Patent Publication Number: US-2012044563-A1

Title: Reduced capacitance display element

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/155,939, filed Jun. 17, 2005, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/613,542, filed on Sep. 27, 2004, both of which are hereby expressly incorporated by reference herein in their entirety. In addition, this application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/613,488, filed on Sep. 27, 2004. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention relates to microelectromechanical systems (MEMS). 
     DESCRIPTION OF THE RELATED TECHNOLOGY 
     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. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In certain embodiments, 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. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. 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 OF CERTAIN EMBODIMENTS 
     The systems, methods, 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 Certain Embodiments” one will understand how the features of this invention provide advantages over other display devices. 
     In some embodiments, a display element comprises: a substantially transparent conductive layer; a partially reflective insulator having a thickness of between about 40 and 150 Angstroms; a moveable reflective layer, the partially reflective insulator being positioned between the conductive layer and the moveable reflective layer, wherein a voltage applied between the conductive layer and the moveable reflective layer induces movement of the moveable reflective layer; and a first dielectric layer positioned between the conductive layer and the partially reflective insulator. 
     In some embodiments, a method of forming a display element comprises: forming a substantially transparent conductive layer; forming a moveable reflective layer; forming a partially reflective insulator having a thickness of between about 40 and 150 Angstroms, the partially reflective insulator being formed between the conductive layer and the moveable reflective layer, wherein a voltage applied between the conductive layer and the moveable reflective layer induces movement of the moveable reflective layer; and forming a first dielectric layer between the conductive layer and the partially reflective insulator. 
     In some embodiments, a display element comprises: first means for transmitting light and conducting electricity; second means for partially reflecting light and insulating, the second means having a thickness of between about 40 and 150 Angstroms; third moveable means for reflecting light, the second means being positioned between the first means and the third means, wherein a voltage applied between the first means and the third means induces movement of the third means; and fourth dielectric means positioned between the first means and the second means. 
    
    
     
       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 relaxed 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 minor 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.times.3 interferometric modulator display of  FIG. 2 . 
         FIGS. 6A and 6B  are system block diagrams illustrating an embodiment of a visual display device comprising a plurality of interferometric modulators. 
         FIG. 7A  is a cross section of the device of  FIG. 1 . 
         FIG. 7B  is a cross section of an alternative embodiment of an interferometric modulator. 
         FIG. 7C  is a cross section of another alternative embodiment of an interferometric modulator. 
         FIG. 7D  is a cross section of yet another alternative embodiment of an interferometric modulator. 
         FIG. 7E  is a cross section of an additional alternative embodiment of an interferometric modulator. 
         FIG. 8  is a cross-section of an exemplary interferometric modulator having a transparent conductor. 
         FIG. 9  is a cross-sectional view of an exemplary reduced capacitance interferometric modulator. 
         FIG. 10  is a cross-sectional view of another exemplary reduced capacitance interferometric modulator. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     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 embodiments 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 embodiments 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 relaxed position, the movable reflective layer is positioned at a relatively large distance from a fixed partially reflective layer. In the second position, referred to herein as the actuated position, the movable reflective 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 reflective layer  14   a  is illustrated in a relaxed position at a predetermined distance from an optical stack  16   a,  which includes a partially reflective layer. In the interferometric modulator  12   b  on the right, the movable reflective layer  14   b  is illustrated in an actuated position adjacent to the optical stack  16   b.    
     The optical stacks  16   a  and  16   b  (collectively referred to as optical stack  16 ), as referenced herein, typically comprise of several fused layers, which can include an electrode layer, such as indium tin oxide (ITO), a partially reflective layer, such as chromium, and a transparent dielectric. The optical stack  16  is thus electrically conductive, partially transparent and partially reflective, and may be fabricated, for example, by depositing one or more of the above layers onto a transparent substrate  20 . In some embodiments, the layers are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective 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 of  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 movable reflective layers  14   a,    14   b  are separated from the optical stacks  16   a,    16   b  by a defined gap  19 . A highly conductive and reflective material such as aluminum may be used for the reflective layers  14 , and these strips may form column electrodes in a display device. 
     With no applied voltage, the cavity  19  remains between the movable reflective layer  14   a  and optical stack  16   a,  with the movable reflective layer  14   a  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 reflective layer  14  is deformed and is forced against the optical stack  16 . A dielectric layer (not illustrated in this Figure) within the optical stack  16  may prevent shorting and control the separation distance between layers  14  and  16 , as illustrated by 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 driver  22 . In one embodiment, the array driver  22  includes a row driver circuit  24  and a column driver circuit  26  that provide signals to a display array or panel  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 relaxed 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 relax 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 relaxed 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 relaxed 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 relaxed pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or relaxed 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 Relaxing 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. 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.times.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 relaxed 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 relaxes 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 relax 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 systems and methods described herein. 
       FIGS. 6A and 6B  are system block diagrams illustrating an embodiment of a display device  40 . The display device  40  can be, for example, a cellular or mobile telephone. However, the same components of display device  40  or slight variations thereof are also illustrative of various types of display devices such as televisions and portable media players. 
     The display device  40  includes a housing  41 , a display  30 , an antenna  43 , a speaker  44 , an input device  48 , and a microphone  46 . The housing  41  is generally formed from any of a variety of manufacturing processes as are well known to those of skill in the art, including injection molding, and vacuum forming. In addition, the housing  41  may be made from any of a variety of materials, including but not limited to plastic, metal, glass, rubber, and ceramic, or a combination thereof. In one embodiment the housing  41  includes removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols. 
     The display  30  of exemplary display device  40  may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display  30  includes a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD as described above, or a non-flat-panel display, such as a CRT or other tube device, as is well known to those of skill in the art. However, for purposes of describing the present embodiment, the display  30  includes an interferometric modulator display, as described herein. 
     The components of one embodiment of exemplary display device  40  are schematically illustrated in  FIG. 6B . The illustrated exemplary display device  40  includes a housing  41  and can include additional components at least partially enclosed therein. For example, in one embodiment, the exemplary display device  40  includes a network interface  27  that includes an antenna  43  which is coupled to a transceiver  47 . The transceiver  47  is connected to a processor  21 , which is connected to conditioning hardware  52 . The conditioning hardware  52  may be configured to condition a signal (e.g. filter a signal). The conditioning hardware  52  is connected to a speaker  45  and a microphone  46 . The processor  21  is also connected to an input device  48  and a driver controller  29 . The driver controller  29  is coupled to a frame buffer  28 , and to an array driver  22 , which in turn is coupled to a display array  30 . A power supply  50  provides power to all components as required by the particular exemplary display device  40  design. 
     The network interface  27  includes the antenna  43  and the transceiver  47  so that the exemplary display device  40  can communicate with one ore more devices over a network. In one embodiment the network interface  27  may also have some processing capabilities to relieve requirements of the processor  21 . The antenna  43  is any antenna known to those of skill in the art for transmitting and receiving signals. In one embodiment, the antenna transmits and receives RF signals according to the IEEE 802.11 standard, including IEEE 802.11(a), (b), or (g). In another embodiment, the antenna transmits and receives RF signals according to the BLUETOOTH standard. In the case of a cellular telephone, the antenna is designed to receive CDMA, GSM, AMPS or other known signals that are used to communicate within a wireless cell phone network. The transceiver  47  pre-processes the signals received from the antenna  43  so that they may be received by and further manipulated by the processor  21 . The transceiver  47  also processes signals received from the processor  21  so that they may be transmitted from the exemplary display device  40  via the antenna  43 . 
     In an alternative embodiment, the transceiver  47  can be replaced by a receiver. In yet another alternative embodiment, network interface  27  can be replaced by an image source, which can store or generate image data to be sent to the processor  21 . For example, the image source can be a digital video disc (DVD) or a hard-disc drive that contains image data, or a software module that generates image data. 
     Processor  21  generally controls the overall operation of the exemplary display device  40 . The processor  21  receives data, such as compressed image data from the network interface  27  or an image source, and processes the data into raw image data or into a format that is readily processed into raw image data. The processor  21  then sends the processed data to the driver controller  29  or to frame buffer  28  for storage. Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation, and gray-scale level. 
     In one embodiment, the processor  21  includes a microcontroller, CPU, or logic unit to control operation of the exemplary display device  40 . Conditioning hardware  52  generally includes amplifiers and filters for transmitting signals to the speaker  45 , and for receiving signals from the microphone  46 . Conditioning hardware  52  may be discrete components within the exemplary display device  40 , or may be incorporated within the processor  21  or other components. 
     The driver controller  29  takes the raw image data generated by the processor  21  either directly from the processor  21  or from the frame buffer  28  and reformats the raw image data appropriately for high speed transmission to the array driver  22 . Specifically, the driver controller  29  reformats the raw image data into a data flow having a raster-like format, such that it has a time order suitable for scanning across the display array  30 . Then the driver controller  29  sends the formatted information to the array driver  22 . Although a driver controller  29 , such as a LCD controller, is often associated with the system processor  21  as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. They may be embedded in the processor  21  as hardware, embedded in the processor  21  as software, or fully integrated in hardware with the array driver  22 . 
     Typically, the array driver  22  receives the formatted information from the driver controller  29  and reformats the video data into a parallel set of waveforms that are applied many times per second to the hundreds and sometimes thousands of leads coming from the display&#39;s x-y matrix of pixels. 
     In one embodiment, the driver controller  29 , array driver  22 , and display array  30  are appropriate for any of the types of displays described herein. For example, in one embodiment, driver controller  29  is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, array driver  22  is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, a driver controller  29  is integrated with the array driver  22 . Such an embodiment is common in highly integrated systems such as cellular phones, watches, and other small area displays. In yet another embodiment, display array  30  is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators). 
     The input device  48  allows a user to control the operation of the exemplary display device  40 . In one embodiment, input device  48  includes a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a touch-sensitive screen, a pressure- or heat-sensitive membrane. In one embodiment, the microphone  46  is an input device for the exemplary display device  40 . When the microphone  46  is used to input data to the device, voice commands may be provided by a user for controlling operations of the exemplary display device  40 . 
     Power supply  50  can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, power supply  50  is a rechargeable battery, such as a nickel-cadmium battery or a lithium ion battery. In another embodiment, power supply  50  is a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell, and solar-cell paint. In another embodiment, power supply  50  is configured to receive power from a wall outlet. 
     In some implementations control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some cases control programmability resides in the array driver  22 . Those of skill in the art will recognize that the above-described optimization may be implemented in any number of hardware and/or software components and in various configurations. 
     The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,  FIGS. 7A-7E  illustrate five different embodiments of the movable reflective layer  14  and its supporting structures.  FIG. 7A  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. 7B , the moveable reflective layer  14  is attached to supports at the corners only, on tethers  32 . In  FIG. 7C , the moveable reflective layer  14  is suspended from a deformable layer  34 , which may comprise a flexible metal. The deformable layer  34  connects, directly or indirectly, to the substrate  20  around the perimeter of the deformable layer  34 . These connections are herein referred to as support posts. The embodiment illustrated in  FIG. 7D  has support post plugs  42  upon which the deformable layer  34  rests. The movable reflective layer  14  remains suspended over the cavity, as in  FIGS. 7A-7C , but the deformable layer  34  does not form the support posts by filling holes between the deformable layer  34  and the optical stack  16 . Rather, the support posts are formed of a planarization material, which is used to form support post plugs  42 . The embodiment illustrated in  FIG. 7E  is based on the embodiment shown in  FIG. 7D , but may also be adapted to work with any of the embodiments illustrated in  FIGS. 7A-7C  as well as additional embodiments not shown. In the embodiment shown in  FIG. 7E , an extra layer of metal or other conductive material has been used to form a bus structure  44 . This allows signal routing along the back of the interferometric modulators, eliminating a number of electrodes that may otherwise have had to be formed on the substrate  20 . 
     In embodiments such as those shown in  FIG. 7 , the interferometric modulators function as direct-view devices, in which images are viewed from the front side of the transparent substrate  20 , the side opposite to that upon which the modulator is arranged. In these embodiments, the reflective layer  14  optically shields the portions of the interferometric modulator on the side of the reflective layer opposite the substrate  20 , including the deformable layer  34 . This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. Such shielding allows the bus structure  44  in  FIG. 7E , which provides the ability to separate the optical properties of the modulator from the electromechanical properties of the modulator, such as addressing and the movements that result from that addressing. This separable modulator architecture allows the structural design and materials used for the electromechanical aspects and the optical aspects of the modulator to be selected and to function independently of each other. Moreover, the embodiments shown in  FIGS. 7C-7E  have additional benefits deriving from the decoupling of the optical properties of the reflective layer  14  from its mechanical properties, which are carried out by the deformable layer  34 . This allows the structural design and materials used for the reflective layer  14  to be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer  34  to be optimized with respect to desired mechanical properties. 
       FIG. 8  is a cross-section of an exemplary interferometric modulator  100 . The interferometric modulator  100  comprises a substrate  120 , a transparent conductor  140 , a partial reflector  116 , a dielectric  112 , a movable mirror  114 , and supports  118 . In the embodiment of  FIG. 8 , the supports  118  support moveable minor  114  and define an air gap  119  between the dielectric layer  112  and the moveable minor. In an advantageous embodiment, the air gap  119  is sized according to the desired optical characteristics of the interferometric modulator. For example, the air gap  119  may be sized in order to reflect a desired color from the interferometric modulator. 
     As described above with respect to  FIGS. 7A ,  7 B, and  7 C, typically a voltage difference is placed across the movable mirror  14  and the partial reflector  16  in order to actuate the interferometric modulator. Thus, in the embodiment of  FIGS. 7A ,  7 B, and  7 C, for example, the movable mirror  14  and the partial reflector  16  are at least partially conductive so that they may be connected to the row and column lines of the display device. In exemplary embodiments where the partial reflector  16  is also an electrode of the interferometric modulator ( FIGS. 7A ,  7 B, and  7 C, for example), the partial reflector may comprise chromium, titanium, and/or molybdenum. 
     In the exemplary interferometric modulator  100 , the transparent conductor  140  is shown positioned between the partial reflector  116  and the substrate  120 . In this embodiment, the transparent conductor  140  is configured as an electrode of the interferometric modulator and, thus, the interferometric modulator  100  may be actuated by placing an appropriate voltage difference, e.g., 10 volts, between the moveable minor  114  and the transparent conductor  140 . In an exemplary embodiment, the transparent conductor  140  comprises Indium Tin Oxide (ITO), Zinc Oxide, Florine doped Zinc Oxide, Cadmium Tin Oxide, Aluminum doped Zinc Oxide, Florine doped Tin Oxide, and/or Zinc Oxide doped with Gallium, Boron or Indium. In this embodiment, the partial reflector  116  is not required to be conductive and, thus, the partial reflector  116  may comprise any suitable partially reflective material, either conductive or nonconductive. 
     In certain embodiments of interferometric modulator, a reflectivity of the partial reflector  116  is within the range of about 30-36%. For example, in one embodiment the reflectivity of the partial reflector  116  is about 31%. In other embodiments, other reflectivities are usable in connection with the systems and methods described herein. In other embodiments, the reflectivity of the partial reflector  116  may be set to other levels according to the desired output criteria for the interferometric modulator  100 . In a typical interferometric modulator, as a thickness of the partial reflector increases, the reflectivity of the partial reflector also increases, thus reducing the effectiveness of a dark state and limiting the contrast of the interferometric modulator. Therefore, in order to achieve a desired reflectivity of the partial reflector, in many embodiments reduction of a thickness of a partial reflector is desired. 
     In the embodiment of  FIG. 8 , the partial reflector  116  may advantageously be thinner due to the fact that the transparent conductor  140  serves as the electrode. Thus, the partial reflector does not need to be conductive, because the transparent conductor serves as the electrode. Accordingly, in embodiments including a transparent conductor, such as transparent conductor  140 , a thickness of a partial reflector may be reduced in order to achieve a desired reflectivity. In one embodiment, the partial reflector  116  has a thickness of about 75 Angstoms. In another embodiment, the partial reflector  116  has a thickness in the range of about 60-100 Angstroms. In yet another embodiment, the partial reflector  116  has a thickness in the range of about 40-150 Angstroms. 
     In one embodiment, the partial reflector comprises silicon nitride, which is a non-conductive, partially reflective material. In other embodiments, oxides of chromium are used, including, but not limited to, CrO 2 , CrO 3 , Cr 2 O 3 , Cr 2 O, and CrOCN. In some embodiments, low conductivity dielectric materials are used as the partial reflector. These low conductivity dielectric materials are generally referred to as “high-k dielectrics”, where “high-k dielectrics” refers to materials having a dielectric constant greater than or equal to about 3.9. High-k dielectrics may include, for example, SiO 2 , Si 3 N 4 , Al 2 O 3 , Y 2 O 3 , La 2 O 3 , Ta 2 O 5 , TiO 2 , HfO 2 , and ZrO 2 , for example. 
     In other embodiments, the partial reflector  116  comprises a dielectric stack having alternating layers of dielectrics with different indices of refraction. As those of skill in the art will recognize, the output characteristics of the interferometric modulator  100 , e.g., the color of light that is reflected from the interferometric modulator  100 , are affected by the reflectivity of the partial reflector  116 . Accordingly, tuning of the reflectivity of the partial reflector  116  may be performed in order to achieve desired output characteristics. In one embodiment, the index of refraction of the partial reflector  116  can be fine-tuned by using a partial reflector  116  comprising a combination of dielectric materials in a stack structure. For example, in one embodiment, the partial reflector  116  may comprise a layer of SiO 2  and a layer of CrOCN. In an exemplary embodiment of an interferometric modulator having a partial reflector comprising a dielectric stack, the material layers above substrate  120  include a layer of ITO that is about 500 Angstroms thick, a layer of SiO 2  that is about 1000 Angstroms thick, a layer of CrOCN that is about 110 Angstroms thick, a layer of SiO 2  that is about 275 Angstroms thick, an air gap that is about 2000 Angstroms thick, and an Al reflector. Thus, in this exemplary embodiment, the partial reflector comprises a layer of SiO 2  that is about 1000 Angstroms thick and a layer of CrOCN that is about 110 Angstroms thick. Those of skill in the art will recognize that there are many other suitable conductive or non-conductive materials that may be used alone, or in combination with other materials, as part of the partial reflector  116 . Use of these materials in combination with the systems and methods described herein is expressly contemplated. 
     In a typical display, as a capacitance of the individual display elements, e.g., interferometric modulators, increases, a power required to change voltages across the display elements also increases. For example, as a capacitance of any actuated display elements in an interferometric modulator display increases, the current required to change voltage levels on the columns of the display also increases. Accordingly, display elements with reduced capacitance are desired. The display elements of  FIGS. 9 and 10  are exemplary embodiments of display elements having reduced capacitance. 
       FIG. 9  is a cross-sectional view of a reduced capacitance interferometric modulator  200 . The interferometric modulator  200  of  FIG. 9  comprises the substrate  120 , the transparent conductor  140 , a dielectric  130 , the partial reflector  116 , the dielectric  112 , movable minor  114 , supports  118 , and air gap  119 . In an exemplary embodiment, the relative thicknesses of these layers are selected so that a thickness of the air gap  119  is larger than a combined thickness of the partial reflector  116 , the dielectric  112 , and the dielectric  130 . In the embodiment of  FIG. 9 , a lower capacitance is achieved by de-coupling the partial reflector  116  from the transparent conductor  140 , thus increasing a distance between electrodes (e.g., moveable minor  114  and transparent conductor  140 ) of the interferometric modulator. More particularly, in the embodiment of  FIG. 9 , the additional dielectric  130  is positioned between the transparent conductor  140  and the partial reflector  116 . The addition of the dielectric  130  does not change a distance between the partial reflector  116  and the movable mirror  114 , but does, however, increase the distance between the transparent conductor  140  and the movable minor  114 . In one embodiment, the dielectric  130  has a thickness of about 1,000 Angstroms. In other embodiments, the dielectric  130  may have a thickness in the range of about 800-3,000 Angstroms. 
     As described above with respect to  FIG. 8 , for example, interferometric modulator embodiments including a transparent conductor  140  may be actuated by placing a voltage between the transparent conductor  140  and the movable minor  114 . In the exemplary embodiment of  FIG. 9 , when the movable mirror  114  collapses against dielectric layer  112 , the resulting distance between the movable minor  114  and the energized transparent conductor  140  is increased by the thickness of dielectric layer  130 . Because capacitance varies inversely to a distance separating capacitive electrodes, by increasing a distance between the electrodes of the interferometric modulator  200 , a capacitance of the interferometric modulator  200  is correspondingly decreased. Thus, the addition of the dielectric  130  does not significantly affect the optical characteristics of the interferometric modulator  200 , but does decrease a capacitance between the electrodes, e.g., the movable minor  114  and the transparent conductor  140 . 
       FIG. 10  is a cross-sectional view of an exemplary reduced capacitance interferometric modulator  300 . The interferometric modulator  300  of  FIG. 10  comprises a substrate  312 , a transparent conductor  310 , a dielectric  308 , a partial reflector  306 , a dielectric  304 , a movable minor  302 , supports  318 , and an air gap  303 . In the embodiment of  FIG. 10 , the movable minor  302  and the partial reflector  306  are separated by the dielectric layer  304  and an air gap  303 . In this embodiment, the air gap  303  and dielectric  308  are sized so that in the released state, e.g., the state shown in  FIG. 10 , the interferometric modulator  300  absorb substantially all light incident on the substrate  312  so that a viewer sees the interferometric modulator  300  as black. When the interferometric modulator  300  is actuated, e.g., the movable minor  302  is collapsed so that it contacts the dielectric  304 , the interferometric modulator  300  reflects substantially all wavelengths of incident light so that the interferometric modulator  300  appears white to a viewer. In certain embodiments, reflection of substantially all wavelengths of light provides white light that is referred to as “broadband white.” Due to the fact that the interferometric modulator  300  operates in a reverse manner when compared to the interferometric modulators  100  and  200  (e.g. the interferometric modulator  300  produces color or white in the released state and black in the actuated state), the interferometric modulator  300  is referred to as a “reverse interferometric modulator.” 
     In one embodiment, an optical gap (including the air gap  303  and the dielectric  306 ) of the reverse interferometric modulator  300  is much smaller than an optical gap of an interferometric modulator that produces black in an actuated state and color or white in a released state (e.g.,  FIG. 100 ). For example, the dielectric  304  may have a thickness of less than about 150 Angstroms and the air gap  304  may have a thickness of about 1,400 Angstroms, while the interferometric modulator  100  may have a dielectric thickness in the range of about 350 to 850 Angstroms and an air gap in the range of about 2,000-3,000 Angstroms. Thus, reverse interferometric modulators, such as the interferometric modulator  300 , have smaller optical gaps than regular interferometric modulators and, accordingly, the electrodes of reverse interferometric modulators are generally closer together. In the exemplary embodiment of  FIG. 10 , the distance between the moveable mirror  302  and the partial reflector  306  is in the range of about 150 to 200 Angstroms when the interferometric modulator  300  is in a collapsed position. This distance comprises the thickness of the dielectric  304  (about 150 Angstroms in the embodiment of  FIG. 10 ) and a small gap of about 0-50 Angstroms that is present because the moveable mirror  302  and dielectric  304  may not be intimately contacting one another in the collapsed position. In other reverse interferometric modulators, the optical gap and distance between electrodes may be greater or smaller than the figures introduced above. 
     Due to the decreased distance between electrodes, the capacitance of reverse interferometric modulators is generally higher than regular interferometric modulators. Accordingly, reverse interferometric modulators may consume additional power when changing voltages across their row and/or column terminals. In order to reduce the capacitance of the reverse interferometric modulator  300 , the dielectric layer  308  is positioned between the terminals of the interferometric modulator. For example, the interferometric modulator  300  includes a dielectric  308  adjacent to the transparent conductor  310 . In the same manner as discussed above with respect to  FIG. 9 , for example, addition of the dielectric  308  does not affect a distance between the partial reflector  306  and the movable minor  302 , but does, however increase the distance between the transparent conductor  310  and the movable mirror  302 , thus decreasing a capacitance of the interferometric modulator  300 . Accordingly, a capacitance of the reverse interferometric modulator  300  may be significantly reduced with the addition of the dielectric layer  308  between the electrodes of the interferometric modulator. 
     The interferometric modulators  100 ,  200 , and  300  each include a movable minor (mirror  114  in  FIGS. 8 and 9 , and minor  302  in  FIG. 10 ). These exemplary moveable minors are deformable so that they collapse against the dielectric  112  ( FIGS. 8 and 9 ),  304  ( FIG. 10 ) when an appropriate voltage is present across the terminals of the interferometric modulators. Those of skill in the art will recognize, however, that the improvements described above with respect to  FIGS. 8 ,  9 , and  10 , may be implemented in other embodiments of interferometric modulators having differently configured movable mirrors. For example, the interferometric modulators  100 ,  200 , and  300 , may be modified to have moveable minors that are attached to supports at the corners only, such as by tethers (e.g.,  FIG. 7B ) or may have moveable minors suspended from deformable layers (e.g.,  FIG. 7C ). Use of the improved systems and methods described with respect to  FIGS. 7 ,  8 , and  9 , are expressly contemplated with these other configurations of movable minors. 
     Various embodiments of the invention have been described above. Although this invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention.