Patent Publication Number: US-2012032692-A1

Title: Mems gas sensor

Description:
TECHNICAL FIELD 
     The present disclosure relates to electromechanical systems (MEMS). 
     DESCRIPTION OF THE RELATED ART 
     Electromechanical systems include devices having electrical and mechanical elements, actuators, transducers, sensors, optical components (e.g., mirrors) and electronics. Electromechanical systems can be manufactured at a variety of scales including, but not limited to, microscales and nanoscales. For example, microelectromechanical systems (MEMS) devices can include structures having sizes ranging from about a micron to hundreds of microns or more. Nanoelectromechanical systems (NEMS) devices can include structures having sizes smaller than a micron including, for example, sizes smaller than several hundred nanometers. Electromechanical elements may be created using deposition, etching, lithography, 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 electromechanical systems 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 include a pair of conductive plates, one or both of which may be transparent and/or reflective, wholly or in part, and capable of relative motion upon application of an appropriate electrical signal. In an embodiment, one plate may include a stationary layer deposited on a substrate and the other plate may include a metallic membrane separated from the stationary layer by an air gap. The position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. Interferometric modulator devices have a wide range of applications, and are anticipated to be used in improving existing products and creating new products, especially those with display capabilities. 
     SUMMARY 
     The systems, methods and devices of the present disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. 
     One innovative aspect of the subject matter described in this disclosure can be implemented in a method for sensing a chemical. The method includes determining a capacitance change between at least two layers in a MEMS device, the capacitance between the at least two layers indicative of a presence of one or more chemicals; and identifying the presence of the one or more chemicals based on a determined electrical response of the at least two layers and the determined capacitance change. In one aspect, determining a capacitance change includes determining an actuation time of the MEMS device in response to a selected voltage. Determining an actuation time can include electronically measuring a change in current, or optically measuring a change in a gap distance between the at least two layers. Optically measuring a change in the gap distance can include detecting a change in the color of light emitted by the MEMS device. In another aspect, determining a capacitance change includes determining the voltage required to collapse the MEMS device. In yet another aspect, determining a capacitance change includes determining a time to open a gate of a field effect transistor. 
     The change in capacitance between the at least two layers can be a function of a change in residual stress in a sensing layer in the MEMS device. The change in residual stress of the sensing layer can be a function of the presence of the one or more chemicals. 
     A sensor is provided in another implementation. The sensor includes a movable layer configured to react to at least one chemical; and a second layer spaced from the movable layer. A change in capacitance between the movable layer and the second layer and a determined electrical response resulting from the change in capacitance are indicative of the presence of the at least one chemical. A distance between the movable layer and the second layer can be a function of the presence of the at least one chemical in one aspect. In another aspect, a distance between the movable layer and the second layer can be a function of a voltage applied between the movable layer and the second layer. 
     The movable layer can include a sensing layer that has a residual stress that is a function of the presence of the at least one chemical. The sensing layer can include a metal oxide. The movable layer can also include a heating layer configured to heat the sensing layer. The heating layer can include indium tin oxide. The movable layer can include aluminum or platinum. 
     The change in capacitance between the movable layer and the second layer may be reversible in one implementation. In another impelementation, the sensor includes a substrate spaced from the second layer. The second layer can be disposed between the substrate and the movable layer. The substrate can include glass. In some aspects, the sensor includes an interferometric modulator. 
     A sensor that includes means for sensing at least one chemical is provided in yet another implementation. The sensor also includes a layer spaced from the sensing means. A change in capacitance between the sensing means and the layer and a determined electrical response resulting from the change in capacitance can be indicative of the presence of the at least one chemical. In one aspect, the sensing means includes a movable layer configured to react to at least one chemical. In another aspect, the sensor includes means for interferometrically modulating light. 
     Note that the relative dimensions of the following figures may not be to scale. 
    
    
     
       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 mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . 
         FIG. 4  is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display. 
         FIGS. 5A and 5B  illustrate one exemplary timing diagram for row and column signals that may be used to write a frame of display data to the 3×3 interferometric modulator display of  FIG. 2 . 
         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 one embodiment of a MEMS gas sensor. 
         FIG. 7B  is a top view of the gas sensor of  FIG. 7A . 
         FIG. 8  is a top view of the MEMS gas sensor of  FIG. 7A  illustrating a heating layer. 
         FIG. 9  is a top view of an alternative embodiment of a MEMS gas sensor. 
         FIG. 10  is an equivalent circuit of the MEMS gas sensor of  FIG. 7A . 
         FIG. 11  is a flowchart illustrating one method for identifying the presence of a gas species of interest in the MEMS gas sensor of  FIG. 7A . 
         FIG. 12  is a cross section of the MEMS gas sensor of  FIG. 7A  connected to a field effect transistor. 
         FIG. 13  is a flowchart illustrating one method of identifying the presence of a gas species of interest using the field effect transistor of  FIG. 12 . 
         FIG. 14  is a cross section of the MEMS gas sensor of  FIG. 7A  connected to an ammeter. 
         FIG. 15  is a flowchart illustrating one method of identifying the presence of a gas species of interest using the ammeter of  FIG. 14  or a spectrometer. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to certain specific embodiments. However, the teachings herein can be applied in a multitude of different ways. In this description, like reference numbers and designations in the various drawings indicate like elements. 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, multimedia Internet enabled cellular telephones, mobile television receivers, wireless devices, personal data assistants (PDAs), wireless electronic mail receivers, hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, electronic reading devices (e.g., e-readers), computer monitors, auto displays (e.g., odometer display, etc.), cockpit controls and/or displays, camera view displays (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. 
     Methods and systems for sensing a chemical are provided herein. For example, embodiments of gas sensors described herein can detect toxic pollutants and inflammable gases. In one implementation, a method of sensing a chemical includes determining a capacitance change between at least two layers in a MEMS device, the capacitance between the at least two layers indicative of a presence of one or more chemicals. The method also includes identifying the presence of the one or more chemicals based on a determined electrical response of the at least two layers and the determined capacitance change. 
     One interferometric modulator display device comprising an interferometric MEMS display element is illustrated in  FIG. 1 . In these devices, the pixels of the MEMS display element are in either a bright or dark state. In the bright (“relaxed” or “open”) state, the display element reflects a large portion of incident visible light to a user. Conversely, in the dark (“actuated” or “closed”) state, the display element reflects little incident visible light. In some implementations, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at particular wavelengths 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 includes 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 gap 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 include 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 . The partially reflective layer can be formed from a variety of materials that are partially reflective such as various metals, semiconductors, and dielectrics. The partially reflective layer can be formed of one or more layers of materials, and each of the layers can be formed of a single material or a combination of materials. 
     In some embodiments, the layers of the optical stack  16  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 ) to form columns 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 , or cavity. 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. Note that  FIG. 1  may not be drawn to scale. For example, in some implementations, the spacing between posts  18  may be on the order of 10-100 um, while the gap  19  may be on the order of &lt;1000 Angstroms (Å). 
     With no applied voltage, the gap  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 (voltage) 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   b  is deformed and is forced against the optical stack  16   b . A dielectric layer (not illustrated in this Figure) within the optical stack  16   b  may prevent shorting and control the separation distance between layers  14   b  and  16   b , as illustrated by actuated pixel  12   b  on the right in  FIG. 1 . The behavior is the same regardless of the polarity of the applied potential difference. 
       FIGS. 2-5  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 interferometric modulators. The electronic device includes a processor  21  which may be any general purpose single- or multi-chip microprocessor such as an ARM®, Pentium®, 8051, MIPS®, Power PC®, or 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 . Note that although  FIG. 2  illustrates a 3×3 array of interferometric modulators for the sake of clarity, the display array  30  may contain a very large number of interferometric modulators, and may have a different number of interferometric modulators in rows than in columns (e.g., 300 pixels per row by 190 pixels per column). 
       FIG. 3  is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices as illustrated in  FIG. 3 . An interferometric modulator may require, for example, about a 10-volt potential difference to cause a movable layer to change 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. Thus, a range of voltage, approximately 3 to 7-volts in the example illustrated in  FIG. 3 , exists where there is 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 near zero volts. After strobing, the pixels are exposed to a steady state or bias voltage difference of approximately 5-volts such that they remain in the previous strobing state. In this example, after being written, each pixel sees a potential difference within the “stability window” of about 3-7-volts. 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 steady voltage within the hysteresis window without consuming or losing power. Essentially no current flows into the pixel if the applied voltage potential remains fixed. 
     In some implementations, a frame of an image may be created by sending a set of data signals (each having a certain voltage level) across 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 a first row electrode, actuating the pixels corresponding to the set of data signals. The set of data signals is then changed to correspond to the desired set of actuated pixels in a second row. A pulse is then applied to the second row electrode, actuating the appropriate pixels in the second row in accordance with the data signals. The first row of pixels are unaffected by the second row pulse, and remain in the state they were set to during the first row 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 image 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 image frames may be used. 
       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 , voltages of opposite polarity than those described above can be used, e.g., actuating a pixel can involve setting the appropriate column to +V bias , and the appropriate row to −ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −V bias , and the appropriate row to the same −ΔV, producing a zero volt potential difference across the pixel. 
       FIG. 5B  is a timing diagram showing a series of row and column signals applied to the 3×3 array of  FIG. 2  which will result in the display arrangement illustrated in  FIG. 5A , where actuated pixels are non-reflective. Prior to writing the frame illustrated in  FIG. 5A , the pixels can be in any state, and in this example, all the rows are initially at zero 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 . The same procedure can be employed for arrays of dozens or hundreds of rows and columns. 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, e-readers, and portable media players. 
     The display device  40  includes a housing  41 , a display  30 , an antenna  43 , a speaker  45 , an input device  48 , and a microphone  46 . The housing  41  is generally formed from any of a variety of manufacturing processes, 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  may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display  30  can include a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD, or a non-flat-panel display, such as a CRT or other tube device. In addition, the display  30  can include an interferometric modulator display, as described herein. 
     The components of one embodiment of the 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 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  can transmit and receive signals. In one embodiment, the antenna  43  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  43  transmits and receives RF signals according to the BLUETOOTH standard. In the case of a cellular telephone, the antenna  43  is designed to receive CDMA, GSM, AMPS, W-CDMA, 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  may include 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 re-formats the raw image data appropriately for high speed transmission to the array driver  22 . Specifically, the driver controller  29  re-formats 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 an 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 re-formats 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 some implementations, 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 some implementations, the driver controller  29  can be integrated with the array driver  22 . Such an implementation 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 rocker, a touch-sensitive screen, or 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 can be used 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. The power supply  50  also can be configured to receive power from a wall outlet. 
     In some implementations, control programmability resides, as described above, in a driver controller  29  which can be located in several places in the electronic display system. In some cases, control programmability resides in the array driver  22 . The above-described optimization may be implemented in any number of hardware and/or software components and in various configurations. 
     MEMS Gas Sensor 
     Methods and systems to detect a chemical or gas species of interest using a MEMS gas sensor are provided herein. For example, embodiments of MEMS gas sensors described herein can use a change in capacitance between two or more layers of a MEMS device to detect toxic pollutants and inflammable gases. Examples of chemicals and gases which can be detected include but are not limited to NO 2 , NO, SO x , CO 2 , and O 3 . 
     Without being bound by any particular theory, changes in capacitance between layers in MEMS devices described herein can be caused by adsorption of gas molecules on the surface of a sensing layer in the MEMS device. The sensitivity of the sensing layer to gas molecules can be a function of temperature. In some embodiments, the sensing layer is activated by heat, bringing the sensor to a required temperature range for detection of a particular gas species. A typical temperature range to activate sensing layers described herein is approximately 200 to 500° C. 
     Embodiments of MEMS gas sensors described herein can be advantageously constructed on non-silicon substrates, function over a wide range of resistances while also supporting large resistance variations, and can be incorporated into a variety of displays, including but not limited to interferometric modulator, OLED, and LCD displays. 
       FIG. 7A  is a cross section of one embodiment of a MEMS gas sensor  700  that can sense the presence of a gas species of interest.  FIG. 7B  is a top view of the exemplary gas sensor  700 . The MEMS gas sensor  700  includes a substrate  702  which may comprise a variety of materials, for example glass or plastic materials. For example, in one embodiment, the substrate  702  comprises glass. In another embodiment, the substrate  702  comprises plastic. In yet another embodiment, the substrate  702  comprises silicon. The substrate is transparent or substantially transparent in some aspects. 
     The MEMS gas sensor  700  comprises an absorber/insulator layer  704  (hereinafter referred to as absorber layer  704 ) disposed over the substrate  702 . The absorber layer  704  can comprise any suitable material, for example molybdenum, chrome, tungsten, tantalum, silicon, titanium, or molychrome. In some embodiments, the absorber layer  704  comprises an optical oxide. For example, the absorber layer  704  can comprise SiO, SiN, AlO, SiNi, or SiON. The absorber layer  704  can be electrically conductive in some aspects. For example, embodiments of the absorber layer  704  described herein can comprise an electrode layer such as that described above with reference to optical stacks  16   a ,  16   b  in  FIG. 1 . The MEMS gas sensor  700  also includes a plurality of supports  706  disposed over the substrate  702 . In some aspects, the supports  706  comprise a dielectric material. 
     The MEMS gas sensor  700  also includes a sensing layer  710  disposed over the supports  706 . Embodiments of the sensing layer  710  can comprise any suitable material, for example a metal oxide. The sensing layer  710  can comprise, for example, tin oxide, tungsten oxide, indium tin oxide, nitrogen oxide, zinc oxide, or any other suitable metal oxide. In some aspects, the material of the sensing layer  710  is chosen based on the particular gas species of interest the MEMS gas sensor  700  is expected to detect. 
     The MEMS gas sensor  700  can optionally comprise a heating layer  708  disposed over the supports  706 . As shown in  FIG. 7A , the heating layer  708  is disposed between the sensing layer  710  and the supports  706  in some embodiments. The heating layer  708  is also reflective in some aspects. For example, embodiments of the heating layer  708  can comprise a reflective layer such as that described above with reference to movable reflective layers  14   a ,  14   b  in  FIG. 1 . The heating layer  708  can comprise a material with high resistivity. For example, the heating layer  708  comprises a metal in some aspects. In one embodiment, the heating layer  708  comprises aluminum. In another embodiment, the heating layer  708  comprises indium tin oxide (ITO). 
     The MEMS gas sensor  700  also includes one or more electrodes  716  disposed on the sensing layer  710 . The electrodes  716  can comprise a metal, for example aluminum or platinum. The heating layer  708 , the sensing layer  710 , and the electrodes  716  can together form a movable element  714 . 
     The gas sensor  700  is actuatable from a relaxed state shown in  FIG. 7A  and an actuated state (not shown) by moving the movable element  714  in a direction generally perpendicular to the substrate  702 . In certain embodiments, the actuation of the gas sensor  700  occurs in response to a voltage difference applied between the absorber layer  704  and the electrodes  716 . In some aspects, selectively moving the movable element  714  with respect to the substrate  702 , for example from the relaxed state illustrated in  FIG. 7A  to an actuated state described with reference to  FIG. 1 , modulates one or more properties of light emitted from the gas sensor  700 . 
     Embodiments of the MEMS gas sensor  700  can form an actuatable element, for example a pixel or a sub-pixel, in a variety of display systems. For example, the MEMS gas sensors described herein can be used in any display device susceptible to contamination by gases, such as OLED and LCD devices. 
     Referring now to  FIG. 7B , the electrodes  716  can be patterned in the shape of squares or rectangles disposed on the sensing layer  710 . It will be understood, however, that embodiments of the electrodes  716  are not limited to square or rectangular shapes, as discussed below with reference to  FIG. 9 . The electrodes  716  can be electrically connected to each other via a probe, another layer of electrically conductive material patterned or deposited over the electrodes  716 , contact points on the electrodes  716 , or any other suitable means. As noted above, it will also be understood that the MEMS gas sensor  700  may comprise one electrode  716  in lieu of a plurality of electrodes  716 . 
       FIG. 8  is a top view of the MEMS gas sensor  700  illustrating one embodiment of the heating layer  708  in dashed lines disposed below the sensing layer  710 . It will be understood that the heating layer  708  can be disposed above the sensing layer  710  and below the electrodes  716  in some embodiments. It will also be understood that embodiments of the heating layer  708  are not limited to the configuration illustrated in  FIG. 9 , and can comprise a layer that has the same or substantially the same surface area as the sensing layer  710 . In some aspects, the heating layer  708  is a resistive heater and comprises a material having high resistivity, for example aluminum or ITO. In one embodiment, the heating layer  708  is patterned on the sensing layer  710  such that, when heated, the heating layer  708  distributes heat across the sensing layer  710 . 
       FIG. 9  is a top view of an alternative embodiment of a MEMS gas sensor  900 . The gas sensor  900  comprises electrodes  916  that are patterned in the shape of a ring and a circle. The electrodes  916  can be disposed on the sensing layer  910 . Other configurations are possible. The electrodes  916  can comprise a metal, for example aluminum or platinum. 
       FIG. 10  is an equivalent circuit of the MEMS gas sensor  700 . As described in detail above, the gas sensor  700  can include a heating layer  708  comprising a resistive heater. The heating layer  708  can comprise an ITO heating layer, for example. In one embodiment, the heating layer  708  is configured to heat the sensing layer  710  to a particular temperature for detection of a specific gas species of interest. In another embodiment, the gas sensor  700  includes a temperature sensor  718 . The temperature sensor  718  can confirm that the heating layer  708  has heated the sensing layer  710  to a desired temperature and the sensing layer  710  is ready to detect the presence of a particular gas species of interest. 
     It will be understood, however, that some embodiments of MEMS gas sensors  700  described herein do not comprise a heating layer  708 . In some embodiments, the sensing layer  710  can sense the presence of a gas species of interest at ambient temperature, and need not be heated in order to sense the presence of the gas. It will also be understood that in some embodiments (not shown), the MEMS gas sensor  700  includes a sensing layer  710  that comprises the heating layer  708 . For example, in one embodiment, the sensing layer  710  comprises indium tin oxide and incorporates the heating layer  708 . 
     Methods of Detecting a Gas Using a MEMS Gas Sensor 
     As described above with reference to  FIG. 7A , a voltage V can be applied between the absorber layer  704  and the electrodes  716 . The presence of a gas species of interest in the device  700  can change the capacitances between at least two layers in the MEMS gas sensor  700 . For example, the presence of a gas species of interest in the MEMS gas sensor  700  can change the capacitance between the absorber layer  704  and the electrodes  716 , the sensing layer  710 , and/or the heating layer  708 . This change in capacitance between at least two layers can be detected in order to identify the presence of a gas species of interest in the MEMS gas sensor  700 . 
       FIG. 11  is a flowchart illustrating one method  1100  for identifying the presence of a gas species of interest using the MEMS gas sensor  700 . The MEMS gas sensor  700  can detect the presence of a gas species of interest when gas molecules are adsorbed by the sensing layer  710 , causing a residual stress change in the sensing layer  710 . The method begins at a block  1102 , in which the MEMS gas sensor  700  is first calibrated before being exposed to a particular gas species. Calibration can be performed to determine the electrical response of the MEMS gas sensor  700  in the absence of the gas. In one embodiment, the calibration procedure is performed during manufacture of the MEMS gas sensor  700 . 
     The MEMS gas sensor  700  can be calibrated using various methods. Calibration methods to determine the electrical response of the MEMS gas sensor  700  in the absence of a gas will be described in greater detail below with reference to  FIGS. 13 and 15 . 
     At a block  1104 , the MEMS gas sensor  700  is exposed to a gas species of interest. In some aspects, for example, the gas sensor  700  may be exposed to a gas after manufacturing is complete and when a display device comprising the gas sensor  700  is in use. 
     Moving to a block  1106 , the sensing layer  710  of the MEMS gas sensor  700  is heated to a temperature falling within the applicable temperature range for a particular gas species of interest. For example, the applicable temperature range to detect the presence of carbon dioxide in the MEMS gas sensor  700  may be about 400° C. to about 600° C. As described above with reference to  FIGS. 8 and 10 , the heating layer  708  of the MEMS gas sensor  700  can be activated in order to heat the sensing layer  710  to a temperature of, for example, 500° C. The sensing layer  710  can be heated to other temperatures as appropriate to detect a particular gas species of interest. 
     It will be understood that the sensing layer  710  can first be heated to a temperature falling within the applicable temperature range for a particular gas species of interest, then the MEMS gas sensor  700  can be exposed to the gas species. In some embodiments, for example, the method moves from block  1102  to block  1106 , in which the sensing layer  710  is heated, then the MEMS gas sensor  700  is exposed to a gas species of interest at block  1104 . 
     The method next moves to a block  1108 , in which an electrical response of the MEMS gas sensor  700  is measured. Various methods for determining the electrical response of the MEMS gas sensor  700  are described in detail below with reference to  FIGS. 12-15 . 
     At a block  1110 , the electrical response determined at block  1108  is compared to the electrical response the MEMS gas sensor  700  would be anticipated to exhibit in the absence of the gas species of interest. In one embodiment, the calibration performed at block  1102  on the MEMS gas sensor  700  provides information on the anticipated electrical response of the MEMS gas sensor  700  in the absence of the gas species of interest. 
     In another embodiment, the anticipated electrical response of the MEMS gas sensor  700  is determined based on known electrical and material property characteristics of MEMS gas sensors  700  at a specific temperature. For example, the electrical response of multiple MEMS gas sensors  700  operating at a particular temperature, in the absence of a gas, can be tested. The resulting test data can yield information on the anticipated electrical response of a specific MEMS gas sensor  700  operating at the given temperature in the absence of a gas. 
     Moving next to a block  1112 , the presence of the gas species of interest is identified. In one embodiment, the difference between the electrical response of the MEMS gas sensor  700  measured or determined at block  1108  and the anticipated electrical response of the MEMS gas sensor  700  is correlated to the presence of a gas species of interest in the device  700 . In another embodiment, the specific electrical response of the MEMS gas sensor  700  in the presence of a gas is used to determine the concentration of the gas species. 
     Determining Capacitance Change Using a Field Effect Transistor 
       FIG. 12  is a cross section of the MEMS gas sensor  700  connected to a field effect transistor  1200 . The field effect transistor includes a gate terminal  1210 , a source terminal  1220 , and a drain terminal  1230 . In operation, electrons flow from the source terminal  1220  toward the drain terminal  1230  if influenced by an applied voltage V 1 . The gate terminal  1210  permits electrons to flow through or blocks their passage by creating or eliminating a channel between the source terminal  1220  and the drain terminal  1230 . 
     The sensing layer  710  can have a residual stress that is a function of the presence of a gas in the MEMS gas sensor  700 . Thus, in some aspects, the presence of a gas in the MEMS gas sensor  700  causes a residual stress change in the sensing layer  710  such that the layer  710  sags slightly in a gap  718  between the absorber layer  704  and the movable element  714 . As a result, in certain embodiments, the distance between the movable element  714  and the absorber layer  704  is a function of the presence of the gas in the MEMS gas sensor  700 . The residual stress change of the sensing layer  710  causes a change in the capacitance between the absorber layer  704  and the electrodes  716 , the capacitance between the absorber layer  704  and the heating layer  708 , and/or the capacitance between the absorber layer  704  and the sensing layer  710 . This change in capacitance is determined or detected as a change in the electrical response from that which would be expected from the MEMS gas sensor  700  in the absence of the gas. Specifically, the presence of the gas species of interest can cause a change in the time it takes for an electrical event to occur in the field effect transistor  1200 , and this change in electrical response can be positively correlated to the presence of a particular gas species in the MEMS gas sensor  700 . 
       FIG. 13  is a flowchart illustrating one method  1300  of identifying the presence of a gas species of interest in the MEMS gas sensor  700  using the field effect transistor  1200  of  FIG. 12 . The method begins at a block  1302 , in which the electrodes  716  are electrically connected to the field effect transistor  1200 . The method next moves to a block  1304 , in which a voltage V is applied between the absorber layer  704  and the electrodes  716 . 
     Moving next to a block  1306 , the time it takes for an electrical event to occur in the field effect transistor  1200  is electrically measured. In one embodiment, the time it takes for current to appear in the drain terminal  1230  of the field effect transistor  1200  is determined through electrical measurement at block  1306 . In another embodiment, the time it takes to open the gate terminal  1210  of the field effect transistor  1200  is determined through electrical measurement at block  1306 . 
     In some embodiments, the blocks  1302 ,  1304 , and  1306  are performed at block  1108  of  FIG. 11 , in which the electrical response of the gas sensor  700  is measured after it has been exposed to a gas species of interest. Persons of skill in the art will also understand that, in other embodiments, the blocks  1302 ,  1304 , and  1306  are performed at block  1102  of  FIG. 11 , in which the MEMS gas sensor  700  is calibrated in the absence of a gas species of interest. For example, the time it takes for current to appear in the drain terminal  1230  (or, alternatively, the time it takes to open the gate terminal  1210 ) can be measured in the absence of a gas during a calibration procedure performed during manufacture of the MEMS gas sensor  700 . Some time later, after exposure to a gas species of interest and heating of the sensor layer  710 , the time for current to appear in the drain terminal  1230  (or, alternatively, the time to open the gate terminal  1210 ) can be measured a second time in order to detect the presence of the gas. 
     The method next moves to a block  1308 , in which the electrical response determined at block  1306  is compared to the electrical response the MEMS gas sensor  700  would be expected to exhibit in the absence of the gas species of interest. In some aspects, a calibration procedure performed at block  1102  of  FIG. 11  provides information on the anticipated electrical response of the MEMS gas sensor  700  in the absence of the gas species of interest. For example, in one embodiment, the time it takes for current to appear in the drain terminal  1230  in the absence of the gas is compared at block  1308  to the time measured at block  1306 . 
     In another embodiment the time it takes for current to appear in the drain terminal  1230  in the absence of the gas is estimated based on known electrical and material property characteristics of MEMS gas sensors  700  at a specific temperature. For example, the time it takes for current to appear in the drain terminal of multiple MEMS gas sensors  700 , operating at a particular temperature in the absence of a gas, can be tested. The anticipated time for current to appear in the drain terminal of a specific MEMS gas sensor  700 , operating at the same temperature in the absence of a gas, can be estimated based on the test data. In this embodiment, the anticipated time is compared to the time it actually took current to appear in the drain terminal  1230  as measured at block  1306  at block  1308 . 
     It will also be understood that, alternatively, the time it takes to open the gate terminal  1210  of the field effect transistor can be used to compare a determined electrical response to an anticipated electrical response in the absence of the gas species of interest. 
     Moving next to a block  1310 , the presence of the gas species of interest is identified. In one embodiment, the difference between the electrical response of the gas sensor  700  (determined at block  1306  by, for example, measuring the time for current to appear in the drain terminal  1230 ) and the anticipated electrical response of the gas sensor  700  (determined, for example, during calibration at block  1102  of  FIG. 11 ) is correlated to the presence of a gas species of interest in the gas sensor  700 . In another embodiment, the specific electrical response of the MEMS gas sensor  700  in the presence of a gas is used to determine the concentration of the gas species. 
     Some time after exposure to the gas species of interest, the residual stress of the sensing layer  710  may return to that which the sensing layer  710  exhibited before exposure to the gas. Thus, in some embodiments, the change in capacitance in the one or more layers of the MEMS gas sensor  700  described above is reversible and nonpermanent. In one embodiment, the MEMS gas sensor  700  is reusable and can detect the presence of a gas species of interest one or more times over the lifetime of the MEMS gas sensor  700 . 
     Determining Capacitance Change by Measuring Actuation Time 
       FIG. 14  is a cross section of the MEMS gas sensor  700  connected to an ammeter  1400 . The ammeter  1400  measures the electric current in the circuit formed between the absorber layer  704  and the electrodes  716  when a voltage V is applied between the absorber layer  704  and the electrodes  716 . 
     As described above with reference to  FIGS. 12-13 , the presence of a gas in the MEMS gas sensor  700  can change the capacitances between various components of the sensor  700 . This change in capacitance can be determined or detected as a change in the electrical response of the MEMS gas sensor  700 . In one embodiment illustrated in  FIG. 14 , the change in electrical response of the MEMS gas sensor  700  is a change in the time it takes to actuate the MEMS gas sensor  700 . This change in actuation time can be positively correlated to the presence of a particular gas species in the MEMS gas sensor  700 . 
       FIG. 15  is a flowchart illustrating one method of identifying the presence of a gas species of interest using the ammeter  1400  of  FIG. 14  or a spectrometer (not shown) to determine a change in the actuation time of the MEMS gas sensor  700 . The method begins at a block  1502 , in which a selected voltage, for example an actuation voltage V A , is applied between the absorber layer  704  and the electrodes  716 . This causes the movable element  714  of the MEMS gas sensor  700  to move toward the absorber layer  704 . Thus, in certain embodiments, the distance between the movable element  714  and the absorber layer  704  is a function of a voltage applied between the movable element  714  and the absorber layer  704 . 
     Moving next to a block  1504 , the gap  718  between the absorber layer  704  and the movable element  714  is collapsed in response to the actuation voltage. Methods and systems for actuating the MEMS gas sensor  700  are described in greater detail above with reference to  FIG. 1 , and in particular the actuated pixel  12   b  on the right in  FIG. 1 . When the MEMS gas sensor  700  is actuated and the gap  718  is collapsed, current in the circuit between the absorber layer  704  and the electrodes  716  will increase or spike. In addition, as described in detail above with reference to  FIG. 7A , one or more properties of light emitted from the sensor  700  can be modulated when the sensor  700  is actuated and the movable element  714  moves toward the substrate  702 . In one embodiment, the color of light emitted by the MEMS gas sensor  700  changes when the sensor  700  is actuated and the gap  718  is collapsed. The spike in current in the circuit and/or the change in the color of light emitted by the sensor  700  can be indicative that the MEMS gas sensor  700  has been actuated and the gap  718  has collapsed. 
     At a block  1506 , the increase or spike in current in the circuit between the absorber layer  704  and the electrodes  716  is electronically measured using the ammeter  1400  connected to the MEMS gas sensor  700 . 
     The method may alternatively move to a block  1508  from block  1504 . At block  1508 , the color of light emitted by the MEMS gas sensor  700  is optically measured in order to detect a change in the color of light emitted by the sensor  700 . In one embodiment, detecting a change in the color of light comprises using a spectrometer (not shown) to optically detect a change in the color of light emitted by the MEMS gas sensor  700 . 
     Moving next to a block  1510 , the time to actuate the MEMS gas sensor  700  is electronically determined. In one embodiment, the actuation time is determined at block  1510  using information gathered at block  1506  when the spike in current in the circuit between the absorber layer  704  and the electrodes  716  was measured electronically. In another embodiment, the actuation time is determined at block  1510  using information gathered at block  1508  when a change in the color of light emitted by the MEMS gas sensor  700  was determined optically. 
     At a block  1512 , the electrical response (in this case, the actuation time) determined at block  1510  is compared to the electrical response the MEMS gas sensor  700  would be expected to exhibit in the absence of the gas species of interest. In some aspects, a calibration procedure performed at block  1102  of  FIG. 11  provides information on the anticipated electrical response of the MEMS gas sensor  700  in the absence of the gas species of interest. For example, in one embodiment, the actuation time of the sensor  700  in the absence of the gas is compared at block  1512  to the actuation time determined at block  1510 . 
     In another embodiment, the actuation time of the sensor  700  in the absence of the gas is estimated based on known electrical and material property characteristics of MEMS gas sensors  700  at a specific temperature. For example, the actuation time of multiple MEMS gas sensors  700  operating at a particular temperature, in the absence of a gas, can be tested. The anticipated actuation time of a specific MEMS gas sensor  700 , operating at the same temperature in the absence of a gas, can be estimated based on the test data. In this embodiment, this anticipated actuation time is compared to the actual actuation time determined at block  1510  at block  1512 . 
     Moving next to a block  1514 , the presence of the gas species of interest is identified. In one embodiment, the difference between the electrical response of the gas sensor  700  (determined at block  1510  by, for example, determining its actuation time) and the anticipated electrical response of the gas sensor  700  (determined, for example, during calibration at block  1102  of  FIG. 11 ) is correlated to the presence of a gas species of interest in the gas sensor  700 . In another embodiment, the specific electrical response of the MEMS gas sensor  700  in the presence of a gas is used to determine the concentration of the gas species. 
     Packaging 
     Embodiments of gas sensors described herein can be packaged using various methods and systems. For example, in one aspect, the MEMS gas sensor  700  is packaged in a device that is chosen or selected based on the anticipated environment the MEMS gas sensor  700  will operate in. It can be anticipated, for example, that the MEMS gas sensor  700  will operate in an environment where the presence of CO 2  gas is of interest. The MEMS gas sensor  700  can be packaged in a device configured to facilitate detection of the gas species of interest, in this case CO 2 , by the gas sensor  700 . 
     In some aspects, the MEMS gas sensor  700  is enclosed in a package comprising epoxy and a piece of glass or film. For example, the piece of glass and/or film can be coupled to the substrate  702  of the gas sensor  700  using epoxy to form a package around the gas sensor  700 . In one embodiment, the epoxy used to seal and/or form the package is permeable to a particular gas species of interest, for example CO 2 . It will also be understood that embodiments of the gas sensors described herein do not require packaging and can be integrated into display devices or any other device without the use of a package. 
     In certain embodiments, the change in capacitance in the layers of the MEMS gas sensors described herein is reversible and non-permanent. Thus, in one embodiment, a non-permanent electrical and/or mechanical response occurs in the MEMS gas sensor  700  as a result of the presence of a gas species of interest in the MEMS gas sensor  700 . After a time, the MEMS gas sensor  700  returns to its original electrical and/or mechanical state prior to exposure to the gas species of interest. Thus, embodiments of the MEMS gas sensor devices described herein can be used repeatedly to detect a gas species of interest. It will be understood that in other embodiments, the capacitance change in the layers of the MEMS gas sensors described herein can be permanent and nonreversible. 
     The skilled artisan will understand that the gas sensing methods and systems described herein are not limited to MEMS devices. The methods and systems described herein can be used in any display device requiring a gas sensor, such as OLED or LCD devices. It will also be understood that use of the gas sensors described herein are not limited to display devices, and can be used in any environment in which detection of a particular gas species of interest is required. 
     One of skill in the art will also understand that embodiments of gas sensors described herein can be included in a variety of displays, such as but not limited to MEMS, LCD, and AMOLED displays. One or more MEMS gas sensors can be fabricated during the manufacture of the display. In one embodiment, the MEMS gas sensor  700  is fabricated on the periphery of an active area of a display. In another embodiment, the MEMS gas sensor  700  is fabricated in place of a sub-pixel in the display. In some aspects, the MEMS gas sensor  700  which has been fabricated in place of a sub-pixel is located in the active area of the display, yet is not visible to the human eye. It will also be understood that one display can include a plurality of MEMS gas sensors  700 , some located in the periphery of the active area of the display and others located in the active area of the display. 
     The hardware and data processing apparatus used to implement the various illustrative logics, logical blocks, modules and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, or, any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In some implementations, particular steps and methods may be performed by circuitry that is specific to a given function. 
     Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the present disclosure is not intended to be limited to the implementations shown herein, but is to be accorded the widest scope consistent with the claims, the principles and the novel features disclosed herein. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other implementations. 
     Certain features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Additionally, other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results.