Patent Publication Number: US-7212328-B2

Title: Electron-beam actuated light modulator with a mechanical stop

Description:
BACKGROUND 
   The invention relates to spatial light modulators. More specifically, the invention relates to spatial light modulators including an array of light-modification elements which are individually controllable, and analogous to image pixels. 
   Micro-devices configured as light modulators have found application in a number of areas of technical endeavor. These include, but are not limited to, computing and data storage, displays, and telecommunications. Arrays of light modifying elements are conventionally fabricated by CMOS-compatible processes on silicon. The cost of CMOS-based circuits is usually high, and the reliability can be low due to a need of many layers of thin-film process-created structure. The size of conventional elements in such an array, and thus the resolution of the device, is typically limited by the manufacturing process. 
   SUMMARY 
   An electron beam actuated light modulator, including an actuatable mirror element carried by a pedestal and hinge portion, having an electrostatically more neutral first position and an electrostatically more charged second position, the mirror being actuatable in a first direction from the first position to the second position by charge accumulation, and is actuatable in a second direction from the second position to the first position by dissipation of the charge accumulation and a stop element positioned adjacent the mirror element in the first position, configured to limit movement of the mirror element in the second direction after the mirror reaches the first position traveling in the second direction. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG. 1  is a schematical illustration of an example environment of an example mirror array spatial light modulator in accordance with an example embodiment; 
       FIG. 2  is a schematic crossectional view of a portion of a spatial light modulator similar to that shown in  FIG. 1 , the relative orientation of modulated light being reversed however, and a second position of the mirror elements being shown in outline; 
       FIG. 3  is a perspective schematic illustration of a broken-out portion of a mirror array of a spatial light modulator in accordance with an example embodiment of the invention such as that shown in  FIGS. 1 and 2 ; 
       FIG. 4  is a schematic crossectional view, taken along line  4 — 4  in  FIG. 3  of a portion of the array shown in  FIG. 3 ; 
       FIG. 5  is a perspective schematical illustration of a broken-out portion of a mirror array of a spatial light modulator in accordance with an example embodiment of the invention, a second position of a mirror element being shown in outline; 
       FIG. 6  is a perspective schematical illustration of a broken-out portion of a mirror array of a spatial light modulator in accordance with an example embodiment of the invention, a configuration of the stop element in a further example embodiment being shown in outline; 
       FIG. 7  is a perspective schematical illustration of a broken-out portion of a mirror array of a spatial light modulator in accordance with another example embodiment of the invention. 
       FIGS. 8–26  are process flow diagrams illustrating an example method of making a spatial light modulator such as that shown in  FIGS. 1–5 , wherein: 
       FIG. 8  is a schematic crossectional view of a glass substrate with a resistor formed thereon in a first step; 
       FIG. 9  is a schematic crossectional view of a glass substrate and resistor and a dielectric layer with openings therein formed thereon in a second step; 
       FIG. 10  is a schematic crossectional view of a glass substrate, resistor, dielectric layer with an ITO conductor layer forming an electrode and conductive pathway through the resistor formed thereon in a third step; 
       FIG. 11  is a schematic crossectional view of a glass substrate, resistor, dielectric layer with an ITO conductor layer forming an electrode, and conductive pathway through the resistor to a post location, with a further dielectric layer formed thereon in a forth step; 
       FIG. 12  is a schematic crossectional view of the example shown in  FIG. 11  with a post formed thereon in contact with the conductor previously formed; 
       FIG. 13  is a schematic crossectional view of the example shown in  FIG. 12  with a SiN diffusion barrier laid down over the structure; 
       FIG. 14  is a schematic crossectional view of the example shown in  FIG. 13  with a first release layer of Si deposited thereon; 
       FIG. 15  is a schematic crossectional view of the example shown in  FIG. 14  after a CMP process forming a planar surface and exposing a top portion of the post; 
       FIG. 16  is a schematic crossectional view of the example shown in  FIG. 15  after deposition of a further SiN diffusion barrier over the planar surface; 
       FIG. 17  is a schematic crossectional view of the example shown in  FIG. 16  with a contact opening at the top of the post etched into said further diffusion barrier; 
       FIG. 18  is a schematic crossectional view of the example shown in  FIG. 17 , after deposition of an Al alloy metal layer for the mirror; 
       FIG. 19  is a schematic crossectional view of the example shown in  FIG. 18 , with the mirror subsequently photo-etched, and then another diffusion barrier of SiN laid down overtop the structure; 
       FIG. 20  is a schematic crossectional view of the example shown in  FIG. 19  after deposition of a second Si release layer; 
       FIG. 21  is a schematic crossectional view of the example shown in  FIG. 20  with an opening etched therein over the post and a mirror hinge portion for attachment of a stop portion to be deposited in a subsequent step; 
       FIG. 22  is a schematic crossectional view of the example shown in  FIG. 21  with another SiN diffusion barrier laid down over the structure; 
       FIG. 23  is a schematic crossectional view of the example shown in  FIG. 22  after deposition of an Al metal layer over the structure, from which the stop for each mirror element will be formed; 
       FIG. 24  is a schematic crossectional view of the example shown in  FIG. 23  after photo etching of the metal layer to form the stop portions; 
       FIG. 25  is a schematic crossectional view of the example shown in  FIG. 24  after etching of the Si release layers to free the mirrors; and 
       FIG. 26  is a schematic crossectional view of the example shown in  FIG. 25  illustrating deflection of the mirrors by deformation of hinge portions thereof. 
   

   DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT(S) 
   With reference to  FIGS. 1 and 2 , a spatial light modulator (SLM)  10  includes a multiplicity of actuatable metal (e.g. Al or AlCu) mirror elements  11  each having a mirror portion  12  of about 10×10 μm area and 0.25 μm thickness. Each mirror element is supported by a conductive pedestal or post  14  adjacent a transparent substrate  16 , for example formed of a glass. The substrate has other transparent structure formed thereon, including an electrode  18  of 10×9 μm area associated with the mirror element and positioned about 1.25 μm ahead of it with respect to a front side  17  of the modulator. The electrode can be formed of Indium-Tin-Oxide (ITO) in one example embodiment. A dielectric layer  19 , which in the illustrated embodiment is a 0.1 μm layer of SiO 2 , overlays the electrode. Each mirror element includes a plurality of hinge portions  20  defined by the outer limits of the mirror element adjacent an opening  22 . 
   When positioned in a vacuum environment within a cathode ray tube (CRT)  21 , the mirror elements  11  are individually addressable by a controlled and focused electron beam  23 , for example one emitted from a cathode  25 . The beam can be turned on and off, and can be conventionally field controlled (e.g. by a controllable magnetic field generator  27 ) to scan through the array of mirrors provided by the modulator  10 , turning each one on or leaving it off as will be described below. For example, the mirror array can be scanned once each frame of a frame-refreshed graphic or video signal. Each mirror element is either charged or left uncharged by the beam in the scan for the frame. The charged mirror elements  11 ′ leave a first un-deflected position, and deflect toward each of the electrodes  18  associated therewith to a second position illustrated in outline. In the illustrated example this second or deflected position ( 11 ′) corresponds to an “on” state of the mirror  12  for the frame, while the first, or an undeflected position corresponds with an “off” state. Light  29  to be modulated is sent to the SLM  10 , for example once each frame, after the scan by the electron beam  23  has completed, and the mirrors have been thus set for the frame. With respect to each mirror  12  in the SLM array, the light  29  is either reflected in a first direction  31  corresponding with an “on” state of each mirror, or in a second direction  32  corresponding with an “off” state of each mirror, depending on the position of each mirror. Thus a pixilated image can be formed in the reflected light traveling along the directions  31 ,  32 , one being a negative and one being a positive image with respect to each other. Either of the modulated light streams can be further processed in the application to which the modulator is being put. It will be appreciated that while a graphics and video display application has been alluded to, and will be further discussed herein, others are contemplated. 
   In one embodiment, the mirrors  12  of the modulator  10  are reset between each frame (scan), so as to be at a first or “off” position unless charged by electrons from the electron beam  23  in the next scan for the next frame. Furthermore, each mirror element  11  has associated with it at least one stop element  24  configured to arrest and damp the motion of the mirror element, and thus to overlap at least a portion of the mirror element. That portion can be very small, just catching an edge of the mirror element, or can overlap a considerable distance, so that it extends well into the interior of an outer surface of the mirror element, for example 10 to 50 percent of a dimensional distance across the mirror element. It can extend even longer in other embodiments. The size and configuration of the stop is chosen so that a mass and length and width of the stop (affecting its performance in its function as a stop) are balanced against the goal of placement and sizing of the stop for minimizing its interception of electrons from the electron beam. The stop can be formed of the same material as the mirror element, or of a different material. For example in another example embodiment the mirror can be formed of the material mentioned above or another metal, and the stop can be formed of a non-conductive material. In this latter example interception of electrons from the electron beam is less of a concern, but ductility and fatigue life of the material chosen for the stop, as compared to a metal, remain a concern. 
   The stop  24  provides a limitation on movement of the mirror portion  12 , preventing movement of the mirror portion in a direction away from the electrode  18  after reaching the first, or un-deflected nominal charge position, which is vertical in the figures, from a second, or deflected, position inclined against the dielectric layer  19  over the electrode  18  (shown by deflected elements  11 ′ in the figures) resulting from an increase in charge differential between the mirror element and the electrode. The second position corresponds with the “on” state mentioned, where the mirror is angled because the mirror element is drawn toward the electrode by electrostatic forces generated by an increase in charge caused by interception of electrons from the electron beam in a scan through the mirror array of the SLM  10 . 
   Since the micro-mirror elements  11  in operation are positioned within a vacuum environment in the CRT  21 , they move easily between the first and second positions. As mentioned, the stop element  24  functions to arrest motion of the mirror element in the second direction and it acts to absorb energy therefrom; as the stop element deflects in response to being struck by the mirror element. This prevents the mirror element from moving substantially past the first position and damps rebound of the mirror element, reducing the time for it to settle into the first position. This can be important in many applications as will be further discussed below. With reference to  FIGS. 3 and 4 , a better appreciation of the overall structure of the SLM  10  in the illustrated example embodiment can be had with reference to a broken-out 16 -pixel portion of the modulator, illustrated showing a back side ( 33  in  FIG. 2 ) of the SLM. The mirror portions ( 12  in  FIG. 2 ) face downward and away, while the mirror elements  11  are seen as they would present to the focused electron beam ( 23  in  FIG. 1 ). It will be appreciated that each of the mirror elements is confined to the area between the stops  24  and the dielectric layer  19  over the structure formed on the substrate  16 . Moreover, it will be appreciated that the hinge portion  20  includes structure adjacent the stop  24  for an adjacent mirror element, and the mirror portion ( 12  in  FIG. 2 ) of the mirror element  11 , and is defined by the outer shape aspect of the mirror element, and the opening  22  formed therein, in each instance, in the illustrated example. 
   With reference again to  FIGS. 1 and 2 , in this example embodiment the SLM  10  is positioned so as to present a bottom or front (depending on how it is viewed) surface  17  of the glass substrate  16  outward, while a top, or back, portion  33  of the SLM is available to be scanned by a focused electron beam  23  inside the CRT  21 . In more detail, the electron beam can be made to scan through the array of mirror elements  1  of the SLM at a selected refresh rate. The focused electron beam is itself modulated in at least one of on/off state, dwell time, and intensity, with respect to each mirror element location in the array of the SLM, so that the charge on individual mirror elements is changed in a controlled way. 
   In one embodiment by means of switching on and off the beam as it moves across the individual mirror elements  11  (here analogous to pixels) the focused electron beam  23  can selectively provide an increase of electrostatic charge, or not provide a change in charge, with respect to each mirror element in the array, during each scan cycle. A cycle here can include a single scan through, or a frame, etc. or multiple scans. Multiple scans may be used, for example, in color graphic applications where each mirror element is a sub-pixel, and a set of sub pixels are used for each color and each color set is created by a separate scan for that color (e.g. a three color system of this type can use three scans per frame). 
   It will be appreciated that when charged, a mirror element  11  having a mirror portion  12  moves in the first direction toward, and then contacts, the dielectric layer  19  over the electrode  18  carried by the substrate  16 . This is due to a capacitive effect, which sets up an oppositely charged electron regime in the electrode. The electrostatic forces thus set up cause the movement of the mirror element, bending it about the hinge portion  20  inducing flexure and torsional strain in the hinge portion. This second or deformed position in contact with the dielectric layer  19  of the substrate  16  adjacent the electrode  18  can also be called a charged, or landed position. The mirror element in the example is returned to the first, nominal, or undeflected (more charge neutral) position by bleeding off the charge differential between the mirror element and the electrode. 
   Decreasing the charge differential to move the mirror element back to the first position from the second position can be done in a number of ways. Moreover, the process is not entirely simple and straightforward conceptually, though in the example embodiment the implementation is advantageously quite simple. As will be appreciated by one skilled in the art, part of the process of coming to charge equilibrium can involve a secondary emission of electrodes from the mirror element. As is known, bombardment of the mirror element  11  by a focused beam  23  will set up a secondary emission of electrons (not shown) from the mirror element, as not all the electrons will be retained in the material and the acceptance of additional electrons excites the atoms of the material, causing other electrons to be ejected (secondary emissions). Likewise, capacitive effects will cause a change in charge on the electrode  18 , which can also involve secondary emissions of electrons. 
   Moreover, as the charge differential between the mirror element  11  and the electrode  18  is dissipated (or in other words is brought toward parity—though of opposite polarity—at a charge level below what is needed to overcome the restoring force in the hinge  20 ), secondary emissions can occur. The charge differential, and thereby the electrostatic forces, acting on the mirror element to actuate it can be controlled by controlling the amount of charge, and the flow of charge. This is done in the illustrated examples by means of: a) the electron beam; b) a controlled conductive pathway, as discussed below in more detail, between the mirror element and the electrode. The controlled conductive pathway in the illustrated embodiments herein may be a simple resistive pathway configured to bleed charge differential off at a known selected rate, thus allowing a material-dependant restoring force to move the mirror element to the first position from the deflected second position. Other methods (not shown) can involve switching devices, and one or more connectable ground potentials (not shown). For example, possible methods involve use of a fixed resistor (e.g.  34  in  FIG. 2 ), a variable resistor (not shown), or an active element such as a transistor, photo diode, or photo transistor (all conventional, not shown) in the conductive pathway(s) in controlling charge bleed off from the electrode  18  and/or mirror element  11  of the device  10 , either relative to each other, or to said other connectable ground(s). In the illustrated embodiment the electron beam can move the mirror in the first direction by increasing charge differential, and then the particular method chosen (for example from the examples just mentioned) to move the mirror back in the second direction does so by decreasing charge differential and thus allowing other forces, such as spring (restoring) forces in the hinge portion  20  of the mirror element, and/or other forces, to overcome electrostatic forces acting in the first direction and move the mirror element back. 
   Returning to the illustrated example shown in  FIGS. 1 through 4  in further detail, as the electrostatic charge difference is reduced as discussed above, and the electrostatic force acting to bias the mirror element  11  in a first direction toward the electrode  18  is reduced, a restoring force set up by the deformative strains (flexural and torsional) in the hinge portion  20  can return the mirror portion to the first, or un-deformed position. Thus it moves a second direction from the second position landed against the dielectric layer  19  to the first position as discussed. Moreover, the SLM  10 , once initially scanned through by the electron beam  23 , and the charge dissipated, can have a charge equilibrium state where the mirror elements are located at the first position. Therefore afterward, as to each mirror after each scan, as the charge differential dissipates, the mirror will return to the first position due to dissipation of electrostatic forces as well as mechanical restoring forces in the material of the hinge portions  20 . 
   However, in another embodiment the mirror element  11  or the electrode  18  in each case can be connected to a source of potential so that relative to the electrode the mirror element is initially nominally charged in the same polarity so to have a slight repulsion from the electrode. In this manner the mirror is biased towards the first position by electrostatic forces, and dissipation of electron beam  23 —added charge can fully occur only after the mirror element has reached its first position returning in the second direction. In one example embodiment, the electrostatic forces can predominate over mechanical restorative forces in moving the mirror from the second back to the first position. In another example embodiment the mechanical forces can predominate over the electrostatic forces in moving the mirror back to the first position. 
   Providing the stop  24 , and thus limiting movement in the second direction once the mirror reaches the first position, reduces oscillation (“ringing”) of the mirror element  11  that may occur in returning to the first or undeformed position. As will be appreciated, the stop prevents the mirror portion moving onward in the second direction past the first position and continuing until a restoring force in the hinge builds up, or charge changes by current flow and secondary emissions accumulate to the point where momentum is checked, then these forces move the mirror back in the first direction past the first or charge-neutral (undeformed) position onward again in the first direction until checked by the building restoring forces, electrostatic forces, etc. in the opposite direction; and then moving back in the second direction, etc., until the oscillation just described dies down, for example due to conversion of momentum to heat energy. The conversion of momentum to heat in the hinge and surrounding air, and regulation of the charge bleed off (and thus the electrostatic force) by regulation of the charge on the electrode are techniques usable to damp the mirror ringing phenomenon without provision of the stop, but the resulting oscillations use time in the frame-refresh cycle, and use up fatigue life of the hinge. Neither the extra time needed to settle the oscillations, nor the extra repeated bending of the hinge, are desirable attributes in a typical application of the SLM  10 . 
   The stop  24  mechanically prevents continued movement of the mirror element  11  in the second direction past the first or un-deformed position. Moreover, impact with, and rebounding from, the stop further dissipates momentum by conversion of impact-imparted energy to heat in the stop and pedestal  14  elements, and a bouncing movement of the mirror element  11  off the stop imparts energy to the surrounding air molecules with less overall travel of the mirror. 
   As will now be more readily appreciated, without the stop a controlled alteration of the rate of charge bleed off (requiring additional complexity in control of charge differential at the electrode  18  and/or mirror element  11 ) and momentum-heat conversion, may be the sole mechanism for damping mirror oscillations. This mirror oscillation is not desirable, particularly in video display applications, as without the stop  24  the “ringing” phenomenon can limit refresh rates in the scanned array, as the time required to settle each mirror  12  into its first, or charge neutral position, prior to the next focused electron beam  23  scan-through can force a delay to when the next scan-through of the array can occur. This delay is undesirable, potentially lowering the frame refresh rate in a video application to unacceptably low levels affecting perceived quality of the presentation of the video program material. It has been found that practical limitations on control of a charge decay function can potentially limit the usefulness of this kind of modulator in display applications due to the need for high cycling rates in video program material display. Hence the stop element  24  can be very useful in increasing the speed of the SLM device  10  so as to be usable in such applications. 
   Moreover, as alluded to, by reducing the amount of ringing oscillation movement of the mirror element  11  carrying the mirror portion  12 , flexural and torsional movement in the hinge areas  20  is very much reduced. Thus, a longer period of time will elapse before a fatigue limit of the metal material in the hinges is reached. This can improve reliability in the device  10  by delaying fatigue failures in the hinges of the mirror elements of the array. Beneficially, it gives the SLM  10  device a longer service life. 
   Returning to the discussion of actuation of the mirror elements  11  in detail, the control of charge on the mirror element in the particular example illustrated in  FIGS. 1–5  first involves controlling a ratio of primary electrons added to each mirror element  11  by the beam  23 , and the number of secondary electrons (not shown) emitted from each mirror as a result of interception of the primary electrons as mentioned above. A more detailed discussion of this can be found in commonly assigned and co-pending U.S. application Ser. No. 10/743,603, filed Dec. 21, 2003, which is hereby incorporated herein by reference for the teachings consistent herewith. The rate of change in the charge on the mirror element can control the timing aspects of change in position of the mirror element and the length of time it remains at a deformed position before returning to the un-deformed or nominal position. 
   In the example embodiment the mirror element  11  can be returned to the first position by dissipation of the accumulated charge through a conductive pathway at a known constant rate. This is done in the illustrated example by bleeding off charge through a resistor  34  ( FIG. 2 ) connecting the electrode  18  to the mirror element  11  via the post  14  that is formed of a metal, or another conductive material. As will be appreciated, in other embodiments (not shown) the charge can be otherwise dissipated, but the illustrated example is simple and straightforward in that each of the mirror elements will return to the undeformed first position (shown by reference  11 ) from the second or deformed position (shown by reference  11 ′) within a period of time known from the relationship of the magnitude of the capacitive charge (given by the ratio mentioned) on the mirror element with respect to the electrode, and the resistance of the resistor  34  and conductive pathway. 
   Thus, as to each mirror element  11 , light  29  applied in the form of a flood pulse or flash, will be reflected by the modulator at each array element, or pixel, in one of at least two directions corresponding with mirror  12  position (angle) as set forth above. In the illustrated embodiment one reflected light direction  31  corresponds with a second position of the mirror element  11 ′ or “on” as light continues in a path from direction  29  through the substrate  16  and electrode  18  and dielectric layer  19 , all of which are formed of transparent material at this scale, and reflects from the mirror  12  and back out through that structure along the direction  31  to be further processed depending on the application. For mirror elements  11  in the nominal or “off” position the light is reflected along the other direction  32 , and as mentioned, and this creates a negative image of the modulated light sent in the other direction  31 . 
   In this way light  29  is modulated, and, moreover, in a graphical application example where still or motion-picture imagery will be the end result, then in accordance with known principles the timing of the modulation can be coordinated with directing the light,  31  and/or  32 , through filters, or changing the wavelength (color) of the light, etc. to provide a color image. This can be done using multiple mirror elements  11  per pixel (each as a subpixel of a color) or resetting and flashing the SLM  10  multiple times per frame (sub-frames for each of several colors) or by using other known ways of providing a color image using a two-state modulator. Moreover, as will be appreciated, even in the example where two positions (only) are allowed for the mirror  12 , the length of time light is reflected to the “on” direction ( 31  in one example) can be used to vary a parameter of the modulated light, for example the amount of light perceived at that pixel (or sub pixel). Thus a grey scale in the array can be created in a frame by the length of time the mirror remains at a location corresponding to an “on” state and is flooded by light  29 . This time can be controlled by the amount of electrons applied to a particular “pixel” (mirror), vs. secondary electron radiation, as mentioned, as the bleed off rate through the resistor  34  is a known function of time and charge differential across the resistor. 
   As will now be appreciated, these properties can be used to create black and white and color images (not shown) for video, still, graphics slide, and other format, information display in one example application. Moreover, the properties can be used in telecommunications switching of individual light beams directed at each mirror location in another example application (not shown), for spatial control of light in holography in another example (not shown), and read/write functionality in holographic memory applications in another example (not shown), and in other applications not included in these examples. 
   Moreover, in another example, the SLM ( 10  in  FIG. 1 ) is reversed within the CRT  21 , so that the electrodes  18  instead of the mirrors  12  intercept the electron beam, and light does not travel through the substrate etc. but is reflected from a “back” side of the mirror elements  11  as discussed in the foregoing illustrated examples. In this example the change in charge is on the electrode, but the device in this example actuates the mirror element by capacitive storage of charge or release of capacitive charge, depending on the relative charge at the electrode plate and the mirror element. 
   Moreover, in another example embodiment the SLM can remain in the same position but light to be modulated enters and leaves the CRT through sidewalls of the tube from the rear, and is thus reflected by the “back” side of the mirror elements, the same side that intercepts the electron beam but in each case otherwise these examples function as described herein. 
   Also, in another example, the charge can be released through another pathway. For example all the electrodes  18 , or alternatively all the mirror elements  11 , can be connected to a common potential which is controllable, or each element connected to one each of two grounds, one ground for the electrodes one for the mirror elements. Moreover, with other configurations the mirror elements can be moved or not moved in successive scans. That charge bleed-off discussed herein, or otherwise configuring the device for controlling the charge between the mirror and the electrode, and thus the mirror position, can be accomplished in other ways will be apparent to those skilled in the art. 
   In another example embodiment, all of the mirror elements can be returned to a default position between each scan, for example returning to the second position ( 11 ′ mirror position in  FIG. 2 ) by a flood electron beam from a flood electron source (not shown) adjacent the focused electron beam  23  cathode  25  source. For example, one scan can be used to charge (set) the array and one to clear it; or one scan to set and one electron flood to clear alternating; or successive scans refreshing the charge holding the mirror or changing the charge so as to change the mirror position as to each mirror successively as the beam proceeds through each and every scan can be used. (It will be appreciated that the positive and negative “images” carried by the light streams ( 31 ,  32  in  FIGS. 1 and 2 ) can be reversed where the second position is made the one the mirrors are made to return to when re-setting the array). These are just some of the ways of controlling the mirror elements that can be given by way of example using the principle that the mirror element will move from the second to the first position after a known time due to charge bleed-off. 
   However, in the illustrated embodiment there is simply a desire to have the mirror elements  11  be able to be brought to the undeformed or first position and remain there until needed to be moved again. Vice versa, it would be desirable for the second position to be a stable and repeatable position in the illustrated embodiment. The later can be accomplished inherently, as the mirror is stopped by the dielectric layer  19  overtop the electrode, and accordingly the angle of the mirror  12  in the second position is set by the height of the post  14  and size of the mirror element  11 . However, the stop element  24  is provided adjacent the first position to keep the mirror element from going well past the first position and “ringing” as discussed above. 
   The stop  24  can take other forms besides that shown in  FIGS. 1–5 . For example with reference to  FIG. 6 , in another embodiment a wider stop  26  can be configured as before described, but to be wider and shorter, so that it contacts the mirror over a wide but shallow area, rather than a thinner but deeper area. In another example embodiment the stop  26 ′ (shown in outline) can be configured to be similarly carried by the mirror pedestal, but to overhang both the mirror portion  12  of the mirror element supported by the same pedestal  14  as well as the mirror portion of the adjacent mirror element  11 . The stop thus assists in providing a motion check on movement in the second direction of the mirror supported by the same pedestal, catching it at a proximal edge adjacent the opening, as well as checking movement of the adjacent mirror, catching it at a distal edge in the same way as in the example shown in  FIGS. 1–5 . Thus, the mirror portion of each mirror element would have a stop adjacent both its distal edge and its proximal edge. 
   With reference to  FIG. 7 , in another example embodiment, instead of a stop carried by an adjacent mirror pedestal  14 , each mirror element  11  has associated with it two stops  28  carried by separate pedestals  30 . Likewise, each stop serves two adjacent mirror elements. This example provides redundancy, but is more complex structurally than that shown in  FIGS. 1–6 , for example. 
   It will be appreciated that other configurations can be employed, but these examples will serve to illustrate the principles involved in providing the functionality discussed herein. 
   The examples discussed above can be made in a number of conventional ways. With reference now to  FIGS. 8–26 , the following description outlines, and the figures illustrate, one process for making the modulator example shown in  FIGS. 1–5 . Fabrication of one mirror element ( 11  in  FIGS. 1–5 ) and associated structure will be described. It will be appreciated that this is repeated for each element by appropriate patterning in the fabrication technique used, so as to lay down/form all the elements, and associated structure, including stops, and current pathways, etc. for the entire SLM ( 10  in  FIGS. 1–5 ). 
   First, with reference to  FIG. 8 , this example fabrication process for the modulator ( 10  in  FIGS. 1–5 ) can include the step of making a resistor  103  ( 34  in  FIGS. 1–5 ) for each mirror. It is through this resistor that the charge will be dissipated at a predetermined rate, so that the mirror ( 12  in  FIGS. 1–5 ) will remain in the tilted position for a predictable period of time based on the charge differential applied, as discussed above. For this period of time to be predictable, the resistor fabrication must be consistent and controlled. A light-transmissive material (such as glass or quartz) substrate  101  ( 16  in  FIGS. 1–5 ) has sheet resistor  103  material deposited on it by a conventional method, including direct deposition methods (e.g. printing methods) and layer deposition and photo/etch methods. The resistor material thus deposited and formed is chosen and configured so as to cooperate with the mirror element  11  and electrode  18  to give a desired time delay range for each mirror element before movement from the second to the first position subsequent to a scan at an energy level within a selected beam energy range. 
   Next, with reference to  FIG. 9 , an insulating layer  105  of a transparent dielectric material, such as Tetra-Ethyl-Ortho-Silicate (TEOS) is then laid down over the substrate and the resistor  103 , with contact openings  107   a ,  107   b  formed therein to accommodate a current pathway to the resistor. These contact openings are approximately the same width as the thickness of the TEOS layer. This TEOS layer formation is by a conventional technique, such as a deposition/photo/etch technique. Referring to  FIG. 10 , a transparent conductive layer  108  of ITO is then conventionally deposited and photo/etch shaped to form the electrode  109  ( 18  in  FIGS. 1–5 ) and a conductor  111  connecting the electrode to the resistor  103  and a conductor  113  which will connect the resistor to the post ( 14  in  FIGS. 1–5 ) and thereby to the mirror element ( 11  in  FIGS. 1–5 ). Turning to  FIG. 11 , another insulating layer  115  of TEOS is then deposition/photo/etch formed (or otherwise formed by conventional methods) over the previous ITO layer, leaving an opening  117  for the post to directly contact the conductor  113  extending from the resistor  103  This insulating layer corresponds to the dielectric layer ( 19  in  FIGS. 1–5 ) discussed above. 
   With reference to  FIG. 12 , metal post material  119 , for example Aluminum or an alloy thereof, is formed into the pedestal, or post ( 14  in  FIGS. 1–5 ) that will support the mirror element ( 11  in  FIGS. 1–5 ). This is done by conventional deposition-photo/etching techniques. The base of the metal post (pedestal) is located over, and in contact with, the ITO conductor  113  coming from the resistor  103 . 
   Next, as shown in  FIG. 13 , a diffusion barrier  121  formed of Silicon Nitride (SiN) is then deposited over the structure. Then, with reference to  FIG. 14 , a release layer  123  having a thickness slightly less in magnitude than the height of the post  121  is deposited over the structure. The structure is then planarized, using a chemical-mechanical planarization (CMP) technique, or other conventional planarization method resulting in the structure shown in  FIG. 15 . As shown in  FIG. 16 , this is then over-coated with a further diffusion layer  125  of SiN. This is then photo-etched to expose the top of the metal post  119  as illustrated by  FIG. 17 . With reference to  FIG. 18  a metal layer  127 , which can be Al or an alloy thereof is laid down. In the example this material is AlCu alloy and comprises the mirror ( 12  in  FIGS. 1–5 ) material in this example. This metal layer is deposited using conventional techniques, such as vapor deposition, sputtering, etc. 
   With reference to  FIG. 19 , a mirror element  129  ( 11  in  FIGS. 1–5 ) of AlCu is photo/etch-formed into the shape discussed above; including the hole  130  ( 22  in  FIGS. 1–5 ) adjacent the post  119 , which hole helps define the hinge portions ( 20  in  FIGS. 1–5 ) of the mirror element  129 . Another diffusion barrier layer  133  of SiN is then deposited over the mirror element using conventional techniques. With reference to  FIG. 20  a second Si release layer  135  is then laid down over the structure. Now, as shown in  FIG. 21 , the second release layer  135  previously laid down is photo-etched over the top of the pedestal (post)  119  to form a hole  137  to expose the mirror element there. With reference to  FIG. 22 , another diffusion barrier layer  139  of SiN is now deposited over the structure. A layer  141  of metal such as Al or an aluminum alloy is then conventionally deposited as shown in  FIG. 23 , and as shown in  FIG. 24  this layer is photo-etched to form the stop element  143  ( 24  in  FIGS. 1–5 ) atop the mirror element  129  over the post  119 . With reference to  FIG. 25  the Si release layers  123 ,  135  are then etched away to free the mirror element  129  at each location of the array. As seen in  FIG. 26 , they are then able to deflect as described above. 
   With reference to all the drawing figures, it will be appreciated that the invention enables a SLM  10  which can be made at relatively low cost and which can be more reliable and provide higher refresh rates. Moreover, the simplicity of mechanical containment of the mirror element to a prescribed range of movement gives advantages described with relatively little increase in complexity or cost. 
   However, while the forgoing descriptions of example embodiments illustrated and described herein give aid in understanding some of the ways in which the invention can be implemented, it should be borne in mind that various modifications can be made consistent with, and within the scope of, the invention set forth in the appended claims; and the examples described above are not to be construed as limiting of the scope of the invention, which is set forth in the appended claims.