Abstract:
A MEMS optical display system includes an illumination source for providing illumination light, a collimating lens for receiving the illumination light and forming from it collimated illumination light, and a microlens array having an array of lenslets for receiving the illumination light from the collimating lens. The converging microlens array directs the illumination light through an array of pixel apertures in an aperture plate to a microelectrical mechanical reflector array positioned opposite the aperture plate. The microelectrical mechanical reflector array includes an array of microelectrical mechanical actuators that support reflectors in alignment with the array of pixel apertures and selectively orients the reflectors to direct the illumination light back through the pixel apertures (to form part of a display image) or against the aperture plate (to be blocked). The illumination light passing back through the pixel apertures passes through the microlens array and a beamsplitter to a display screen.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and is a continuation of application Ser. No. 10/959,496, filed Oct. 5, 2004, which is a divisional of application Ser. No. 10/086,397, filed Mar. 1, 2002, all of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to optical display systems and, in particular, to a reflective display system that employs a microelectrical mechanical system (MEMS) optical modulator.  
       BACKGROUND AND SUMMARY OF THE INVENTION  
       [0003]     Flat panel optical display systems, such as liquid crystal displays, are well known and widely used. Many such displays (e.g., liquid crystal displays) require polarized illumination light. Typically, polarization of illumination light greatly attenuates the light, thereby resulting in displays with decreased brightness, or require relatively expensive optical components. Moreover, such displays commonly have relatively low contrast ratios, which decreases image clarity and overall image quality. Furthermore, such displays typically require complex or difficult manufacturing processes.  
         [0004]     To address such shortcomings, the present invention includes a microelectrical mechanical optical display system that employs microelectrical mechanical system (MEMS) actuators to modulate light. As is known in the art, MEMS actuators provide control of very small components that are formed on semiconductor substrates by conventional semiconductor (e.g., CMOS) fabrication processes. MEMS systems and actuators are sometimes referred to as micromachined systems-on-a-chip.  
         [0005]     In one implementation, a MEMS optical display system according to the present invention includes an illumination source for providing illumination light, a collimating lens for receiving the illumination light and forming from it collimated illumination light, and a microlens array having an array of lenslets and receiving the illumination light from the collimating lens. The converging microlens array directs the illumination light an array of pixel apertures in an aperture plate to a microelectrical mechanical reflector array positioned opposite the aperture plate.  
         [0006]     The microelectrical mechanical reflector array includes an array of microelectrical mechanical actuators that support reflectors in alignment with the array of pixel apertures. The array of microelectrical mechanical actuators orient the reflectors selectively to direct the illumination light back through the pixel apertures (to form part of a display image) or against the aperture plate (to be blocked). The illumination light passing back through the pixel apertures pass through the microlens array and a beamsplitter to a display screen.  
         [0007]     A MEMS optical display system according to the present invention is operable without polarized illumination light, thereby eliminating the light attenuation or expense of the polarizing illumination light. In addition, light can be completely blocked or modulated by cooperation between the MEMS reflectors and the aperture plate, thereby providing display images with very high contrast ratios. Furthermore, such MEMS actuators can be manufactured by conventional CMOS circuit manufacturing processes.  
         [0008]     Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIGS. 1-15  are cross-section views of a general multi-user MEMS process known in the prior art for fabricating microelectrical mechanical devices. Cross-hatching is omitted to improve clarity of the prior art structure and process depicted.  
         [0010]      FIG. 16  an implementation of a microelectrical mechanical (MEMS) optical display system according to the present invention.  
         [0011]      FIG. 17  is another implementation of a microelectrical mechanical (MEMS) optical display system according to the present invention.  
         [0012]      FIGS. 18 and 19  are schematic side views of an exemplary MEMS actuator in respective activated and relaxed states for controlling a MEMS reflector.  
         [0013]      FIG. 20  is a plan view of a MEMS actuator used in optical display system of  FIG. 16 .  
         [0014]      FIGS. 21 and 22  are side views of the MEMS actuator of  FIG. 19  in respective activated and relaxed states.  
         [0015]      FIG. 23  is a schematic diagram of a 2×2 array of actuators having a storage or memory capability.  
         [0016]      FIG. 24  is a schematic diagram of a 50×50 array of actuators having a storage or memory capability.  
         [0017]      FIG. 25  is a flow diagram of a row-sequential addressing method.  
         [0018]      FIG. 26  is a graph illustrating hysteresis characteristics of a MEMS actuator with respect to applied voltage differentials.  
         [0019]      FIG. 27  is a schematic sectional side view of a mirror portion of a MEMS actuator having a composite structure.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]     To assist with understanding the present invention, the general procedure for fabricating micromechanical devices using the MUMPs process is explained with reference to  FIGS. 1-15 .  
         [0021]     The MUMPs process provides three-layers of conformal polysilicon that are etched to create a desired physical structure. The first layer, designated POLY  0 , is coupled to a supporting wafer, and the second and third layers, POLY  1  and POLY  2 , respectively, are mechanical layers that can be separated from underlying structure by the use of sacrificial layers that separate layers and are removed during the process.  
         [0022]     The accompanying figures show a general process for building a micro-motor as provided by the MEMS Technology Applications Center, 3021 Cornwallis Road, Research Triangle Park, N.C.  
         [0023]     The MUMPs process begins with a 100 mm n-type silicon wafer  10 . The wafer surface is heavily doped with phosphorus in a standard diffusion furnace using POCI  3  as the dopant source. This reduces charge feed-through to the silicon from electrostatic devices subsequently mounted on the wafer. Next, a 600 nm low-stress Low Pressure Chemical Vapor Deposition (LPCVD) silicon nitride layer  12  is deposited on the silicon as an electrical isolation layer. The silicon wafer and silicon nitride layer form a substrate.  
         [0024]     Next, a 500 nm LPCVD polysilicon film—POLY  0   14 —is deposited onto the substrate. The POLY  0  layer  14  is then patterned by photolithography; a process that includes coating the POLY  0  layer with a photoresist  16 , exposing the photoresist with a mask (not shown) and developing the exposed photoresist to create the desired etch mask for subsequent pattern transfer into the POLY  0  layer ( FIG. 2 ). After patterning the photoresist, the POLY  0  layer  14  is etched in a Reactive Ion Etch (RIE) system ( FIG. 3 ).  
         [0025]     With reference to  FIG. 4 , a 2.0 μm phosphosilicate glass (PSG) sacrificial layer  18  is deposited by LPCVD onto the POLY  0  layer  14  and exposed portions of the nitride layer  102 . This PSG layer, referred to herein as a First Oxide, is removed at the end of the process to free the first mechanical layer of polysilicon, POLY  1  (described below) from its underlying structure; namely, POLY  0  and the silicon nitride layers. This sacrificial layer is lithographically patterned with a DIMPLES mask to form dimples  20  in the First Oxide layer by RIE ( FIG. 5 ) at a depth of 750 nm. The wafer is then patterned with a third mask layer, ANCHOR 1 , and etched ( FIG. 6 ) to provide anchor holes  22  that extend through the First Oxide layer to the POLY  0  layer. The ANCHOR  1  holes will be filled in the next step by the POLY  1  layer  24 .  
         [0026]     After the ANCHOR 1  etch, the first structural layer of polysilicon (POLY  1 )  24  is deposited at a thickness of 2.0 μm. A thin 200 nm PSG layer  26  is then deposited over the POLY  1  layer  24  and the wafer is annealed ( FIG. 7 ) to dope the POLY  1  layer with phosphorus from the PSG layers. The anneal also reduces stresses in the POLY  1  layer. The POLY  1  and PSG masking layers  24 ,  26  are lithographically patterned to form the structure of the POLY  1  layer. After etching the POLY  1  layer ( FIG. 8 ), the photoresist is stripped and the remaining oxide mask is removed by RIE.  
         [0027]     After the POLY  1  layer  24  is etched, a second PSG layer (hereinafter “Second Oxide”)  28  is deposited ( FIG. 9 ). The Second Oxide is patterned using two different etch masks with different objectives.  
         [0028]     First, a POLY 1 _POLY 2 _VIA etch (depicted at  30 ) provides for etch holes in the Second Oxide down to the POLY  1  layer  24 . This etch provide a mechanical and electrical connection between the POLY  1  layer and a subsequent POLY  2  layer. The POLY 1 _POLY 2 _VIA layer is lithographically patterned and etched by RIE ( FIG. 10 ).  
         [0029]     Second, an ANCHOR 2  etch (depicted at  32 ) is provided to etch both the First and Second Oxide layers  18 ,  28  and POLY  1  layer  24  in one step ( FIG. 11 ). For the ANCHOR 2  etch, the Second Oxide layer is lithographically patterned and etched by RIE in the same way as the POLY 1 _POLY 2 _VIA etch.  FIG. 11  shows the wafer cross section after both POLY 1 _POLY 2 _VIA and ANCHOR 2  etches have been completed.  
         [0030]     A second structural layer, POLY  2 ,  34  is then deposited at a thickness of 1.5 μm, followed by a deposition of 200 nm of PSG. The wafer is then annealed to dope the POLY  2  layer and reduce its residual film stresses. Next, the POLY  2  layer is lithographically patterned with a seventh mask and the PSG and POLY  2  layers are etched by RIE. The photoresist can then be stripped and the masking oxide is removed ( FIG. 13 ).  
         [0031]     The final deposited layer in the MUMPs process is a 0.5 μm metal layer  126  that provides for probing, bonding, electrical routing and highly reflective mirror surfaces. The wafer is patterned lithographically with the eighth mask and the metal is deposited and patterned using a lift-off technique. The final, unreleased exemplary structure is shown in  FIG. 14 .  
         [0032]     Lastly, the wafers undergo sacrificial release and test using known methods.  FIG. 15  shows the device after the sacrificial oxides have been released.  
         [0033]     In preferred embodiments, the device of the present invention is fabricated by the MUMPs process in accordance with the steps described above. However, the device of the present invention does not employ the specific masks shown in the general process of  FIGS. 1-15 , but rather employs masks specific to the structure of the present invention. Also, the steps described above for the MUMPs process may change as dictated by the MEMS Technology Applications Center. The fabrication process is not a part of the present invention and is only one of several processes that can be used to make the present invention.  
         [0034]      FIG. 16  is a diagrammatic side view of a microelectrical mechanical structure (MEMS) optical display system  50  according to the present invention. Display system  50  includes a light source  52  and reflector  54  that direct illumination light to a condenser lens  58 . A beam splitter  60  receives the illumination light from condenser lens  58  and reflects the light toward a microlens array  62  having a two-dimensional array of lenslets  64  (only one dimension shown). Lenslets  64  of microlens array  62  receive the illumination light and focus it through apertures  66  in an aperture plate  68  toward a microelectrical mechanical structure (MEMS) reflective modulator  70 . Microlens array  62  could be formed as a molded array of plastic lenses or an array of holographic lenses, also referred to as hololenses, or may be an assembled array of conventional glass lenses.  
         [0035]     MEMS reflective modulator  70  has a two-dimensional array of microelectrical mechanical structure (MEMS) reflectors  72  that are positioned opposite corresponding apertures  66  in aperture plate  68 . Each MEMS reflector  72  corresponds to a picture element or pixel and is separately controllable by a display controller  78  to selectively reflect illumination light back through an aperture  66  according to an image control signal, thereby to form a display image. For example, each MEMS reflector  72  would direct light back through its aperture  66  for an amount of time in proportion to the brightness of the corresponding pixel for a given pixel period. Light reflected by MEMS reflectors  72  through apertures  66  passes through lenslets  64  and beam splitter  60  to a rear surface  84  of a transmissive display screen  86  for viewing by an observer  88 . In an alternative implementation, a projecting lens array may be positioned between beam splitter  60  and transmissive display screen  86  to enlarge or reduce the optical field so that it provides a desired image size on transmissive display screen  86 . MEMS reflective modulator  70 , aperture plate  68 , and microlens array  62  may be considered a display engine  90  that may be compactly and efficiently manufactured for a wide range of applications.  
         [0036]     MEMS optical display system  50  has a number of advantages over commonly available liquid crystal displays. For example, MEMS reflective modulator  70  does not require that the illumination light be polarized, in contrast to the typical operation of liquid crystal cells. This eliminates the expense and light attenuation that typically accompanies polarization. Moreover, MEMS reflective modulator  70  can pass unmodulated light with virtually no attenuation, whereas typical liquid crystal cells significantly attenuate light. Similarly, MEMS reflective modulator  70  can provide much higher contrast ratios than liquid crystal cells because light is either losslessly reflected through apertures  66  or completely blocked by aperture plate  68  to provide complete modulation of the light. Finally, MEMS reflective modulator  70  can be manufactured by conventional CMOS circuit techniques without requiring the complex processes typically required for liquid crystal displays.  
         [0037]     In one implementation, for example, MEMS reflective modulator  70  could include a 200×200 array of MEMS reflectors  72  for controlling light passing through a corresponding 200×200 array of apertures  66 . In this implementation, for example, microlens array  62  could include 200×200 lenslets  64  that each have a focal length of about 1 mm, and apertures  66  may be positioned in a right, regular array with separations of about 50 μm between them. MEMS reflective modulator  70  in such an implementation could have dimensions of 1 cm×1 cm. With lenslets  64  of projection microlens array  62  providing magnification of about 2.5, display screen  86  could have dimensions of about 2.5 cm×2.5 cm, or about 1 inch×1 inch.  
         [0038]      FIG. 17  is a diagrammatic side view of a microelectrical mechanical structure (MEMS) optical display system  150  showing one implementation of a polychromatic illumination source  152  and an associated reflector  154 . Components of MEMS optical display system  150  that are generally the same as those of display system  50  are indicated by the same reference numerals.  
         [0039]     Illumination source  152  includes multiple (e.g., three) color component light sources (e.g., lamps)  156 R,  156 G, and  156 B that are positioned generally in a line and generate red, green, and blue light, respectively. A display controller  158  that separately controls MEMS reflectors  72  also activates color component light sources  156 R,  156 G, and  156 B separately. During times that it successively activates color component light sources  156 R,  156 G, and  156 B, display controller  158  applies control signals to MEMS reflectors  72  corresponding to red, green, and blue image components, thereby to form color component images in a field-sequential manner.  
         [0040]     For example, color component images that are generated at a rate of 180 Hz can provide an image frame rate of 60 Hz. In one exemplary implementation, a display of 200×200 multi-color pixels could employ microlens arrays  62  with a 204×204 array of lenslets  64  to compensate for different optical paths taken by different color components of light forming the display gamut. Aperture plate  68  and MEMS reflective modulator  70  would include corresponding arrays of apertures  66  and reflectors  72 , respectively. As an alternative implementation, it will be appreciated that multiple successive colors of illumination could be obtained by a spinning color wheel and a white light source, as is known in the art.  
         [0041]      FIGS. 18 and 19  are schematic side views of an exemplary MEMS actuator  170  in respective activated and relaxed states for controlling MEMS reflector  72 .  FIG. 18  shows MEMS reflector  72  with an orientation behind an associated aperture  66  generally perpendicular to a light propagation direction  172 . In this activated, display ON state, illumination light directed through aperture  66  is reflected by MEMS reflector  72  back through aperture  66  to be included in a display image.  FIG. 19  shows MEMS reflector  72  with an inclined or a tilted orientation relative to light propagation direction  172 . In this relaxed, display OFF state, illumination light directed through aperture  66  is reflected by MEMS reflector  72  toward a solid region of aperture plate  68  to be blocked and excluded from a display image.  
         [0042]      FIG. 20  is a plan view of MEMS actuator  170 , and  FIGS. 21 and 22  are side views of MEMS actuator  170  in its respective activated and relaxed states. MEMS actuator  170  is one of a variety of MEMS actuators that could be used to control MEMS reflector  72 .  
         [0043]     MEMS actuator  170  is an implementation of an electrostatic actuator and includes a structural anchor  174  ( FIGS. 21 and 22 ) that is secured to a substrate  176  (e.g., substrate  10  or nitride layer  12 , not shown). One end  177  of a cantilevered flexible arm  178  is secured to or formed integrally from anchor  174  and extends to a free or floating paddle end  180  that supports MEMS reflector  72  (e.g., formed of gold). Flexible arm  178  includes a semiconductor (e.g., polysilicon) arm base  181  and a residual stress layer  182  of a material other than the semiconductor (e.g., polysilicon) of arm base  181 .  
         [0044]     Residual stress layer  182  is formed of a material (e.g., gold) that is selected to have a coefficient of expansion different from that of the semiconductor (e.g., polysilicon) material of arm base  181 . In the illustrated implementation, residual stress layer  182  is formed on top surface of arm base  181 . The differing thermal coefficients of expansion of arm base  181  and residual stress layer  182  characterize flexible arm  178  as a bimorph.  
         [0045]     Optional flex scores  184  may extend into a top surface of arm base  181  and extend across its length, either part way or completely (the former shown). Flex scores  184  are spaced apart from each other and are generally perpendicular to the length of flexible arm  178 . In one implementation, residual stress layer  182  is formed between optional flex scores  184  when they are present.  
         [0046]     MEMS actuator  170  includes one or more electrostatic activation electrodes  190  that are formed in or on substrate  176  at spaced-apart intervals along and underneath flexible arm  178 . Activation electrodes  190  and flexible arm  178  are electrically connected to respective actuator controllers  192  and  194 . An optional memory or lock electrode  196  is formed under floating paddle end  180  and electrically connected to an optional memory controller  198 .  
         [0047]     In the activated, display ON state illustrated in  FIG. 21 , complementary signals or electrical states are applied by actuator controllers  192  and  194  to respective activation electrodes  190  and flexible arm  178  to impart electrostatic attraction between them. The electrostatic attraction between activation electrodes  190  and flexible arm  178  functions to hold flexible arm  178  generally flat against substrate  176 . Separate activation of optional memory controller  198 , connected to a memory electrode  196 , can then serve to hold flexible arm  178  generally flat against substrate  176  even after the complementary signals provided to activation electrodes  190  and flexible arm  178  are relaxed.  
         [0048]     Stand-off dimples  202  extending from flexible arm  178  toward substrate  176  hold flexible arm  178  in spaced-apart relation to substrate  176  in the activated, display ON state. Dimples  202  contact the electrically insulating (e.g., nitride layer) of substrate  176 . A dimple  202  at the end of paddle end  180  also keeps reflector  72  flat (i.e., parallel to substrate  176 ) in the activated, display ON state, as well as keeping flexible arm  178  spaced apart from memory electrode  196 .  
         [0049]     In the relaxed, display OFF state illustrated in  FIG. 22 , complementary signals or electrical states are not applied by actuator controllers  192  and  194  to respective activation electrodes  190  and flexible arm  178 , or the complementary signals are insufficient to activate actuator  170 . Likewise, optional memory controller  198  is not activated. Accordingly, residual stress between arm base  181  and residual stress layer  182  serves to bend, tilt, or “curl” flexible arm  178  out of the plane of the underlying substrate  176 , as illustrated in  FIG. 21 .  
         [0050]     In one implementation, reflector  72  in the relaxed, display OFF state rests at an orientation of about 12 degrees with respect to substrate  176 . In one implementation, a transition time of about 1 ms is needed to activate or release actuator  170  (i.e., change between the relaxed, display OFF state and the activated, display ON state). It will be appreciated that this transition time can be changed, and substantially reduced.  
         [0051]      FIG. 23  is a schematic diagram of a 2×2 array  210  of actuators  170  having a storage or memory capability to illustrate the operation of actuators  170 . The operation of array  210  is described with reference to the following activation or control signals: 
        Vse=storage electrode voltage     Ry=mirror arm voltage for Row-y     Cx=actuation electrode voltage for Column-x          
         [0055]     As one exemplary implementation, actuation of a single actuator  170  at a location CxRy in array  210  (e.g., location C 1 R 2 ) to activated, display ON state is accomplished by applying a row activation voltage (e.g., +60 volts) to a row electrode Ry (e.g., R 2 ), which delivers the row activation voltage to the flexible arm  178  of each actuator in the row. A column activation voltage (e.g., −60 volts) is applied to a column electrode Cx (e.g., C 1 ), which delivers the column activation voltage to the activation electrodes  190  of each actuator  170  in the column. These exemplary row and column activation voltages establish at actuator  170  at a location CxRy in array  210  (e.g., location C 1 R 2 ) a voltage differential of 120 volts between flexible arm  178  and activation electrodes  190 . With a voltage differential of at least 114 volts needed for actuation, the 120 volt differential is sufficient to activate the actuator  170 .  
         [0056]     The activation voltages need be applied only temporarily if a memory or storage voltage is applied to memory electrode  200  to establish a differential relative to the row activation voltage delivered to flexible arm  178 . In particular, the activation voltages need be applied only long enough (e.g., 1 ms in one implementation) for flexible arm  178  to be deflected to and held by memory electrode  200 .  
         [0057]     With row and column electrodes other than row electrode Ry and column electrode Cx held at a median potential (e.g., 0 volts in one implementation), application of the row activation voltage to activation row electrode Ry and column activation voltage to column electrode Cx will function to activate only actuator  170  at location CxRy. With Ry=+60 volts and Cx=−60 volts, for example, other actuators  170  in row Ry and column Cx will each receive a voltage differential of only 60 volts, which is insufficient for actuation. Moreover, with storage or memory electrode  200  energized, all actuators  170  not specifically being activated will retain their previous states. For example, storage or memory electrode  200  may be energized with a voltage of +60 volts, for example. Such a memory or storage potential establishes between all storage electrodes  200  and actuators  170  that are not specifically being activated or addressed a differential of at least 25 volts that is sufficient to hold them in the display ON state.  
         [0058]     Actuators  170  may be considered to have an actuation (or activation) state in which reflector  72  is moved into a flat (i.e., parallel to substrate  176 ) position for the activated, display ON state, a release state in which reflector  72  is released from the flat (i.e., parallel to substrate  176 ) position and curls out-of-plane for the relaxed, display OFF state, and a storage state in which in which reflector  72  is held in the flat (i.e., parallel to substrate  176 ) position after actuation. The actuation state and the storage state may be represented by the following equations: 
 
 A   xy   =|R   y   −C   x |=actuation potential difference for mirror RxCy 
 
 H   xy   =|R   y   −V   se |=hold potential difference via storage 
 
 electrode for mirrors on Ry 
 
         [0059]     Actuation (From the Released State)  
         [0060]     The mirror will transition to the actuated (down) state only if 
 
 A   xy &gt;114 volts. 
 
         [0061]     Release (From the Actuated State)  
         [0062]     The mirror will transition to the released (up) state only if 
 
 A   xy 21 53 volts and  H   xy &lt;25 volts 
 
         [0063]     Storage  
         [0064]     The mirror will retain its current state only if 
 
 A   xy &gt;53 volts or  H   xy &gt;25 volts if state=actuated 
 
 A   xy &lt;114 volts if state=released 
 
         [0065]      FIG. 24  is a fragmentary schematic diagram of a 50×50 array  230  of actuators  170  having a storage or memory capability. Array  230  employs 50 row electrodes  232  that are coupled to corresponding row drivers  234 , 50 column electrodes  236  that are coupled to corresponding column drivers  238 , and a common storage electrode  240  for all actuators  170  connected to a storage driver  242 . It will be appreciated that row drivers  234  and column drivers  238  may include an individual driver for each of respective electrodes  232  and  236 , or may include a lesser number of drivers that are multiplexed among the electrodes. Drivers  234 ,  238 , and  242  and other electronics can be formed on substrate  176  or as separate devices, depending on complexity, packaging and economics. A display processor  244  receives display signals from a display input  246  and provides corresponding control signals to drivers  234 ,  238 , and  242 .  
         [0066]     In this implementation, row drivers  234  switch between 0 and +60 volts, and column drivers  238  switch between 0, +60 and −60 volts. Storage electrode driver  242  switches between 0 and +60 volts. If the actuators  170  are sequentially addressed, a period of about 50×50×1 ms, or 2.5 seconds, will be needed to address all actuators  170  in array  230 . It will be appreciated that only those actuators  170  with a reflector  72  requiring a state change need be addressed. Accordingly, less time would be required if fewer than all actuators  170  were to be addressed. If 50 row drivers  234  and 50 column drivers  238  are employed, whole rows or columns can be addressed simultaneously, and the actuators  170  in the rows or columns activated in parallel, thereby decreasing the array addressing period to about 50 ms.  
         [0067]      FIG. 25  is a flow diagram of a row-sequential addressing method  250  described with reference to the exemplary implementation of actuator  170  and array  230  described above. It will be appreciated that row-sequential addressing method  250  could be readily adapted to other implementations of actuators  170  and arrays  230 .  
         [0068]     Step  252  indicates a “Clear All” step in which the reflectors  72  of all actuators  170  in array  250 , for example, are returned to a relaxed, no display state. The Clear All of step  252  is optional and may be represented by the following drive voltages: 
 
 Vse=Ry=Cx= 0 volts (for all x and y). 
 
         [0069]     Step  254  indicates an “Arm Storage” step in which the storage electrodes  240  for all actuators  170  are activated or energized by storage driver  242 . The Arm Storage of step  254  may be represented by the following drive voltage: 
 
 Vse=+ 60 volts. 
 
 Step  254  applies +60 volts to storage electrodes  240  so any activated actuator  170  is held in its activated, display ON state until released. This step does not affect actuators in the relaxed, display OFF because the activation of storage electrodes  240  is insufficient to activate an actuator  170  in the relaxed state. 
 
         [0070]     Step  256  indicates a “Begin” step in which a counter of rows or columns (e.g., rows) is initialized at a value “1.” Step  258  sets the row counter “i” to a first row of actuators  170  in array  230 . The Begin of step  256  may be represented as: 
 
Set“i”=1. 
 
         [0071]     Step  258  indicates a “Row-i Set” step in which the storage electrodes  240  for all actuators  170  are activated or energized by storage driver  242 . The Row-i Set of step  258  may be represented by the following drive voltages: 
 
Set Ry=i=+60 volts, 
 
Cx=data states for Ry=iC 1 , Ry=iC 2 , . . . ,Ry=iC 50 . 
 
 To activate actuators  170  within Row-i (Ry=i=+60 volts), set Cx=−60 volts, giving an activation differential: 
 
( Ax,y=i=| 60−(−60)|=120 volts =&gt;mirror is actuated). 
 
 To release actuators  170  within Row-I, (Ry=i=+60 volts), set Cx=+60 volts, giving a release differential: 
 
( Ax,y=j=| 60−60|=0 volts,  Hx,y=j=| 60−60|=0 volts (no hold) 
 
 For mirrors other than in Row-i (Ry≠i=0 volts) 
 
 Ax,y≠i=| 0 −Rx| =(0 or 60) volts insufficient for actuation. 
 
 Hx,y≠i=| 0−60|=60 volts sufficient for holding states 
 
 It will be noted that there is no hold function within Row-i in this step, since Hx,y=i=0 volts. Accordingly, step  258  sets (or releases previously actuated and held) the states of actuators  170  in Row-i to their required new positions. 
 
         [0072]     Step  260  indicates a “Hold All” step in which all row and column electrodes are set to a neutral potential (e.g., 0 volts) and the storage electrodes  240  for all actuators  170  are activated or energized by storage driver  242  sufficient for holding the current states. The Hold All of step  260  may be represented by the following drive voltages: 
 
All Ry=0 volts and all Cx=0 volts 
 
Hxy 60 volts 
 
         [0073]     Step  262  indicates an “Increment Row” step in which counter “i” is incremented by a count of one. Step  262  then proceeds to step  264 .  
         [0074]     Step  264  indicates a “Repeat” step that returns to step  256 . Step  264  returns to step  258  repeatedly until all rows are addressed (e.g., until count “I”=50.  
         [0075]     In the exemplary implementation described above, an activation potential of greater than 114 volts between flexible arm  178  and activation electrodes  190  establishes sufficient electrostatic force to pull curled flexible arm  178  in its relaxed state toward substrate  176 . As a result, dimples  202  are pulled against substrate  176  and reflector  72  lies flat, nominally at 0 degrees, in an activated, display ON state.  
         [0076]      FIG. 26  is a graph  280  illustrating hysteresis characteristics of an actuator  170  with respect to applied voltage differentials. As shown in  FIG. 26 , actuator  170  exhibits hysteresis effects such that after activation at about 114 volts, the applied voltage differential needs to be reduced below 53 volts for actuator  170  to be released.  
         [0077]     A bottom horizontal segment  282  illustrates that the voltage differential between activation electrodes  190  and flexible arm  178  needs to be increased to about 114 volts to move flexible arm  178  from a relaxed upward state to an activated downward state, the transition being illustrated by vertical segment  284 . A top horizontal segment  286  illustrates that the voltage differential then needs to be decreased to about 53 volts to move flexible arm  178  from an activated downward state to a relaxed upward state, the transition being illustrated by vertical segment  288 .  
         [0078]     A storage activated segment  290  illustrates activation of storage electrodes  240  and shows that flexible arm  178  remains in the activated downward state even as the voltage differential between activation electrodes  190  and flexible arm  178  is reduced to below 53 volts. In the illustrated implementation, activation of storage electrodes  240  includes a voltage difference of greater than 25 volts (e.g., 60 volts) between storage electrode  240  and flexible arm  178 . In contrast, a storage not-activated segment  292  illustrates absence of activation of storage electrodes  240  (e.g., 0 volt) and shows flexible arm  178  returning to its relaxed upward state when the voltage differential between activation electrodes  190  and flexible arm  178  is reduced to below 53 volts.  
         [0079]     It will be appreciated that the hysteresis of actuator  170  allows it to be operated and held in its activated downward state without use of storage electrodes  240 . Such an operation requires closer tolerances as to the row and column drive voltages and can be more sensitive to process or manufacturing variations that may cause operation failure. Nevertheless, actuator  170  can be operated without storage electrodes  240  to provide a simpler layout and to eliminate the requirement for storage drivers.  
         [0080]     With reference to the exemplary MUMPS manufacturing process, arm base  181  may be formed as a singly clamped cantilever of 1.5 μm thick polysilicon patterned on the Poly 2  layer. Activation electrode  190  may be formed from the Poly 0  layer. Dimples  202  may be formed in the Poly 2  layer with Anchor 1  (i.e., a hole in the 2 μm Oxide 1  layer) to provide a stand-off of 2 μm, thereby to prevent the major bottom surface of flexible arm  178  from electrically contacting activation electrode  190  when actuated. Residual stress layer  182  may be formed as a 0.5 μm thick layer of gold. Reflector  72  may also be coated with gold to increase optical reflectance.  
         [0081]     Paddle end  180  of flexible arm  178  may be formed to resist the curling from residual stress characteristic of the rest of arm  178 .  FIG. 27  is a schematic sectional side view of one implementation paddle end  180  formed as a relatively thick composite structure  300  of, for example, a 1.5 μm thick layer  302  of Poly 2  (i.e., the material of arm base  181 ), as well as a 2 μm thick layer  304  of Poly 1  (which includes dimples  202 ) and a 0.75 μm thick layer  306  of trapped Oxide 2  between layers  302  and  304 .  
         [0082]     Parts of the description of the preferred embodiment refer to steps of the MUMPs fabrication process described above. However, as stated, MUMPs is a general fabrication process that accommodates a wide range of MEMS device designs. Consequently, a fabrication process that is specifically designed for the present invention will likely include different steps, additional steps, different dimensions and thickness, and different materials. Such specific fabrication processes are within the ken of persons skilled in the art of photolithographic processes and are not a part of the present invention.  
         [0083]     In view of the many possible embodiments to which the principles of our invention may be applied, it should be recognized that the detailed embodiments are illustrative only and should not be taken as limiting the scope of our invention. Rather, I claim as my invention all such embodiments as may come within the scope and spirit of the following claims and equivalents thereto.