Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/027,862, filed Feb. 7, 2008, which is a divisional of U.S. application Ser. No. 11/197,885, filed Aug. 5, 2005, which claims the priority benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/613,451, filed Sep. 27, 2004. 
         [0002]    This application is related to U.S. patent application Ser. No. 10/251,196, filed Sep. 20, 2002; and U.S. patent application Ser. No. 11/090,911, filed Mar. 25, 2005. 
     
    
     BACKGROUND 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to microelectromechanical systems (MEMS) and, more particularly, to devices and methods for selectively creating constructive and/or destructive interference of light waves. 
         [0005]    2. Description of the Related Technology 
         [0006]    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 
       [0007]    In accordance with one aspect of the invention, a microelectromechanical system is provided. The microelectromechanical system comprises a conductor layer, a mechanical layer and a charged layer. The mechanical layer is separated by a cavity from the conductor layer and is configured to move relative to the conductor layer. The charged layer comprises an incorporated charged species and is disposed between the conductor layer and the mechanical layer. 
         [0008]    In accordance with another aspect of the invention, a method is provided for modulating electromagnetic radiation. The method comprises providing a plurality of micromechanical devices. Each device comprises a conductor layer, a reflective layer and a charged layer between the conductor layer and the reflective layer. The charged layer has an incorporated charged species. The reflective layer is parallel to and spaced a distance from the conductor layer while in a relaxed state. The distance for some of the micromechanical devices is different from other of the micromechanical devices. The reflective layer is configured to move relative to the conductor layer upon being switched to an actuated state. A voltage is applied to the conductor layers and the reflective layers of the micromechanical devices to actuate the micromechanical devices. 
         [0009]    In accordance with yet another aspect of the invention, a method is provided for fabricating a micromechanical device. The method comprises forming a first conductive layer, forming a dielectric layer over the first conductive layer, adding charge to the dielectric layer and forming a second conductive layer over the dielectric layer. In another aspect, the invention provides a micromechanical device formed by this method. 
         [0010]    In accordance with another aspect of the invention, an interferometric modulator is provided. The modulator comprises a conductor layer, a movable layer and a means for modifying a voltage actuation threshold of the movable layer with charged species. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      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. 
           [0012]      FIG. 2  is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display. 
           [0013]      FIG. 3  is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . 
           [0014]      FIG. 4  is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display. 
           [0015]      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 . 
           [0016]      FIGS. 6A and 6B  are system block diagrams illustrating an embodiment of a visual display device comprising a plurality of interferometric modulators. 
           [0017]      FIG. 7A  is a cross section of the device of  FIG. 1 . 
           [0018]      FIG. 7B  is a cross section of an alternative embodiment of an interferometric modulator. 
           [0019]      FIG. 7C  is a cross section of another alternative embodiment of an interferometric modulator. 
           [0020]      FIG. 7D  is a cross section of yet another alternative embodiment of an interferometric modulator. 
           [0021]      FIG. 7E  is a cross section of an additional alternative embodiment of an interferometric modulator. 
           [0022]      FIG. 8  is a cross section of an interferometric modulator having an incorporated charged species. 
           [0023]      FIG. 9  is a cross section of another interferometric modulator having an incorporated charged species. 
           [0024]      FIG. 10  is a cross section of yet another interferometric modulator having an incorporated charged species. 
           [0025]      FIGS. 11-12  illustrate the effect of charge incorporation on the hysteresis window of interferometric modulators. 
           [0026]      FIG. 13  illustrates a cross section of additional alternative embodiments of interferometric modulators. 
           [0027]      FIG. 14  illustrates the effect of charge incorporation on the hysteresis windows of interferometric modulators of  FIG. 13 . 
           [0028]      FIGS. 15-16  are flowcharts showing steps in processing sequences for making an interferometric modulator. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    As discussed in greater detail below, interferometric modulators can be switched between a bright and a dark state by moving a reflective part (the “reflective layer”) relative to a partly-transmissive and partly-reflective part (the “optical stack”), which is spaced from the reflective layer. The movement is actuated by creating electrostatic attraction between the two parts, which causes at least one of the parts to move relative to the other part. In an actuated position, one of the parts has a net positive charge, while the other part has a net negative charge, thereby causing the parts to be drawn close together. In a relaxed position, the net charge between the parts is not sufficient to overcome the mechanical resistance of the parts to movement, and the parts are spaced relatively far apart. The voltage needed to generate sufficient electrostatic attraction to draw the parts into the actuated position may be referred to as the actuation voltage. 
         [0030]    According to some preferred embodiments, the actuation voltage can be altered by incorporating positively and/or negatively charged species, such as ions, into the reflective layer and/or the optical stack. The reflective layer and/or the optical stack are preferably provided with a dielectric layer, situated between the reflective layer and the optical stack, into which the charged species can be embedded. The charged species create a constant, baseline level of charge, which augment and/or cancel part of the electrostatic attraction that is generated when applying a voltage to the optical stack and the reflective layer. As a result, a higher or lower actuation voltage may be needed to generate the net level of electrostatic attraction necessary to, e.g., cause the reflective layer to move to an actuated position. For example, if the reflective layer and the optical stack are wired so as to have a positive and a negative charge, respectively, then the actuation voltage can be increased by implanting positively charged ions, which can repel the positively charged reflective layer. Conversely, the actuation voltage can be decreased by implanting negatively charged ions, which help to attract the positively charged reflective layer. Thus, the incorporation of the charged species can be used to alter the actuation voltage as desired. 
         [0031]    It will also be appreciated that, as discussed further below, the interferometric modulators exhibit a hysteresis behavior in which they remain in a particular state over a range of applied voltages. This range of applied voltages is referred to as the “hysteresis window.” For example, the interferometric modulator remains stable in the relaxed position until the applied voltage is increased to the actuation voltage, when it switches to the actuated position. The interferometric modulator then remains stable in the actuated state until the applied voltage drops below a certain voltage. Pre-charging, e.g., by ion implantation preferably leaves the hysteresis window substantially unchanged and shifts the window with the actuation voltage. Advantageously, this shifting allows the windows to be centered as desired, allowing for the simplification of driver and control systems, as discussed below. 
         [0032]    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. 
         [0033]    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. 
         [0034]      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. 
         [0035]    The depicted portion of the pixel array in  FIG. 1  includes two adjacent interferometric modulators  12 , which may be referred to separately as  12   a  and  12   b . The interferometric modulators  12  each include a movable mechanical layer  14 , which is preferably reflective, and an optical stack  16 , which may be referred to separately as movable reflective layers  14   a  and  14   b  and optical stacks  16   a  and  16   b . In the interferometric modulator  12   a  on the left, the movable reflective layer  14   a  is illustrated in a relaxed position at a predetermined distance from the 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.    
         [0036]    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. 
         [0037]    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 , in the actuated position, 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. 
         [0038]      FIGS. 2 through 5  illustrate one exemplary process and system for using an array of interferometric modulators in a display application. 
         [0039]      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. 
         [0040]    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 panel or display array (display)  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. 
         [0041]    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. 
         [0042]      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. 
         [0043]      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 relaxed states. 
         [0044]    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. 
         [0045]      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. 
         [0046]    The display device  40  includes a housing  41 , a display  30 , an antenna  43 , a speaker  45 , a microphone  46 , and an input device  48 . 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. 
         [0047]    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. 
         [0048]    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 the 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 the 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. 
         [0049]    The network interface  27  includes the antenna  43  and the transceiver  47  so that the exemplary display device  40  can communicate with one or 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 . 
         [0050]    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. 
         [0051]    The 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. 
         [0052]    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. 
         [0053]    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 . 
         [0054]    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. 
         [0055]    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, the driver controller  29  is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, the array driver  22  is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, the 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, the display array  30  is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators). 
         [0056]    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 . 
         [0057]    The power supply  50  can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, the 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, the power supply  50  is configured to receive power from a wall outlet. 
         [0058]    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. 
         [0059]    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  18  at the corners only, on tethers  32 . In  FIG. 7C , the functions of movement and reflectivity are separated. 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  18 . The deformable layer  34  constitutes the mechanical layer and the layer  14  is the reflective surface. In the embodiment illustrated in  FIG. 7D  the support posts  18  include 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 unlike  FIG. 7B-7C , the deposition of 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  18  are at least partially formed of a separately deposited 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 . 
         [0060]    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 (i.e., viewed from the substrate side). In these embodiments, the reflective layer  14  optically shields some portions of the interferometric modulator on the side of the reflective layer opposite the substrate  20 , including the deformable layer  34  and the bus structure  44 . This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. 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. 
         [0061]    With reference to  FIG. 8 , a cross section of the interferometric modulator  12  is shown in an isolated view. In the exemplary illustrated embodiment, a conductor layer  100 , having a fixed position, is deposited onto the glass substrate  20 . As noted above, the fixed conductor layer  100  is preferably partially reflective and transparent to desired wavelengths of light, and can be made from, e.g., layers of ITO and chromium. A dielectric layer  102  is deposited onto conductor layer  100 . The dielectric layer  102  can comprise silicon oxide, although other dielectric materials, such as aluminum oxide, known in the art are equally applicable. For example, the layer  102  can comprise charge trapping materials, and particularly materials that trap both positive and negative charges, e.g., Al 2 O 3 , AlO x  (non-stoichiometric aluminum oxide), Si 3 N 4 , SiN x  (non-stoichiometric silicon nitride), Ta 2 O 5  and TaO x  (non-stoichiometric tantalum oxide). The fixed conductor layer  100  and the dielectric layer  102  form the optical stack  16 . 
         [0062]    Support posts  18  are provided to support the movable, reflective layer, or mechanical/mirror element,  14  a predetermined distance (in the relaxed mode) above the dielectric layer  102 . The support posts  18  are preferably formed of a stable material with sufficient structural integrity to support the movable reflective layer  14 . For example, the support posts  18  can be fabricated from an organic material, such as photoresist, or from spin-on glass. The movable layer  14  is preferably formed of a flexible, conductive and highly reflective material, for example, a metal such as aluminum, nickel, chromium or combinations or alloys thereof. 
         [0063]    With continued reference to  FIG. 8 , a region  104  within the dielectric layer  102  incorporates a charged species, preferably implanted ions and/or dopants, to alter the optical response of the modulator  12 . Ions implanted in the region  104  can be positively charged (p-type) ions or negatively charged (n-type) ions according to the desired effect on the threshold and/or hysteresis properties of the modulator  12 . For example, potassium can be used as a positively charged ion and phosphorous can be used as a negatively charged ion. Examples of other positively charged ions, include without limitation, sodium ions and lithium ions. 
         [0064]    It will be appreciated that the ions can be disposed in various other positions in the interferometric modulator  12 . To prevent the dissipation of charge from the ions, the ions are preferably embedded within a non-conducting material, such as a dielectric, or are surrounded by or embedded within in a material surrounded by an insulator.  FIGS. 9-10  illustrate other non-limiting examples of positions for the ions. With reference to  FIG. 9 , a dielectric layer  106  can be formed adjacent the movable layer  14  and the ion implanted region  104  can be disposed within that layer  106 . In such an arrangement, the dielectric layer  102 , which typically serves to space and prevent shorting between the movable layer  14  and the fixed conductive layer  100 , can optionally be omitted. Preferably, the dielectric layer  106  is formed of a flexible material that does not significantly impede the movement of the layer  14 . As noted above, the dielectric layer  106  can be formed of silicon oxide and aluminum oxide. 
         [0065]    With reference to  FIG. 10 , where two dielectric layers, layers  102  and  106  are provided, both layers can be implanted with ions to form ion implanted regions  104  and  108 . Depending on the desired effect, both regions can be implanted with ions of the same polarity or with ions of different polarities. Implantation of both the layers  102  and  106  can increase the effect of the implantation. For example, where regions  104  and  108  are implanted with ions of different polarities, a constant level of attraction can be established between the layers  102  and  106 , thereby reducing the actuation voltage in cases where the layers  100  and  14  are wired to have the same polarity as the regions  104  and  108 , respectively. 
         [0066]    Along with changes in the actuation voltage, the introduction of charged ions between the movable layer  14  and the fixed layer  16  can shift the optical response curves of the modulators  12 . For example, the optical response curves of  FIGS. 11-13  plot the displacement of movable layer  14  against the voltage applied to the movable layer  14  and the fixed layer  16  of the interferometric modulator  12  of  FIG. 8 . In the positive voltage region of the graph, the movable layer  14  is connected to a voltage source to generate a positive charge in that layer  14  and the fixed conductor layer  100  is connected to the voltage source to generate a negative charge in that layer  100 . In the negative voltage region of the graph, the movable layer  14  is connected to a voltage source to generate a negative charge in that layer  14  and the fixed conductor layer  100  is connected to the voltage source to generate a positive charge in that layer  100 . At the bottom of the curves, the movable layer  14  is in the relaxed position and, at the top of the curves, the movable layer  14  is in the actuated position. 
         [0067]      FIG. 11  illustrates the effect of implanting positively charged or p-type ions into the dielectric layer  102 . The introduction of the positively charged ions into the dielectric layer  102  causes the optical response curves to shift to the right. In the positive voltage region, the p-type ions repel the positively charged movable layer  14 , thus requiring a larger voltage and greater electrostatic attraction to be applied before the movable layer  14  can be made to collapse. In the negative voltage region, the p-type ions attract the negatively charged movable layer  14 , thus requiring a smaller voltage and lesser electrostatic attraction to be applied before the movable layer  14  can be made to collapse. Optical response curves  302   a  and  302   b  represent the optical response characteristics for an interferometric modulator without ion implantation. The same interferometric modulator with positively charged ions in the dielectric layer displays optical response characteristics represented by optical response curves  304   a  and  304   b . Because the level of charge introduced by the ions is constant, the ions augment or reduce the net electrostatic attraction by the same amount, so that both the positive and negative response curves will shift to the right by the same amount. The amount of the shift in the optical response characteristics is determined by the total charge of the ions introduced into dielectric layer  102 , which may be proportional to the amount of ions implanted. 
         [0068]      FIG. 12  illustrates the effect of implanting negatively charged or n-type ions into the dielectric layer  102 . The introduction of negatively charged ions into dielectric layer  102  causes the optical response curves to shift to the left. In the positive voltage region, the n-type ions attract the movable layer  14  to the actuated position, while in the negative voltage region, the n-type ions repel the movable layer  14  to maintain that layer in the relaxed position. Optical response curves  312   a  and  312   b  represent the optical response characteristics for an interferometric modulator  12  without ion implantation. The same interferometric modulator  12  with negatively charged ions in the dielectric layer displays optical response characteristics represented by the optical response curves  314   a  and  314   b . As in  FIG. 11 , both the positive and negative response curves shift to the left by the same amount and the amount of the shift in the optical response characteristics is determined by the amount of ions introduced into dielectric layer  102 . 
         [0069]    It will be appreciated that interferometric modulators can be formed with a hysteresis curve centered away from the zero voltage line. For example, the inteferometric modulators can be formed having a particular level of charge between the layers  14  and  16 , even without ion implantation. For example, structural defects or structural modifications in the dielectric layer  102  can result in such a charge. As a result of this charge, the hysteresis window for these interferometric modulators may not be centered relative to the zero voltage line. Moreover, different interferometric modulators may exhibit a different level of charge. The charges and the different levels of charges may adversely affect the behavior of the interferometric modulators by reducing predictability and control over the actuation and release of the movable layers of the interferometric modulators. Advantageously, depending upon the charge already present, ion implantation can allow the hysteresis behavior of the interferometric modulators to be re-centered about the zero voltage line by, e.g., neutralizing the already present charge. As a result, predictability and control over the actuation and release of the movable layers of the interferometric modulators can be increased. 
         [0070]    While discussed above with reference to incorporated charged species in the dielectric layer  102 , it will be appreciated that similar affects can be achieved by incorporation of charged species in the layer  106  (when present, as illustrated in  FIG. 9 ) or incorporation of charged species in both the layers  102  and  106  (when present, as illustrated in  FIG. 10 ). For example, charged species can be incorporated in the layer  106  to achieve the effects illustrated in  FIGS. 11 and 12 . Where a voltage source is connected to the movable layer  14  and to the fixed layer  16  to generate positive and negative charges in those layers, respectively, incorporation of a negative charged species in the layer  106  will result in the rightward shift of the hysteresis curve illustrated in  FIG. 11 . Conversely, in a similar arrangement, but with a positive charged species in the layer  106 , the hysteresis curve will shift to the left, as shown in  FIG. 12 . 
         [0071]    Moreover, both the layers  102  and  106  can be provided in an interferometric modulator and each can be incorporated with charged species. For example, with the movable layer  14  and the fixed layer  16  again configured to be positively and negatively charged, respectively, an effect similar to that illustrated in  FIG. 11  can be achieved by incorporation of a positive charged species in the layer  102  and a negative charged species in the layer  106 . In addition, the hysteresis curves can be shifted to the left by incorporation of a negative charged species in the layer  102  and a positive charged species in the layer  106 . 
         [0072]    It will be appreciated that the effect of the incorporation of the charged species can be reversed by reversing the polarities of the layers  102  and  106 . For example, if a certain arrangement of charged species shifts the hysteresis curves to the left when the movable layer  14  and the fixed layer  16  are connected to a voltage source to generate positive and negative charges, respectively, in those layers, the same arrangement of charged species will shift the hysteresis curves to the right if the voltage source is connected to the layers  102  and  106  in reverse, i.e., so that the polarities in those layers is reversed. 
         [0073]    With reference to  FIG. 13 , in some arrangements, interferometric modulators can be formed to generate constructive interference centered at a plurality of different frequencies to generate different perceived colors, e.g., two or more, or three or more different colors. These interferometric modulators can be grouped to form, e.g., the individual red, green and blue picture elements of a display. It will be appreciated that the interference behavior of the interferometric modulators is determined by the spacing between the movable layer  14  and the fixed layer  16 . Thus, interferometric modulators  100   a ,  100   b , and  100   c  may be forming having a different spacing  110   a ,  110   b ,  110   c  between the movable layers  14   a ,  14   b ,  14   c  and the fixed layer  16 , thereby allowing each of the different colors to be generated. Because of the different spacing, each interferometric modulator  100   a ,  100   b , and  100   c  can have a different actuation voltage and hysteresis curve. 
         [0074]    Such a situation is illustrated in  FIG. 14 , which shows the hysteresis curves,  322   a ,  322   b  and  322   c , for the three interferometric modulators  100   a ,  100   b ,  100   c , respectively, each having a different spacing designed to give a different color. It will be appreciated that the interferometric modulators in a display can be implanted with different ions and/or with different levels of ions. Advantageously, all or some of the interferometric  100   a ,  100   b ,  100   c  modulators can be implanted with ions to shift the curves  322   a - 322   c  so that they overlap. By overlapping the curves, the threshold and release voltages can be made similar, advantageously reducing the number of voltages needed for operating the interferometric modulators and thus simplifying the driver and control systems associated with the interferometric modulators. 
         [0075]    For example, in the arrangement shown in  FIG. 14 , the curve  322   b  is used as a reference and the interferometric modulators associated with the curves  322   a  and  322   c  are implanted with different ions, so that the curve  322   a  (e.g., with a dielectric layer, between the layer  34   a  and the conductor layer in the optical stack  16  of the interferometric modulator  100   a , implanted with p-type ions) shifts to the right and the curve  322   c  (e.g., with a dielectric layer, between the layers  34   c  and the conductor layer in the optical stack  16  of the interferometric modulator  100   c , implanted with n-type ions) shifts to the left, thereby allowing both to overlap the curve  322   b . As a result, all three curves  322   a - 322   c  can advantageously be driven, in the positive voltage region, using the same threshold and release voltages. 
         [0076]    With reference to  FIG. 15 , flowchart  350  illustrates generally steps in the formation of an interferometric modulator  12 . A first conductive part is formed  360 , e.g., on a substrate, which can be, e.g., glass. A dielectric is formed  370  over the first conductive part and charged species is added  380  into the dielectric layer. 
         [0077]    While various process steps are illustrated as separate blocks in  FIGS. 15 and 16 , it will be appreciated that the separate blocks do not indicate that the steps are necessarily temporally separated. For example, dielectric formation and charge addition can occur simultaneously, so that charged species, e.g., ions, are formed as as-deposited species in the dielectric. An example of a suitable process for simultaneous dielectric formation and charge addition is co-sputtering, in which ionic species and dielectric precursors are simultaneously sputtered on a substrate. In other embodiments, charge addition occurs after the formation of the dielectric. In this case, charge addition can be accomplished by various processes known in the art. In some embodiments, the ions can be implanted into the dielectric or can be diffused into the dielectric. For example, the dielectric layer can be doped, e.g., diffusion doped. 
         [0078]    With continued reference to  FIG. 15 , a second conductive part is formed  390  over the ion implanted dielectric. It will be appreciated that the first conductive part can correspond to the one of the conductive layers of the interferometric modulator  12  ( FIG. 8 ), e.g., the fixed conductive layer  16 . The second conductive part can correspond to the other illustrated conductive layer of the interferometric modulator  12 , e.g., the movable layer  14 . 
         [0079]    With reference to  FIGS. 8 and 16 , flowchart  400  describes certain steps of a fabrication sequence used to make the exemplary interferometric modulator  12  illustrated in  FIG. 8 . The conductor layer  100 , typically comprising ITO and chromium, is deposited  402  onto the substrate  20 . The conductor layer  100  is patterned and etched  404  to form rows of the interferometric modulators  12 . The dielectric layer  102  is deposited  406  on conductor layer  100 . This dielectric layer  102  can be formed of SiO 2 , although other dielectrics compatible with the other materials and process steps for forming the interferometric modulator  12  can be used. A layer of photoresist is deposited and patterned  407  to provide a mask shielding some areas of the dielectric layer  102  and having openings allowing for implantation in desired areas of the dielectric  102 . The charged ions are implanted through the patterned photoresist and into the dielectric layer  102 , thereby forming the implanted regions  104  of the dielectric layers  102 . The charge of the implanted ions and the degree of the implantation is selected in accordance with the desired effect on optical response characteristics, as described above. For example, the polarity of the ions can be chosen based upon the direction in which a shift in a hysteresis curve ( FIG. 11-13 ) is desired and the degree of the implantation can be chosen based upon the magnitude of the desired shift. 
         [0080]    It will be appreciated that the ion implantation can disrupt the structure of the dielectric. As a result, the ion implantation can be followed by an anneal to reorient the dielectric structure, to improve the optical characteristics of the implanted dielectric and to more evenly distribute the ions within the dielectric. 
         [0081]    It will be appreciated that, in some embodiments, no photoresist is needed and the step  407  can be omitted if all interferometric modulators  12  are to be uniformly implanted with the same ion or ions. In other embodiments, the patterned photoresist allows interferometric modulators  12  to be selectively implanted with ions, thereby allowing different ions to be implanted or different levels of implantation to be achieved. It will be appreciated that multiple photoresist depositions and/or patterning steps can be used to selectively implant a plurality of different ions or to selectively implant different quantities of ions. For example, the photoresist can be deposited and patterned to implant ions in some particular interferometric modulators  12 , additional photoresist can be deposited and patterned to implant ions in other interferometric modulators  12 , and so on. After the ion implantation, the photoresist is preferably removed. 
         [0082]    Moreover, the charge incorporation can occur at a later step than that illustrated. For example, the ion implantation can be performed after deposition  410  of a non-metal sacrificial layer (e.g., silicon), discussed below, and preferably before the formation of additional metal layers. 
         [0083]    In step  410 , a sacrificial layer (which will later be removed to form the optical cavity of the interferometric modulator  12 ) is deposited. The sacrificial layer is formed of a solid material that can later be removed, e.g., by etching, without disrupting the other materials of the interferometric modulator  12 . An example of a preferred material for the sacrificial layer is molybdenum. Other suitable sacrificial materials include silicon and tungsten, which advantageously can also be selectively or preferentially removed using XeF 2  without etching aluminum or silicon oxide. The sacrificial layer is patterned and etched  412  to provide voids into which materials to form the support posts  18  will be deposited. In step  414 , the post material is deposited, thereby forming the support posts  18 . The post material can be, e.g., photoresist or some other organic compound or spin on glass. In step  416 , a mechanical/mirror film is deposited. As noted above, the film can be made of, e.g., aluminum or other flexible metals. In step  418 , the mechanical film is patterned and etched to form the mechanical/mirror layer  14 . The sacrificial layer is then removed  420 . 
         [0084]    Interferometric modulators according to the preferred embodiments offer numerous advantages. For example, charge incorporation allows the actuation voltage and/or hysteresis curve for a particular interferometric modulator to be shifted as desired. As a result, it is possible to reduce the voltages required to drive the interferometric modulators, thereby lowering the power requirements and power consumption of displayers utilizing the interferometric modulators. In addition, hysteresis curves can be shifted by charge incorporation to center the curves about the zero voltage line. This can be achieved, for example, by neutralizing charges that may form in the dielectric layer of between the conductive layers of the interferometric modulators. Centering the hysteresis curves can make control over the states of the interferometric modulators more predictable, e.g., by setting the actuation voltages at expected values. Moreover, in cases in which multiple interferometric modulators, each naturally having a different actuation voltage and shifted hysteresis curves, are present, some or all of the interferometric modulators can be doped so that the hysteresis curves substantially overlap. As a result, the actuation voltages and the release voltages of each of the interferometric modulators overlap, thereby reducing the number of different voltages that is generated to control the interferometric modulators. Thus, the driver and control systems can be simplified. 
         [0085]    While the charged species are discussed above as “implanted ions,” it will be appreciated that the charged species can be any charged species incorporated in a material disposed between the movable conductive layer and the fixed conductive layer. In other embodiments, the charged species can simply be deposited on a dielectric substrate and preferably has a charge as deposited. The dielectric layer, while preferably disposed on a conductive layer for simplicity of fabrication and structure, can be spaced from the conductive layers. Moreover, while discussed as having one movable and one fixed conductive layer for ease of description and illustration, in some embodiments, the positions of the movable and fixed layers can be reversed from that illustrated, or both layers can be made to move. 
         [0086]    Accordingly, it will be appreciated by those skilled in the art that various other omissions, additions and modifications may be made to the methods and structures described above without departing from the scope of the invention. All such modifications and changes are intended to fall within the scope of the invention, as defined by the appended claims.

Technology Category: 3