μm towards the substrate  104 . These standoffs limit the ribbon deflection to 0.2 μM in the event of snap-down. Standoff features under the static ribbons  106  are optional and are not needed. In a different configuration, the standoffs may be located on the substrate, instead of the underside of the ribbon. 
   In  FIG. 2  both ribbons are undeflected, and 0th order light is transmitted at a maximum. With a sacrificial layer of 0.3 μm, the ribbon  108  may typically be deflected to ⅓ of that value, without snapping to the substrate  104 . A deflection of 0.1 μm at 820 nm would create diffraction sufficient to reduce the 0th order intensity to 50% relative to the undeflected state. This 50% attenuation state is illustrated in  FIG. 3 . 
   The deflection up to 0.1 μm is continuous and monotonic, and such a deflection may be used for uniformity correction in the thermal printing application. At voltages higher than that needed for 0.1 μm deflection, the ribbon will snap to the substrate. This condition is illustrated in  FIG. 4 . Here, the standoffs  208  limit the deflection to 0.2 μm, which is precisely the requirement of extinguishing the 0th order light at ˜820 nm. In particular, the height difference between the top surface of the undeflected ribbons and the top surface of the deflected ribbons in snap-down is one fourth of the wavelength of the incident light. One quarter of 820 nm is 205 nm or approximately 0.2 μm. More generally, the height difference between the top surface of the undeflected ribbons and the top surface of the deflected ribbons in snap-down may be an odd multiple of one fourth of the wavelength of the incident light. 
   A graph of the expected intensity-voltage (IV) characteristic is shown in  FIG. 5 . Lower voltages are used to attenuate the 0th order intensity. A step increase in voltage is then used to snap the ribbon down into contact, and into the OFF state. A contact device has intrinsic hysteresis in the IV characteristic due to the electromechanical instability. However, for the large standoffs, this hysteresis is small, and the device is never operated near the snap-down or snap up regions. 
     FIG. 6  is a flow chart of a method of operating a pixel of a MEMS based SLM in accordance with an embodiment of the invention. This method may to each pixel of a MEMS based SLM array. 
   Per this method, a corrected intensity level may be determined  602  for an intensity of reflected light by a pixel of the MEMS-based SLM when the pixel is in an ON state. In correspondence to this corrected intensity level, an analog voltage is determined  604  to apply between movable ribbons of the pixel and a substrate thereunder. When this analog voltage is applied to the movable ribbons of a pixel and the substrate, the movable ribbons are deflected from an undeflected state by an analog distance so as to reduce the intensity of the reflected light by the pixel to the corrected intensity level. 
   In a practical implementation, a calibration procedure for the SLM may be used to pre-determine  602  and  604  the corrected intensity level and the corresponding analog voltage per pixel. Thereafter, the analog voltage per pixel in the SLM array may be stored  605  in memory accessible by the SLM driver circuit. 
   Subsequently, when the SLM pixel array is being operated, a determination  606  may be periodically (for example, once per image frame or once per refresh period) made as to whether the pixel is to be ON or OFF. In other words, is the pixel to be in a state where it constructively reflects light (ON) or in a state where it destructively diffracts light (OFF). 
   If the pixel is to be in an ON state, then that particular pixel is controllably driven  608  by its corresponding predetermined analog voltage. In other words, the corresponding analog voltage is applied  608  to that pixel so as to achieve a pixel reflecting the corrected intensity level. 
   If the pixel is to be in an OFF state, then that particular pixel is controllably driven  610  to the snap-down voltage. In other words, a voltage difference sufficient to snap down the movable ribbons is applied  608  to that pixel so as to achieve a pixel which destructively diffracts light. More particularly, the snap-down voltage deflects a top surface of the movable ribbons to a predetermined height above a top surface of the substrate, where the predetermined height may be determined by stand-off features between the ribbons and the substrate. 
     FIG. 7  is a schematic diagram of an apparatus in accordance with an embodiment of the invention. The apparatus includes a MEMS based SLM pixel array  702 . For example, the array  702  may comprise GLV™ pixels. The SLM pixel array  702  is controllably driven by an SLM driver circuit  704 . 
   The SLM driver circuit  704  may include accessible data storage. The accessible data storage may include, for example, semiconductor memory configured as one or more look-up tables. 
   In accordance with one embodiment, the data storage includes a look-up table or other data structure  706  which stores digital values (or digital approximations) of the analog voltage levels for the ON states of the pixels in the array  702 . In other words, the driver circuit  704  is able to access, for each pixel, a digital value which indicates the analog voltage level which should be driven for that pixel to achieve the corrected intensity level of its ON state. Digital-to-analog conversion (DAC) circuitry  710  in the driver circuit  704  may be used to convert the digital value to an analog voltage level. 
   In addition, the snap-down voltage level may also be stored as a digital value  708  in the driver circuit  704 . DAC circuitry  710  in the driver circuit  704  may also be used to convert the digital value to an analog voltage level. 
   The advantages of the above-disclosed technique over previous or conventional SLMs include the ability to reduce the thickness of the sacrificial layer to be about one third (⅓) that of the prior art devices, thereby reducing the thermal resistance by about ⅓, leading to either a low ribbon temperature and longer lifetime, or to a much higher power handling capability. Potentially, the power handling capability can be increased by as much as a factor of three (3). 
   The thinner sacrificial layer also leads to higher damping, which can lead to faster switching speeds. The expected improvement in damping is the sacrificial layer thickness ratio to the 3rd power—in this case, as much as 27 times. The thinner sacrificial allows for lower operating voltages at the same ribbon length, or, shorter (i.e. faster) ribbons operating at a similar voltage. Finally, it should be noted that a thinner sacrificial layer is generally more manufacturable than a thicker one. 
   Also the device can be operated to as much as fifty percent (50%) attenuation in a continuously variable manner, and the device is also compatible for standard pulse-width-modulation techniques for grayscale control as well. 
   An additional advantage is that operation in the region near snap-down, which is the most sensitive to charging or electronics drifts, is avoided. The OFF-state is in contact with the substrate and thus is independent of charging for medium amounts of charging. 
   Although, the invention has been described with reference to the FLV embodiment, it will be appreciated that the structure and design technique of the present invention can work equally well with a GLV™-style device, i.e., one having only moving ribbons supported above a reflective surface of the substrate with an approximately equal reflective area. It will also be appreciated that the above described embodiment is but one example of sacrificial thickness and standoff size given for illustrative means. Depending upon an attenuation budget needed for uniformity correction, other values of sacrificial and standoffs may offer optimum performance. 
   The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been described and illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications, improvements and variations within the scope of the invention are possible in light of the above teaching. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents.