# Patent

## Title
Method and system for driving MEMS display elements

## Abstract

Systems and methods for driving MEMS display elements are disclosed. In one embodiment, a display comprises an array of MEMS display elements, and a driving circuit coupled to said array, wherein said driving circuit configured to provide at lease a row signal and a column signal to drive said array, and only one of said row and column signals is adjusted for temperature change. In another embodiment, a method of driving an array of MEMS display elements is disclosed, where the method comprises sensing a temperature at a predetermined location, generating one of a row signal and a column signal having a level based at least in part on the sensed temperature and the other not based on the sensed temperature, and providing said row and column signals to said array.

## Background
RELATED APPLICATIONS
The present Application for Patent is a Continuation in Part of patent application Ser. No. 11/218,887 entitled “METHOD AND SYSTEM FOR DRIVING INTERFEROMETRIC MODULATORS” filed Sep. 2, 2005, and assigned to the assignee hereof and hereby expressly incorporated by reference herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention relates to microelectromechanical systems (MEMS).
2. Description of the Related Technology
Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. One type of MEMS device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In certain embodiments, an interferometric modulator may comprise a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed.
SUMMARY OF CERTAIN EMBODIMENTS
The system, method, and devices of the invention each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of Certain Embodiments” one will understand how the features of this invention provide advantages over other display devices.
In one embodiment, a display comprises an array of MEMS display elements, and a driving circuit coupled to said array and configured to provide actuation signals to drive said array, wherein said actuation signals comprise at lease a row signal and a column signal, and wherein only one of said row and column signals is adjusted for temperature change.
In another embodiment, a method of driving an array of MEMS display elements comprises sensing a temperature at a predetermined location, generating one of a row signal and a column signal having a level based at least in part on the sensed temperature and the other of the row and column signals not based on the sensed temperature, and providing said row and column signals to said array.
In another embodiment, a display comprises means for sensing a temperature at a predetermined location, means for generating one of a row signal and a column signal having a level based at least in part on the sensed temperature and the other of the row and column signals not based on the sensed temperature, means for displaying image data, and means for providing said row and column signals to said displaying means.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view depicting a portion of one embodiment of an interferometric modulator display in which a movable reflective layer of a first interferometric modulator is in a relaxed position and a movable reflective layer of a second interferometric modulator is in an actuated position.
FIG. 2 is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display.
FIG. 3 is a diagram of movable 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.
FIG. 5A illustrates one exemplary frame of display data in the 3×3 interferometric modulator display of FIG. 2.
FIG. 5B illustrates one exemplary timing diagram for row and column signals that may be used to write the frame of FIG. 5A.
FIGS. 6A and 6B are system block diagrams illustrating an embodiment of a visual display device comprising a plurality of interferometric modulators.
FIG. 7A is a cross section of the device of FIG. 1.
FIG. 7B is a cross section of an alternative embodiment of an interferometric modulator.
FIG. 7C is a cross section of another alternative embodiment of an interferometric modulator.
FIG. 7D is a cross section of yet another alternative embodiment of an interferometric modulator.
FIG. 7E is a cross section of an additional alternative embodiment of an interferometric modulator.
FIG. 8 is a perspective view of an interferometric modulator illustrating multiple layers of the movable reflective layer.
FIG. 9 is a graph illustrating the relationship between operational voltage of a interferometric modulator and temperature.
FIG. 10 is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display when temperature changes.
FIG. 11 is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display when temperature changes.
FIG. 12 is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display and temperature sensor.
FIG. 13 is a system block diagram illustrating another embodiment of an electronic device incorporating a 3×3 interferometric modulator display and temperature sensor.
FIG. 14 is a system block diagram illustrating another embodiment of an electronic device incorporating a 3×3 interferometric modulator display and temperature sensor.
FIG. 15 is a system block diagram illustrating an embodiment of an electronic device incorporating a 3×3 interferometric modulator display and a test interferometric modulator.
FIG. 16 is a graph of time (x-axis) verses capacitance and voltage (y-axis), and illustrates the capacitance of an interferometric modulator resulting from an applied voltage.
FIG. 17 is a flow diagram illustrating a process of driving an array based on a sensed temperature.



DETAILED DESCRIPTION OF THE CERTAIN EMBODIMENTS OF THE INVENTION
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.
The amount of voltage applied by a control system that is necessary to place an modulator into an actuated state (the “actuation voltage”) can change due to a number of detrimental operating factors that affect the interferometric modulator, including, for example, temperature, changes in the electro-mechanical properties of the interferometer, electrical charge buildup, and physical wear of the mechanical mirror. As described in more detail below, the actuation voltage applied to an interferometric modulator as a combination of two voltages, a column bias voltage (Vbias) and a row voltage. Changes in the electro-mechanical properties of the interferometer, electrical charge buildup, and physical wear of the mechanical mirror typically affect the actuation voltage only after a significant amount of use or after the passage of a certain amount of time. The operating temperature of the interferometric modulator affects characteristics of the movable reflective layer 14 immediately such that a large change in temperature can cause a significant change in the actuation voltage. Depending on the environmental conditions in which the interferometric modulator is used, for example, incorporated in a display on a device placed on the dashboard of an automobile in Arizona during the summer, or on a device exposed to sub-zero winter temperatures, a significant temperature change of the interferometric modulator can occur within hours or even minutes. In one embodiment of the invention, a sensor monitors a temperature existing at a location in a device having a display incorporating interferometric modulators and provides a signal related to the temperature to the driving circuit for the display. The driving circuit, using predetermined information that correlates the sensed temperature to the necessary voltage required to operate the display at various temperatures, drives the display to operate over a wide range of temperatures by adjusting the bias voltage based on the signal it receives from the sensor.
One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated in FIG. 1. In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“off” or “closed”) state, the display element reflects little incident visible light to the user. Depending on the embodiment, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at selected colors, allowing for a color display in addition to black and white.
FIG. 1 is an isometric view depicting two adjacent pixels in a series of pixels of a visual display, wherein each pixel comprises a MEMS interferometric modulator. In some embodiments, an interferometric modulator display comprises a row/column array of these interferometric modulators. Each interferometric modulator includes a pair of reflective layers positioned at a variable and controllable distance from each other to form a resonant optical cavity with at least one variable dimension. In one embodiment, one of the reflective layers may be moved between two positions. In the first position, referred to herein as the relaxed position, the movable reflective layer is positioned at a relatively large distance from a fixed partially reflective layer. In the second position, referred to herein as the actuated position, the movable reflective layer is positioned more closely adjacent to the partially reflective layer. Incident light that reflects from the two layers interferes constructively or destructively depending on the position of the movable reflective layer, producing either an overall reflective or non-reflective state for each pixel.
The depicted portion of the pixel array in FIG. 1 includes two adjacent interferometric modulators 12a and 12b. In the interferometric modulator 12a on the left, a movable reflective layer 14a is illustrated in a relaxed position at a predetermined distance from an optical stack 16a, which includes a partially reflective layer. In the interferometric modulator 12b on the right, the movable reflective layer 14b is illustrated in an actuated position adjacent to the optical stack 16b. 
The optical stacks 16a and 16b (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. 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 are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective layers 14a, 14b may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of 16a, 16b) 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 14a, 14b are separated from the optical stacks 16a, 16b by a defined gap 19. A highly conductive and reflective material such as aluminum may be used for the reflective layers 14, and these strips may form column electrodes in a display device.
With no applied voltage, the cavity 19 remains between the movable reflective layer 14a and optical stack 16a, with the movable reflective layer 14a in a mechanically relaxed state, as illustrated by the pixel 12a in FIG. 1. However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel becomes charged, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable reflective layer 14 is deformed and is forced against the optical stack 16. A dielectric layer (not illustrated in this Figure) within the optical stack 16 may prevent shorting and control the separation distance between layers 14 and 16, as illustrated by pixel 12b on the right in FIG. 1. The behavior is the same regardless of the polarity of the applied potential difference. In this way, row/column actuation that can control the reflective vs. non-reflective pixel states is analogous in many ways to that used in conventional LCD and other display technologies.
FIGS. 2 through 5B illustrate one exemplary process and system for using an array of interferometric modulators in a display application.
FIG. 2 is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor 21 which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium®, PentiumII®, PentiumIII®, Pentium IV®, Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor 21 may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application.
In one embodiment, the processor 21 is also configured to communicate with an array driver 22. In one embodiment, the array driver 22 includes a row driver circuit 24 and a column driver circuit 26 that provide signals to a display array or panel 30. The cross section of the array illustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices illustrated in FIG. 3. It may require, for example, a 10 volt potential difference to cause a movable layer to deform from the relaxed state to the actuated state. However, when the voltage is reduced from that value, the movable layer maintains its state as the voltage drops back below 10 volts. In the exemplary embodiment of FIG. 3, the movable layer does not relax completely until the voltage drops below 2 volts. There is thus a range of voltage, about 3 to 7 V in the example illustrated in FIG. 3, where there exists a window of applied voltage within which the device is stable in either the relaxed or actuated state. This is referred to herein as the “hysteresis window” or “stability window.” For a display array having the hysteresis characteristics of FIG. 3, the row/column actuation protocol can be designed such that during row strobing, pixels in the strobed row that are to be actuated are exposed to a voltage difference of about 10 volts, and pixels that are to be relaxed are exposed to a voltage difference of close to zero volts. After the strobe, the pixels are exposed to a steady state voltage difference of about 5 volts such that they remain in whatever state the row strobe put them in. After being written, each pixel sees a potential difference within the “stability window” of 3-7 volts in this example. This feature makes the pixel design illustrated in FIG. 1 stable under the same applied voltage conditions in either an actuated or relaxed pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or relaxed state, is essentially a capacitor formed by the fixed and moving reflective layers, this stable state can be held at a voltage within the hysteresis window with almost no power dissipation. Essentially no current flows into the pixel if the applied potential is fixed.
In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row 1 electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row 2 electrode, actuating the appropriate pixels in row 2 in accordance with the asserted column electrodes. The row 1 pixels are unaffected by the row 2 pulse, and remain in the state they were set to during the row 1 pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used in conjunction with the present invention.
FIGS. 4, 5A, and 5B 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 −Vbias, 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 +Vbias, 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 +Vbias, or −Vbias. 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 +Vbias, and the appropriate row to −ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −Vbias, and the appropriate row to the same −ΔV, producing a zero volt potential difference across the pixel.
FIG. 5B is a timing diagram showing a series of row and column signals applied to the 3×3 array of FIG. 2 which will result in the display arrangement illustrated in FIG. 5A, where actuated pixels are non-reflective. Prior to writing the frame illustrated in FIG. 5A, the pixels can be in any state, and in this example, all the rows are at 0 volts, and all the columns are at +5 volts. With these applied voltages, all pixels are stable in their existing actuated or relaxed states.
In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) are actuated. To accomplish this, during a “line time” for row 1, columns 1 and 2 are set to −5 volts, and column 3 is set to +5 volts. This does not change the state of any pixels, because all the pixels remain in the 3-7 volt stability window. Row 1 is then strobed with a pulse that goes from 0, up to 5 volts, and back to zero. This actuates the (1,1) and (1,2) pixels and relaxes the (1,3) pixel. No other pixels in the array are affected. To set row 2 as desired, column 2 is set to −5 volts, and columns 1 and 3 are set to +5 volts. The same strobe applied to row 2 will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again, no other pixels of the array are affected. Row 3 is similarly set by setting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3 strobe sets the row 3 pixels as shown in FIG. 5A. After writing the frame, the row potentials are zero, and the column potentials can remain at either +5 or −5 volts, and the display is then stable in the arrangement of FIG. 5A. It will be appreciated that the same procedure can be employed for arrays of dozens or hundreds of rows and columns. It will also be appreciated that the timing, sequence, and levels of voltages used to perform row and column actuation can be varied widely within the general principles outlined above, and the above example is exemplary only, and any actuation voltage method can be used with the systems and methods described herein.
FIGS. 6A and 6B are system block diagrams illustrating an embodiment of a display device 40. The display device 40 can be, for example, a cellular or mobile telephone. However, the same components of display device 40 or slight variations thereof are also illustrative of various types of display devices such as televisions and portable media players.
The display device 40 includes a housing 41, a display 30, an antenna 43, a speaker 44, an input device 48, and a microphone 46. The housing 41 is generally formed from any of a variety of manufacturing processes as are well known to those of skill in the art, including injection molding, and vacuum forming. In addition, the housing 41 may be made from any of a variety of materials, including but not limited to plastic, metal, glass, rubber, and ceramic, or a combination thereof. In one embodiment the housing 41 includes removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols.
The display 30 of exemplary display device 40 may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display 30 includes a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD as described above, or a non-flat-panel display, such as a CRT or other tube device, as is well known to those of skill in the art. However, for purposes of describing the present embodiment, the display 30 includes an interferometric modulator display, as described herein.
The components of one embodiment of exemplary display device 40 are schematically illustrated in FIG. 6B. The illustrated exemplary display device 40 includes a housing 41 and can include additional components at least partially enclosed therein. For example, in one embodiment, the exemplary display device 40 includes a network interface 27 that includes an antenna 43 which is coupled to a transceiver 47. The transceiver 47 is connected to a processor 21, which is connected to conditioning hardware 52. The conditioning hardware 52 may be configured to condition a signal (e.g. filter a signal). The conditioning hardware 52 is connected to a speaker 45 and a microphone 46. The processor 21 is also connected to an input device 48 and a driver controller 29. The driver controller 29 is coupled to a frame buffer 28, and to an array driver 22, which in turn is coupled to a display array 30. A power supply 50 provides power to all components as required by the particular exemplary display device 40 design.
The network interface 27 includes the antenna 43 and the transceiver 47 so that the exemplary display device 40 can communicate with one ore more devices over a network. In one embodiment the network interface 27 may also have some processing capabilities to relieve requirements of the processor 21. The antenna 43 is any antenna known to those of skill in the art for transmitting and receiving signals. In one embodiment, the antenna transmits and receives RF signals according to the IEEE 802.11 standard, including IEEE 802.11(a), (b), or (g). In another embodiment, the antenna transmits and receives RF signals according to the BLUETOOTH standard. In the case of a cellular telephone, the antenna is designed to receive CDMA, GSM, AMPS or other known signals that are used to communicate within a wireless cell phone network. The transceiver 47 pre-processes the signals received from the antenna 43 so that they may be received by and further manipulated by the processor 21. The transceiver 47 also processes signals received from the processor 21 so that they may be transmitted from the exemplary display device 40 via the antenna 43.
In an alternative embodiment, the transceiver 47 can be replaced by a receiver. In yet another alternative embodiment, network interface 27 can be replaced by an image source, which can store or generate image data to be sent to the processor 21. For example, the image source can be a digital video disc (DVD) or a hard-disc drive that contains image data, or a software module that generates image data.
Processor 21 generally controls the overall operation of the exemplary display device 40. The processor 21 receives data, such as compressed image data from the network interface 27 or an image source, and processes the data into raw image data or into a format that is readily processed into raw image data. The processor 21 then sends the processed data to the driver controller 29 or to frame buffer 28 for storage. Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation, and gray-scale level.
In one embodiment, the processor 21 includes a microcontroller, CPU, or logic unit to control operation of the exemplary display device 40. Conditioning hardware 52 generally includes amplifiers and filters for transmitting signals to the speaker 45, and for receiving signals from the microphone 46. Conditioning hardware 52 may be discrete components within the exemplary display device 40, or may be incorporated within the processor 21 or other components.
The driver controller 29 takes the raw image data generated by the processor 21 either directly from the processor 21 or from the frame buffer 28 and reformats the raw image data appropriately for high speed transmission to the array driver 22. Specifically, the driver controller 29 reformats the raw image data into a data flow having a raster-like format, such that it has a time order suitable for scanning across the display array 30. Then the driver controller 29 sends the formatted information to the array driver 22. Although a driver controller 29, such as a LCD controller, is often associated with the system processor 21 as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. They may be embedded in the processor 21 as hardware, embedded in the processor 21 as software, or fully integrated in hardware with the array driver 22.
Typically, the array driver 22 receives the formatted information from the driver controller 29 and reformats the video data into a parallel set of waveforms that are applied many times per second to the hundreds and sometimes thousands of leads coming from the display's x-y matrix of pixels.
In one embodiment, the driver controller 29, array driver 22, and display array 30 are appropriate for any of the types of displays described herein. For example, in one embodiment, driver controller 29 is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, array driver 22 is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, a driver controller 29 is integrated with the array driver 22. Such an embodiment is common in highly integrated systems such as cellular phones, watches, and other small area displays. In yet another embodiment, display array 30 is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators).
The input device 48 allows a user to control the operation of the exemplary display device 40. In one embodiment, input device 48 includes a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a touch-sensitive screen, a pressure- or heat-sensitive membrane. In one embodiment, the microphone 46 is an input device for the exemplary display device 40. When the microphone 46 is used to input data to the device, voice commands may be provided by a user for controlling operations of the exemplary display device 40.
Power supply 50 can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, power supply 50 is a rechargeable battery, such as a nickel-cadmium battery or a lithium ion battery. In another embodiment, power supply 50 is a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell, and solar-cell paint. In another embodiment, power supply 50 is configured to receive power from a wall outlet.
In some implementations control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some cases control programmability resides in the array driver 22. Those of skill in the art will recognize that the above-described optimization may be implemented in any number of hardware and/or software components and in various configurations.
The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example, FIGS. 7A-7E illustrate five different embodiments of the movable reflective layer 14 and its supporting structures. FIG. 7A is a cross section of the embodiment of FIG. 1, where a strip of metal material 14 is deposited on orthogonally extending supports 18. In FIG. 7B, the moveable reflective layer 14 is attached to supports at the corners only, on tethers 32. In FIG. 7C, the moveable reflective layer 14 is suspended from a deformable layer 34, which may comprise a flexible metal. The deformable layer 34 connects, directly or indirectly, to the substrate 20 around the perimeter of the deformable layer 34. These connections are herein referred to as support posts. The embodiment illustrated in FIG. 7D has support post plugs 42 upon which the deformable layer 34 rests. The movable reflective layer 14 remains suspended over the cavity, as in FIGS. 7A-7C, but the deformable layer 34 does not form the support posts by filling holes between the deformable layer 34 and the optical stack 16. Rather, the support posts are formed of a planarization material, which is used to form support post plugs 42. The embodiment illustrated in FIG. 7E is based on the embodiment shown in FIG. 7D, but may also be adapted to work with any of the embodiments illustrated in FIGS. 7A-7C as well as additional embodiments not shown. In the embodiment shown in FIG. 7E, an extra layer of metal or other conductive material has been used to form a bus structure 44. This allows signal routing along the back of the interferometric modulators, eliminating a number of electrodes that may otherwise have had to be formed on the substrate 20.
In embodiments such as those shown in FIG. 7, the interferometric modulators function as direct-view devices, in which images are viewed from the front side of the transparent substrate 20, the side opposite to that upon which the modulator is arranged. In these embodiments, the reflective layer 14 optically shields the portions of the interferometric modulator on the side of the reflective layer opposite the substrate 20, including the deformable layer 34. This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. Such shielding allows the bus structure 44 in FIG. 7E, which provides the ability to separate the optical properties of the modulator from the electromechanical properties of the modulator, such as addressing and the movements that result from that addressing. This separable modulator architecture allows the structural design and materials used for the electromechanical aspects and the optical aspects of the modulator to be selected and to function independently of each other. Moreover, the embodiments shown in FIGS. 7C-7E have additional benefits deriving from the decoupling of the optical properties of the reflective layer 14 from its mechanical properties, which are carried out by the deformable layer 34. This allows the structural design and materials used for the reflective layer 14 to be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer 34 to be optimized with respect to desired mechanical properties.
The amount of voltage applied by a control system that is necessary to place the movable mirror of an interferometric modulator in an actuated state is referred to as the actuation voltage. For example, as illustrated in FIG. 3, the actuation voltage is about 9-10 volts, so that the application of about −10 volts or about +10 volts actuates the movable reflective layer 14b (as illustrated in FIG. 1) of an interferometric modulator and the application of about 0 volts relaxes the movable reflective layer 14a (as illustrated in FIG. 1) of the interferometric modulator. The actuation voltage can change over time due to a number of factors including, for example, temperature, changes in the electro-mechanical properties of the interferometer, and physical wear of the mechanical mirror.
Some of these factors (e.g., changes in the electro-mechanical properties of the interferometric modulator, and physical wear of the mechanical mirror) typically affect the bias voltage only after a significant amount of use or after the passage of a certain amount of time. Temperature, however, affects characteristics of the movable reflective layer 14 in a short period of time and cause a significant change in the voltage required to operate the interferometric modulator. Depending on the environmental conditions in which the interferometric modulator is used, for example, as incorporated in a display on a device placed on the dashboard of an automobile in Arizona during the summer, or in a device exposed to sub-zero winter temperatures, a significant temperature change of the interferometric modulator can occur within hours or even minutes. Sensing a temperature existing at a location in such a device and, using predetermined information that correlates the sensed temperature to the necessary voltage required to operate the interferometric modulators at that temperature, the display can be efficiently driven to operate over a wide range of temperatures by adjusting the bias voltage as a function of the temperature.
FIG. 8 is a perspective illustration of one embodiment of an interferometric modulator 60 in a released (or relaxed) state. The interferometric modulator 60 includes an optical stack 16 that typically includes an electrode layer, an absorber layer and an electrode layer (not shown separately) on a transparent substrate 20. The relative thickness of the substrate 20 is much greater than the thickness of the optical stack 16. For example, in some embodiments the substrate 20 is about 700 μm thick and the optical stack 16 is about 1 μm or less thick. In some embodiments, the substrate 20 is glass. Supports 18 provide support for the movable reflective layer 14 which is separated from the optical stack 16 by a cavity 19.
The movable reflective layer 14 includes a relatively thin layer of a first material 11 and a relatively thick layer of a second material 13. In the embodiment of FIG. 8, the first material 11 is aluminum disposed in a layer which is about 300 Angstroms thick, and the second material 13 is Nickel disposed in a layer that is about 1000 Angstroms thick. In other embodiments, the first material 11 and the second material 13 can comprise other materials, for example, an aluminum alloy. The thickness of the first material 11 and the second material 13 can also be different in other embodiments. In some embodiments, the movable reflective layer 14 can be monolithic, including only a uniform single layer comprised of, for example, nickel, nickel alloy, aluminum, or an aluminum alloy. In other embodiments, the movable reflective layer 14 can include more than two layers of materials. In some embodiments, the layer of the first material 11 can be thicker than the layer of the second material 13, which can change the relationship of the dominant material for stress and strain.
The stress which is introduced in an interferometric modulator through a change in temperature of an interferometric modulator, and the resulting strain, can significantly affect the movement of the movable reflective layer 14. Stress is the force exerted per unit area by a body upon an adjoining part, and strain is the deformation or change in dimension occasioned by stress. Both the resistance to stress and the elastic limit depend on the composition of the solid. When a body is subjected to pull, it is said to be under tension, or tensile stress, and when it is being pushed, it is under compression, or compressive stress. Tensile stress is generally considered to be positive, while, compressive stress is considered to be negative. As the temperature of material changes, the body expands or contracts in accordance with the coefficient of thermal expansion (CTE) of the materials from which it is made. The normal operating temperature of an interferometric modulator can be, for example, about −40° C. to +70° C. As temperature changes, the substrate 20, the first material 11 and the second material 13 of the movable the reflective layer 14 expand and contract differently in accordance with their respective CTE's. This expansion and contraction of the two different materials introduces strain in the movable reflective layer 14, which causes a corresponding change in stress in the movable reflective layer 14.
Although both the layer of the first material 11 and the layer of the second material 13 expand and contract as a function of temperature as expressed by their respective CTE's, the CTE of the thicker layer (e.g., the second material 13), dominates the amount of expansion or contraction. The amount of expansion and contraction of the substrate 20 and optical stack 16 is dominated by the expansion and contraction of the substrate 20 due to its much greater thickness. Typically, the CTE of the substrate 20 is less than the CTE of the layer of second material 13, so that the layer of second material 13 expands and contracts more than that the substrate 20 as the reference temperature changes. However, the supports 18 constrain the expansion and contraction of the movable reflective layer 14 relative to the substrate 20. Accordingly, as the temperature changes, the movable reflective layer 14 experiences a change in strain in the planar x and y direction of the movable reflective layer 14 and a corresponding change in stress (σ) occurs also in the x and y direction of the movable reflective layer 14. Stress of the movable reflective layer 14 affects its ability to move between an actuated and an unactuated position, and accordingly affects the bias voltage. In one embodiment, the substrate 20 comprises display grade Corning 1737, an aluminosilicate glass with a CTE of 3.76×10−6/° C. A typical composition of aluminosilicate glass is 55.0% SiO2, 7.0% B2O3, 10.4% Al2O3, 21.0% CaO, and 1.0% Na2O.
FIG. 9 is a graph illustrating the relationship between temperature (x-axis) and bias voltage (y-axis) of an interferometric modulator, according to one embodiment. As shown in FIG. 9, the bias voltage of an interferometric modulator over a certain temperature range is approximately inversely related to the temperature of the interferometric modulator, e.g., as the temperature of the interferometric modulator increases, the bias voltage decreases. Even a small change of the bias voltage (e.g., about a 0.25 volts or less, in some embodiments) can be significant to affect the interferometric modulator's operation depending on hysteresis characteristics of the interferometric modulator. In the graph of FIG. 9, bias voltage changes by about 0.25 volts during a temperature change of about 25° C.
As FIG. 9 exemplifies, a change in temperature causes an increase or decrease in stress in a planar x and y direction of the movable reflective layer 14 which affects the bias voltage. Temperature based compensation of the voltage applied to control the interferometric modulator 60 can be advantageously used to keep the interferometric modulator 60 operating consistently. That is, when the temperature of the interferometric modulator increases, a lower activation voltage is provided, and when temperature decreases, a higher activation voltage is provided.
As described above, the actuation voltage applied to an interferometric modulator as a combination of two voltages that are applied to the interferometric modulator, a column bias voltage (Vbias) and a row voltage. In the embodiment described herein, the row voltage does not change from its values of either +ΔV or −ΔV (e.g., see FIG. 4). The bias voltage can be adjusted by the array driver 22, for example, as a function of temperature thus providing an actuation voltage that is compensates for temperature. The relationship between the bias voltage, also referred to herein as the operational voltage (VOpp), stress (σ), and temperature (T) is illustrated in the following equations:




  
    
      
        
          V
          Opp
        
        ∝
        
          σ
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        1
      
    
  
  
    
      
        σ
        =
        
          
            σ
            o
          
          +
          
            k
            ⁢
            
              
            
            ⁢
            Δ
            ⁢
            
              
            
            ⁢
            T
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        2
      
    
  




wherein_σ0 is the residual stress at, for example, a reference temperature, and k is a constant. A typical reference temperature is a room temperature of about 25 degrees Celsius. As an example of the relationship between these parameters in one embodiment, every one degree Celsius increase in temperature results in a 2 MPa change in stress in the movable reflective layer and  ̃11 mV shift in operational voltage. In a common embodiment, the stress (σ) within layer 14 of the interferometric modulator 60 is a tensile stress, which means that σ is greater than or equal to zero.

The residual stress, σ0, in layer 14 refers to the stress at a reference temperature when in a relaxed (unactuated) state, which is a result of a process used to manufacture interferometric modulator 60. Manufacturing processes influence the residual stress, σ0, since interferometric modulator 60 is exposed to various processing temperatures and since layer 14 is initially formed on a sacrificial layer that is ultimately removed.
In FIG. 8, the stresses, σx and σy, along the respective x and y axes, within layer 14 are shown with respect to a unit area 17. The change in actuation voltage resulting from a change in temperature of the interferometric modulator can be shown by the following equation:




  
    
      
        
          V
          act
        
        ∝
        
          
            (
            
              
                h
                
                  3
                  /
                  2
                
              
              L
            
            )
          
          ⁢
          
            
              
                σ
                ⁡
                
                  (
                  T
                  )
                
              
              *
              t
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        3
      
    
  




where L is distance between the supports of an interferometric modulator, h is the air gap thickness through which the reflective layer 14 moves, σ(T) is the stress in the movable reflective layer 14 which is a function of the reference temperature T, and t is the thickness of the movable reflective layer 14. The air gap, the thickness of the movable reflective layer, and the distance between the supports are selected during the design of the interferometric modulator and thus are not subject to change once the modulator is fabricated.

The temperature dependence of the stress σ can be described as σ=σ0−σT(T) where σ0 is the residual stress, at a reference temperature, in the movable reflective layer 14 after fabrication, which is dominated by the CTE of the second material 13, as described above. In some embodiments, the reference temperature is the reference temperature.
The thermal expansion mismatch between the moveable reflective layer 14 and substrate 20 causes a thermal strain and resulting thermal stress that is a function of the thermal expansion mismatch. For example, where the moveable reflective layer 14 is nickel and the substrate 20 is Corning Glass No. 1737, the thermal mismatch (ΔCTE) can be described as

ΔCTE=α1−α2  Equation 4

where α1=13.0×10−6/° C. (the CTE of Nickel), and α2 =3.76×10−6/° C. (the CTE of Corning Glass No. 1737). The thermal strain εT can then be described as

εT=(ΔCTE)(ΔT)  Equation 5

wherein ΔT is the temperature change with respect to a reference temperature. The resulting thermal stress can them be described as

σT(T)=E1εT=E1(ΔCTE)(ΔT)  Equation 6

where E1 is the elastic modules of nickel, and ΔT is the temperature change with respect to a reference temperature. The actuation voltage can then be described as a function of temperature as shown in either of the following equations:





  
    
      
        
          
            
              V
              act
            
            ⁡
            
              (
              T
              )
            
          
          ∝
          
            
              (
              
                
                  h
                  
                    3
                    /
                    2
                  
                
                L
              
              )
            
            ⁢
            
              
                
                  (
                  
                    (
                    
                      
                        σ
                        o
                      
                      -
                      
                        
                          
                            E
                            1
                          
                          ⁡
                          
                            (
                            
                              
                                ∝
                                1
                              
                              ⁢
                              
                                -
                                
                                  ∝
                                  2
                                
                              
                            
                            )
                          
                        
                        ⁢
                        Δ
                        ⁢
                        
                          
                        
                        ⁢
                        T
                      
                    
                    )
                  
                  )
                
                *
                t
              
            
          
        
        ⁢
        
          

        
        ⁢
        or
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        7
      
    
  
  
    
      
        
          
            V
            act
          
          ⁡
          
            (
            T
            )
          
        
        ≅
        
          
            k
            1
          
          ⁢
          
            
              
                σ
                o
              
              ⁢
              t
            
          
          ⁢
          
            (
            
              1
              -
              
                
                  
                    k
                    2
                  
                  ⁢
                  Δ
                  ⁢
                  
                    
                  
                  ⁢
                  T
                
                
                  2
                  ⁢
                  
                    σ
                    o
                  
                
              
            
            )
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        8
      
    
  




wherein ΔT is the temperature change with respect to a reference temperature. Equation 8 shows the actuation voltage as a linear approximation of the Equation 7. FIG. 9 is a graph illustrating the relationship between temperature and bias voltage in a particular embodiment, and illustrating that this relationship is nearly linear over a certain temperature range. Note that k1 and k2 are constants that simplify the representation of the equation.

The residual stress of the movable reflective layer 14 is controllable to some extent during fabrication by the selection of variables that minimize the mismatch of CTE's between the movable reflective layer 14 and the substrate 20, the thickness of the layers of each material used (e.g., the first material 11 and the second material 13), and the modulator fabrication technique.
The interferometric modulator is driven by the difference between the row and column voltages. Only the column voltage needs to be adjusted to make the modulator work properly when the temperature changes. It will be appreciated that the terms “columns” and “rows” are geometrically arbitrary in the sense that either can be oriented in the vertical or horizontal direction. In this disclosure, the “columns” will be considered the set of display inputs receiving signals that are image data dependent. The “rows” will be considered to be the set of display inputs receiving signals that do not vary with the image data, such as the sequential row strobe input signals described above.
FIG. 3 illustrates hysteresis windows of the MEMS display elements at a particular temperature. These hysteresis windows shift when the temperature changes, as in shown in FIG. 10. The hysteresis windows at a reference temperature T0 are shown in solid line in FIG. 10. The locations of the hysteresis windows can be characterized by the respective middle points of the right and left windows, Vbias0 and −Vbias0. A typical reference temperature is a room temperature of about 25 degrees Celsius, but it can be any temperature. When the temperature lowers from T0 to T, the hysteresis windows moves away from each other as represented in dotted line. The new locations of the hysteresis windows can similarly be characterized by the respective middle points of the right and left windows, Vbias and −Vbias. Vbias and −Vbias can be calculated from Vbias0 and −Vbias0 as follows:

−Vbias(left hysteresis window)=−Vbias0−Kt*(T−T0)  Equation 9

Vbias(right hysteresis window)=Vbias0+Kt*(T−T0)  Equation 10

where Kt is a negative constant. As shown in FIG. 10, when the temperature decreases, the hysteresis windows move away from each other. If the temperature increases, the hysteresis windows move toward each other.
Once the locations of hysteresis windows are determined, the proper column voltages to drive the modulator can then be determined using the scheme shown in FIG. 11, which are repeated here as follows:

Vhigh=Vbias0+Kt*(T−T0)  Equation 11

Vlow=−Vbias0−Kt*(T−T0)  Equation 12

The control of the column voltages can be realized by various means, such as software control or an analog circuit using operational amplifiers.
The column voltages may be adjusted to compensate other factors such as electrical charge buildup in addition to temperature shift. As shown in FIG. 10, temperature change causes the hysteresis windows to move toward or away from each other, while these windows continue to be symmetrical around zero volts. Unlike the temperature shift, electrical charge buildup causes a unidirectional shift of both hysteresis windows, such that the two windows are no longer symmetrical around zero volts. In one embodiment, the column voltages may be adjusted as follows:

Vhigh=VrowMid+Voffset+Vbias0+Kt*(i−T0)  Equation 13

Vlow=VrowMid+Voffset−Vbias0−Kt*(T−T0)  Equation 14

VrowMid is the row voltage between strobe applications. As shown in FIG. 5B, the typical value for VrowMid is zero. Voffset represents the shift of the hysteresis windows caused by other factors such as electrical charge buildup.
FIG. 12 is a system block diagram similar to FIG. 2 that schematically illustrates one embodiment of an electronic device incorporating a 3×3 interferometric modulator display, and where the driving circuit is configured to provide actuation signals to drive the array 30 based on a temperature present. The block diagram of FIG. 12 illustrates a sensor 62 coupled to the circuit that drives the array 30. The sensor 62 senses a temperature condition and provides a signal to the array driver 22 that is based on the sensed temperature. The sensor 62 can include various embodiments of sensor circuitry, for example, circuitry that senses temperature and generates a corresponding signal, or circuitry that is influenced by temperature so that signals from the sensor correspond to the temperature. For example, in one embodiment the sensor 62 includes a thermistor whose resistance changes with temperature. Because of the known dependence of resistance on temperature, the resistor can be used as a temperature sensor. In some embodiments, the thermistor is fabricated on silicon in conjunction with fabricating the array of interferometric modulators. In some embodiments, the sensor 62 comprises a thermocouple.
In the embodiment illustrated in FIG. 12, the sensor 62 is located outside of the driving circuit and coupled to the array driver 22. The array driver 22 is configured to use the signal it receives from the sensor 62 to provide signals to drive the array 30 that correspond to the temperature. In one embodiment, the array driver 22 uses a pre-determined look-up table stored in memory to determine the appropriate signal to provide a signal to the array based on the received temperature-based signal. In other embodiments where the sensor 62 is disposed in the array driver 22 (e.g., FIG. 14) or in the processor (e.g., FIG. 13), a look-up table can also be used to determine the appropriate signal to provide a signal to the array based on the received temperature-based signal. In another embodiment, circuitry in the array driver 22 (or the processor 21) can approximate the curve illustrated in FIG. 9 (for example, approximate the relationship between temperature and operational voltage as linear) and then provide signals to the array 30 that are proportional to the received temperature-based signal using the defined relationship between temperature and operational voltage.
The temperature sensed by the sensor 62 can be a temperature at the array 30, at a location substantially proximal to the array 30, or at a location other than that of the array 30. For example, in various embodiments, the sensor 62 senses the temperature of the array driver 22, the processor 21, or at the sensor 62 itself. In some embodiments the sensor 62 is configured to sense the temperature at a predetermined location in a display which includes the array 30, or to sense the temperature at a predetermined location in an electronic device that includes the array 30.
In some embodiments, the sensor 62 also includes a sensing element 68 placed at a particular location to sense the temperature, where the location is preferably determined as it relates to the temperature under which the interferometric modulators of the array 30 operate. In this embodiment, the sensing element is located near the array 30. In other embodiments, the sensing element 68 can be placed, for example, in the driving circuit, anywhere in a display that includes the array 30, or anywhere in an electronic device that includes the array 30. Circuitry in the sensor 62 detects the influence of a temperature on the sensing element 68, and communicates a signal to the driving circuit (e.g., array driver 22) based on the temperature.
The sensor 62 can be located in various places, depending on the particular implementation desired. FIG. 13 is a system block diagram illustrating another embodiment of an electronic device incorporating a 3×3 interferometric modulator display and the sensor 62. In FIG. 13, the sensor is located in the processor 21. In one embodiment, the sensor 62 senses a temperature in the processor that is related to the temperature of the array 30, and a signal based on the sensed is used to drive the array 30. In some embodiments, the processor 21 can have a connection to connect a sensing element (not shown) so that the sensor 62 can sense the temperature at a location outside of the processor 21.
FIG. 14 is a system block diagram illustrating another embodiment of an electronic device incorporating a 3×3 interferometric modulator display and sensor 60. Here, the sensor 60 is located in the array driver 22. In one embodiment, the sensor 62 senses a temperature in the array driver 22 that is related to the temperature of the array 30, and a signal based on the sensed temperature is used to drive the array 30. In some embodiments, the array driver 22 can have a connection to a sensing element (not shown) so that the sensor 62 can sense the temperature at a location outside of the array driver 22.
FIG. 15 is a system block diagram schematically illustrating an embodiment of an electronic device incorporating a 3×3 interferometric modulator display and circuitry to control the amount of voltage applied by the array driving system to that which is necessary to place the interferometric modulators in an actuated or an released state. This embodiment includes test circuit 64 that is connected to the driving circuit, e.g., in particular array driver 22, and can comprise one or more test interferometric modulator(s) 66, (or test modulator). To account for the effect of temperature, the test circuit 64 determines the amount of voltage that is required to place the mirror of the test modulator 66 in an actuated and/or released state, and sends a signal corresponding to the determined voltage to the driving circuit, e.g., the array driver 22. The array driver 22 then adjusts the drive voltages to achieve an appropriate operational voltage based on the signal from the test circuit 64. As a result of monitoring the actuation and/or release of the one or more test modulators, the array driver 22 can provide drive signals to the array 30 based on the measured actuation/release voltage. In some embodiments, the drive signals provided to the array are proportional or substantially equal to the measured actuation voltage. In some embodiments, a second driving circuit is included in the display to drive one or more test modulator(s) 66.
In one embodiment, the test interferometric modulator 66 is an interferometric modulator having a similar structural configuration as an interferometric modulator found in array 30. The test modulator 66 serves as a platform upon which test drive signals can be applied and from which measurements are recorded. Usually, such test interferometric modulators are not used to output light for display purposes. The overall dimensional scale of the test interferometric modulator 66 can be similar to or different than that of interferometric modulators within array 30. The overall or specific dimensions of the test interferometric modulator 66 can be varied with respect to the interferometric modulators of array 30 depending upon the intended test measurement objectives. In alternative embodiments, the test interferometric modulators 66 have different structural configurations from those of array 30.
In some embodiments, two or more test modulators can be used (not shown). The test modulators can be located in various places in the display, including at the end of each row and/or column of the modulators in the array. Typically, the test modulators are positioned such that they are not visible to a viewer of the display, e.g., they do not receive or output any visible light to a viewer.
The test circuit 64 can determine the voltage that is required to actuate the test modulator 66 by applying a voltage to the test modulator 66 to “toggle” the modulator between an actuated state and a release state, while monitoring the test modulator 66 to determine at what voltage the modulator changes states. In some embodiments, the test circuit 64 is driven by a triangular-shaped voltage wave, as illustrated by signal 90 in FIG. 16. In another embodiment, a test modulator is driven with a signal having a triangle waveform and an amplitude that is proportional to a voltage level required to actuate the test modulator.
The test circuit 64 is preferably driven at the same frequency at which the display is being driven, but other frequencies can also be used to drive the test circuit 64. Also, the test circuit 64 is preferably driven at the same frame rate as the array 30, but other frame rates can be used too. For example, in some embodiments, the test modulator is driven with a signal having a frequency that is equal to or proportional to the display frame rate. In another embodiment, the frequency of the signal used to drive the test modulator is approximately one-half of the display frame rate.
In some embodiments, the voltage amplitude of the driving signal 90 is greater than the expected actuation voltage to ensure the actuation voltage is reached. In some embodiments, the voltage amplitude of the drive signal 90 may increase when the reference temperature decreases. In other embodiments, the signal used to drive the test modulator is periodic and the drive signals to drive the array are image content specific.
In one embodiment, the capacitance of the test modulator 66 is monitored to determine at what voltage the modulator changes states, and this information is used to change the drive voltage based on the applied voltage level at the time of the change in capacitance and the sign of the change in capacitance. FIG. 16 shows a graph of time (x-axis) verses capacitance and voltage (y-axis), and includes signal 90 and capacitance curve 95. The signal 90 represents the voltage applied across the test modulator 66 to actuate and release the movable reflective layer 14, according to one embodiment. The capacitance curve 95 represents the measured capacitance of the test modulator 66 that results from applying the voltage illustrated by the signal 90.
Here, the test modulator 66 begins in a released position. The voltage applied to the test modulator 66 begins at a negative value in state 70, increases to a positive peak value at state 74, decreases to a negative peak value at state 80, and increases again to a small negative value at state 82. The capacitance curve 95 reflects the measured capacitance of the test modulator 66 while the voltage changes in accordance with signal 90. The measured capacitance curve 95 starts at a low value at state 70 and then changes to a high value at state 72 as the voltage is increased, indicating the test modulator 66 actuated. At state 76, the capacitance curve 95 changes back to a low value indicating the test modulator 66 released. At state 78 the capacitance curve 95 changes to a high value, indicating the test modulator 66 again is actuated. Finally at state 82 the capacitance curve 95 changes back to a low value indicating the test modulator 66 is again released. In other embodiments, the current flow to the test modulator 66 is monitored to determine when it actuates or releases. When the test modulator 66 actuates or releases, the current will spike and the capacitance will increase or decrease.
In one embodiment, the test modulator 66 is “toggled,” e.g., a series of voltages are applied to the test modulator 66 so that the voltage switches from a positive to a negative voltage, or from a negative to a positive voltage. In this condition, the capacitance is monitored as the voltage is toggled to determine the level of voltage which causes the test modulator 66 to actuate and release, and the actuation signal provided to drive the array 30 is adjusted accordingly. Such toggling can be performed upon startup of the modulator, and then periodically to account for changes that occur during its use. In some embodiments, the process is performed as a result of an input received by a user or by an automatic process, e.g., diagnostics.
The electromechanical response of a test modulator can be configured to have a predetermined relationship with the electromechanical response of the interferometric modulators of the array 30. For example, the predetermined relationship can be such that the electromechanical responses are substantially proportional, substantially equal, or that they have substantially the same electromechanical behavior. By knowing the relationship between the electromechanical response of the test interferometric modulators 66 with respect to the interferometric modulators of array 30, measurement of the voltage levels required to actuate and release the test interferometric modulators 66 allows for adjustment of the drive signals sent to array 30 to compensate for various factors that affect performance. As previously mentioned, one factor is that of temperature. Note that the use of the test interferometric modulators 66 allows for compensation of the drive signals to array 30 without requiring the measurement of temperature.
Test interferometric modulators 66 can also be used to measure long term drift of offset voltage in the electrical and mechanical performance of the interferometric modulators of array 30. Drift of offset voltage can be a result of, for example, long term exposure to adverse temperature, mechanical or structural changes in the device, or electrical charge buildup in the optical stack and/or the moving mirror layer 14.
The test modulators can have different electromechanical behavior to, for example, measure severe temperature changes, voltage spikes, or other conditions that would warrant the display to initiate a diagnostic procedure, such as shutting down and restarting.
Test interferometric modulators 66 can also be used to measure the offset voltage, which is the voltage level at approximately the midpoint between a positive and negative hysteresis window of an interferometric modulator system. A recursive algorithm of applying a corrective voltage pulse to the test and array interferometric modulators and measuring the offset voltage of the test interferometric modulator can be used to adjust or reset an offset voltage.
In some embodiments, the interferometric modulators in the array and the test modulators each have an electrical response function that is related in a predetermined relationship. For example, a test modulator can have a different resistance in its column electrode than the interferometric modulators in the array so that the electrical behavior of the test modulator is different from the interferometric modulators in the array but its mechanical behavior is the same. In other embodiments, the interferometric modulators in the array and the test modulators each have a mechanical response function that is related in a predetermined relationship. For example, the test modulator can have different physical or mechanical properties, e.g., higher post density, than the interferometric modulators in the array so that the mechanical behavior of the test modulator is different but its electrical behavior is the same.
In another embodiment, circuitry coupled to the driving circuit (e.g., the array driver 22, the processor 21, or the driver circuits 24, 26) has temperature influenced circuitry, e.g., it has one or more electronic characteristics that change in a predetermined manner corresponding to a change in the temperature of the display. Based on the changed electronic characteristic, the driving circuit generates actuation signals for the array 30 that correspond to the change in temperature so that the interferometric modulators of the array 30 operate with an appropriate operational voltage. This temperature influenced circuitry can be coupled to the driving circuit, embodied in the driving circuit, or embodied in the sensor 62.
FIG. 17 illustrates a process 100 of driving an array 30 (e.g., FIG. 2) having a plurality of interferometric modulators that can be used with the previously described embodiments. In state 102, a temperature is sensed at a predetermined location in the display. The temperature can be sensed using a sensor, test circuitry, a test interferometric modulator, or temperature influenced circuitry.
In state 104, a sensor signal, which is based on the sensed temperature, is communicated to a display driver. Subsequently, in state 106, the process 100 generates an actuation signal based on the sensor signal that was communicated to the display driver. The level of the generated actuation signal is adjusted according to the temperature sensed such that as the temperature of the interferometric modulators in the array 30 increases, the voltage applied to the interferometric modulators as dictated by the actuation signal decreases so that the driving circuit provides the proper operational voltage to the array 30. Conversely, as the temperature of the interferometric modulators in the display decreases, the voltage applied to the interferometric modulators as dictated by the actuation signal increases. Finally, in state 108, the process 100 provides the actuation signal to the array 30.
In some embodiments, the measurements made upon test interferometric modulators 66 can also be performed upon the interferometric modulators of array 30 such that a dedicated test interferometric modulator 66 would be optional. For example, a small number of interferometric modulators of array 30 (for example, one or more) can be used as both test and display interferometric modulators. In many cases, it would be desirable to perform the tests upon interferometric modulators located to one side or within one corner of a display screen so as to minimize any adverse optical effects of the test procedures. Also, in many cases, the dimensional and structural configuration of the interferometric modulators within an array 30 used for testing would be substantially the same as the interferometric modulators in the remaining portion of the array 30.
The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention can be practiced in many ways. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the invention with which that terminology is associated.

## Claims

1. A display, comprising:
an array of microelectromechanical system (MEMS) display elements; and
a driving circuit coupled to said array and configured to provide actuation signals to drive said array, wherein said actuation signals comprise at lease a row signal and a column signal, and wherein only one of said row and column signals is adjusted for temperature change.

2. The display of claim 1, wherein only one of said row and column signals comprises a data signal representing images to be displayed in said array, and wherein only said signal comprising a data signal is adjusted for temperature change.

3. The display of claim 2, wherein at least one of the MEMS display elements comprises an interferometric modulator.

4. The display of claim 3, wherein said row and column signals comprise voltage signals.

5. The display of claim 4, wherein said row or column voltage signal to be adjusted for temperature change is designed to take one of two values, Vhigh and Vlow, at a temperature T, which can be calculated by the following equations:


Vhigh=Vbias0+Kt*(T −T0)



Vlow=−Vbias0−Kt*(T−T0)

where Vbias0 and −Vbias0 are the voltage values chosen to drive the array at a reference temperature T0, and Kt is a negative constant.

6. The display of claim 1, further comprising:
a processor that is configured to communicate with said display, said processor being configured to process image data; and
a memory device that is configured to communicate with said processor.

7. The display of claim 6, further comprising a controller configured to send at least a portion of said image data to said driver circuit.

8. The display of claim 6, further comprising an image source module configured to send said image data to said processor.

9. The display of claim 8, wherein said image source module comprises at least one of a receiver, transceiver, and transmitter.

10. The display of claim 6, further comprising an input device configured to receive input data and to communicate said input data to said processor.

11. A method of driving an array of microelectromechanical system (MEMS) display elements, the method comprising:
sensing a temperature at a predetermined location;
generating one of a row signal and a column signal having a level based at least in part on the sensed temperature and the other of the row and column signals not based on the sensed temperature; and
providing said row and column signals to said array.

12. The method of claim 11, wherein only one of said row and column signals comprises a data signal representing images to be displayed in said array, and wherein only said signal comprising a data signal is generated based on the sensed temperature.

13. The method of claim 12, wherein at least one of the MEMS display elements comprises an interferometric modulator.

14. The method of claim 13, wherein said row and column signals comprise voltage signals.

15. The method of claim 14, wherein generating one of said row or column voltage signals based on temperature further comprises calculating at least one of two values, Vhigh and Vlow, at a sensed temperature T, by the following equations:


Vhigh=Vbias0+Kt*(T−T0)



Vlow=−Vbias0−Kt*(T−T0)

where Vbias0 and −Vbias0 are the voltage values chosen to drive the array at a reference temperature T0, and Kt is a negative constant.

16. A display, comprising:
means for sensing a temperature at a predetermined location;
means for generating one of a row signal and a column signal having a level based at least in part on the sensed temperature and the other of the row and column signals not based on the sensed temperature;
means for displaying image data; and
means for providing said row and column signals to said displaying means.

17. The display of claim 16, wherein only one of said row and column signals comprises a data signal representing images to be displayed in said displaying means, and wherein only said signal comprising a data signal is generated based on the sensed temperature.

18. The display of claim 17, wherein the displaying means comprises an interferometric modulator.

19. The display of claim 18, wherein said row and column signals comprise voltage signals.

20. The display of claim 19, wherein means for generating one of said row or column voltage signals based on temperature further comprises means for calculating at least one of two values, Vhigh and Vlow, at a sensed temperature T, by the following equations:


Vhigh=Vbias0+Kt*(T−T0)



Vlow=−Vbias0−Kt*(T−T0)

where Vbias0 and −Vbias0 are the voltage values chosen to drive the array at a reference temperature T0, and Kt is a negative constant.