Abstract:
In one embodiment, a light modulator includes a substrate and a number of modulator elements disposed above and spaced apart from the substrate. Each modulator element has an optically active portion adapted to receive light incident thereon, and a support portion on either side of the active portion to support the modulator element above the substrate. The modulator elements include at least one deflectable modulator element. To shorten damping time, the deflectable modulator element has a lower surface in the support portion that is closer to the substrate than a lower surface under the optically active portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/609,733, filed Sep. 14, 2004, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to micro electro-mechanical systems (MEMS), and more particularly but not exclusively to MEMS light modulators. 
   2. Description of the Background Art 
   MEMS devices typically include micro mechanical structures that may be actuated using electrical signals. MEMS devices may be employed in various applications including light modulation for printing, video display, optical networks, and maskless lithography. Example MEMS light modulators include the Grating Light Valve™ (GLV™) light modulators available from Silicon Light Machines, Inc. of Sunnyvale, Calif. (Grating Light Valve™ and GLV™ are trademarks of Silicon Light Machines). A light modulator may employ movable ribbon-like structures. A ribbon may be deflected to modulate light incident thereon. 
     FIG. 1A  schematically shows a top view of a portion of a conventional ribbon-type diffractive spatial light modulator  100 . Light modulator  100  includes ribbon pairs  110 , with each ribbon pair  110  consisting of a deflectable active ribbon  112 A and a stationary bias ribbon  112 B. In some applications,  3  ribbon pairs  110  are employed to represent one pixel of information (e.g. a pixel of a video image). The ribbons  112  (i.e.  112 A and  112 B) are symmetrical about a symmetry line  102 . The right hand portion of the ribbons  112  are not shown for clarity of illustration. In operation, a light source illuminates the optically active area  114  of the ribbons  112 . The optically active area is also referred to as a “sweet spot” as it is the portion of the ribbons  112  configured to be illuminated by a light source. In the example of  FIG. 1A , active ribbons  112 A are configured to deflect, while bias ribbons  112 B are configured to remain relatively stationary or fixed. Light modulator  100  represents a particular implementation where the ribbons are used as modulator elements, as opposed to embodiments wherein the ribbons and the gaps between the ribbons are used as the modulating elements. 
     FIG. 1B  schematically shows a side cross-sectional view of the light modulator  100  taken at section A—A of  FIG. 1A . Each ribbon  112  (i.e.  112 A or  112 B) comprises a reflective material  120  supported by a resilient structure  121 . A gap separates the ribbons  112  from the substrate  122 . On top of the substrate is a drive electrode  131 , also referred to as a “bottom electrode.” The reflective materials  120  may be configured as actuator electrodes, also referred to as “top electrodes.” Applying a potential difference between the drive electrode  131  and the actuator electrodes creates an electrostatic force that deflects the actuator electrodes toward the substrate  122 . 
     FIG. 1C  schematically shows the ribbons  112  of  FIG. 1B  when the active ribbons  112 A are actuated. As shown in  FIG. 1C , a height difference between adjacent ribbons can be changed by controllably deflecting the active ribbons  112 A towards the substrate  122  by up to about 9λ/4 and more typically about 5λ/4, where λ is the wavelength of the incident light. If, upon reflection, the light from adjacent ribbons is in phase, then the 0 th  order light reflection is effectively maximized and the light modulator  100  is in an ON state. To minimize the 0 th  order light reflection; the active ribbons  112 A are deflected by an odd multiple of the wavelength. When the 0 th  order light reflection is minimized, the light modulator  100  is in an OFF state. The ribbons  112 A may be actuated such that the light modulator  100  is ON, OFF, or in between to modulate incident light. 
   The speed of currently available devices employing ribbon-type diffractive spatial light modulators is limited by damping time “T” (tau), which is the time required for a ribbon to transition from an OFF state to an ON state, or from a first deflected state to an undeflected or a second deflected state.  FIG. 2  shows a graph  200  illustrating the impact of damping time on transition from an OFF state to an ON state in a conventional ribbon-type spatial light modulator, such as light modulator  100 . Plot  204  shows a simulated response of a conventional ribbon-type spatial light modulator having a response time of about 4 microseconds, while plot  202  shows the simulated response of a conventional ribbon-type spatial light modulator having a response time of about 6 microseconds. As shown in  FIG. 2 , the transition from OFF to ON (region I to II) results in oscillation such that the integrated intensity is lower in region II than in region III for both plots  204  and  202 . Thus, the minimum pulse time for the transition (i.e. the maximum speed of the device) is limited by the maximum allowable variation. 
     FIG. 3  shows a diagram and a formula illustrating the impact of various characteristics of a ribbon-type diffractive spatial light modulator on damping time. The factors that affect damping time include: gap thickness (G) that is the distance separating a lower surface of the ribbon from an upper surface of the supporting substrate; ribbon density (ρ); ribbon thickness (t); ribbon width (w); and the gas effective viscosity (η eff ) of gas enveloping the light modulator and filling spaces between the ribbons and substrate. As the gas becomes more viscous, the damping time is increased. Although many, if not all, of these factors can be optimized for speed (i.e. to minimize damping time), there is typically a compromise of other device performance parameters including wavelength of modulated light, illumination efficiency or fill-factor, diffraction angle, die or modulator size, sweet spot size, snap-down margin, or operating voltages.  FIG. 4  shows a chart illustrating the tradeoff between optimization of speed realized through decreased damping time and spatial light modulator performance in a conventional ribbon type spatial light modulator. 
   SUMMARY 
   In one embodiment, a light modulator includes a substrate having an upper surface and a number of modulator elements disposed above the upper surface of the substrate and in spaced apart relation thereto. Each modulator element has an optically active portion adapted to receive light incident on the light modulator and a support portion on either side of the optically active portion to support the modulator element above the substrate. The modulator elements include at least one deflectable modulator element having a lower surface in the support portion at least a section of which is separated in an undeflected state from the upper surface of the substrate by a gap distance less than that of a lower surface of the optically active region. Generally, the substrate further includes a bottom or drive electrode, the deflectable modulator element further includes a top or actuator electrode on a top surface thereof, and the light modulator further includes a means for applying an electrostatic force between the drive and actuator electrodes to deflect the deflectable modulator element relative to the upper surface of the substrate. 
   In one embodiment, the support portion of the deflectable modulator element further includes at least one projection extending from the lower surface thereof, and the section of the lower surface that is separated by a gap distance less than the lower surface of the optically active region is on the projection. Preferably, the support portion of the deflectable modulator element further includes at least one depression or indentation in the top surface thereof, and the depression is located and sized to complement the projection. More preferably, the projection and depression are located and sized relative to one another to maintain a substantially constant average cross-sectional thickness across a width of the deflectable modulator element in the support portion. Most preferably, the actuator electrode on the top surface of the deflectable modulator element does not extend into or cover the depression, thereby substantially eliminating snap down of the deflectable modulator element under normal operating conditions. 
   In one embodiment, the projection includes a single central projection with a long axis parallel to the long axis of the deflectable modulator element, and the complementary depression includes a single central depression also having a long axis parallel with that of the deflectable modulator element. In one version of this embodiment, the modulator elements include at least one stationary or bias modulator element, and the support portion of the bias modulator element further includes a cross-sectional thickness across a width thereof less than that of the deflectable modulator element. 
   In one embodiment, the projection includes a pair of projections proximal to sides of the deflectable modulator element, the pair of projections having long axes parallel with the long axis of the deflectable modulator element to define two projecting or lowered portions of the lower surface in the support portion along sides thereof. The support portion of the deflectable modulator element may further include a pair of depressions proximal to sides of the deflectable modulator element located and sized to complement the pair of projections. The modulator elements may also include at least one stationary or bias modulator element, and wherein the support portion of the bias modulator element further includes a cross-sectional thickness across a width thereof less than that of the deflectable modulator element. 
   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 
       FIGS. 1A ,  1 B, and  1 C schematically show various views of an example conventional light modulator. 
       FIG. 2  shows a graph illustrating the impact of damping time on transition from an OFF state to an ON state in a conventional ribbon-type diffractive spatial light modulator. 
       FIG. 3  shows a diagram and a formula illustrating the impact of various characteristics of a ribbon-type diffractive spatial light modulator on damping time. 
       FIG. 4  shows a chart illustrating the tradeoff between optimization of speed realized through decreased damping time and spatial light modulator performance in a conventional ribbon-type diffractive spatial light modulator. 
       FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E schematically show various views of a light modulator in accordance with a first embodiment of the present invention. 
       FIGS. 6A ,  6 B,  6 C,  6 D,  6 E, and  6 F schematically illustrate the fabrication of a light modulator in accordance with an embodiment of the present invention. 
       FIG. 7  shows a graph illustrating the minimization of oscillation and the impact on damping and transition time in a light modulator according to an embodiment of the present invention. 
       FIGS. 8A and 8B  schematically show various views of a light modulator in accordance with a second embodiment of the present invention. 
       FIGS. 9A and 9B  schematically show various views of a light modulator in accordance with a third embodiment of the present invention. 
       FIGS. 10A and 10B  schematically show various views of a light modulator in accordance with a fourth embodiment of the present invention. 
       FIG. 11  schematically shows a side view of a light modulator in accordance with a fifth embodiment of the present invention. 
   

   The use of the same reference label in different drawings indicates the same or like components. The drawings are not to scale unless otherwise noted. 
   DETAILED DESCRIPTION 
   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. 
   Referring now to  FIG. 5A , there is schematically shown a top view of a portion of a ribbon-type diffractive spatial light modulator  500  in accordance with an embodiment of the present invention. Light modulator  500  includes modulator elements in the form of ribbon pairs  510 , with each ribbon pair  510  comprising a deflectable active ribbon  512 A and a stationary bias ribbon  512 B. In some applications,  3  ribbon pairs  510  are employed to represent one pixel of information (e.g. a pixel of a video image). The ribbons  512  (i.e.  512 A and  512 B) are symmetrical about a symmetry line  502 . That is, a mirror image of the ribbons  512  extends to the right hand side of symmetry line  502 . Portions of the ribbons  512  on the right hand side of symmetry line  502  are not shown for clarity of illustration. In operation, a light source illuminates the optically active area  514  of the ribbons  512 . The optically active area is also referred to as a “sweet spot” as it is the portion of the ribbons  512  configured to be illuminated by a light source. Portions of the ribbons  512  not on the optically active area, such as depressions  508 , are also referred to as “support portions” in that they support the optically active area  514  over the substrate. 
   In the example of  FIG. 5A , active ribbons  512 A are configured to deflect, while bias ribbons  512 B are configured to remain relatively stationary or fixed. Active ribbons  512 A and bias ribbons  512 B typically have uniform dimensions and are covered with the same reflective material. The ribbons  512  also include long anchor regions  504 , short anchor regions  506 , and depressions  508 . Anchor regions  504  and  506  secure the ribbons  512  to the substrate. As will be more apparent below, depressions  508  form corresponding projections  509  (see  FIG. 5C ) that reduce the gap between the bottom surface of the ribbons  512  and the substrate to reduce damping time. In light modulator  500 , each depression  508  and projection  509  has a long axis that is substantially in parallel with a long axis of the ribbon  512 . 
     FIG. 5B  schematically shows a side view of the light modulator  500  as seen in the direction indicated by an arrow  516  in  FIG. 5A . As shown in  FIG. 5B , portions of a ribbon  512  are suspended over substrate  522 . A gap  520  is formed under the suspended portions of the ribbon  512  to allow the ribbon to deflect towards the substrate  522  in the case of an active ribbon  512 A. Gap  520  is typically filled with gas formed during fabrication of the light modulator  500 . In some embodiments, a reflective material formed on a top surface of the ribbon  512  serve as a top or actuator electrode. A bottom or drive electrode  541  is formed on the substrate  512 . A potential difference may be applied between the actuator and drive electrodes to electrostatically deflect ribbons  512 A toward the substrate  522 . 
     FIG. 5C  schematically shows a side cross-sectional view of the light modulator  500  taken at section B—B of  FIG. 5A . The ribbons  512 A are in the undeflected state in  FIG. 5C . Note that only some components shown in  FIG. 5C  are labeled for clarity of illustration. As shown in  FIG. 5C , each ribbon  512  ( 512 A or  512 B) may comprise a reflective material  520  supported by a resilient structure  521 . Reflective material  520  may comprise aluminum, while the resilient structure  521  may comprise silicon nitride. The gap  520  separates the ribbons  512  from the substrate  522 . On top of the substrate  522  is the drive electrode  541 . The reflective materials  520  may be configured as actuator electrodes. Applying a potential difference between the drive electrode  541  and the reflective materials  520  creates an electrostatic force that deflects the active ribbons  512 A toward the substrate  522 . In one embodiment, each active ribbon  512 A is moved less than about ⅓ of the distance  532  between the lower surface of the ribbon  512 A and the bottom portion of the gap  520 , which is the top surface of the drive electrode  541  in this example. The distance  532  may be about 1 μm, for example. In that case, a projection  509  may formed such that the distance  533  is about 0.31 μm, for example. 
   Still referring to  FIG. 5C , each depression  508  may have a complementary projection  509  that makes the lower surface of a ribbon  512  in the support portions closer to the substrate. The projections  509  reduce the gap distance under the resilient structures  521 , thereby advantageously shortening damping time. In one embodiment, each resilient structure  521  comprises a single layer of resilient material, such as silicon nitride. 
   The depressions  508  and projections  509  are preferably outside the optically active area. This results in a projection  509  having a gap distance less than that of a lower surface of the ribbon  512  under the optically active area. That is, the exposed surface of the projection  509 , which is in the support portion of the ribbon  512 , is closer to the substrate  522  than the lower surface (i.e. surface facing the substrate) of the ribbon  512  under the optically active area.  FIG. 5D  schematically shows a side view of the light modulator  500  as seen in the direction indicated by an arrow  517  in  FIG. 5A . As shown in  FIG. 5D , a projection  509  results in the gap distance  542  being shorter than a gap distance  543  under the optically active area  514 . 
   A depression on the top surface of the ribbons may be located and sized to correspond to or complement the projection. More preferably, the depressions and projections are located and sized relative to one another to maintain a substantially constant average cross-sectional thickness across a width of the ribbon, in the support portion. Most preferably, the actuator electrode (e.g. reflective material) on the top surface of the ribbon does not extend into or cover the depression in the support portion thereof. Removing the actuator electrodes from the depressions substantially eliminates the potential for snap down of the ribbon under normal operating conditions, which could arise from moving the actuator electrode closer to the substrate or the drive electrode, and allowing near contact movement of the ribbon. 
     FIG. 5E  schematically shows the light modulator  500  when the ribbons  512 A are actuated (i.e. in the deflected state). A height difference between adjacent ribbons  512  can be changed by controllably deflecting the active ribbons  512 A towards the substrate  522  by up to about 9λ/4 and more typically about 5λ/4, where λ is the wavelength of the incident light. If, upon reflection, the light from adjacent ribbons  512  is in phase, then the 0 th  order light reflection is effectively maximized and the light modulator  500  is in an ON state. To minimize the 0 th  order light reflection, the active ribbons  512 A are deflected by an odd multiple of the wavelength. When the 0 th  order light reflection is minimized, the light modulator  500  is in an OFF state. The ribbons  512 A may be actuated such that the light modulator  500  is ON, OFF, or in between to modulate incident light. 
     FIGS. 6A–6F  schematically illustrate the fabrication of a ribbon-type diffractive spatial light modulator  500  in accordance with an embodiment of the present invention. The fabrication steps of  FIGS. 6A–6F  are provided herein merely for illustration purposes, not as a limitation. As can be appreciated, the previously described light modulator  500  may be fabricated using any appropriate fabrication process without detracting from the merits of the present invention.  FIGS. 6A–6F  omit well known steps, such as masking steps, in the interest of clarity. 
   In  FIG. 6A , a conductive layer serving as the drive electrode  541  is formed over the substrate  522 . The substrate  522  may comprise a semiconductor (e.g. silicon) substrate, while the drive electrode  541  may comprise a layer of metal or doped polysilicon. In one embodiment, the drive electrode  541  comprises doped polysilicon formed to a thickness of about 3000 to 10000 Angstroms. A relatively thin oxide layer (not shown) may be formed between the drive electrode  541  and the substrate  522 . A sacrificial layer  602  is formed over the drive electrode  541 . Sacrificial layer  602  may be formed to a thickness of about 2000 to 20,000 Angstroms. Sacrificial layer  602  may comprise a material that is preferentially etched by a noble gas fluoride (e.g. xenon difluoride). For example, sacrificial layer  602  may comprise amorphous silicon. 
   In  FIG. 6B , the sacrificial layer  602  is etched to form a pattern for depressions  508 . The etching of the sacrificial layer  602  in  FIG. 6B  may be a timed etch using masks (not shown) formed over the sacrificial layer  602 , for example. The sacrificial layer  602  may be etched to a depth of about 1500 to 15,000 Angstroms, for example. 
   In  FIG. 6C , a layer of resilient material serving as resilient structures  521  is formed over the sacrificial layer  602 . The resilient material may comprise silicon nitride formed to a thickness of about 500 to 2000 Angstroms, for example. 
   In  FIG. 6D , reflective materials  120  are formed on the material of the resilient structures  521 . Reflective materials  120  may comprise aluminum formed to a thickness of about 500 to 1000 Angstroms, for example. 
   In  FIG. 6E , release holes  644  are formed through the reflective materials  520  and the material of resilient structures  521 . Release holes  644  allow the sacrificial layer  602  to be exposed to an etchant that will etch the sacrificial layer  602  and release the ribbons  512 . 
   In  FIG. 6F , the sample of  FIG. 6E  is exposed to an etchant to isotropically etch the sacrificial layer  602 . In one embodiment, a sacrificial layer  602  comprising amorphous silicon is etched using an etchant comprising a noble gas fluoride, such as xenon difluoride. Etching the entirety of sacrificial layer  602  forms air gap  520  and releases the ribbons  512 . Relative to the substrate  522 , the gap distance to a projection  509  is shorter compared to that of a lower surface of a ribbon  512  under an optically active area. 
   It will be appreciated that because damping time has a cubic relationship with gap distance, as illustrated in  FIG. 3 , the light modulator  500  will provide shorter or faster damping times in transitions from an OFF state to an ON state, and vice-versa.  FIG. 7  shows a graph  700  illustrating the minimization of oscillation and the impact on damping and transition time in a light modulator according to an embodiment of the present invention. Plot  704  shows a simulated response of a light modulator with reduced gap distance and having a response time of about 4 microseconds, while plot  702  shows a simulated response of a light modulator with reduced gap distance and having a response time of about 6 microseconds. From  FIG. 7 , it is seen that regions II and III have similar integrated intensity, thereby providing faster device damping and enabling more uniform pulses at higher speeds. 
   In light of the present disclosure, those of ordinary skill in the art can appreciate that gap distance under a modulator element may be reduced using other configurations without departing from the scope and spirit of the present invention. For example,  FIGS. 8–10  show alternative embodiments for reducing gap distance under a modulator element, and thereby shorten damping time. As can be appreciated, the embodiments shown in  FIGS. 8–10  may be fabricated using process steps similar to those of the light modulator  500  or using other suitable fabrication processes without detracting from the merits of the present invention. 
     FIG. 8A  schematically shows a top view of a portion of a ribbon-type diffractive spatial light modulator  700  in accordance with an embodiment of the present invention. Light modulator  700  includes modulator elements in the form of ribbon pairs  710 , with each ribbon pair  710  consisting of a deflectable active ribbon  712 A and a stationary bias ribbon  712 B. The ribbons  712  (i.e.  712 A and  712 B) are symmetrical about a symmetry line  702 . Portions of the ribbons  712  on the right hand side of symmetry line  702  are not shown for clarity of illustration. In operation, a light source illuminates the optically active area  714  (i.e. sweet spot) of the ribbons  712 . 
   In the example of  FIG. 8A , active ribbons  712 A are configured to deflect, while bias ribbons  712 B are configured to remain relatively stationary or fixed. Active ribbons  712 A and bias ribbons  712 B have uniform dimensions and are covered with the same reflective material. The ribbons  712  also include long anchor regions  704 , short anchor regions  706 , and depressions  708 . Anchor regions  704  and  706  secure the ribbons  712  to the substrate. As will be more apparent below, depressions  708  have complementary projections  709  (see  FIG. 8B ) that reduce the gap distance between the substrate and the lower surface of the ribbons  712  to shorten damping time. 
     FIG. 8B  schematically shows a side cross-sectional view of the light modulator  700  taken at section C—C of  FIG. 8A . Note that only some components shown in  FIG. 8B  are labeled for clarity of illustration. As shown in  FIG. 8B , each ribbon  712  ( 712 A or  712 B) may comprise a reflective material  720  supported by a resilient structure  721 . A gap separates the ribbons  712  from a substrate  722 . On top of the substrate is a drive electrode  741 . The reflective materials  720  may be configured as actuator electrodes. Applying a potential difference between the drive electrode  741  and the reflective materials  720  creates an electrostatic force that deflects the active ribbons  712 A toward the substrate  722 . 
   Still referring to  FIG. 8B , depressions  708  on a ribbon  712  have a complementary pair of projections  709  formed proximal to the sides of the ribbon  712 . Depressions  708  are also formed proximal to the sides of the ribbons  712  and are located and sized to complement their corresponding pair of projections  709 . Each pair of projections  709  has long axes in parallel with a long axis of the ribbon  712  to define two projecting or lowered portions of the lower surface in the support portion along sides thereof. Depressions  708  and projections  709  are preferably outside the optically active area. This results in a projection  709  having a gap distance less than that of a lower surface of the ribbon  712  under the optically active area. That is, the lower surface of a ribbon  712  under the optically active is higher than the projections  709  relative to the substrate  722 . The projections  709  reduce the gap distance under the resilient structures  721 , thereby advantageously shortening damping time. In the example of  FIG. 8B , each resilient structure  721  comprises a single layer of resilient material. 
   In the embodiments of  FIGS. 5 and 8 , all ribbons of the light modulator are identical to improve 1 st  order dark state. However, as will be appreciated by those skilled in the art, this is not necessary and one or more of the active or bias ribbons may comprise a non-identical area, cross-section or shape to achieve other additional functionalities, without departing from the scope or spirit of the invention. The difference between the ribbons can be implemented in either the support portion or the optically active area, for example.  FIGS. 9 and 10  show alternative embodiments where the active and bias ribbons are not identical yet still achieve relatively short damping time due to the reduction of the gap distance in portions of the light modulator other than the optically active area. 
     FIG. 9A  schematically shows a top view of a portion of a ribbon-type diffractive spatial light modulator  900  in accordance with an embodiment of the present invention. Light modulator  900  includes modulator elements in the form of deflectable active ribbons  912 A and stationary bias ribbon  912 B. The ribbons  912  (i.e.  912 A and  912 B) are symmetrical about a symmetry line  902 . Portions of the ribbons  912  on the right hand side of symmetry line  902  are not shown for clarity of illustration. In operation, a light source illuminates the optically active area  914  (i.e. sweet spot) of the ribbons  912 . 
   In the example of  FIG. 9A , active ribbons  912 A are configured to deflect, while bias ribbons  912 B are configured to remain relatively stationary or fixed. Active ribbons  912 A and bias ribbons  912 B are typically covered with the same reflective material. The ribbons  912  also include long anchor regions  904  and short anchor regions  906 . Anchor regions  904  and  906  secure the ribbons  912  to the substrate. Because bias ribbons  912 B remain relatively stationary, they may include additional short anchor regions (labeled as  906 A) for additional structural support without affecting mechanics. 
   In light modulator  900 , not all of the ribbons  912  include projections  909  (see  FIG. 9B ) or depressions  908 . Instead the bias ribbons  912 B, which do not move, do not include projections or depressions and can be made smaller or thinner in the support portion, thereby enabling the support portion of adjacent active ribbons  912 A to be wider. In active ribbons  912 A, the depressions  908  have complementary projections  909  that reduce the gap distance between the lower surface of the active ribbons  912 A and the substrate to reduce damping time. The lowered and wider support portions of the active ribbons  912 A have a more pronounced effect, thereby enabling higher operating speeds. 
     FIG. 9B  schematically shows a side cross-sectional view of the light modulator  900  taken at section D—D of  FIG. 9A . Note that only some components shown in  FIG. 9B  are labeled for clarity of illustration. As shown in  FIG. 9B , each ribbon  912  ( 912 A or  912 B) may comprise a reflective material  920  supported by a resilient structure  921 . A gap separates the ribbons  912  from a substrate  922 . On top of the substrate is a drive electrode  941 . The reflective materials  920  may be configured as actuator electrodes. Applying a potential difference between the drive electrode  941  and the reflective materials  920  creates an electrostatic force that deflects the ribbons  912 A toward the substrate  922 . 
   Still referring to  FIG. 9B , a depression  908  on an active ribbon  912 A has a complementary projection  909  that makes the lower surface of the active ribbon  912 A closer to the substrate  922  to shorten damping time. The depressions  908  and projections  909  are preferably outside the optically active area. This results in a projection  909  having a gap distance less than that of a lower surface of the ribbon  912 A under the optically active area. That is, the bottom surface of a ribbon  912 A under the optically active area is higher than a projection  909  relative to the substrate  922 . 
   Referring now to  FIG. 10A , there is schematically shown a top view of a portion of a ribbon-type diffractive spatial light modulator  990  in accordance with an embodiment of the present invention. Light modulator  990  includes modulator elements in the form of deflectable active ribbons  952 A and stationary bias ribbon  952 B. The ribbons  952  (i.e.  952 A and  952 B) are symmetrical about a symmetry line  962 . Portions of the ribbons  952  on the right hand side of the symmetry line  962  are not shown for clarity of illustration. In operation, a light source illuminates the optically active area  954  (i.e. sweet spot) of the ribbons  952 . 
   In the example of  FIG. 10A , active ribbons  952 A are configured to deflect, while bias ribbons  952 B are configured to remain relatively stationary or fixed. Active ribbons  952 A and bias ribbons  952 B are typically covered with the same reflective material. The ribbons  952  also include long anchor regions  954  and short anchor regions  956 . Anchor regions  954  and  956  secure the ribbons  912  to the substrate. Because bias ribbons  952 B remain relatively stationary, they may include additional short anchor regions (labeled as  956 A) for additional structural support without affecting mechanics. 
   In light modulator  990 , not all of the ribbons  952  include projections  959  (see  FIG. 10B ) or depressions  958 . Instead the bias ribbons  952 B, which do not move, do not include projections or depressions and can be made smaller or thinner in the support portion, thereby enabling the support portion of adjacent active ribbons  952 A to be wider. In active ribbons  952 A, the depressions  958  have complementary projections  959  that reduce the gap distance between the lower surface of the active ribbons  952 A and the substrate to reduce damping time. The lowered and wider support portions of the active ribbons  952 A have a more pronounced effect, thereby enabling higher operating speeds. 
   Still referring to  FIG. 10B , depressions  958  on an active ribbon  952 A have a complementary pair of projections  959  formed proximal to the sides of the active ribbon  952 A. Depressions  958  are also formed proximal to the sides of the active ribbon  952 A and are located and sized to complement their corresponding pair of projections  959 . Each pair of projections  959  has long axes in parallel with a long axis of the active ribbon  952 A to define two projecting or lowered portions of the lower surface in the support portion along sides thereof. Depressions  958  and projections  959  are preferably outside the optically active area. This results in a projection  959  having a gap distance less than that of a lower surface of the active ribbon  952 A under the optically active area. That is, the bottom surface of an active ribbon  952 A under the optically active is higher than the projections  959  relative to the substrate  972 . The projections  959  reduce the gap distance under a resilient structure  971  of an active ribbon  952 A, thereby advantageously shortening damping time. In the example of  FIG. 10B , each resilient structure  971  comprises a single layer of resilient material. 
   In light of the present disclosure, yet more embodiments are possible without departing from the scope and spirit of the present invention. For example, gap distance may be reduced to shorten dampen time by a projection formed on the substrate rather than on an active ribbon. That is, the upper surface of a substrate may comprise at least one projection extending toward the section of the lower surface of the support portion of a ribbon, thereby reducing the gap separating the lower surface of the ribbon in the support portion from the substrate. A side view of an example ribbon  12  with such a feature is schematically shown in  FIG. 11 , where a projection  9  extends toward the support portion of the ribbon  12 . Ribbon  12  comprises a reflective material  20  supported by a resilient structure  21 . In the example of  FIG. 11 , the projection  9  is part of a drive electrode  41 , which is formed over the substrate  22 . Projection  9  is under the support portion (not the optically active area) of the ribbon  12  to reduce the gap distance and thereby shorten damping time. 
   While specific embodiments of the present invention 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.