Patent Publication Number: US-8970939-B2

Title: Method and device for multistate interferometric light modulation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/940,884, filed Nov. 5, 2010, now U.S. Pat. No. 8,213,075, which is a continuation of U.S. patent application Ser. No. 12/115,829, filed May 6, 2008, now U.S. Pat. No. 7,839,557, which is a continuation of U.S. patent application Ser. No. 11/112,734, filed Apr. 22, 2005, now U.S. Pat. No. 7,372,613, which claims the benefit of U.S. Provisional Application No. 60/613,486 filed Sep. 27, 2004, and U.S. Provisional Application No. 60/613,499 filed Sep. 27, 2004. Each of the foregoing applications is incorporated in its entirety by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The field of the invention relates to microelectromechanical systems (MEMS). 
     2. Background 
     Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. One type of MEMS device is called an interferometric modulator. An interferometric modulator may comprise a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. One plate may comprise a stationary layer deposited on a substrate, the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed. 
     SUMMARY 
     The system, method, and devices of the invention each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of Preferred Embodiments” one will understand how the features of this invention provide advantages over other display devices. 
     One embodiment is a light modulator. The light modulator includes a fixed reflector comprising an electrically conductive layer and a partially reflective layer. The light modulator further comprises an electrode positioned at a distance from the fixed reflector and defining a first cavity therebetween. The light modulator further comprises a movable reflector comprising an electrically conductive material. The movable reflector is positioned between the fixed reflector and the electrode. The movable reflector is movable between an undriven position, a first driven position, and a second driven position. The first driven position is closer to the fixed reflector than is the undriven position and the second driven position is farther from the fixed reflector than is the undriven position. 
     Another embodiment is a light modulator comprising a first reflector, a first electrode positioned at a distance from the first reflector, and a second reflector positioned between the first reflector and the first electrode. The second reflector is movable between an undriven position, a first driven position, and a second driven position. The first driven position is closer to the first reflector than is the undriven position and the second driven position is farther from the first reflector than is the undriven position. 
     Another embodiment is a method of driving a MEMS device comprising a first electrode, a second electrode, and a movable electrode positioned between the first electrode and the second electrode and configured to move to at least two positions therebetween. The method includes applying a first voltage potential difference between the first electrode and the movable electrode so as to drive the movable electrode to a position substantially in contact with a dielectric layer, wherein an attractive force is created between the movable electrode and the dielectric layer. The method further includes applying a second voltage potential difference between the first electrode and the movable electrode and a third voltage potential difference between the second electrode and the movable electrode so as to overcome the attractive force between the movable electrode and the dielectric layer and to drive the movable electrode away from the dielectric layer. 
     Another embodiment is a method of fabricating a multistate light modulator. The method includes forming a first reflector. The method further includes forming a first electrode positioned at a distance from the first reflector. The method further includes forming a second reflector positioned between the first reflector and the first electrode. The second reflector is made movable between an undriven position, a first driven position, and a second driven position, wherein the first driven position is closer to the first reflector than is the undriven position and wherein the second driven position is farther from the first reflector than is the undriven position. 
     Another embodiment is a display comprising a plurality of display elements. Each of the display elements includes a first reflective member, a first conductive member positioned at a distance from the first reflective member, and a second reflective member positioned between the first reflective member and the first conductive member. The second reflective member is movable between an undriven position, a first driven position, and a second driven position. The first driven position is closer to the first reflective member than is the undriven position and the second driven position is farther from the first reflective member than is the undriven position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view depicting a portion of one embodiment of an interferometric modulator display in which a movable reflective layer of a first interferometric modulator is in a released position and a movable reflective layer of a second interferometric modulator is in an actuated position. 
         FIG. 2  is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display. 
         FIG. 3  is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . 
         FIG. 4  is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display. 
         FIGS. 5A and 5B  illustrate one exemplary timing diagram for row and column signals that may be used to write a frame of display data to the 3×3 interferometric modulator display of  FIG. 2 . 
         FIG. 6A  is a cross section of the device of  FIG. 1 . 
         FIG. 6B  is a cross section of an alternative embodiment of an interferometric modulator. 
         FIG. 6C  is a cross section of another alternative embodiment of an interferometric modulator. 
         FIG. 7  is a side cross-sectional view of an exemplary interferometric modulator that illustrates the spectral characteristics of produced light. 
         FIG. 8  is a graphical illustration of reflectivity versus wavelength for mirrors of several exemplary interferometric modulators. 
         FIG. 9  is a chromaticity diagram that illustrates the colors that can be produced by a color display that includes exemplary sets of red, green, and blue interferometric modulators. 
         FIG. 10  is a side cross-sectional view of an exemplary multistate interferometric modulator. 
         FIG. 11A-11C  are side cross-sectional views of another exemplary multistate interferometric modulator. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An interferometric modulator has a reflector which is movable between three positions. In an undriven state of the modulator, the movable mirror is in an undriven position. In a first driven state of the modulator, the movable mirror is deflected toward a fixed mirror to a first driven position which is closer to the fixed mirror than is the undriven position. In a second driven state of the modulator, the movable mirror is deflected away from the fixed mirror to a second driven position which is farther from the fixed mirror than is the undriven position. In one embodiment, the modulator is non-reflective, e.g., black, when the movable mirror is in the undriven position, reflects white light when the movable mirror is in the first driven position, and reflects a selected color of light when the movable mirror is in the second driven position. A color display including such modulators thus reflects relatively intense white light while having a large color gamut. 
     The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the invention may be implemented in any device that is configured to display an image, whether in motion (e.g., video) or stationary (e.g., still image), and whether textual or pictorial. More particularly, it is contemplated that the invention may be implemented in or associated with a variety of electronic devices such as, but not limited to, mobile telephones, wireless devices, personal data assistants (PDAs), hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, computer monitors, auto displays (e.g., odometer display, etc.), cockpit controls and/or displays, display of camera views (e.g., display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, packaging, and aesthetic structures (e.g., display of images on a piece of jewelry). MEMS devices of similar structure to those described herein can also be used in non-display applications such as in electronic switching devices. 
     One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated in  FIG. 1 . In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“off” or “closed”) state, the display element reflects little incident visible light to the user. Depending on the embodiment, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at selected colors, allowing for a color display in addition to black and white. 
       FIG. 1  is an isometric view depicting two adjacent pixels in a series of pixels of a visual display, wherein each pixel comprises a MEMS interferometric modulator. In some embodiments, an interferometric modulator display comprises a row/column array of these interferometric modulators. Each interferometric modulator includes a pair of reflective layers positioned at a variable and controllable distance from each other to form a resonant optical cavity with at least one variable dimension. In one embodiment, one of the reflective layers may be moved between two positions. In the first position, referred to herein as the released state, the movable layer is positioned at a relatively large distance from a fixed partially reflective layer. In the second position, the movable layer is positioned more closely adjacent to the partially reflective layer. Incident light that reflects from the two layers interferes constructively or destructively depending on the position of the movable reflective layer, producing either an overall reflective or non-reflective state for each pixel. 
     The depicted portion of the pixel array in  FIG. 1  includes two adjacent interferometric modulators  12   a  and  12   b . In the interferometric modulator  12   a  on the left, a movable and highly reflective layer  14   a  is illustrated in a released position at a predetermined distance from a fixed partially reflective layer  16   a . In the interferometric modulator  12   b  on the right, the movable highly reflective layer  14   b  is illustrated in an actuated position adjacent to the fixed partially reflective layer  16   b.    
     The fixed layers  16   a ,  16   b  are electrically conductive, partially transparent and partially reflective, and may be fabricated, for example, by depositing one or more layers each of chromium and indium-tin-oxide onto a transparent substrate  20 . The layers are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable layers  14   a ,  14   b  may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes  16   a ,  16   b ) deposited on top of posts  18  and an intervening sacrificial material deposited between the posts  18 . When the sacrificial material is etched away, the deformable metal layers are separated from the fixed metal layers by a defined air gap  19 . A highly conductive and reflective material such as aluminum may be used for the deformable layers, and these strips may form column electrodes in a display device. 
     With no applied voltage, the cavity  19  remains between the layers  14   a ,  16   a  and the deformable layer is in a mechanically relaxed state as illustrated by the pixel  12   a  in  FIG. 1 . However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel becomes charged, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable layer is deformed and is forced against the fixed layer (a dielectric material which is not illustrated in this Figure may be deposited on the fixed layer to prevent shorting and control the separation distance) as illustrated by the pixel  12   b  on the right in  FIG. 1 . The behavior is the same regardless of the polarity of the applied potential difference. In this way, row/column actuation that can control the reflective vs. non-reflective pixel states is analogous in many ways to that used in conventional LCD and other display technologies. 
       FIGS. 2 through 5  illustrate one exemplary process and system for using an array of interferometric modulators in a display application.  FIG. 2  is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor  21  which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor  21  may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application. 
     In one embodiment, the processor  21  is also configured to communicate with an array controller  22 . In one embodiment, the array controller  22  includes a row driver circuit  24  and a column driver circuit  26  that provide signals to a pixel array  30 . The cross section of the array illustrated in  FIG. 1  is shown by the lines  1 - 1  in  FIG. 2 . For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices illustrated in  FIG. 3 . It may require, for example, a 10 volt potential difference to cause a movable layer to deform from the released state to the actuated state. However, when the voltage is reduced from that value, the movable layer maintains its state as the voltage drops back below 10 volts. In the exemplary embodiment of  FIG. 3 , the movable layer does not release completely until the voltage drops below 2 volts. There is thus a range of voltage, about 3 to 7 V in the example illustrated in  FIG. 3 , where there exists a window of applied voltage within which the device is stable in either the released or actuated state. This is referred to herein as the “hysteresis window” or “stability window.” For a display array having the hysteresis characteristics of  FIG. 3 , the row/column actuation protocol can be designed such that during row strobing, pixels in the strobed row that are to be actuated are exposed to a voltage difference of about 10 volts, and pixels that are to be released are exposed to a voltage difference of close to zero volts. After the strobe, the pixels are exposed to a steady state voltage difference of about 5 volts such that they remain in whatever state the row strobe put them in. After being written, each pixel sees a potential difference within the “stability window” of 3-7 volts in this example. This feature makes the pixel design illustrated in  FIG. 1  stable under the same applied voltage conditions in either an actuated or released pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or released state, is essentially a capacitor formed by the fixed and moving reflective layers, this stable state can be held at a voltage within the hysteresis window with almost no power dissipation. Essentially no current flows into the pixel if the applied potential is fixed. 
     In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row 1 electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row 2 electrode, actuating the appropriate pixels in row 2 in accordance with the asserted column electrodes. The row 1 pixels are unaffected by the row 2 pulse, and remain in the state they were set to during the row 1 pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used in conjunction with the present invention. 
       FIGS. 4 and 5  illustrate one possible actuation protocol for creating a display frame on the 3×3 array of  FIG. 2 .  FIG. 4  illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves of  FIG. 3 . In the  FIG. 4  embodiment, actuating a pixel involves setting the appropriate column to −V bias , and the appropriate row to +ΔV, which may correspond to −5 volts and +5 volts respectively Releasing the pixel is accomplished by setting the appropriate column to +V bias , and the appropriate row to the same +ΔV, producing a zero volt potential difference across the pixel. In those rows where the row voltage is held at zero volts, the pixels are stable in whatever state they were originally in, regardless of whether the column is at +V bias , or −V bias . As is also illustrated in  FIG. 4 , it will be appreciated that voltages of opposite polarity than those described above can be used, e.g., actuating a pixel can involve setting the appropriate column to +V bias , and the appropriate row to −ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −V bias , and the appropriate row to the same −ΔV, producing a zero volt potential difference across the pixel. 
       FIG. 5B  is a timing diagram showing a series of row and column signals applied to the 3×3 array of  FIG. 2  which will result in the display arrangement illustrated in  FIG. 5A , where actuated pixels are non-reflective. Prior to writing the frame illustrated in  FIG. 5A , the pixels can be in any state, and in this example, all the rows are at 0 volts, and all the columns are at +5 volts. With these applied voltages, all pixels are stable in their existing actuated or released states. 
     In the  FIG. 5A  frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) are actuated. To accomplish this, during a “line time” for row 1, columns 1 and 2 are set to −5 volts, and column 3 is set to +5 volts. This does not change the state of any pixels, because all the pixels remain in the 3-7 volt stability window. Row 1 is then strobed with a pulse that goes from 0, up to 5 volts, and back to zero. This actuates the (1,1) and (1,2) pixels and releases the (1,3) pixel. No other pixels in the array are affected. To set row 2 as desired, column 2 is set to −5 volts, and columns 1 and 3 are set to +5 volts. The same strobe applied to row 2 will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again, no other pixels of the array are affected. Row 3 is similarly set by setting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3 strobe sets the row 3 pixels as shown in  FIG. 5A . After writing the frame, the row potentials are zero, and the column potentials can remain at either +5 or −5 volts, and the display is then stable in the arrangement of  FIG. 5A . It will be appreciated that the same procedure can be employed for arrays of dozens or hundreds of rows and columns. It will also be appreciated that the timing, sequence, and levels of voltages used to perform row and column actuation can be varied widely within the general principles outlined above, and the above example is exemplary only, and any actuation voltage method can be used with the present invention. 
     The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,  FIGS. 6A-6C  illustrate three different embodiments of the moving mirror structure.  FIG. 6A  is a cross section of the embodiment of  FIG. 1 , where a strip of metal material  14  is deposited on orthogonally extending supports  18 . In  FIG. 6B , the moveable reflective material  14  is attached to supports at the corners only, on tethers  32 . In  FIG. 6C , the moveable reflective material  14  is suspended from a deformable layer  34 . This embodiment has benefits because the structural design and materials used for the reflective material  14  can be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer  34  can be optimized with respect to desired mechanical properties. The production of various types of interferometric devices is described in a variety of published documents, including, for example, U.S. Published Application 2004/0051929. A wide variety of well known techniques may be used to produce the above described structures involving a series of material deposition, patterning, and etching steps. 
     Embodiments of interferometric modulators described above operate in one of a reflective state, which produces white light, or light of a color determined by the distance between the mirrors  14  and  16 , or in a non-reflective, e.g., black, state. In other embodiments, for example, embodiments disclosed in U.S. Pat. No. 5,986,796, the movable mirror  14  may be positioned at a range of positions relative to the fixed mirror  16  to vary the size of the resonant gap  19 , and thus the color of reflected light. 
       FIG. 7  is a side cross-sectional view of an exemplary interferometric modulator  12  that illustrates the spectral characteristics of light that would be produced by positioning the movable mirror  14  at a range of positions  111 - 115 . As discussed above, a potential difference between a row and column electrode causes the movable mirror  14  to deflect. The exemplary modulator includes a conductive layer  102  of indium-tin-oxide (ITO) acting as a column electrode. In the exemplary modulator, the mirror  14  includes the row conductor. 
     In one embodiment, a dielectric layer  104  of a material such as alumina (Al 2 O 3 ) is positioned over a layer of chrome that forms a reflective surface of the mirror  16 . As discussed above with reference to  FIG. 1 , the dielectric layer  104  prevents shorting and controls the separation distance between the mirrors  14  and  16  when the mirror  14  deflects. The optical cavity formed between the mirrors  14  and  16  thus includes the dielectric layer  104 . The relative sizes of items in  FIG. 7  have been selected for purposes of conveniently illustrating the modulator  12 . Thus, such distances are not to scale and are not intended to be representative of any particular embodiment of the modulator  12 . 
       FIG. 8  is a graphical illustration of reflectivity versus wavelength for the mirrors  16  of several exemplary optical stacks. The horizontal axis represents a range of wavelengths of visible light incident on the optical stacks. The vertical axis represents the reflectivity of the optical stack as a percentage of incident light at a particular wavelength. In one embodiment, in which the optical stack does not include the dielectric layer  104 , the reflectivity of the mirror  16  formed of a layer of chrome is approximately 75%. An optical stack including a dielectric layer  104  comprising a 100 Å layer of alumina results in 65% reflectivity and a dielectric layer  104  comprising a 200 Å layer of alumina results in 55% reflectivity. As shown, reflectivity does not vary according to wavelength in these particular embodiments. Accordingly, by adjusting the thickness of an Al 2 O 3  layer, the reflectivity of the mirror  16  can be controlled consistently across the visible spectrum to allow specific properties of interferometric modulators  12  to be selected. In certain embodiments, the dielectric layer  104  is a layer of Al 2 O 3 , having a thickness in the range of 50-250 Å. In other embodiments, the dielectric layer  104  comprises a thin layer of Al 2 O 3 , having a thickness in the range of 50-100 Å and a layer of bulk SiO 2 , having a thickness in the range of 400-2000 Å. 
     As discussed above, the modulator  12  includes an optical cavity formed between the mirrors  14  and  16 . The characteristic distance, or effective optical path length, L, of the optical cavity determines the resonant wavelengths, λ, of the optical cavity and thus of the interferometric modulator  12 . The resonant wavelength, λ, of the interferometric modulator  12  generally corresponds to the perceived color of light reflected by the modulator  12 . Mathematically, the distance L=½ N λ, where N is an integer. A given resonant wavelength, λ, is thus reflected by interferometric modulators  12  having distances L of ½λ (N=1), λ (N=2), 3/2λ (N=3), etc. The integer N may be referred to as the order of interference of the reflected light. As used herein, the order of a modulator  12  also refers to the order N of light reflected by the modulator  12  when the mirror  14  is in at least one position. For example, a first order red interferometric modulator  12  may have a distance L of about 325 nm, corresponding to a wavelength λ of about 650 nm. Accordingly, a second order red interferometric modulator  12  may have a distance L of about 650 nm. Generally, higher order modulators  12  reflect light over a narrower range of wavelengths and thus produce colored light that is more saturated. 
     Note that in certain embodiments, the distance, L, is substantially equal to the distance between the mirrors  14  and  16 . Where the space between the mirrors  14  and  16  comprises only a gas (e.g., air) having an index of refraction of approximately 1, the effective optical path length is substantially equal to the distance between the mirrors  14  and  16 . In embodiments that include the dielectric layer  104 , which has an index of refraction greater than one, the optical cavity is formed to have the desired optical path length by selecting the distance between the mirrors  14  and  16  and by selecting the thickness and index of refraction of the dielectric layer  104 , or of any other layers between the mirrors  14  and  16 . In one embodiment, the mirror  14  may be deflected one or more positions within a range of positions to output a corresponding range of colors. For example, the voltage potential difference between the row and column electrodes may be adjusted to deflect the mirror  14  to one of a range of positions in relation to the mirror  16 . In general, the greatest level of control of the position of the mirror by adjusting voltage is near the undeflected position of the path of the mirror  14  (for example, for smaller deflections, such as deflections within about ⅓rd of the maximum deflection from the undeflected position of the mirror  14 ). 
     Each of a particular group of positions  111 - 115  of the movable mirror  14  is denoted in  FIG. 7  by a line extending from the fixed mirror  16  to an arrow point indicating the positions  111 - 115 . Thus, the distances  111 - 115  are selected so as to account for the thickness and index of refraction of the dielectric layer  104 . When the movable mirror  14  deflects to each of the positions  111 - 115 , each corresponding to a different distance, L, the modulator outputs light to a viewing position  101  with a different spectral response that corresponds to different colors of incident light being reflected by the modulator  12 . Moreover, at position  111 , the movable mirror  14  is sufficiently close to the fixed mirror  16 , that the effects of interference are negligible and modulator  12  acts as a mirror that reflects substantially all colors of incident visible light substantially equally, e.g., as white light. The broadband mirror effect is caused because the small distance L is too small for optical resonance in the visible band. The mirror  14  thus merely acts as a reflective surface with respect to visible light. 
     As the gap is increased to the position  112 , the modulator  12  exhibits a shade of gray as the increased gap distance between the mirrors  14  and  16  reduces the reflectivity of the mirror  14 . At the position  113 , the distance L is such that the cavity operates interferometrically but reflects substantially no visible wavelengths of light because the resonant wavelength is outside the visible range. 
     As the distance L is increased further, a peak spectral response of the modulator  12  moves into visible wavelengths. Thus, when the movable mirror  14  is at position  114 , the modulator  12  reflects blue light. When the movable mirror  14  is at the position  115 , the modulator  12  reflects green light. When the movable mirror  14  is at the non-deflected position  116 , the modulator  12  reflects red light. 
     In designing a display using interferometric modulators  12 , the modulators  12  may be formed so as to increase the color saturation of reflected light. Saturation refers to the intensity of the hue of color light. A highly saturated hue has a vivid, intense color, while a less saturated hue appears more muted and grey. For example, a laser, which produces a very narrow range of wavelengths, produces highly saturated light. Conversely, a typical incandescent light bulb produces white light that may have a desaturated red or blue color. In one embodiment, the modulator  12  is formed with a distance L corresponding to higher order of interference, e.g., 2nd or 3rd order, to increase the saturation of reflected color light. 
     An exemplary color display includes red, green, and blue display elements. Other colors are produced in such a display by varying the relative intensity of light produced by the red, green, and blue elements. Such mixtures of primary colors such as red, green, and blue are perceived by the human eye as other colors. The relative values of red, green, and blue in such a color system may be referred to as tristimulus values in reference to the stimulation of red, green, and blue light sensitive portions of the human eye. In general, the more saturated the primary colors, the greater the range of colors that can be produced by the display. In other embodiments, the display may include modulators  12  having sets of colors that define other color systems in terms of sets of primary colors other than red, green, and blue. 
       FIG. 9  is a chromaticity diagram that illustrates the colors that can be produced by a color display that includes two sets of exemplary red, green, and blue interferometric modulators. The horizontal and vertical axes define a chromaticity coordinate system on which spectral tristimulus values may be depicted. In particular, points  120  illustrate the color of light reflected by exemplary red, green, and blue interferometric modulators. White light is indicated by a point  122 . The distance from each point  120  to the point  122  of white light, e.g., the distance  124  between the point  122  for white and the point  120  for green light, is indicative of the saturation of light produced by the corresponding modulator  12 . The region enclosed by the triangular trace  126  corresponds to the range of colors that can be produced by mixing the light produced at points  120 . This range of colors may be referred to as the color gamut of the display. 
     Points  128  indicate the spectral response of another set of exemplary modulators  12 . As indicated by the smaller distance between the points  128  and the white point  122  than between points  120  and point  122 , the modulators  12  corresponding to the points  128  produce less saturated light that do the modulators  12  corresponding to the points  120 . The trace  130  indicates the range of colors that can be produced by mixing the light of points  128 . As is shown in  FIG. 9 , the trace  126  encloses a larger area than does the trace  130 , graphically illustrating the relationship between the saturation of the display elements and the size of the color gamut of the display. 
     In a reflective display, white light produced using such saturated interferometric modulators tends to have a relatively low intensity to a viewer because only a small range of incident wavelengths is reflected to form the white light. In contrast, a mirror reflecting broadband white light, e.g., substantially all incident wavelengths, has a greater intensity because a greater range of incident wavelengths is reflected. Thus, designing reflective displays using combinations of primary colors to produce white light generally results in a tradeoff between color saturation and color gamut and the brightness of white light output by the display. 
       FIG. 10  is a side cross-sectional view of an exemplary multistate interferometric modulator  140  that can produce highly saturated color light in one state and relatively intense white light in another state. The exemplary modulator  140  thus decouples color saturation from the brightness of output white light. The modulator  140  includes a movable mirror  14  that is positioned between two electrodes  102  and  142 . The modulator  140  also includes a second set of posts  18   a  that are formed on the opposite side of the mirror  14  as the posts  18 . 
     In certain embodiments, each of the mirrors  14  and  16  may be part of a stack of layers defining a reflector or reflective member that perform functions other than reflecting light. For example, in the exemplary modulator of  FIG. 10 , the mirror  14  is formed of one or more layers of a conductive and reflective material such as aluminum. Thus, the mirror  14  may also function as a conductor. Similarly, the mirror  16  may be formed of one or more layers of reflective material and one or more layers of an electrically conductive material so as to perform the functions of the electrode  102 . Furthermore, each of the mirrors  14  and  16  may also include one or more layers having other functions, such as to control the mechanical properties affecting deflection of the mirror  14 . In one embodiment, the moveable mirror  14  is suspended from an additional deformable layer such is described in connection with  FIG. 6C . 
     In one embodiment that includes modulators that reflect red, green, and blue light, different reflective materials are used for modulators that reflect different colors so as to improve the spectral response of such modulators  12 . For example, the movable mirror  14  may include gold in the modulators  12  configured to reflect red light. 
     In one embodiment, dielectric layers  144  may be positioned on either side of the conductor  142 . The dielectric layers  144   a  and  104  advantageously prevent electrical shorts between conductive portions of the mirror  14  and other portions of the modulator  140 . In one embodiment, the mirror  16  and the electrode  102  collectively form a reflective member. 
     In the exemplary embodiment, the distance between fixed mirror  16  and the movable mirror  14  in its undriven position corresponds to the optical path length L in which the modulator  140  is non-reflective or “black.” In the exemplary embodiment, the optical path length between the fixed mirror  16  and the movable mirror  14  when driven towards the fixed mirror  16  corresponds to the optical path length L in which the modulator  140  reflects white light. In the exemplary embodiment, the distance between the fixed mirror  16  and the movable mirror  14  when driven towards the conductor  142  corresponds to the optical path length L in which the modulator  140  reflects light of a color such as red, blue, or green. In certain embodiments, the distance between the undriven movable mirror  14  and the fixed mirror  16  is substantially equal to the distance between the undriven movable mirror  14  and the electrode  142 . Such embodiments may be considered to be two modulators positioned around the single movable mirror  14 . 
     When a first voltage potential difference is applied between the mirror  14  and the electrode  102 , the mirror  14  deflects towards the mirror  16  to define a first optical path length, L, that corresponds to a first driven state. In this first driven state, the movable mirror  14  is closer to the mirror  16  than in the undriven state. When a second voltage potential difference is applied between the mirror  14  and the electrode  142 , the mirror  14  is deflected away from the mirror  16  to define a second optical path length, L, that corresponds to a second driven state. In this second driven state, the movable mirror  14  is farther from the mirror  16  than in the undriven state. In certain embodiments, at least one of the first driven state and second driven state is achieved by applying voltage potential differences both between the mirror  14  and the electrode  102  and between the mirror  14  and the electrode  142 . In certain embodiments, the second voltage difference is selected to provide a desired deflection of the mirror  14 . 
     As illustrated in  FIG. 10 , in the first driven state, the mirror  14  deflects to a position indicated by the dashed line  152 . In the exemplary modulator  140 , the distance between the mirrors  14  and  16  in this first driven state corresponds to the thickness of the dielectric layer  104 . In the exemplary modulator  140 , the mirror  14  acts as a broadband mirror in this position, substantially reflecting all visible wavelengths of light. As such, the modulator  140  produces a broadband white light when illuminated by broadband white light. 
     In the second driven state, the mirror  14  deflects to a position indicated by the dashed line  154 . In the exemplary modulator  140 , this distance corresponds to a color of light, e.g., blue light. In the undriven state, the mirror  14  is positioned as shown in  FIG. 10 . In the undeflected position, the mirror  14  is spaced at a distance from the mirror  16  so that substantially no visible light is reflected, e.g., an “off” or non-reflective state. Thus, the modulator  140  defines an interferometric modulator having at least three discrete states. In other embodiments, the positions of the movable mirror  14  in the three states may be selected so as to produce different sets of colors, including black and white, as desired. 
     In one embodiment, light enters the modulator  12  through the substrate  20  and is output to a viewing position  141 . In another embodiment, the stack of layers illustrated in  FIG. 10  is reversed, with layer  144  closest to the substrate  20  rather than layer  102 . In certain such embodiments, the modulator  12  may be viewed through the opposite side of the stack from the substrate  20  rather than through the substrate  20 . In one such embodiment, a layer of silicon dioxide is formed on the ITO layer  102  to electrically isolate the ITO layer  102 . 
     As noted above, having a separate state for outputting white light in a modulator  140  decouples the selection of the properties of the modulator controlling color saturation from the properties affecting the brightness of white output. The distance and other characteristics of the modulator  140  may thus be selected to provide a highly saturated color without affecting the white light produced in the first state. For example, in an exemplary color display, one or more of the red, green, and blue modulators  12  may be formed with optical path lengths L corresponding to a higher order of interference. 
     The modulator  140  may be formed using lithographic techniques known in the art, and such as described above with reference to the modulator  12 . For example, the fixed mirror  16  may be formed by depositing one or more layers of chromium onto the substantially transparent substrate  20 . The electrode  102  may be formed by depositing one or more layers of a transparent conductor such as ITO onto the substrate  20 . The conductor layers are patterned into parallel strips, and may form columns of electrodes. The movable mirror  14  may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the column electrodes  102 ) deposited on top of posts  18  and an intervening sacrificial material deposited between the posts  18 . Vias through one or more of the layers described above may be provided so that etchant gas, such as xenon diflouride, can reach the sacrificial layers. When the sacrificial material is etched away, the deformable metal layers are separated from the fixed layers by an air gap. A highly conductive and reflective material such as aluminum may be used for the deformable layers, and these strips may form row electrodes in a display device. The conductor  142  may be formed by depositing posts  18   a  over the movable mirror  14 , depositing an intervening sacrificial material between the posts  18   a , depositing one or more layers of a conductor such as aluminum on top of the posts  18   a , and depositing a conductive layer over the sacrificial material. When the sacrificial material is etched away, the conductive layer can serve as the electrode  142  which is separated from the mirror  14  by a second air gap. Each of the air gaps provides a cavity in which the mirror  14  may move to achieve each of the states described above. 
     As further illustrated in  FIG. 10 , in the exemplary modulator  140 , the conductive mirror  14  is connected to the row driver  24  of the array controller  22 . In the exemplary modulator  140 , the conductors  102  and  142  are connected to separate columns in the column driver  26 . In one embodiment, the state of the modulator  140  is selected by applying the appropriate voltage potential differences between the mirror  14  and the column conductors  102  and  142  according to the method described with reference to  FIGS. 3 and 4 . 
       FIGS. 11A-11C  illustrates another exemplary interferometric modulator  150  that provides more than two states. In the exemplary modulator  150 , the mirror  16  includes both a reflective layer and a conductive layer so as to perform the function of the electrode  102  of  FIG. 10 . The conductive layer  142  can also be protected by a second dielectric layer  144   a  and supported by a support surface  148  that is maintained some distance above the movable mirror  14  through a second set of supports  18   a.    
       FIG. 11A  illustrates the undriven state of the modulator  150 . As with the modulator  140  of  FIG. 10 , the mirror  14  of the exemplary modulator  150  of  FIGS. 11A-11C  is deflectable towards the dielectric layer  104  (e.g., downwards), as in the driven state illustrated  FIG. 11B , and is deflectable in the reverse or opposite direction (e.g., upwards), as illustrated in  FIG. 11C . This “upwardly” deflected state may be called the “reverse driven state.” 
     As will be appreciated by one of skill in the art, this reverse driven state can be achieved in a number of ways. In one embodiment, the reverse driven state is achieved through the use of an additional charge plate or conductive layer  142  that can electrostatically pull the mirror  16  in the upward direction, as depicted in  FIG. 11C . The exemplary modulator  150  includes what is basically two interferometric modulators positioned symmetrically around a single movable mirror  14 . This configuration allows each of the conductive layer of the mirror  16  and the conductive layer  142  to attract the mirror  14  in opposite directions. 
     In certain embodiments, the additional conductive layer  142  may be useful as an electrode in overcoming stictional forces (static friction) that may develop when the mirror  14  comes in close proximity, or contacts, the dielectric layer  104 . These forces can include van der Waals or electrostatic forces, as well as other possibilities as appreciated by one of skill in the art. In one embodiment, a voltage pulse applied to the conductive layer of the mirror  16  may send the movable mirror  14  into the “normal” driven state of  FIG. 11B . Similarly, the next voltage pulse can be applied to the conductive layer  142  to attract the movable mirror  14  away from the mirror  16 . In certain embodiments, such a voltage pulse applied to the conductive layer  142  can be used to accelerate the recovery of the movable mirror  14  back to the undriven state illustrated in  FIG. 11A  from the driven state illustrated in  FIG. 11B  by driving the movable mirror  14  towards the reverse driven state. Thus, in certain embodiments, the modulator  150  may operate in only two states, the undriven state of  FIG. 11A  and the driven state of  FIG. 11B , and can employ the conductive layer  142  as an electrode to help overcome stictional forces. In one embodiment, the conductive layer  142  may be driven as described above each time that the modulator  150  changes from the driven position of  FIG. 11C  to the undriven position of  FIG. 11A . 
     As will be appreciated by one of skill in the art, not all of these elements will be required in every embodiment. For example, if the precise relative amount of upward deflection (e.g., as shown in  FIG. 11C ) is not relevant in the operation of such embodiments, then the conductive layer  142  can be positioned at various distances from the movable mirror  14 . Thus, there may be no need for support elements  18   a , the dielectric layer  144   a , or a separate support surface  148 . In these embodiments, it is not necessarily important how far upward the movable mirror  14  deflects, but rather that the conductive layer  142  is positioned to attract the mirror  14  at the appropriate time, such as to unstick the modulator  12 . In other embodiments, the position of the movable mirror  14  as shown in  FIG. 11C , may result in altered and desirable optical characteristics for the interferometric modulator. In these embodiments, the precise distance of deflection of the movable mirror  14  in the upward direction can be relevant in improving the image quality of the device. 
     As will be appreciated by one of skill in the art, the materials used to produce the layers  142 ,  144   a , and support surface  148  need not be similar to the materials used to produce the corresponding layers  16 ,  105  and  20 . For example, light need not pass through the layer  148 . Additionally, if the conductive layer  142  is positioned beyond the reach of the movable mirror  14  in its deformed upward position, then the modulator  150  may not include the dielectric layer  144   a . Additionally, the voltages applied to the conductive layer  142  and the movable mirror  14  can be accordingly different based on the above differences. 
     As will be appreciated by one of skill in the art, the voltage applied to drive the movable mirror  14  from the driven state of  FIG. 11B , back to the undriven state of  FIG. 11A , may be different than that required to drive the movable mirror  14  from the undriven state of  FIG. 11A  to the upward or reverse driven state of  FIG. 11C , as the distance between the conductive layer  142  and movable mirror  14  may be different in the two states. Such requirements can depend upon the desired application and amounts of deflection, and can be determined by one of skill in the art in view of the present disclosure. 
     In some embodiments, the amount of force or duration that a force is applied between the conductive layer  142  and the movable mirror  14  is such that it only increases the rate at which the interferometric modulator transitions between the driven state and the undriven state. Since the movable mirror  14  can be attracted to either conductive layer  142  or the conductive mirror  16 , which are located on opposite sides of movable mirror  14 , a very brief driving force can be provided to weaken the interaction of movable mirror  14  with the opposite layer. For example, as the movable mirror  14  is driven to interact with fixed conductive mirror  16 , a pulse of energy to the opposite conductive layer  142  can be used to weaken the interaction of the movable mirror  14  and the fixed mirror  16 , thereby make it easier for the movable mirror  14  to move to the undriven state. 
     While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.