WO2009041950A1

WO2009041950A1 - Method of manufacturing a mems package using partially reactivated desiccant

Info

Publication number: WO2009041950A1
Application number: PCT/US2007/020967
Authority: WO
Inventors: Lauren Palmateer; Khurshid Alam
Original assignee: Qualcomm Mems Technologies, Inc.
Priority date: 2007-09-28
Filing date: 2007-09-28
Publication date: 2009-04-02

Abstract

A method of packaging a MEMS device in an ambient environment may include partial regeneration of a desiccant within the MEMS package during the assembly process. The desiccant may be a multilayer desiccant configured to retain moisture primarily in the exposed outer layers during the assembly process. A method of packaging a MEMS device may also include heating of a desiccant during the assembly process to remove water vapor retained within a polymer binding in the desiccant.

Description

METHOD OF MANUFACTURING A MEMS PACKAGE USING PARTIALLY

REACTIVATED DESICCANT

BACKGROUND OF THE INVENTION Field of the Invention

|0001] This invention relates to small scale electromechanical devices, such as microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS) device. More specifically, the invention relates to the use of reactivated desiccartts in such devices. Description of the Related Art

[0002] MEMS include micro mechanical elements, actuators, and electronics. Although the term MEMS is used through the specification for convenience, it will be understood that the term is intended to encompass smaller-scale devices, such as NEMS. Micromechanical elements may be created using deposition, etching, and/or other micromachining processes that etch away parts of substrates and/or deposited materia! 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 THE INVENTION

[0003] In one aspect, a method of packaging an electromechanical device is provided, the method including providing a backplate, the backplate supporting a layer of desiccant, performing an additional manufacturing step, reactivating the desiccant, where reactivating the desiccant is done after performing an additional manufacturing step, and sealing the backplate to a substrate to form a package, where the substrate supports an electromechanical structure, and where the electromechanical structure and the desiccant are located within the package.

[0004] In another aspect, an electromechanical device is provided, including a substrate, where the substrate includes an electromechanical structure, a backplate, a partially reactivated desiccant supported by the substrate, and a seal adhering the substrate to the backplate.

[0005] In another aspect, a method of packaging an electromechanical device is provided, the method including providing a backplate, the backplate supporting a multilayer desiccant, the multilayer desiccant including: an upper desiccant sublayer, an additional layer, where the additional layer is located between the upper desiccant sublayer and the backplate, and a lower desiccant sublayer, where the lower desiccant sublayer is located between the additional layer and the backplate, performing an additional manufacturing step, and sealing the backplate to a substrate to form a package, where the substrate supports an electromechanical structure, and where the electromechanical structure and the desiccant are located within the package.

[0006] In another aspect, an electromechanical device is provided, including a substrate, where the substrate includes an electromechanical structure, a backplate, a multilayer desiccant supported by the substrate, the multilayer desiccant including: an upper desiccant sublayer, an additional layer, where the additional layer is located between the upper desiccant sublayer and the backplate, and a lower desiccant sublayer, where the lower desiccant sublayer is located between the additional layer and the backplate, and a seal adhering the substrate to the backplate.

[0007] In another aspect, an electromechanical device is provided, including means for absorbing moisture, first means for supporting the absorbing means, means for inhibiting moisture flux, the inhibiting means being disposed between the absorbing means and the first supporting means, second means for supporting an electromechanical structure, and means for sealing the second supporting means to the first supporting means.

BRIEF DESCRIPTION OF THE DRAWIlMGS

10008] Figure 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.

[0009J Figure 2 is a system block diagram illustrating one embodiment of an electronic device incorporating a 3x3 interferometric modulator display.

[0010] Figure 3 is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of Figure 1.

[0011] Figure 4 is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display.

[0012] Figure 5A illustrates one exemplary frame of display data in the 3x3 interferometric modulator display of Figure 2.

[0013] Figure 5B illustrates one exemplary timing diagram for row and column signals that may be used to write the frame of Figure 5A.

[0014] Figures 6A and 6B are system block diagrams illustrating an embodiment of a visual display device comprising a plurality of interferometric modulators.

[0015] Figure 7 A is a cross section of the device of Figure 1 .

[0016] Figure 7B is a cross section of an alternative embodiment of an interferometric modulator.

[0017] Figure 7C is a cross section of another alternative embodiment of an interferometric modulator.

[0018] Figure 7D is a cross section of yet another alternative embodiment of an interferometric modulator.

[0019] Figure 7E is a cross section of an additional alternative embodiment of an interferometric modulator. [0020] Figure 8 is a cross-section of an embodiment of a MEMS device package comprising a desiccant supported by a shaped backplate.

[0021] Figure 9 is a flowchart illustrating an exemplary process for assembling a MEMS device package.

[0022] Figure 1OA is one embodiment of a backplate supporting a multilayer desiccant.

[0023] Figure 1 OB is another embodiment of a backplate supporting a multilayer desiccant.

DETAILED DESCRIPTION

[0024] 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.

[0025] In certain embodiments, it is desirable to assemble a MEMS package in an ambient environment, due to significant possible reductions in cost and complexity. However, for MEMS packages comprising a desiccant, such assembly processes may significantly constrain the types of usable desiccants, due to rapid absorption of moisture from the ambient environment. Partial regeneration of fast-acting desiccants during the assembly process can serve to limit the amount of moisture absorbed, as well as to limit moisture absorption to those portions of the desiccant which can be easily regenerated. In some embodiments, this partial regeneration may be in the form of in-line heating of the desiccant during the assembly process. In some embodiments, a multilayer desiccant may be provided to similarly limit moisture absorption to easily regenerated portions. Such a desiccant may include an exposed desiccant over a slow permeation layer, and may also include a protected layer of desiccant beneath the slow permeation layer. Heating of a desiccanl during the assembly process can also be used to dry out polymer binding in a slow- acting desiccant to increase the lifetime of the desiccant. In some embodiments, the desiccant may be exposed to vacuum or a moisture free environment such as a Nitrogen environment. This exposure to vacuum or a moisture-free environment may be sufficient to drive off moisture without heating, but in other embodiments, this exposure may be combined with a heating process.

[0026] One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated in Figure 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.

[0027] Figure 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 inlerferometric 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.

[0028] The depicted portion of the pixel array in Figure 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.

[0029] The optical stacks 16a and 16b (collectively referred to as optical stack 16), as referenced herein, typically comprise 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.

[0030] 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 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.

[00311 With no applied voltage, the gap 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 Figure 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 Figure 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.

|0032] Figures 2 through 5B illustrate one exemplary process and system for using an array of interferometric modulators in a display application.

[0033] Figure 2 is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor 21 which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium^®, Pentium II^®, Pentium III^®, Pentium IV^®, Pentium^® Pro, an 8051 , a MIPS^®, a Power PC^®, an ALPHA^®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor 21 may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application.

[0034] 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 Figure 1 is shown by the lines 1-1 in Figure 2. For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices illustrated in Figure 3. H 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 Figure 3, the movable layer does not relax completely until the voltage drops below 2 volts. Thus, there exists a window of applied voltage, about 3 to 7 V in the example illustrated in Figure 3, 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 Figure 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 Figure 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.

[0035] 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.

[0036] Figures 4, 5A, and 5B illustrate one possible actuation protocol for creating a display frame on the 3x3 array of Figure 2. Figure 4 illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves of Figure 3. !n the Figure 4 embodiment, actuating a pixel involves setting the appropriate column to -V_bi_as, 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 ⁺Vbi_as, 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 -V_bi_as- As is also illustrated in Figure 4, it will be appreciated that voltages of opposite polarity than those described above can be used, e.g., actuating a pixel can involve setting the appropriate column to +V^" _bi_as, and the appropriate row to — ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to -Vbi_as, and the appropriate row to the same -ΔV, producing a zero volt potential difference across the pixel.

[0037] Figure 5B is a timing diagram showing a series of row and column signals applied to the 3x3 array of Figure 2 which will result in the display arrangement illustrated in Figure 5A, where actuated pixels are non-reflective. Prior to writing the frame illustrated in Figure 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.

|0038J In the Figure 5 A 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 Figure 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 Figure 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.

J0039] Figures 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.

|0040] 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 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.

|0041] 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.

[0042] The components of one embodiment of exemplary display device 40 are schematically illustrated in Figure 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.

[0043] The network interface 27 includes the antenna 43 and the transceiver 47 so that the exemplary display device 40 can communicate with one or more devices over a network. In one embodiment, the network interface 27 may also have some processing capabilities to relieve requirements of the processor 21. The antenna 43 is any antenna known to those of skill in the art for transmitting and receiving signals. In one embodiment, the antenna transmits and receives R.F signals according to the IEEE 802.1 1 standard, including IEEE 802.1 1 (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. [0044] 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.

[0045] 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.

[0046] 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.

[0047] 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 (1C), 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. |0048] 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.

10049] 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).

[0050] 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, 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 may be provided by a user for controlling operations of the exemplary display device 40.

[0051] 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 reneλvable 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.

[0052] In some embodiments, control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some embodiments, control programmability resides in the array driver 22. Those of skill in the art will recognize that the above-described optimizations may be implemented in any number of hardware and/or software components and in various configurations.

[0053] The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example, Figures 7A-7E illustrate five different embodiments of the movable reflective layer 14 and its supporting structures. Figure 7A is a cross section of the embodiment of Figure 1, where a strip of metal material 34 is deposited on orthogonally extending supports 18. In Figure 7B, the moveable reflective layer 14 is attached to supports at the corners only, on tethers 32. In Figure 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 Figure 7D has support post plugs 42 upon which the deformable layer 34 rests. The movable reflective layer 14 remains suspended over the gap, as in Figures 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 arc formed of a planarization material, which is used to form support post plugs 42. The embodiment illustrated in Figure 7E is based on the embodiment shown in Figure 7D, but may also be adapted to work with any of the embodiments illustrated in Figures 7A- 7C, as well as additional embodiments not shown. In the embodiment shown in Figure 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.

|0054] In embodiments such as those shown in Figure 1, 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 Figure 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 Figures 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.

[0055] Interferometric modulators, and other MEMS devices — particularly those having large surfaces which come into contact with one another — are particularly sensitive to failure due to humidity and moisture buildup. In order to protect such MEMS devices from humidity, and other forms of environmental or mechanical interference, such MEMS devices are often sealed within a protective package. In many embodiments, a substrate comprising an array of MEMS devices is sealed to a back plate in order to provide such a package. Although the below embodiments are primarily discussed with respect to interferometric modulators, it will be understood that a wide variety of MEMS devices may benefit from the protection afforded by such a package, and can be used in conjunction with the structures and processes described herein.

{0056] Figure 8 depicts an embodiment of a package 100, which may, for example, form a part of a display device.

[0057] Figure 8 depicts an embodiment of a package 100 which may form a part of a display device. The package 100 comprises a light-transmissive substrate 110, which may preferably be a substantially transparent substrate, through which a viewer may view an array 120 of interferometric modulators. The substrate thus provides one means for supporting an electromechanical structure, such as the array of interferometric modulators. The light-transmissive substrate 110 is sealed to a backplate 130 via seal 140, providing a cavity 150 in which the interferometric modulator array 120 resides. Also within the cavity 150 is a layer of desiccant 160, which in the illustrated embodiment is positioned within a US2007/020967

recess 170 in the backplate 130. The desiccant 160 thus provides one means for absorbing moisture, the backplate 130 provides one means for supporting the desiccant, and the seal 140 thus provides one means for sealing the backplate to the substrate. Because the backplate 130 is a shaped backplate comprising recess 170, the height (a) of the seal 140 can be advantageously minimized while still providing sufficient clearance for the desiccant 160 to be positioned without substantial risk of mechanical interference with the interferometric modulator array 120. It will be understood, however, that the desiccant 160 may be placed at any of a variety of locations in the package 100, and may be placed at multiple locations throughout the package 100.

[0058] A wide variety of materials can be used to form the backplate 130, including glass, metal, foil, polymer, plastic, ceramic, or semiconductor materials (e.g. silicon). The substrate 110 may comprise, for example, glass, plastic, or transparent polymer. In other embodiments, the substrate may comprise a plastic or polymer along with an embedded adhesive, so as to provide an additional measure of protection. The seal 140 may be a non-hermetic seal, and comprise a material such as a conventional epoxy-based adhesive. In other embodiments, the seal 140 may be a polyisobutylene (sometimes called butyl rubber and other times PEB), o-rings, polyurethane, thin film metal weld, liquid spin-on glass, glass frit, solder, polymers, or plastics, among other types of seals that may have a range of permeability of water vapor of about 0.2-4.7 g mm/m²kPa day. In other embodiments, the seal 140 may be a hermetic seal.

[0059] MEMS devices, and in particular MEMS devices such as interferometric modulators, are sensitive to environmental conditions such as humidity. Generally, it is desirable to minimize the permeation of water vapor into the package structure and thus control the environment inside the package 100 and hermetically seal it to ensure that the environment remains constant. When the humidity within the package 100 exceeds a level beyond which surface tension from the moisture becomes higher than the restoration force of a movable element (e.g., the movable mirrors 14a, 14b described above with respect to Figure 1 ) in a MEMS device such as the interferometric modulator array 120, the movable element may become stuck to an adjacent surface for a prolonged period of time, and may become permanently stuck. Because of the large contact areas and comparatively low restoration forces of the movable mirrors in an interferometric modulator, interferometric modulators are particularly susceptible to failure due to permanent adhesion brought about by high humidity levels. Humidity within the package 100 can contribute to other undesirable effects, such as the development of discoloration, which is particularly undesirable in an optical device such as an interferometric modulator display.

[0060] A desiccant such as desiccant 160 may be used to control moisture resident within the package 100. Desiccants may be used for packages that have either hermetic or non-hermetic seals. In packages having a hermetic seal, desiccants are typically used to control moisture resident within the interior of the package. In packages having a non-hermetic seal, a desiccant may be used to control moisture moving into the package from the environment. The skilled artisan will appreciate that a desiccant may not be necessary for a hermetically scaled package, but may be desirable to control moisture resident within the package or to capture outgases materials or materials from surfaces inside the package.

[0061 ] According to the embodiments described herein, the desiccant preferably is configured to absorb water molecules that permeate the display package structure once it has been manufactured as well as after sealing. As can be appreciated, the desiccant maintains a low humidity environment within the package and prevents water vapor from adversely affecting the operation of the MEMS devices and any associated display electronics.

[0062] Generally, any substance that can trap moisture while not interfering with the optical properties of the interferometric modulator array may be used as the desiccant material 160. Suitable desiccant materials include, but are not limited to, zeolites, calcium sulfate, calcium oxide, silica gel, molecular sieves, surface adsorbents, bulk adsorbents, and chemical reactants. Other desiccant materials inclue indicating silica gel, which is silica gel with some of its granules coated with cobalt chloride. The silica changes color as it becomes saturated with water. Calcium oxide is a material that relatively slowly absorbs water.

[0063] The desiccant may be in different forms, shapes, and sizes. In addition to being in solid or gel form, the desiccant material may alternately be in powder form. These powders may be inserted into a water vapor permeable pouch, or directly into the package without a pouch, or may be mixed with an adhesive for application. In an alternative embodiment, the desiccant may be formed into different shapes, such as cylinders or sheets, before being applied inside the package. It should be realized that the desiccant 160 may take any form, and can be of any thickness that provides the proper desiccating function for the package 100.

|0064) The desiccant 160 may be applied within the package in a variety of other ways, as well. In one embodiment, the desiccant 160 may be deposited as part of the interferometric modulator array 120. In another embodiment, the desiccant material is applied inside the package as a spray or a drip coat. The desiccant may also be printed or sprayed onto a surface of the interior of the package, or may be brushed on. The desiccant may be provided in a patch form, and may be adhered to the interior of the package via an adhesive, such as a pressure sensitive adhesive. The portions of the backplate which are not intended to be covered by desiccant may be protected by a mask layer.

[0065] Typically, in packages containing desiccants, the lifetime expectation of the device may depend on the lifetime of the desiccant. When the desiccant is fully consumed, the interferometric modulator array 120 may fail to operate as sufficient moisture enters the cavity 150 and causes damage to the array 120. The theoretical maximum lifetime of the display device is determined by the water vapor flux into the cavity 150 as well as the amount and type of desiccant material.

[0066] The theoretical lifetime of the device may be calculated with the following equations:

. li.f„eti .me = desiccant = — capacit ±y±(g°s) water _ vapor _ flυx{g I area I day) * perimeter _ seal _ area

water vapor flux = -P — dt where P is the water vapor permeation coefficient for the perimeter seal 280 and — di is the water vapor pressure gradient across the width of the seal 280. [0067] In the embodiment of a display having a hermetic seal, the lifetime of the device is not as dependent on the desiccant capacity, or the geometry of the seal. In display devices wherein the seal 140 is not hermetic, the lifetime of the device is more dependent on the capacity of the desiccant to absorb and retain moisture.

[0068] In one embodiment, a method for assembling the package 100 includes fabricating the interferometric modulator array 120 on the light-transmissive substrate 110. In certain embodiments, the backplate 130 may be shaped via a sandblasting or etching process in order to form recess 170. In other embodiments, the backplate 130 may be deformed to form a recess 170, or a pre-shaped backpate provided. The desiccant 160 may then be applied in the recess 170, or elsewhere in the package. The seal 140 is then put into place, and the backplate 130 and the light transmissive substrate 110 may be brought together to form the cavity 150 which encapsulates both the desiccant 160 and the interferometric modulator array 120. In other embodiments where the desiccant is sufficiently thin, the backplate may not combine a recess, as a sufficiently thin desiccant in conjunction with a sufficiently thick seal will reduce the possibility of mechanical interference with the MEMS device.

[0069] It will be understood that, while the desiccant 160 may be protected from absorption of water and other materials after production of the desiccant by maintaining the desiccant in hermetically sealed packaging, the desiccant 160 may be exposed to the surrounding environment during the packaging process. If the desiccant 160 is exposed to an ambient environment prior to or during the manufacture or assembly of the device, the desiccant will absorb humidity from the ambient environment, shortening the lifetime of the desiccant, as a finite amount of moisture can be absorbed by a given amount of desiccant. In certain embodiments, this problem may be avoided by using a controlled environment (e.g., a glove box) to apply the desiccant 160 to the backplate 130 and seal the backplate 130 to the light transmissive substrate 110. However, the use of a controlled environment may add additional cost and time to the packaging process.

10070] In a process in which the packaging process is done in an ambient environment, the choice of desiccant may be limited by exposure time and the humidity of the ambient environment. Desiccants which absorb moisture too quickly may become saturated in a matter of minutes. Moreover, the exposure time of the desiccant to the environment can shorten the lifetime of the operation of the MEMS devices within that package by many times that exposure time. In certain embodiments, slow-acting getters have been used as the desiccant, in order to minimize the effect of exposure to the ambient environment. An example of a slow acting getter is produced by Cookson Electronics, which comprises CaO in a polymer binding. It will be understood that the moisture absorption rate of a desiccant such as a getter will vary depending on the temperature, humidity, and the amount of moisture already absorbed by the desiccant. An exemplary slow acting getter, the Cookson product, loses about 2% of its capacity in 24 hours, at 30° C and 60% humidity.

[0071] While slow acting getters enable packaging in an ambient environment while still providing effective moisture absorption after packaging, fast-acting getters may provide better protection against moisture accumulation once the package is sealed. This is due to the fact that a fast acting getter may absorb water vapor quickly, before it can affect the operation of the device. An example of a fast-acting getter is produced by Dynic, which comprises CaO as the active ingredient in a polytetrafluorocthelyne (PTFE) binding. Both the exemplary fast-acting getter and slow-acting getter utilize calcium oxide as the active ingredient. However, the amount and size of the calcium oxide particles, as well as the binder material used, may affect the moisture absorption rate, and whether the getter is considered fast or slow-acting. Although the absorbing materials described in various portions of the specifications are often referred to as getters, it will be understood that the embodiments discussed herein may be used in conjunction with any of the desiccant materials discussed herein, and may also be applicable to other materials which would otherwise absorb materials from an ambient environment at a higher rate than is desired.

[0072] In certain embodiments, it is desirable to package a MEMS device in an ambient environment, as doing so can simplify and reduce the cost of the manufacturing process. Many MEMS devices, such as interferometric modulators, can be packaged in an ambient environment, in contrast to display devices such as OLED displays, which may require packaging in a controlled environment, such as Nitrogen. This packaging process may expose the components to the ambient environment for an extended period of time, as long as several hours in certain embodiments. Any desiccant exposed to the ambient environment may absorb moisture from the ambient, reducing the useful lifetime of the desiccant. In particular, if a fast acting desiccant is used in the packaging of a MEMS device in an ambient environment, the fast acting desiccant may absorb, within a matter of minutes, too much moisture to function as an effective desiccant in the sealed package. Nevertheless, when packaging in an ambient environment, it may be desirable to utilize a fast acting desiccant, as such a desiccant, as the ambient air sealed within the package will comprise moisture, and the fast-acting desiccanl can quickly absorb any such moisture.

[0073] In certain embodiments discussed in greater detail below, a layer of fast- acting desiccant may be provided during an assembly process under ambient conditions, and the packaging process may be controlled so as to partially regenerate this desiccant at one or more points throughout the packaging process. As used throughout this application, the removal of retained moisture from a desiccant may be referred to herein as activation, reactivation, or regeneration. In other embodiments, a slow acting desiccant comprising a polymer or binder matrix may be treated to remove the absorbed water from the polymer or binder matrix prior to the end of the packaging process, so as to restore the desired lifetime of the desiccant.

10074] Figure 9 illustrates an exemplary process flow 900 for one embodiment of a manufacturing process. The process begins at a step 902 wherein a desiccant is applied to a backplatc. The desiccant applied is one which is capable of being regenerated, such as, for example, a molecular sieve. In certain embodiments, the desiccant may be applied via a printing process. In other embodiments, this step 902 may be performed at a location distinct from the eventual assembly site wherein the package is assembled, such as when the desiccant comes from a vendor pre-applied to a backplate, such as a solid patch desiccant. In other embodiments, the desiccant may be provided in a form or package which permits the desiccant to be extruded or dispensed. As will be discussed in greater detail below, in certain embodiment, the desiccant may advantageously be provided in a recessed portion of a backplate.

[0075] The process then moves to a step 904 wherein the printed desiccant is cured and reactivated. The curing and reactivation process may be done via any suitable high-throughput equipment, such as, for example, via heating in ovens, via infrared heating, or via microwave heating. In other embodiments, a heat stage or hot plate may be used. Advantageously, the prepared backplates may be cured and activated in batches so as to maximize throughput and minimize the amount of time for which the high-throughput heating equipment is required to be active.

[0076] In certain embodiments, the prepared backplates containing the cured desiccant may move to a step 906 where the prepared backplates can be stored for later use. In certain embodiments, the prepared backplate may be moved to a different facility at this point, such as being delivered to the end user by a desiccant vendor. In certain embodiments, this storage may be done in ambient conditions. In such embodiments, the desiccant moves to a step 908 in which it undergoes a full reactivation process prior to subsequent loading onto an assembly line. As the backplate may comprise a durable material, the full reactivation process of step 908 may advantageously be done under conditions particularly suited for activation of desiccant, and in particular may be done at high temperatures which could damage other components of the eventual MEMS package. In certain embodiments, the full reactivation process may comprise exposing the desiccant to vacuum and temperatures of about 35O°C for a duration of about 30 minutes, although the particular parameters may vary depending upon a variety of factors.

[0077] The conditions required to fully activate or reactivate the desiccant depend on the amount and type of desiccant being reactivated as well as three significant process conditions: temperature, pressure and duration. The process conditions are interdependent, such that increased temperatures will shorten the necessary duration, and decreasing the pressure will lower the necessary temperatures and durations. In certain embodiments, the pressure may be varied through the use of a vacuum chamber. In other embodiments, exposing the desiccant to a flow of moisture-free gas, such as nitrogen, may serve to decrease the necessary temperature and duration during an ambient manufacturing process without the use of a vacuum chamber.

[0078] For Zeolite-based systems, high temperatures may be required to remove the water from the molecular sieves so as to reactivate the desiccant. At standard pressures, temperatures as high as about 340⁰C to about 450°C may be used to reactivate the desiccant. If the reactivation occurs in a vacuum chamber with pressures as low as -1OkPa, -5OkPa, or even -10OkPa₇ the temperature may be reduced to about 200⁰C, and the desiccant may be fully reactivated in about 30 minutes. In other embodiments, a nitrogen gas flow over the desiccant during the heating process may similarly reduce the necessary temperature and time.

[0079] For this and other embodiments, as will be discussed in greater detail below, this process may be used to drive water from a polymer matrix within the desiccant. The necessary conditions for removal of water may be significantly less stringent than those needed to remove water from molecular sieves.

|0080] Thus, the process conditions during the reactivation process may be tailored to fit particular situations. A desiccant which would be reactivated after exposure to temperatures of 350⁰C for 30 minutes may also be reactivated after exposure to a lower temperature for an increased duration, such as through exposure to 200⁰C temperatures for roughly 2 hours. These times may be further reduced if vacuum pressures are used, such as pressures ranging from -1OkPa to -10OkPa or greater.

[0081] After the full activation process, the system then moves to a step 910 where the backplate containing the fully activated desiccant is loaded onto a conveyor belt for use in an LCD-like assembly process. In embodiments in which the prepared backplates are not stored in ambient temperature as discussed above, the prepared backplates may be transferred directly from the curing and activation machinery used in step 904 to loading step 910. In such embodiments, the curing and activation process 904 may be done under process conditions sufficient to ensure full activation of the desiccant, such as those discussed with respect to step 908.

(0082] In order to ensure that the exposure time to the ambient air during the assembly process is substantially uniform across a batch of prepared backplates loaded onto the assembly line, the process may include a step 912 during the loading process in which the prepared backplates arc purged with Clean Dry House Air at high pressure, such as through purging a load cassette or other transport container holding a batch of prepared backplates. Such purging ensures that the first and last plates to be loaded on the assembly line see similar exposure times to the ambient air, which may be at 25 C and at 65% relative humidity. The uniformity in exposure time and conditions allows optimization of the reactivation processes discussed below.

|0083) The process may then move to a transport step 914 wherein the prepared backplate is moved along the assembly line. At any point during the transport step or other transport steps, as well as during suitable process steps, inline equipment such as IR heaters or microwave heaters may be provided to expose the prepared backplate to radiation and partially reactivate the desiccant during the transport step. In some embodiments, the desiccanl may be partially reactivated, while in others, the rate of absorption of liquid may merely be slowed.

[0084J In certain embodiments, the reactivation of the desiccant at this stage may take the place of the full activation of the desiccant discussed with respect to step 908. In a particular embodiment, the reactivation fully reactivates the desiccant just prior to the mating process of the backplate to the MEMS substrate. Because the full reactivation process may require high temperatures depending on other process conditions, as noted above, in some embodiments, full reactivation is done prior to application of the mating seal or the inclusion of other components of the package which may be more temperature-sensitive than the backplate and desiccant.

(0085] In certain embodiments, the process may move to an optional step 916 in which the prepared backplate is cleaned via exposure to UY radiation and ozone prior to application of the sealant which will seal the backplate to the array substrate, in order to ensure adhesion between the backplate and the sealant. This cleaning process may be done in a controlled environment so as to avoid additional saturation of the desiccant during this process.

[0086] The process then moves to a step 920 in which the thermal glue which will be used to seal the backplate to the array substrate is printed onto the backplate around the periphery of the desiccant. Although this process flow is described with respect to thermal glue, it will be understood that a wide variety of other sealants may be used to seal the prepared backplate to the array substrate, such as, for example, UV glue. The time between loading onto the assembly conveyor until the glue is printed may in this embodiment be roughly 2-3 minutes, although it will be understood that the time may vary widely depending on the specifics of a given assembly process, and in particular the inclusion or exclusion of particular process steps, such as the UV cleaning step 914 discussed above. As will be discussed in greater detail below, the thermal glue may be printed or otherwise deposited to form a perimeter sea! which comprises one or more small endseal openings, which will be sealed in a subsequent step to complete the encapsulation process.

[0087] The process may then move to a step 922 in which the thermal glue printed onto the desiccant-bearing backplate is pre-cured. This pre-curing step may comprise exposing the backplate and the applied thermal glue to a temperature of about 170⁰C for about 3 to 5 minutes. It will be understood that slight reactivation of the desiccant may occur during this pre-curing stage. A glue having desirable pre-bake requirements may be used to optimize the reactivation of the desiccant. If a significant amount of reactivation is desired at this stage, a glue with a higher pre-bake temperature, such as 200°C may be used. If less reactivation is necessary, such as because the desiccant was fully reactivated very recently, a lower pre-bake temperature may be used to minimize process cost and heating of components, such as 100^DC.

[0088J The process moves to a step 924 wherein an array substrate comprising an array of MEMS devices is aligned with and adhered to the backplale via the thermal glue printed on the backplate and pre-cured. The alignment and assembly process may take on the order of one minute, although it will be understood that depending on the sealing materials and process, this time may vary significantly. In some embodiments, UV glue may be dispensed in an ambient environment, and may be assembled with an open or closed seal. In other embodiments, UV or thermal glue, may be used and the assembly may be performed in a vacuum chamber so as to permit control of the pressure to assist with moisture evacuation, Similar to the reactivation processes discussed above, any of the control parameters of time, temperature, and pressure may be modified to optimize the encapsulation process.

[0089] Once the backplate has been mated to the array substrate, the process moves to a step 930 wherein the package is exposed to a thermal back process to fully cure the thermal glue. In this curing process, the package is exposed to a temperature of roughly 17O⁰C for roughly 4 hours, although the time and temperature may again vary significantly. The temperature required to cure the glue, which may range from 50°C to 200⁰C will typically be the primary consideration in selecting the temperature for the curing process. However, the upper range of usable temperatures for this process is dependent at least in part on the nature of the MEMS devices being packaged and the material composition and intended use of those MEMS devices. Similarly, the time necessary to cure the thermal glue or other sealant may vary depending on the amount of sealant used and the environment in which the curing process takes place. If the process takes place in a vacuum, the time required may decrease significantly. It will be understood that the desiccant may undergo significant reactivation during this process, due to the high temperature, the extended exposure, and the possibility of performing this step in a controlled environment such as a vacuum or other low-humidity environment. Water vapor released during this reactivation can escape the package though the endseal openings in the perimeter seal formed by the thermal glue.

[0090] Finally, the process moves to a step 932 wheren an endseal process is used to finish the encapsulation of the package. In an LCD-type manufacturing process, the perimeter seal formed via the thermal glue in step 920 may comprise one or more endseal openings which permit, for example, pressure equalization between the exterior and the interior of the package. As the increased temperatures during the curing process may result in an increase in pressure within the package, the equalization of pressure via the endseal openings prevents possible damage to the seal during the curing process. The endseal process may include the sealing of these endseal openings once the curing process has completed. The endseal may comprise a variety of materials and be applied via a variety of techniques, but in one embodiment, the endseal may be sealed with a polymer which is cured either thermally or via UV exposure. The endseal process may be in certain embodiments be combined with a vacuum pump and purge system so as to reactivate additional desiccant by removal of retained water vapor. The temperature during this process may also be increased to further accelerate the evacuation of water through the endseal. As noted above, this heating may also serve to remove moisture from a polymer matrix in a desiccant.

[0091] Thus, the above process illustrates an exemplary process which may be utilized to partially reactivate desiccant during a package assembly process, so as to enable packaging in an ambient environment. In-line heating elements arranged along transport 20967

paths of the desiccant-containing components may be used to minimize and even reverse the desiccant consumption resulting from exposure to an ambient environment. Such partial reactivation of the desiccant may be used to not only control the amount of desiccant consumption but also the type and location of the desiccant consumption. In particular, reactivation of the desiccant during the assembly process can be used to at least partially limit desiccant exposure to the portion of the desiccant close to the exposed surface of the desiccant, This greatly increases the efficacy of later reactivation processes, such as the thermal glue curing process which occurs near the end of the packaging process, as the desiccant near the surface of the desiccant layer can be more easily regenerated.

[0092] It will be understood, however, that the above process is exemplary, and may be modified in a variety of ways. In certain embodiments, additional manufacturing steps may be included. For example, as will be discussed below, the effective environment may be modified during a process step such as an in-line heating step via exposing the prepared backplate to a high-pressure flow of a desired gas. In other embodiments, process steps may be left out or replaced with other process steps in order to accommodate different components or assembly techniques. Other modifications to the above process are contemplated.

[0093] In another embodiment, which may be utilized in conjunction with the above process, the desiccant layer itself may be formed so as to at least partially localize the moisture absorption, such that the desiccant near the exposed surface absorbs more moisture than the desiccant located away from the surface. Figure 1OA illustrates an embodiment of a prepared backplate 1000 comprising a shaped backplate 1040 and desiccant 1060. It can be seen that the desiccant 1060 comprises multiple sublayers disposed so as to at least partially overlie other sublayers. In the illustrated embodiment, desiccant 1060 comprises a exposed desiccant layer 1010 overlying a slow permeation layer 1020, which in turn overlies a protected desiccant layer 1030.

[0094} Advantageously, it can be seen that the desiccant 1060 has been deposited fully within the recessed portion 1070 of the backplate 1040. Thus, only the outer surface of the exposed desiccant layer 1010 will be exposed to the ambient environment during a packaging process such as that described with respect to Figure 9. In particular, it can be seen that the protected desiccant layer 1030 will not be exposed to an ambient environment, and is surrounded on all sides by either the backplate or the slow permeation layer 1020. Figure 1OB illustrates an alternate arrangement in which the slow permeation layer 1020 extends over the sides of the protected desiccant layer 1030. An arrangement such as that of Figure 1OB may be utilized when a non-indented backplate is used, or simply when the desiccant does not fill the entire recessed portion of the backplate, as shown in Figure 1 OB.

|009S] The exposed desiccant layer 1010 preferably comprises a desiccanl which is capable of being reactivated, such as a molecular sieve. Advantageously, a fast acting desiccant may be utilized. In certain embodiments, the exposed desiccant layer 1010 may comprise the Dynic ZA desiccant made by Dynic Corporation, which includes a Zeolite embedded in a porous Teflon matrix, or the Zcogel made by Sud Chemi. Slow acting desiccants capable of regeneration may also be used. For example, by altering the properties of the polymer matrix, the desiccant absorption rate can be slowed down significantly, such as by an order of magnitude or greater. Such slow-acting desiccants include desiccants made by Multisorb. To the extent that the polymer matrix absorbs water, the polymer matrix can be dried out by the reactivation heating steps, as will be discussed in greater detail below.

[0096] In some embodiments, a combination of CaO desiccant and Zeolite-based desiccants may be used, either within a single sublayer or within separate sublayers. In other embodiments, a layer of materials may be provided adjacent the desiccant layer which enhance the activation and transport of moisture and other material to the desiccant, and otherwise improve desiccant operation. For example, a titanium oxide layer may be used as a getter, and may be provided over all or part of the exposed surface of the desiccant layer. Such a layer may be used to control moisture penetration by inhibiting or accelerating moisture penetration or evacuation. Other getters which may absorb moisture or volatile moleculcd may be provided within one of the sublayers of the desiccant layers, so as to further provide an environment free of moisture and other volatile molecules. In certain embodiments, desiccants including nanoparticles — which can be particularly easily regenerated — may be used as the exposed desiccant sublayer 1010 layer or on the exposed surface of the layer. An example of a nanoparticle based desiccant which may be used as the layer 1010 is desiccant made by ^'Nanoscape. (0097) The slow permeation layer 1020 comprises a material which has a lower rate of moisture permeation than the exposed desiccant layer 1010, so as to at least partially inhibit moisture flux into and through the slow permeation layer 1020. In certain embodiments, the slow permeation layer 1020 may comprise a layer of slow acting desiccant. In other embodiments, the slow permeation layer may comprise a non-desiccant material such as a layer of epoxy. In still further embdoments, the slow permeation layer may comprise multiple sublayers, such as, for example, a layer of slow-acting desiccant and a layer of epoxy. In another embodiment, the slow permeation layer may comprise titanium oxide or a titanium oxide sublayer which absorbs water and inhibits moisture transmission therethrough. The slow permeation layer 1020 thus provides one means for inhibiting moisture flux.

[0098] The protected desiccant layer 1030 covered by the slow permeation layer 1020 may comprise any suitable desiccant. In certain embodiments, the protected desiccant layer 1030 may comprise the same desiccant as the exposed desiccant layer 1010. In embodiments in which the slow permeation layer 1020 comprises a desiccant, the protected desiccant layer 1030 may comprise the same desiccant as the slow permeation layer 1020. Because of the encapsulation of the protected desiccant layer 1030 by the slow permeation layer 1020, the protected desiccant layer 1030 will absorb little if any moisture during an ambient environment packaging process, but will still function as a desiccant in the final encapsulated package, as the moisture flux rate into the sealed package will usually be significantly less than the flux through the slow permeation layer 1020 during the assembly process. It will be understood that, depending on the composition of the various layers and the intended use and lifetime of the packaged MEMS device, the thicknesses of the various layers may vary significantly in certain embodiments.

[00991 While the thickness of the overall desiccant layer 1060 prevents the lower layers such as protected desiccant layer 1030 from being easily reactivated by later reactivation processes in the process flow of Figure 9, it will be understood that the initial full reactivation step 908 may be done at sufficiently high temperatures and for sufficient periods of time to activate all desiccant in the desiccant stack. Such high temperatures can be used at this initial stage as the only components being heated are the backplate and the desiccant applied thereto, both of which may be designed to tolerate such high temperatures. At later stages in the fabrication process, and particularly once the MEMS device has been integrated into the package, the use of such high temperatures for extended periods of time may not be possible due to potential damage to the MEMS device. The use of a striated desiccant structure such as the one seen in Figures 1OA and 1OB serves to contain a large portion of the absorbed moisture in the outer layer of the desiccant, enabling the recovery of more usable desiccant than would be possible had this moisture been absorbed and retained deeper within the desiccant.

[0100] In other embodiments, slow acting desiccants may be utilized in an ambient packaging process and may be dried out prior to assembly of the package in order to recover capacity of the desiccant. Slow acting desiccants are typically used when packaging MEMS devices in ambient environments, but the desiccants which work at the desired low dew points (usually between -30C and -70C) are scarce. In order to provide a desiccant which has the desired properties, a desiccant is typically mixed with a binder or polymer which is tailored to provide slow uptake of moisture by the desiccant. Calcium oxide is an exemplary desiccant which can be modified in this manner to provide a slow acting desiccant.

[0101] it has been discovered that when used in such a manner during an ambient manufacturing process, the polymer, which transmits water vapor to the calcium oxide binding sites, itself collects a significant amount of water vapor when prepared in an ambient environment. Assembly techniques using such a desiccant typically utilize the desiccant in such a state, as the package is capable of tolerating such moisture levels and the moisture will eventually be absorbed by the calcium oxide. However, in certain embodiments, an ambient assembly process may be modified to include a process step which dries out the polymer prior to assembly of the package.

(0102) In some embodiments, prolonged exposure to a moisture-free environment will remove a sufficient amount of moisture from the polymer. Thus, if the assembly process is performed at least partially in an controlled moisture-free environment, and the portion of the assembly process which takes place in the controlled environment is of sufficient duration, the moisture will be outgassed from the polymer. The dryness (or dew point) of the environment will determine the amount of water outgassed from the polymer during the assembly process, and if the process time in a controlled environment is sufficiently long, the desired amount of moisture will be outgassed.

[0103] However, if the assembly is performed or concluded in an ambient environment, or if the assembly process in a controlled environment is not of sufficient length to remove the moisture, the outgassing process can be accelerated. This process can be accelerated, for example, by heating the desiccant, whether in an ambient or controlled environment.

10104) Advantageously, this heating step is performed as close to the final alignment and assembly process as possible. In certain embodiments of ambient assembly processes, the heating step may comprise an inline heating step performed immediately prior to the alignment and mating of the backplate to the array substrate bearing the MEMS device. In an exemplary embodiment, the backplate and desiccant are subjected to heating at 150⁰C for roughly five minutes. These time and temperature ranges are compatible with in-line incorporation of this step into an LCD-like assembly process discussed above. In certain embodiments, heating may be provided by a ceramic heating element, although other types of heating such as IR heating and microwave heating may alternately be utilized. In addition, in particular embodiments the heating site may be inundated with a flow of dry gas such as nitrogen in order to facilitate this drying process, although this heating step can also be performed in ambient environments.

[0105] By heating the desiccant in such a manner, the dew point of the desiccant can be reduced to a desired level, such as below -60⁰C or -65°C in certain embodiments. The reduction in moisture may come about as a result of both evacuation of water vapor from the polymer, as well as chemical reaction between the calcium oxide and the water to chemically transform the water content. By selecting proper process conditions such as temperature and duration, the outgassing of water vapor can be optimized so that as much water vapor as possible is being removed from the polymer to the environment, increasing the lifetime of the desiccant to a desired level.

[0106] As discussed above, the necessary conditions for the temperature, process, and duration of the heating process are interdependent. If heated to above the glass transition temperature, for example 15O⁰C, the duration may be further shorted, for example to as little as 30 seconds. If the heating is done in a vacuum chamber, the pressure can be reduced as discussed above so as to further assist with the water removal, or in other embodiments the desiccant may be heated in a nitrogen flow.

10107) While the above detailed description has shown, described and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others.

Claims

WHAT IS CLAIMED IS:

1. A method of packaging an electromechanical device, the method comprising: providing a backplate, the backplate supporting a layer of desiccant; performing an additional manufacturing step; reactivating the desiccant, wherein reactivating the desiccant is done after performing an additional manufacturing step; and sealing said backplate to a substrate to form a package, wherein said substrate supports an electromechanical structure, and wherein said electromechanical structure and said desiccant are located within said package.

2. The method of Claim 1, wherein reactivating the desiccant comprises heating said desiccant to remove a portion of the water vapor retained therein.

3. The method of Claim 1, wherein said desiccant comprises a polymer matrix.

4. The method of Claim 3, wherein said reactivating comprises heating said desiccanl to release water from said polymer matrix.

5. The method of Claim 1, wherein providing a backplate additionally comprises providing an additional layer disposed between the desiccant and the backplate, wherein said additional layer permits a lower rate of moisture flux than the desiccant layer.

6. The method of Claim 5, wherein the additional layer comprises an epoxy layer.

7. The method of Claim 5, wherein the additional layer comprises a desiccant.

8. The method of Claim 5, wherein providing a backplate additionally comprises providing a second desiccant layer, wherein said second desiccant layer is located between said additional layer and said backplate.

9. The method of Claim 1, wherein reactivating the desiccant comprises reactivating only a portion of the desiccant.

10. The method of Claim 1, wherein the electromechanical structure comprises an interferometric modulator.

1 1. The method of Claim 1 , wherein performing an additional manufacturing step comprises exposing said desiccant to an ambient environment.

12. An electromechanical device, comprising: a substrate, wherein said substrate comprises an electromechanical structure; a backplate; a partially reactivated desiccant supported by said substrate; and a seal adhering said substrate to said backplate.

13. The device of Claim 12, wherein said desiccant comprises a polymer matrix.

14. The device of Claim 12, wherein the desiccant comprises: a first sublayer, said first sublayer comprising a desiccant which is capable of regeneration; and a second sublayer disposed between the first sublayer and the backplate, said second sublayer comprising a material which permits a lower rate of moisture flux therethrough than the first sublayer.

15. The device of Claim 14, wherein the second sublayer comprises a layer of epoxy.

16. The device of Claim 14, wherein the second sublayer comprises an additional layer of desiccant.

17. The device of Claim 14, additionally comprising a third sublayer disposed between the second sublayer and the backplate, said third sublayer comprising an additional layer of desiccant.

18. The electromechanical device of Claim 12, wherein the electromechanical structure comprises a display element, said electromechanical device additionally comprising: a processor that is configured to communicate with said electromechanical structure, said processor being configured to process image data; and a memory device that is configured to communicate with said processor.

19. The electromechanical device of Claim 18, further comprising a driver circuit configured to send at least one signal to the electromechanical structure.

20. The electromechanical device of Claim 19, further comprising a controller configured to send at least a portion of the image data to the driver circuit.

21. The electromechanical device of Claim 18, further comprising an image source module configured to send said image data to said processor.

22. The electromechanical device of Claim 21 , wherein the image source module comprises at least one of a receiver, transceiver, and transmitter.

23. The electromechanical device of Claim 18, further comprising an input device configured to receive input data and to communicate said input data to said processor.

24. The electromechanical device of Claim 12, wherein the electromechanical structure comprises an interferometric modulator.

25. A method of packaging an electromechanical device, the method comprising: providing a backplate, the backplate supporting a multilayer desiccant, the multilayer desiccant comprising: an upper desiccant sublayer; an additional layer, wherein the additional layer is located between the upper desiccant sublayer and the backplate; and a lower desiccant sublayer, wherein the lower desiccant sublayer is located between the additional layer and the backplate; performing an additional manufacturing step; and sealing said backplate to a substrate to form a package, wherein said substrate supports an electromechanical structure, and wherein said electromechanical structure and said desiccant are located within said package.

26. The method of Claim 25, wherein the upper desiccant sublayer comprises a desiccant which is capable of reactivation.

27. The method of Claim 26, additionally comprising at least partially reactivating the desiccant after performing said additional manufacturing step.

28. The method of Claim 25, wherein the additional layer comprises a material which permits a lower rate of moisture flux therethrough than the upper dessicant sublayer.

29. The method of Claim 25, wherein the additional layer comprises an epoxy.

30. The method of Claim 25, wherein the additional layer comprises a desiccant.

31. An electromechanical device, comprising: a substrate, wherein said substrate comprises an electromechanical structure; a backplate; a multilayer desiccant supported by said substrate, said multilayer desiccant comprising: an upper desiccant sublayer; an additional layer, wherein the additional layer is located between the upper desiccant sublayer and the backplate; and a lower desiccant sublayer, wherein the lower desiccant sublayer is located between the additional layer and the backplate; and a seal adhering said substrate to said backplate.

32. The electromechanical device of Claim 31, wherein the upper desiccant sublayer comprises a desiccant which is capable of reactivation.

33. The electromechanical device of Claim 32, wherein the upper desiccant sublayer is at least partially reactivated.

34. The electromechanical device of Claim 31, wherein the additional layer comprises a material which permits a lower rate of moisture flux therethrough than the upper dessicant sublayer.

35. The electromechanical device of Claim 31, wherein the additional layer comprises an epoxy,

36. The electromechanical device of Claim 31 , wherein the additional layer comprises a desiccant.

37. An electromechanical device, comprising: means for absorbing moisture; first means for supporting said absorbing means; means for inhibiting moisture flux, said inhibiting means being disposed between said absorbing means and said first supporting means; second means for supporting an electromechanical structure; and means for sealing said second supporting means to said first supporting means.

38. The electromechanical device of Claim 37, wherein said absorbing means comprises a desiccant. 2007/020967

39. The electromechanical device of Claim 37, wherein said first supporting means comprises a backplate.

40. The electromechanical device of Claim 37, wherein said inhibiting means comprises a layer permitting lower moisture flux therethrough than said absorbing means.

41. The electromechanical device of Claim 37, wherein said second supporting means comprises a substrate for supporting a electromechanical structure.

42. The electromechanical device of Claim 37, wherein said sealing means comprises a perimeter seal.