Abstract:
An electro-mechanical grating device for diffracting an incident light beam has a base which defines a surface. A spacer layer is provided above the base, said spacer layer defining an upper surface of said spacer layer. A longitudinal channel is formed in said spacer layer. A plurality of spaced apart deformable ribbon elements are disposed parallel to each other and span the channel. The deformable ribbon elements define a ribbon structure which comprises at least one layer of a single material. The ribbon elements and the electro-mechanical grating device are formed so that charge build-up in layers of the electro-mechanical grating device can be avoided during operation of the device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to U.S. Ser. No. 09/216,202, filed Dec. 18, 1998, entitled “Process for Manufacturing an Electro-Mechanical Grating Device”; U.S. Ser. No. 09/216,289, filed Dec. 18, 1998, entitled “A Mechanical Grating Device”; U.S. Ser. No. 09/215,106, now U.S. Pat. No. 6,144,481, filed Dec. 18, 1998, entitled “Method and System for Actuating Electro-Mechanical Ribbon Elements in Accordance to a Data Stream”; and U.S. Ser. No. 09/216,054, filed Dec. 18, 1998, entitled “A Mechanical Grating Device with Optical Coating and a Method of Making Mechanical Grating Device with Optical Coating.” 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of modulation of an incident light beam by the use of a mechanical grating device. More particularly, this invention is for a movable diffraction grating and the use of improved conducting ribbon structures. 
     BACKGROUND OF THE INVENTION 
     Advances in micromachining technology have given rise to a variety of Micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (kHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. No. 5,311,360, issued May 10, 1994 to Bloom et al., entitled “Method and Apparatus for Modulating a Light Beam.” This modulator is a micromachined reflective phase grating. It consists of a plurality of equally spaced deformable elements in the form of beams suspended at both ends above a substrate thereby forming a grating. The deformable elements have a metallic layer that serves both as an electrode, and as reflective surface for incident light. The substrate is also reflective and contains a separate electrode. Typically the deformable elements are supported a distance of λ/4 above, and parallel to, the substrate, where λ is the wavelength of the incident light source. When the deformable elements are actuated (for example, when a sufficient switching voltage is applied), they are pulled down and the incident light is diffracted. Optical systems can intercept the diffracted light. For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image. 
     U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, to Bloom et al., entitled “Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate” discloses a method of making a deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate. The deformable elements are patterned on top of a sacrificial silicon dioxide film. In order to achieve free-standing beams, the silicon dioxide film is etched away until a supporting frame remains. The last fabrication step provides an aluminum film in order to enhance the reflectance of both the beams and the substrate. To enhance the optical performance of the grating element, dielectric optical coatings may be added using methods that minimize their mechanical effects. The stress can be minimized by alternating materials with compressive and tensile stresses, as discussed by Ennos in “Stresses developed in optical film coatings,” published in  Applied Optics  vol. 5 (1966). Silicon dioxide (SiO 2 ) evaporated in vacuum at room temperature can be used as a low-index material with compressive stress, and titanium dioxide (TiO 2 ) evaporated in the same way can be used as a high-index material with tensile stress. 
     According to the prior art, for operation of the Grating Light Valve (GLV) device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. By modulating the voltage waveform, it is possible to modulate the diffracted optical beam as needed by the specific application. However, a single polarity voltage waveform can lead to device operation difficulties if leakage or injection of charge occurs into the intermediate dielectric layers between the ground plane and the conductor on the ribbons. 
     The preferred material for the deformable ribbons in the prior art has been silicon nitride. It is a film that can support an intrinsic tensile stress controlled by the deposition process and that is stable, meaning it does not degrade due to stress relaxation or oxidation over time. However one problem encountered with this material is its tendency to trap charge when a bias is applied across it. If this dielectric charge does not dissipate quickly enough after the actuation voltage is turned off, a significant charge accumulation can occur that leads to deterioration in the performance of the device with repetitive actuation. 
     Charge injection and trapping into insulating dielectric films, such as silicon nitride and silicon dioxide, on semiconductors is well known to occur in various microelectronic devices. Charging effects in silicon dioxide can be minimized by proper deposition, as described, for example, in “Charge transport and transport phenomena in off-stoichiometric silicon dioxide films,” J. Appl. Phys. 54, 1983, pp. 5801-5827, by D. J. Marie et al. On the other hand, charge injection into silicon nitride can be used beneficially in non-volatile memories such as the device described by R. T. Bate; see U.S. Pat. No. 4,360,900, “Non-volatile semiconductor memory elements,” issued Nov. 23, 1982. Charge trapping phenomena in rolling contact micro-electromechanical actuators have also been reported by C. Cabuz et al. in “High reliability touch-mode electrostatic actuators (TMEA),” Proc. of Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 8-11, 1998, pp. 296-299. 
     One method to alleviate this problem is to provide an alternating bipolar voltage to the ribbons. A DC-free bipolar waveform produces nearly the same temporal modulation of the diffracted optical beam as the corresponding single polarity waveform while minimizing charge accumulation in the dielectric layers. Stable device operation is thus achieved. However, this complicates the driving circuitry requiring bipolar rather than unipolar driving capability. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an electro-mechanical grating device which eliminates a potential drop across charging layers, which are part of the electro-mechanical grating device. 
     The above object is accomplished by an electro-mechanical grating device comprising: 
     a base having a surface; 
     a channel formed above said base; and 
     a ribbon structure defining a plurality of spaced apart deformable ribbon elements spanning said channel, said deformable ribbon elements are tensile and reflective, define a top ribbon surface and a bottom ribbon surface and being fabricated of at least one layer opposing the base wherein said layer is conductive. 
     It is a further object of the present invention to form the ribbon structure of the ribbon elements of the electro-mechanical grating device so that the ribbon structure as a whole is tensile, conductive and reflective and thereby avoids a potential drop across charging layers. 
     The above object is accomplished by an electro-mechanical grating device comprising: 
     a base having a surface; 
     a channel formed above said base; and 
     a plurality of spaced apart deformable ribbon elements spanning the channel, said deformable ribbon elements are formed by a single material layer which is tensile, reflective, and conductive. 
     This object is further accomplished by an electro-mechanical grating device comprising: 
     a base having a surface; 
     a channel formed above said base; and 
     a ribbon structure defining a plurality of spaced apart deformable ribbon elements spanning the channel, said deformable ribbon elements are formed by a three layer structure defining a layer opposing the base, an intermediate layer, and a top layer, wherein the layer opposing the base is conductive, the intermediate layer is tensile, and the top layer is reflective. 
     This object is also accomplished by an electro-mechanical grating device comprising: 
     a base having a surface; 
     a channel formed above said base; and 
     a ribbon structure defining a plurality of spaced apart deformable ribbon elements spanning the channel, said deformable ribbon elements are formed by a three layer structure defining a layer opposing the base, an intermediate layer on top of the layer opposing the base, and a top layer, wherein the layer opposing the base is conductive and reflective, the intermediate layer is transparent, and the top layer is transparent and tensile. 
     It is another object of the present invention to provide an electro-mechanical grating device which is improved to eliminate a potential drop across charging layers, which are part of the electro-mechanical grating device. 
     The above object is accomplished by an electro-mechanical grating device comprising: 
     a base having a surface; 
     a channel formed above said base; 
     a ribbon structure defining a plurality of spaced apart deformable ribbon elements spanning said channel, said deformable ribbon elements are tensile and reflective, define a top ribbon surface and a bottom ribbon surface and being fabricated of at least one layer opposing the base is conductive; and 
     a plurality of conductive standoffs patterned on said base in relation to said deformable ribbon elements. 
     An advantage of the electro-mechanical grating device of the present invention is that a ribbon structure can be constructed to substantially reduce the effects of charge injection into non-conducting layers. The significant effects of charge injection, beam stiction and unstable electro-mechanical operation, will be eliminated. 
     A further advantage is the need for a simple two-level unipolar drive circuit to achieve standard pulse-width modulation with substantially reduced effect of charge injection. 
     Additionally, the structure and materials of the device are selected to be compatible with standard CMOS fabrication methods and allow a fabrication process sequence that make the fabrication of the electro-mechanical grating device compatible with the integration of CMOS circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention is described with reference to the embodiments shown in the drawing. 
     FIG. 1 is a perspective, partially cut-away view of the electro-mechanical grating device of the present invention; 
     FIG. 2 is a perspective, partially cut-away view of a second embodiment of the electro-mechanical grating device of the present invention; 
     FIG. 3 is a perspective, partially cut-away view of an electro-mechanical grating device from prior art; 
     FIGS. 4 a - 4   b  show the change in ribbon element behavior due to charging for devices in the prior art with actuation by a unipolar triangular waveform; 
     FIG. 5 is a cross-sectional view of the electromechanical grating device having a ribbon structure composed of a single layer; 
     FIG. 6 is a cross-sectional view of the electromechanical grating device having a ribbon structure composed of a tensile-conductive layer and a reflective layer; 
     FIG. 7 is a cross-sectional view of the electromechanical grating device having a ribbon structure composed of a tensile-conductive layer, a reflective layer, and optical coatings; 
     FIG. 8 is a cross-sectional view of the electromechanical grating device having a ribbon structure composed of a conductive-reflective layer and optical coatings with tensile properties; 
     FIG. 9 is a cross-sectional view of the electromechanical grating device having a ribbon structure composed of a conductive layer, a tensile layer, and a reflective layer; 
     FIG. 10 is a cross-sectional view of the electromechanical grating device having a ribbon structure composed of a conductive layer, a tensile layer, a reflective layer, and optical coatings; 
     FIG. 11 is a cross-sectional view of the electromechanical grating device having a ribbon structure composed of a tensile-conductive layer, a reflective layer, and optical coatings that have been patterned; 
     FIG. 12 is a cross-sectional view of the electro-mechanical grating device of FIG. 2 showing the stationary ribbons suspended over conductive screening features; and 
     FIG. 13 is a cross-sectional view of the mechanical grating device of FIG. 2 showing the movable ribbons suspended over conductive standoffs. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a perspective, partially cut-away view of the electro-mechanical grating device  100  of the present invention. The electro-mechanically deformable structures of the electro-mechanical grating device  100  are formed on top of a base  50 . The present embodiment as shown in FIG. 1 discloses an electro-mechanical grating device  100  which can be operated by the application of an electrostatic force. Because the actuation force of the electro-mechanical grating device  100  is electrostatic, the base  50  comprises several layers of different materials. The base  50  comprises a substrate  52  that is chosen from the materials glass and silicon. The substrate is covered by a bottom conductive layer  56 . In this embodiment the thin bottom conductive layer  56  is necessary because it acts as an electrode for applying the voltage to actuate the electro-mechanical grating device  100 . The thin bottom conductive layer  56  is covered by a protective layer  58 . The bottom conductive layer  56  is selected from the group consisting of aluminum, titanium, gold, silver, tungsten, silicon alloys, and indium tin oxide. Above the protective layer  58  a spacer layer  65  is formed. On top of the spacer layer  65 , a ribbon structure  74  is formed which has the desired characteristics of conductivity, high reflectivity and tensile stress, with a surface opposing the base being conductive. 
     The spacer layer  65  has a longitudinal channel  67  formed therein. The longitudinal channel  67  comprises a first and second side wall  67   a  and  67   b , and a bottom  67   c . The channel  67  is open to the top and covered by a first and a second set of deformable ribbon elements  72   a  and  72   b , each deformable ribbon element  72   a  and  72   b  of the first and second set encompasses the ribbon structure  74 . Each deformable ribbon element  72   a  and  72   b  spans the channel  67  and is secured to the surface of the spacer layer  65  on either side of the channel  67 . The bottom  67   c  of the channel  67  is covered by a protective layer  58 . The ribbon structure  74  is patterned such that there is a first and a second region  78   a  and  78   b , each with a comb-like structure, arranged at the surface of the electro-mechanical grating device  100  in an inter-digitated pattern. The first and second region  78   a  and  78   b  are mechanically and electrically isolated from one another. As a result there are the first and second set of deformable ribbon elements  72   a  and  72   b  spanning the channel  67  and in the direction of the channel  67  are arranged such that every other deformable ribbon element belongs to one set. 
     For operation of the device, an attractive electrostatic force is produced by a voltage difference between the ground plane  56  and the deformable ribbon elements  72   a  or  72   b . In the unactuated state, with no voltage difference, all of the ribbons in the device are suspended above the substrate at the same height as shown in FIG. 1. A periodic grating is obtained by applying a voltage to fully actuate the ribbons  72   a  or  72   b  such that they come in contact with the protective layer  58 . When the applied voltage is removed, the force due to the tensile stress and the bending moment restores the ribbons to their original unactuated state. 
     The embodiment shown in FIG. 1 eliminates the effect of charge build-up in dielectrics, primarily silicon nitride, that are part of the ribbon structure  74 , which appears to be the most common scenario for charging. However, it is conceivable that charge can be injected into the protective layer  58  when a conductive ribbon element is used. Alternatively, surface charges can be formed on the surface of the dielectric protective layer through contact with the ribbon elements  72   a  or  72   b . Either scenario would significantly degrade device performance if a sufficient charge is built up. For simplicity all the elements which are similar in both embodiments (FIG.  1  and FIG. 2) are marked with the same reference numeral. FIG. 2 shows a second embodiment of the present invention. Here, the bottom  67   c  of the channel  67  is provided with a plurality of standoffs  64  and screening features  62 . The standoffs  64  and screening features  62  are formed on top of the protective layer  58  and are conductive. The electro-mechanical deformable structures of the electro-mechanical grating device  100  are formed on top of the base  50 . 
     The present embodiment as shown in FIG. 2 discloses an electro-mechanical grating device  100  which can be operated by the application of an electrostatic force. Because the actuation force of the electro-mechanical grating device  100  is electrostatic, the base  50  comprises several layers of different materials. The base  50  comprises a substrate  52  that is chosen from the materials glass and silicon. The substrate is covered by a bottom conductive layer  56 . In this embodiment the thin bottom conductive layer  56  is necessary because it acts as an electrode for applying the voltage to actuate the electro-mechanical grating device  100 . The thin bottom conductive layer  56  is covered by a protective layer  58 . The bottom conductive layer  56  is selected from the group consisting of aluminum, titanium, gold, silver, tungsten, silicon alloys, and indium tin oxide. Above the protective layer  58  a conductive standoff layer  60  is formed of a material selected from the group of aluminum, titanium, gold, silver, tungsten, silicon alloys, and indium tin oxide. Above the conductive standoff layer  60  a spacer layer  65  is formed. On top of the spacer layer  65 , a ribbon structure  74  is formed which has the desired characteristics of conductivity, high reflectivity and tensile stress, with the surface opposing the base being conductive. 
     The spacer layer  65  has a longitudinal channel  67  formed therein. The longitudinal channel  67  comprises a first a second side wall  67   a  and  67   b  and a bottom  67   c . At the bottom  67   c  of the channel, the conductive standoff layer  60  is patterned to form conductive screening features  62  and standoffs  64  that take the form of wide lines along the width of the channel connected by narrow lines running in the longitudinal direction of the channel. The conductive standoffs  62  held at the same potential as the ribbon elements  72   a  and  72   b . The actuated device would then induce contact between two conductive surfaces, and injection of surface charge from ribbon elements  72   a  and  72   b  to dielectric materials on the substrate  50  would be avoided. 
     The channel  67  is open to the top and covered by a first and a second set of deformable ribbon elements  72   a  and  72   b . Each deformable ribbon element  72   a  and  72   b  spans the channel  67  and is secured to the surface of the spacer layer  65  on either side of the channel  67 . The ribbon structure  74  is patterned such that there is a single conducting region  80 , with the two alternating sets of ribbon elements  72   a  and  72   b , arranged at the surface of the electro-mechanical grating device  100  and mechanically isolated from one another. As a result there are the first and the second set of deformable ribbon elements  72   a  and  72   b  spanning the channel  67  and in the direction of the channel  67  are arranged such that every other deformable ribbon element belongs to one set. 
     An electrical interconnect between the conducting region  80  and the conductive standoff layer  60  is formed by etching a via hole  122  through the spacer layer  65  and filling with a conductive material  124  before the deposition of the ribbon structure  74 . The conductive screening features  62  and standoffs  64  are held at the same potential as the ribbon elements  72   a  and  72   b . The actuated device would then induce contact between two equipotential conductive surfaces, and injection of surface charge from ribbon elements  72   a  and  72   b  to dielectric materials on the base  50  is avoided. 
     FIG. 3 shows a prior art electro-mechanical grating device  210  as described by Bloom et al. in U.S. Pat. No. 5,311,360, issued May 10, 1994, entitled “Method and Apparatus for Modulating a Light Beam.” The figure shows a cross-section from a view plane normal to the length of the ribbon elements. The grating device  210  comprises a base  216  above which a plurality of ribbon elements  218  are suspended. The ribbon elements are silicon nitride deposited with a low tensile stress. A thin conductive layer  222  is deposited on top and between the ribbon elements  218  to provide a top electrode and to enhance the reflectivity of the device. 
     For operation of the device  210 , an attractive electrostatic force is produced by a voltage difference between the base  216  and conductive layer  222 . This voltage difference leads to a large electric field in the device, typically 10 8  to 10 9  V/m depending on material thicknesses and applied voltage. This electric field can cause injection and trapping of charge into any dielectric materials between the top conductive layer  222  and the base  216 . In microelectronic devices, charging is known to occur in a variety of insulating thin films including silicon nitride and silicon dioxide. Silicon nitride in particular is very prone to charge injection at modest electric fields, and can be used as a charge storage layer in non-volatile memories because of its long-term charging capability. Charge accumulation on the ribbons of a grating light valve can cause detrimental variations in the operation of the device from one actuation to the next. 
     Charge accumulation with repeated actuation was observed on a prior art grating light valve device with a ribbon structure as shown in FIG. 3. A 5 kHz 50V triangular voltage waveform with positive polarity was used to actuate the ribbons into contact with the substrate. A triangular waveform was chosen because it allows the charge accumulation to be quantified by measuring the length of time the ribbons remain in contact with the substrate during each actuation, i.e., the ribbon down time. FIG. 4 a  shows the applied waveform  160  and the time-dependent position of a single ribbon for two different time delays, 5 and 120 seconds, after the onset of actuation. The behavior of the ribbons is significantly different 120 seconds after actuation  163  than at 5 seconds  162 . Measurement of the two time intervals t U  and t D  with a time interval analyzer (TIA) provides a convenient means for monitoring the change in response of the ribbon from one actuation to the next. FIG. 4 b  shows the measured TIA data as a function of time delay after initial actuation. Before any significant charging occurs  165 , the ribbon down time t U  is approximately 130 μsec and t D  is zero because the ribbons do not actuate during the downward portion of the voltage waveform. After approximately 3 seconds and roughly 15000 actuations, ribbon charging becomes large enough to fully actuate the ribbon when the applied voltage is near zero and t D  becomes non-zero  167 . If the applied voltage waveform is turned off past this point, the ribbon will become fully actuated because of the accumulated charge present. 
     The ribbon structure  74  is one of the unique characteristics of this invention. In order to eliminate the effects of charge trapping by dielectric materials in the beam, ribbon structures are disclosed herein which do not include a charge-trapping dielectric layer on the ribbon structure  74  across which the actuation potential is applied. This is accomplished by using either a completely conductive ribbon structure  74  or a conductive layer on the bottom of the ribbon structure  74 . The actuation voltage is applied from an appropriate source  110  to the bottom conductive layer  56 , which is adjacent to the bottom  67   c  of the channel  67  and faces the beam  50 . 
     In this invention it is necessary to provide a dielectric layer on the base (protective layer  58  see FIG. 1) across which a large field will exist due to the actuation voltage. The protective layer  58  can be a high quality thermally-grown silicon dioxide, though, which can withstand a substantially higher electric field before any charge injection occurs as compared to dielectric ribbon materials such as silicon nitride. The various ribbon structures  74  that make up the embodiments of this invention are shown in FIG.  5  through FIG.  11 . 
     FIG. 5 is a cross-sectional view of an electro-mechanical device  100  showing a structural drawing that includes the elements for electro-mechanical actuation. The bottom conductive layer  56  is covered by a protective layer  58 . Above the protective layer  58  a spacer layer  65  is formed. The spacer layer  65  has a longitudinal channel  67  formed therein. A ribbon structure  74  is formed which includes the desired characteristics of a conductive surface opposing the base  50 , reflectivity and tensile stress in a single layer  82 . The layer  82  material is selected from a group of materials that includes titanium aluminide, titanium, tantalum, and diamond-like carbon. The thickness of layer  82  is selected to produce the desired mechanical properties of the completed ribbon structure  74 . The ribbon structure  74  is patterned as discussed above to produce the useful electro-mechanical device  100 . 
     The thickness and tensile stress of the layer  82  is chosen to optimize performance by influencing the electrostatic force required for actuation and the returning force, which affects the speed, resonance frequency and voltage requirements of the device. Typical characteristics are tensile intrinsic stress greater than 300 MPa and thickness necessary to achieve full actuation with 15 V or less, which corresponds to between 1000 Å and 5000 Å. 
     FIG. 6 is a cross-sectional view of another electro-mechanical grating device  100  showing a structural drawing that includes the elements for electro-mechanical actuation. The device structure is the same as that of FIG. 5 except for the ribbon structure  74 . The ribbon structure  74  is formed to include the desired characteristics of a conductive surface opposing the base  50 , reflectivity and tensile stress using two layers. A first layer  85  opposing the bottom  67   c  of the channel  67  provides conductive and tensile properties of the ribbon structure  74  and a second layer  87  atop the layer opposing the bottom  67   c  of the channel  67  provides the reflective property. The first layer  84  consists of a material that is selected from a group that includes titanium aluminide, titanium, tantalum, diamond-like carbon, tungsten silicide, titanium nitride, and tantalum nitride. The second layer  87  is selected from a group of materials that includes aluminum, chrome, gold, silver and other standard materials or alloys used for optical mirrors. 
     The thickness and tensile stress of the first layer  85  is chosen to optimize performance by influencing the electrostatic force required for actuation and the returning force, which affects the speed, resonance frequency and voltage requirements of the device. Typical characteristics are tensile intrinsic stress greater than 300 MPa and thickness necessary to achieve full actuation with 15 V or less, which corresponds to between 1000 Å and 5000 Å. Thickness of the second layer  87  is chosen to provide substantially the largest possible reflection, and is chosen based on the extinction ratio of the material at the wavelength of the incident light. For example, if Al (aluminum) is used for the second layer  87 , the thickness should be between 300 Å and 2000 Å for visible wavelengths. The ribbon structure  74  is patterned as discussed above to produce the electro-mechanical device  100  with the electrically and mechanically isolated sets of ribbon elements. 
     FIG. 7 is a cross-sectional view of another electro-mechanical grating device  100  showing a structural drawing that includes the elements for electro-mechanical actuation. The device structure is the same as that of FIG. 6 except for the ribbon structure  74 . The ribbon structure  74  is formed to include the desired characteristics of a conductive surface opposing the base  50 , reflectivity and tensile stress using four layers. A first layer  84  opposing the bottom  67   c  of the channel  67  provides conductive and tensile properties of the ribbon structure  74  and a second layer  86  atop the first layer  84  provides the reflective property and thus intrinsically will be conductive. The first layer  84  consists of a material that is selected from a group that includes titanium aluminide, titanium, tantalum, diamond-like carbon, tungsten silicide, titanium nitride, and tantalum nitride. The second layer  86  is selected from a group of materials that includes aluminum, chrome, gold, silver and other standard materials or alloys used for optical mirrors. A third layer  88  on top the second layer  86  is a first optical coating having a lower index of refraction than a fourth layer  90 , which is a second optical coating on top the third layer  88 . The thicknesses of these layers are optimized according to optical thin-film coating design rules as applied to a grating device. The materials for layers  88  and  90  are expected to be from a group of materials used for standard optical coatings and not significantly contribute to the conductivity or tension of the ribbon structure  74 . The ribbon structure  74  is patterned as discussed above to produce the useful electro-mechanical grating device  100 . Although the electro-mechanical grating device  100  is depicted with two layers added to enhance the reflectivity properties of the ribbon structure  74 , additional layers may be used to further improve the design according to standard design rules for optical thin film coatings. 
     The thickness and tensile stress of the first layer  84  is chosen to optimize performance by influencing the electrostatic force required for actuation and the returning force, which affects the speed, resonance frequency and voltage requirements of the device. Typical characteristics are tensile intrinsic stress greater than 300 MPa and thickness necessary to achieve full actuation with 15 V or less, which corresponds to between 1000 Å and 5000 Å. The thickness of the second layer  86  is chosen to provide substantially the largest possible reflection, and is chosen based on the extinction ratio of the material for the wavelength of the incident light. For example, if Al is used for the second layer  86 , the thickness should be between 300 Å and 2000 Å for visible wavelengths. The third and fourth layers  88  and  90  are chosen based on optical think film design techniques that are well known to one skilled in the art. The optical thickness of these layers will be approximately λ/4, and thus, the layers will be approximately 1000 E thick. 
     FIG. 8 is a cross-sectional view of another electro-mechanical grating device  100  showing a structural drawing that includes the elements for electro-mechanical actuation. The device structure is the same as that of FIG. 7 except for the ribbon structure  74 . The ribbon structure  74  is formed to include the desired characteristics of a conductive surface opposing the base  50 , reflectivity and tensile stress using three layers. A first layer  92  opposing the bottom  67   c  of the channel  67  provides conductive and reflective properties of the ribbon structure  74 . The first layer  92  is selected from a group of materials that includes aluminum, chrome, gold, silver and other standard materials or alloys used for optical mirrors. A second layer  94  on top the first layer  92  is a first optical coating having a lower index of refraction than a third layer  96  atop the second layer. The third layer  96  is a second optical coating that also provides the tensile properties needed in the ribbon structure  74  and could be a material such as silicon nitride. The thicknesses of these layers  94  and  96  are optimized according to optical thin-film coating design rules as applied to a grating device. The ribbon structure  74  is patterned as discussed above to produce the useful electro-mechanical device  100 . Although the electro-mechanical device  100  is depicted with two layers added to enhance the reflectivity properties of the ribbon structure  74 , additional layers may be used to further improve the design according to standard design rules for optical thin film coatings. 
     The thickness of the first layer  92  is chosen to provide substantially the largest possible reflection, and is chosen based on the extinction ratio of the material for the wavelength of the incident light. For example, if Al is used for the first layer  92 , the thickness should be between 300 Å and 2000 Å for visible wavelengths. The second and third layers  94  and  96  are chosen based on optical thin film design techniques that are well known to one skilled in the art. The optical thickness of these layers will be approximately λ/4, and thus, the layers will be approximately 1000 E thick. The tensile stress of the third layer  96  should be chosen to provide the desired mechanical properties of the ribbon structure  74 . 
     FIG. 9 is a cross-sectional view of another electro-mechanical grating device  100  showing a structural drawing that includes the elements for electro-mechanical actuation. The ribbon structure  74  is formed to include the desired characteristics of a conductive surface structure opposing the base  50 , reflectivity and tensile stress using three layers. A first layer  98  opposing the bottom  67   c  of the channel  67  provides the conductivity of the ribbon structure  74 , a second layer  102  on top the first layer  98  provides the tensile property, and a third layer  104  on top the second layer  102  provides the reflectivity. The first layer  98  is selected from a group of materials that is conductive and compatible with the subsequent layer addition (e.g., adhesion and high temperature processing) that includes indium tin oxide, tungsten, tungsten silicide, titanium nitride, and tantalum nitride. The second layer  102  is selected from a group of materials that are tensile but not conductive including silicon nitride, titanium dioxide, and magnesium fluoride. The third layer  104  is selected from a group of materials that includes aluminum, chrome, gold, silver and other standard materials or alloys used for optical mirrors. The ribbon structure  74  is patterned as discussed above to produce the useful electro-mechanical grating device  100 . 
     The first layer  98  should be thin in order to have a negligible effect on the mechanical properties of the device. The only requirement is that a sufficient thickness to possess a uniform potential be maintained, and thus a couple hundred angstroms is sufficient. The thickness and tensile stress of the second layer  102  are chosen to optimize performance by influencing the electrostatic force required for actuation and the returning force, which affects the speed, resonance frequency and voltage requirements of the device. Typical characteristics are tensile intrinsic stress greater than 300 MPa and thickness necessary to achieve full actuation with 15 V or less, which corresponds to between 1000 Å and 5000 Å. The thickness of the third layer  104  is chosen to provide substantially the largest possible reflection, and is chosen based on the extinction ratio of the material for the wavelength of the incident light. For example, if Al is used for the third layer  104 , the thickness should be between 300 Å and 2000 Å for visible wavelengths. 
     FIG. 10 is a cross-sectional view of another electro-mechanical grating device  100  showing a structural drawing that includes the elements for electro-mechanical actuation. The ribbon structure  74  is formed to include the desired characteristics of a conductive surface opposing the base  50 , reflectivity and tensile stress using five layers. A first layer  106  opposing the bottom  67   c  of the channel  67  provides the conductivity of the ribbon structure  74 , and a second layer  107  on top the first layer  106  provides the tensile property, and a third layer  108  on top the second layer  107  provides the reflectivity. The first layer  106  is selected from a group of materials that is conductive and compatible with the subsequent layer addition (e.g., adhesion and high temperature processing) that includes indium tin oxide, tungsten, tungsten silicide, titanium nitride, and tantalum nitride. The second layer  107  is selected from a group of materials that are tensile but not conductive including silicon nitride, titanium dioxide, and magnesium fluoride. The third layer  108  is selected from a group of materials that includes aluminum, chrome, gold, silver and other standard materials or alloys used for optical mirrors. A fourth layer  111  on top the third layer  108  is a first optical coating having a lower refractive index than a fifth layer  112  on top the fourth layer  111 , which constitutes a second optical coating. The thicknesses of these layers  111  and  112  are optimized according to optical thin-film coating design rules as applied to a grating device. The ribbon structure  74  is patterned as discussed above to produce the useful electro-mechanical grating device  100 . Although the electro-mechanical grating device  100  is depicted with two layers added to enhance the reflectivity properties of the ribbon structure  74 , additional layers may be used to further improve the design according to standard design rules for optical thin film coatings. 
     The first layer  98  should be thin in order to have a negligible effect on the mechanical properties of the device. The only requirement is that a sufficient thickness to possess a uniform potential be maintained, and thus a couple hundred angstroms is sufficient. The thickness and tensile stress of the second layer  102  is chosen to optimize performance by influencing the electrostatic force required for actuation and the returning force, which affects the speed, resonance frequency and voltage requirements of the device. Typical characteristics are tensile intrinsic stress greater than 300 MPa and thickness necessary to achieve full actuation with 15 V or less, which corresponds to between 1000 Å and 5000 Å. The thickness of the third layer  104  is chosen to provide substantially the largest possible reflection, and is chosen based on the extinction ration of the material for the wavelength of the incident light. For example, if Al is used for the third layer  104 , the thickness should be between 300 Å and 2000 Å for visible wavelengths. The fourth and fifth layers  111  and  112  are chosen based on optical thin film design techniques that are well known to one skilled in the art. The optical thickness of these layers will be approximately λ/4, and thus, the layers will be approximately 1000 E thick. 
     The embodiments shown in FIGS. 9 and 10 can also be operated with a voltage applied between the first and third layers  98  and  104  and  106  and  108  respectively of the ribbon structure  74 . The electrical contact shown in FIGS. 9 and 10 between layers  98  and  104  and  106  and  108  respectively would then be absent. This provides a binding force to the ribbon structure  74  that can prevent delamination of the first layer  98  and  106  from the second layer  102  and  107  respectively. The first layer  98  and  106  respectively can delaminate from the second layer  102  and  107  respectively if the adhesion force at the interface between layers  98  and  102  and  106  and  107  respectively is less than the force required for the actuation of the ribbon structure  74 , which is applied by the voltage source  110  to the first layer  98  and  106  respectively. Charge can be injected into the second layer  102  and  107  a respectively as a result of this voltage, but the trapped charge is screened from the bottom conductor  56  by the conductive first layer  98  and  106  respectively, and therefore the electro-mechanical performance of the device  100  is not affected. 
     One characteristic of the embodiments shown in FIGS. 7,  8  and  10  is the existence of a stack of dielectric layers  88  and  90 ,  94  and  96 , and  111  and  112  respectively, above the reflector layer  86 ,  92 , and  108  respectively that acts as an optical coating. The purpose of the dielectric optical coating is to alter the reflective properties of the reflective layer  86 ,  92  and  108  respectively. Likely alterations are enhancement of the reflectivity (or reduction of absorption) at the wavelength of interest, or a shift in the phase accumulated by the reflected light. The former is useful for electro-mechanical grating elements being subjected to high-intensity illumination. The latter can be useful in novel embodiments of GLV structures. 
     The electro-mechanical grating structure places some limitations on the allowable thickness of the dielectric optical coating stack. Because the width of the ribbon elements  72   a  and  72   b  in the preferred embodiment is a few microns or less, the total thickness of the dielectric optical coating must be less than a micron to minimize the effects of the edges of the optical coating. 
     The effects on the mechanical characteristics of the ribbon elements  72   a  and  72   b  must also be considered in the design of GLV devices with optical coatings. The increased thickness will typically increase the electrostatic force required to actuate the ribbon elements  72   a  or  72   b , because of an increase in the bending moment at the ends of the ribbon elements  72   a  or  72   b . In theory, compensation for this effect can be realized by depositing optical films with compressive stress. 
     An alternative embodiment that reduces the effects of the optical coating on the electromechanical performance of the ribbon elements  72   a  and  72   b  is illustrated in FIG.  11 . The device  100  in FIG. 11 is similar to the device in FIG. 6, except the optical film stack comprising layers  118  and  120  is patterned to cover only the central region of the ribbon structure  74 . The optical film stack is centered with respect to the channel  67 . Thus, the coating will produce the desired optical effects when illuminated at the center with a line focus. It will not as significantly affect the electro-mechanical behavior of the ribbon elements  72   a  and  72   b , because the bending occurs at the portions of the ribbon elements closest to the ends, which are not covered by the optical coating. The only remaining effect is on the restoring force due to the alteration of the tension in the ribbon in the region covered by the optical coating. 
     In the embodiment illustrated in FIG. 11, the overall stress of the optical coating should preferably be tensile or negligible. A significant compressive stress degrades the device performance by causing the ribbon elements  72   a  and  72   b  to bow due to the strain mismatch between the tensile material comprising a first layer  114 , opposing the bottom  67   c  of the channel  67 , and the optical coating layers  118  and  120 . Ideally, the technique of stress compensation would be used in the optical coating stack, so that its presence would have negligible effect on the performance of the electro-mechanical grating device  100 . 
     The embodiment shown in FIG. 11 illustrates the use of the patterning method on a structure with the ribbon structure  74  composition disclosed in FIG.  7 . It is easily understood that the same patterning method can be used to form an embodiment with the ribbon structure  74  composition disclosed in FIG. 10 with reduced electromechanical effects from the optical coating layers  88  and  90 . 
     The embodiments shown in FIGS. 5 through 11 eliminate the effect of charge build-up in dielectrics, primarily silicon nitride, that are part of the ribbon structure  74 . This appears to be the most common scenario for charging. However, it is conceivable that charge can be injected into the protective layer  58  when a conductive beam is used. Alternatively, surface charges can be formed on the surface of the dielectric protective layer  58  through contact with the ribbon elements  72   a  or  72   b . Either scenario would significantly degrade device performance if a sufficient charge is built up. The electro-mechanical grating device  100  illustrated in FIG. 2 includes a series of conductive screening elements  62  and standoffs  64  that can be used to prevent charge injection from the ribbon elements  72   a  and  72   b  into the protective layer  58  or onto the top surface of the base  50 . 
     The deformable ribbon elements  72   b  are aligned to the pattern in the conductive standoff layer  60  such that they are directly over the screening features  62 , as illustrated in the cross-sectional view in FIG.  12 . Because the ribbon elements  72   b  and the screening features  62  are at the same potential, no electrostatic force is exerted on the ribbon elements  72   b  when a voltage is applied by the voltage source  10  between the conductive ribbon structure  74  and the bottom conductive layer  56 . Therefore, the deformable ribbon elements  72   b  remain stationary regardless of the applied voltage. 
     The deformable ribbon elements  72   a  are aligned to the pattern in the conductive standoff layer  60  such that they are directly over conductive standoffs  64 . This is illustrated in the cross-sectional view in FIG. 13. A voltage applied by the voltage source  110  between the conductive ribbon structure  74  of the deformable ribbon elements  72   a  and the bottom conductive layer  56  results in an electrostatic force being applied to the deformable ribbon elements  72   a  in the large gaps between conductive standoffs  64 . Thus, these ribbon elements  72   a  can be actuated until contact is made between the conductive bottom surface of the deformable ribbon elements  72   a  and the top surface of the conductive standoffs  64 . 
     The inclusion of the conductive standoffs  64  improves the device for two reasons. First, no metal-dielectric interface is created during the actuation of ribbon elements  72   a  that could lead to creation of surface charges or injection of charge into the protective layer  58  from the ribbon elements  72   a . It is possible that charge could be injected into the protective layer  58  from the screening elements  62  and the conductive standoffs  64 . However, the effect of this charge on the electro-mechanical performance of the device should be insignificant due to the electric field screening provided by those conductive features  62  and  64 . The reduction in contact surface area that results from the use of the conductive standoffs  64  also improves the reliability of the device by reducing the probability of stiction. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       50  base 
       52  substrate 
       56  bottom conductive layer 
       58  protective layer 
       60  conductive standoff layer 
       62  conductive screening element 
       64  conductive standoff 
       65  spacerlayer 
       67  channel 
       67   a  first sidewall of the channel 
       67   b  second sidewall of the channel 
       67   c  bottom of the channel 
       74  ribbon structure 
       72   a  first set of deformable ribbon elements 
       72   b  second set of deformable ribbon elements 
       78   a  first conducting region 
       78   b  second conducting region 
       80  conductive region 
       82  single layer 
       84  first layer 
       85  first layer 
       86  second layer 
       87  second layer 
       88  third layer (optical coating) 
       90  fourth layer (optical coating) 
       92  first layer 
       94  second layer (optical coating) 
       96  third layer (optical coating) 
       98  first layer 
       100  electro-mechanical grating device 
       102  second layer 
       104  third layer 
       106  first layer 
       107  second layer 
       108  third layer 
       110  voltage source 
       111  fourth layer (optical coating) 
       112  fifth layer (optical coating) 
       114  first layer 
       116  second layer 
       118  first optical coating layer 
       120  second optical coating layer 
       122  via hole 
       124  thick conductive layer 
       160  applied waveform 
       162  graph 
       163  graph 
       165  time at which charging occurs 
       167  time at which t D  becomes non-zero