# Patent

## Title
Method and device for packaging a substrate

## Abstract

A package structure and method of packaging for an interferometric modulator. A thin film material is deposited over an interferometric modulator and transparent substrate to encapsulate the interferometric modulator. A gap or cavity between the interferometric modulator and the thin film provides a space in which mechanical parts of the interferometric modulator may move. The gap is created by removal of a sacrificial layer that is deposited over the interferometric modulator.

## Background
RELATED APPLICATION
This application claims priority to U.S. Provisional Application No. 60/613,318, filed Sep. 27, 2004, the contents of which are hereby incorporated by reference in their entirety.


BACKGROUND
1. Field of the Invention
The field of the invention relates to microelectromechanical systems (MEMS) and the packaging of such systems. More specifically, the field of the invention relates to interferometric modulators and methods of fabricating such modulators with thin film backplanes.
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. 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. One plate may comprise a stationary layer deposited on a substrate, the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. 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
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.
An embodiment provides a package structure for an interferometric modulator display device that eliminates the need for a separate backplane, desiccant, and seal. The display device includes a transparent substrate, an interferometric modulator configured to modulate light transmitted through the transparent substrate, and a thin film backplane disposed on the modulator and sealing the modulator within a package between the transparent substrate and the thin film backplane. A gap exists between the modulator and the thin film and is created by the removal of a sacrificial layer.
In accordance with another embodiment, a method of manufacturing a display device is provided. According to this method, a transparent substrate is provided and an interferometric modulator is formed on the transparent substrate. A thin film backplane is then deposited over the interferometric modulator and the transparent substrate to seal the modulator between the transparent substrate and the thin film backplane. A sacrificial layer is deposited on the interferometric modulator prior to deposition of the thin film backplane. The sacrificial layer is removed after deposition of the thin film backplane to create a gap between said interferometric modulator and the thin film backplane.
In accordance with yet another embodiment, a microelectromechanical systems display device is provided, comprising a transparent substrate, an interferometric modulator formed on the transparent substrate, and a thin film backplane sealed to the transparent substrate to encapsulate the interferometric modulator between the transparent substrate and the thin film backplane. A cavity exists between the interferometric modulator and the thin film backplane. The cavity is created by removing a sacrificial layer between the interferometric modulator and the thin film backplane.
According to another embodiment, a display device is provided, comprising a transparent substrate, an interferometric modulator, a thin film backplane deposited over the interferometric modulator, and a cavity between the modulator and the thin film backplane. The interferometric modulator is configured to modulate light transmitted through the transparent substrate, and is formed on the transparent substrate. The thin film backplane is deposited over the interferometric modulator to seal the modulator within a package between the transparent substrate and the thin film backplane. The cavity is formed by removing a sacrificial material.
According to yet another embodiment, a display device is provided. The display device includes a transmitting means for transmitting light therethrough, a modulating means configured to modulating light transmitted through the transmitting means, and a sealing means for sealing the modulating means within a package between the transmitting means and the sealing means. The modulating means comprises an interferometric modulator, and the sealing means comprises a thin film.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the invention will be readily apparent from the following description and from the appended drawings (not to scale), which are meant to illustrate and not to limit the invention, and wherein:
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 released position and a movable reflective layer of a second interferometric modulator is in an actuated position.
FIG. 2 is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display.
FIG. 3 is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of FIG. 1.
FIG. 4 is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display.
FIGS. 5A and 5B illustrate one exemplary timing diagram for row and column signals that may be used to write a frame of display data to the 3×3 interferometric modulator display of FIG. 2.
FIG. 6A is a cross section of the device of FIG. 1.
FIG. 6B is a cross section of an alternative embodiment of an interferometric modulator.
FIG. 6C is a cross section of another alternative embodiment of an interferometric modulator.
FIG. 7 schematically illustrates a package structure in which an interferometric modulator is packaged without a conventional backplane, according to an embodiment.
FIG. 8 is a flow chart of an embodiment of a method to package interferometric modulators.
FIG. 9 schematically illustrates a package structure in which a sacrificial layer has been deposited over the interferometric modulator, according to an embodiment.
FIG. 10 schematically illustrates a package structure in a thin film has been deposited over the sacrificial layer.
FIG. 11 is a top view of an embodiment of the package structure 800 after the thin film 820 has been deposited and patterned and before the sacrificial layer 850 is released.
FIG. 12 schematically illustrates a package structure in which an interferometric modulator is packaged according to an embodiment and having an overcoat layer.



DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the invention 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 invention may be implemented in or associated with a variety of electronic devices such as, but not limited to, mobile telephones, wireless devices, personal data assistants (PDAs), hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, computer monitors, auto displays (e.g., odometer display, etc.), cockpit controls and/or displays, display of camera views (e.g., display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, packaging, and aesthetic structures (e.g., display of images on a piece of jewelry). MEMS devices of similar structure to those described herein can also be used in non-display applications such as in electronic switching devices.
One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated in FIG. 1. In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“off” or “closed”) state, the display element reflects little incident visible light to the user. Depending on the embodiment, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at selected colors, allowing for a color display in addition to black and white.
FIG. 1 is an isometric view depicting two adjacent pixels in a series of pixels of a visual display, wherein each pixel comprises a MEMS interferometric modulator. In some embodiments, an interferometric modulator display comprises a row/column array of these interferometric modulators. Each interferometric modulator includes a pair of reflective layers positioned at a variable and controllable distance from each other to form a resonant optical cavity with at least one variable dimension. In one embodiment, one of the reflective layers may be moved between two positions. In the first position, referred to herein as the released state, the movable layer is positioned at a relatively large distance from a fixed partially reflective layer. In the second position, the movable 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 and highly reflective layer 14a is illustrated in a released position at a predetermined distance from a fixed partially reflective layer 16a. In the interferometric modulator 12b on the right, the movable highly reflective layer 14b is illustrated in an actuated position adjacent to the fixed partially reflective layer 16b. 
The fixed layers 16a, 16b are electrically conductive, partially transparent and partially reflective, and may be fabricated, for example, by depositing one or more layers each of chromium and indium-tin-oxide onto a transparent substrate 20. The layers are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable layers 14a, 14b may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes 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 deformable metal layers are separated from the fixed metal layers by a defined air gap 19. A highly conductive and reflective material such as aluminum may be used for the deformable layers, and these strips may form column electrodes in a display device.
With no applied voltage, the cavity 19 remains between the layers 14a, 16a and the deformable layer is 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 layer is deformed and is forced against the fixed layer (a dielectric material which is not illustrated in this Figure may be deposited on the fixed layer to prevent shorting and control the separation distance) as illustrated by the 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 5 illustrate one exemplary process and system for using an array of interferometric modulators in a display application. FIG. 2 is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor 21 which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor 21 may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application.
In one embodiment, the processor 21 is also configured to communicate with an array controller 22. In one embodiment, the array controller 22 includes a row driver circuit 24 and a column driver circuit 26 that provide signals to a pixel array 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 released 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 release 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 released 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 released 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 released pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or released state, is essentially a capacitor formed by the fixed and moving reflective layers, this stable state can be held at a voltage within the hysteresis window with almost no power dissipation. Essentially no current flows into the pixel if the applied potential is fixed.
In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row 1 electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row 2 electrode, actuating the appropriate pixels in row 2 in accordance with the asserted column electrodes. The row 1 pixels are unaffected by the row 2 pulse, and remain in the state they were set to during the row 1 pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used in conjunction with the present invention.
FIGS. 4 and 5 illustrate one possible actuation protocol for creating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment, actuating a pixel involves setting the appropriate column to −Vbias, and the appropriate row to +ΔV, which may correspond to −5 volts and +5 volts respectively 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. 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.
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 released 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 releases 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 release 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 present invention.
The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example, FIGS. 6A–6C illustrate three different embodiments of the moving mirror structure. FIG. 6A 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. 6B, the moveable reflective material 14 is attached to supports at the corners only, on tethers 32. In FIG. 6C, the moveable reflective material 14 is suspended from a deformable layer 34. This embodiment has benefits because the structural design and materials used for the reflective material 14 can be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer 34 can be optimized with respect to desired mechanical properties. The production of various types of interferometric devices is described in a variety of published documents, including, for example, U.S. Published Application 2004/0051929. A wide variety of well known techniques may be used to produce the above described structures involving a series of material deposition, patterning, and etching steps.
FIG. 7 illustrates a package structure 800 in which an interferometric modulator 830 is packaged on a transparent substrate 810 without a conventional backplane or cap. The package structure 800 illustrated in FIG. 7 may eliminate the need for not only a backplane but also a separate seal as well as a desiccant.
In accordance with the embodiment shown in FIG. 7, instead of sealing a backplane to the transparent substrate to encapsulate the interferometric modulator 830, as discussed above, a thin film or superstructure 820 is deposited over the transparent substrate 810 to encapsulate the interferometric modulator 830 within the package structure 800. The thin film 820 protects the interferometric modulator 830 from harmful elements in the environment.
A method of packaging an interferometric modulator according to the embodiment shown in FIG. 7 will be discussed in more detail below. The packages and packaging methods described herein may be used for packaging any interferometric modulator, including, but not limited to, the interferometric modulators described above.
As discussed above, the interferometric modulator 830 is configured to reflect light through the transparent substrate and includes moving parts, such as the movable mirrors 14a, 14b. Therefore, to allow such moving parts to move, a gap or cavity 840 is preferably created between such moving parts and the thin film 820. The gap or cavity 840 allows the mechanical parts, such as the movable mirrors 14a, 14b, of the interferometric modulator 830 to move. It will be understood that before the thin film 820 can be deposited to encapsulate the interferometric modulator 830, a sacrificial layer 850 (shown in FIG. 9) is preferably deposited over the interferometric modulator 830 and the transparent substrate 810, and then removed, to create a cavity 840 between the interferometric modulator 830 and the thin film 820. This will be described in more detail below.
FIG. 8 shows one embodiment of a method of packaging an interferometric modulator without a conventional backplane or cap. A transparent substrate 810 is first provided at Step 900 and the interferometric modulator 830 is formed on the transparent substrate 810 at Step 910. The interferometric modulator 830 is preferably formed in accordance with the processes described with reference to FIGS. 1–6. The transparent substrate 810 may be any transparent substance capable of having thin film, MEMS devices built upon it. Such transparent substances include, but are not limited to, glass, plastic, and transparent polymers. Images are displayed through the transparent substrate 810, which serves as an imaging surface.
After the interferometric modulator 830 has been formed on the transparent substrate 810, a sacrificial layer 850 is preferably deposited over the upper surfaces of the interferometric modulator 830 and the transparent substrate 810 in Step 920. The sacrificial layer 850 is then patterned in Step 930, using photolithographic techniques. This patterning process preferably localizes the sacrificial layer 850 to the interferometric modulator 830, exposing the transparent substrate 810 around the periphery of the interferometric modulator 830. After the sacrificial layer 850 has been deposited and patterned, a thin film 820 is then deposited over the entire structure, in Step 940. The thin film 820 is then patterned in Step 950, using photolithographic techniques. This patterning process localizes the thin film 820 to the sacrificial layer 850. This patterning step also provides features in the thin film 820 that enable the subsequent removal of the sacrificial layer 850. It should be noted that, at this point in the process, additional sacrificial layers may or may not remain within the interferometric modulator structure. The patterning step 930 allows for removal of sacrificial layer 850 as well as for removal of any sacrifical layers remaining within the interferometric modulator 830. In Step 960, the sacrificial layer 850 and any sacrificial layers within the interferometric modulator 830 are removed, leaving a cavity 840 between the interferometric modulator 830 and the thin film 820, completing processing of the interferometric modulator 830. In Step 970, the features or openings in the thin film 820 are sealed.
In accordance with an embodiment, an interferometric modulator 830 is preferably formed on a transparent substrate 810. It will be understood that the fixed mirrors 16a, 16b of the interferometric modulator 830 are adjacent the transparent substrate 810 and the movable mirrors 14a, 14b are formed over the fixed mirrors 16a, 16b such that the movable mirrors 14a, 14b may move within the cavity 840 of the package structure of the embodiment shown in FIG. 7.
To form the interferometric modulator 830, the transparent substrate 810 in one embodiment is covered with indium tin oxide (ITO). The ITO may be deposited by standard deposition techniques, including chemical vapor deposition (CVD) and sputtering, preferably to a thickness of about 500 Å. A relatively thin layer of chrome is preferably deposited over the ITO. The ITO/chrome bilayer is then etched and patterned into columns to form the column electrodes 16a, 16b. A layer of silicon dioxide (SiO2) is preferably formed over the ITO/chrome columns to create partially reflective fixed mirrors 16a, 16b. A sacrificial layer of silicon (Si) is preferably deposited (and later released) over the structure to create a resonant optical cavity between the fixed mirrors 16a, 16b and the movable mirrors 14a, 14b. In other embodiments, this sacrificial layer may be formed of molybdenum (Mo), tungsten (W), or titanium (Ti).
Another mirror layer, preferably formed of aluminum, is deposited over the sacrificial layer of silicon to form the movable mirrors 14a, 14b of the interferometric modulator 830. This mirror layer is deposited and patterned into rows orthogonal to the column electrodes 16a, 16b to create the row/column array described above. In other embodiments, this mirror layer may comprise highly reflective metals, such as, for example, silver (Ag) or gold (Au). Alternatively, this mirror layer may be a stack of metals configured to give the proper optical and mechanical properties.
The sacrificial layer of silicon is removed, preferably using a gas etching process, after the movable mirrors 14a, 14b are formed to create the optical cavity between the fixed mirrors 16a, 16b and the movable mirrors 14a, 14b. In an embodiment, this sacrificial layer is etched away after the thin film 820 is formed. Standard etching techniques may be used to remove the sacrificial layer of silicon. The particular release etching will depend on the material to be released. For example, xenon diflouride (XeF2) may be used to remove the silicon sacrificial layer. In one embodiment, the sacrificial layer of silicon between the mirrors 16a, 16b, 14a, 14b is removed after the thin film 820 is formed. The skilled artisan will appreciate that each layer of the interferometric modulator 830 is preferably deposited and patterned using standard deposition techniques and standard photolithographic techniques.
As shown in FIG. 9, after the interferometric modulator 830 is formed on the transparent substrate 810, another sacrificial layer 850 is deposited over the upper surfaces of the interferometric modulator 830 and the transparent substrate 810. The sacrificial layer 850 may be formed of a material, such as, for example, molybdenum (Mo), silicon (Si), tungsten (W), or titanium (Ti), which is capable of being released after deposition of the thin film 820. In an embodiment, the sacrificial layer 850 is formed of a material, such as a polymer, spin-on glass, or oxide. The removal processes, which may differ depending on the material of the sacrificial layer, will be described in more detail below.
The skilled artisan will appreciate that the upper sacrificial layer 850 may be formed of any of molybdenum (Mo), silicon (Si), tungsten (W), titanium (Ti), polymer, spin-on glass, or oxide so long as the material provides sufficient step coverage and can be deposited to the desired thickness. The thickness of the sacrificial layer 850 should be sufficient to separate the thin film 820 and the interferometric modulator 830. In one embodiment, the upper sacrificial layer 850 is deposited to a thickness in the range of about 1000 Å to 1 μm, and more preferably in a range of about 1000 Å to 5000 Å. In one embodiment, the sacrificial layer 850 is patterned and etched using standard photolithographic techniques.
In one embodiment, the thin film 820 can be deposited over the entire upper surface of the sacrificial layer 850, as shown in FIG. 10. The thin film 820 may be formed over the sacrificial layer 850 using known deposition techniques. After the thin film 820 is patterned and etched, the sacrificial layer 850 is released to form a cavity 840 in which the movable mirrors 14a, 14b may move, as shown in FIG. 8.
The thin film 820 is preferably patterned and etched to form at least one opening therein through which a release material, such as xenon diflouride (XeF2), may be introduced into the interior of the package structure 800 to release the sacrificial layer 850. The number and size of these openings depend on the desired rate of release of the sacrificial layer 850. The openings may be positioned anywhere in the thin film 820. In certain embodiments, the sacrificial layer 850 and the sacrificial layer within the interferometric modulator (between the fixed mirrors 16a, 16b and the movable mirrors 14a, 14b) may be released at the same time. In other embodiments, the sacrificial layer 850 and the sacrificial layer within the interferometric modulator are not removed at the same time, with the sacrificial layer 850 being removed prior to the removal of the sacrificial layer within the interferometric modulator.
An alternative release technique is shown by the embodiment in FIG. 11. FIG. 11 is a top view of an embodiment of the package structure 800 after the thin film 820 has been deposited and patterned and before the sacrificial layer 850 is released. As shown in FIG. 11, the sacrificial layer 850 is deposited and patterned such that it has a plurality of protrusions 855. The thin film 820 is then deposited over the sacrificial layer 850 and the transparent substrate 810. After the thin film 820 is deposited, it is then preferably etched back on each side, as shown in FIG. 11. The package structure 800 can then be exposed to the release material, such as xenon diflouride (XeF2), which reacts first with the exposed sacrificial layer 850 material and then enters the package structure 800 through the openings created at the protrusions 855 by the removal of the sacrificial layer 850 on the sides of the package structure. It will be understood that the number and size of the protrusions 855 will depend on the desired rate of release of the sacrificial layer 850.
To remove a sacrificial layer of molybdenum (Mo), silicon (Si), tungsten (W), or titanium (Ti), xenon diflouride (XeF2) may be introduced into the interior of the package structure 800 through an opening or openings in the thin film 820. Such openings in the thin film 820 are preferably created by etching an opening in the thin film 820. The xenon diflouride (XeF2) reacts with the sacrificial layer 850 to remove it, leaving a cavity 840 between the interferometric modulator 830 and the thin film 820. A sacrificial layer 850 formed of spin-on glass or oxide is preferably gas etched or vapor phase etched to remove the sacrificial layer 850 after the thin film 820 has been deposited. The skilled artisan will appreciate that the removal process will depend on the material of the sacrificial layer 850.
The skilled artisan will also appreciate that the cavity 840 is necessary behind the interferometric modulator 830 to allow the mechanical parts, such as the movable mirrors 14a, 14b, of the interferometric modulator 830 to be free to move. The resulting height h of the cavity 840 depends on the thickness of the sacrificial layer 850.
In some embodiments, the thin film 820 may be any type of material that is hermetic or hydrophobic, including, but not limited to, nickel, aluminum, and other types of metals and foils. The thin film 820 may also be formed of an insulator, including, but not limited to, silicon dioxide, aluminum oxide, or nitrides.
Alternatively, the thin film 820 may be formed of a non-hermetic material. Suitable non-hermetic materials include polymers, such as, for example, PMMA, epoxies, and organic or inorganic spin-on glass (SOG) type materials. If non-hermetic materials are used for the thin film 820, an overcoat layer 860, as shown in FIG. 12, is preferably formed over the non-hermetic thin film to provide additional protection to the interferometric modulator 830 after the sacrificial layer 850 is removed, as shown in FIG. 12. Such an overcoat layer 860 is preferably formed of a vapor barrier and has a thickness of about 1000 Å to about 10,000 Å. In one embodiment, the overcoat layer 860 is BarixTM, a thin film coating commercially available from Vitex Systems, Inc. in San Jose, Calif. Such an overcoat may be multi-layered in which some layers may serve gas hermeticity purposes, and some layers, as described below, may serve mechanical purposes.
In certain embodiments in which the thin film 820 is a hydrophobic material, it does not necessarily create a hermetic seal, but may nevertheless eliminate the need for a conventional backplane. It will be appreciated that any further moisture barrier required can be incorporated in the next step of packaging at the module level.
The thin film 820 can be deposited by chemical vapor deposition (CVD) or other suitable deposition methods to a thickness of about 1 μm. The skilled artisan will understand that the thickness of the thin film 820 may depend on the particular material properties of the material selected for the thin film 820.
The thin film 820 may be either transparent or opaque. Because images are not displayed through the thin film 820, but rather through the transparent substrate 810, it is understood that the thin film 820 need not be transparent. The skilled artisan will appreciate that transparent materials, such as spin-on glass, may be used to form the thin film 820 as they may have material properties that are suitable for use as a thin film 820 for protection of the interferometric modulator 830. For example, a material such as spin-on glass, which is transparent, may provide more strength and protection to the interferometric modulator 830 within the package structure 800.
After the sacrificial layer 850 is released, the opening(s) in the thin film 820 are preferably sealed. In an embodiment, epoxy is used to seal these openings. The skilled artisan will appreciate that other materials may be used as well and that materials having high viscosity are preferred. If the openings are sufficiently small (e.g., less than 1μ), another layer of the thin film 820 material may be used to seal the openings.
In some embodiments, including, but not limited to, certain embodiments having a hermetic thin film 820, an overcoat layer 860 may be deposited over the thin film 820 after the sacrificial layer 850 has been removed, as shown in FIG. 12. The overcoat layer is preferably formed of a polymer and preferably has a thickness of about 1 μm to several millimeters. The overcoat layer 860 provides additional strength and stiffness to the thin film 820. In certain embodiments where the opening(s) in the thin film 820 are sufficiently small (e.g., less than 1μ), the overcoat layer 860 may be used to seal the openings rather than another layer of the thin film 820, as described above.
The thin film 820 preferably hermetically seals the interior the package structure 800 from the ambient environment, as shown in FIG. 7. As the thin film 820 may provide a hermetic seal, the need for a desiccant is therefore eliminated as the hermetic seal prevents moisture from entering the package structure 800 from the ambient environment. In another embodiment, the thin film 820 provides a semi-hermetic seal and a desiccant is included within the package structure 800 to absorb excess moisture.
A desiccant may be used to control moisture resident within the package structure 800. However, as the thin film 820 may provide a hermetic seal, depending on the material selected, a desiccant is not necessary to prevent moisture from traveling from the atmosphere into the interior of the package structure 800. In the case of a semi-hermetic thin film 820, the amount of desiccant required is reduced.
In an embodiment, the method of packaging an interferometric modulator according to this embodiment integrates the sealing of the package structure 800 into the front-end processing and eliminates the need for a separate backplane, desiccant, and seal, thereby lowering the cost of packaging. In another embodiment, the thin film 820 reduces the amount of desiccant required rather than eliminating the need for a desiccant. Packaging in accordance with these embodiments reduces the material constraints with respect to both the desiccant and seal, thereby allowing a greater choice or materials, geometries, and opportunities to reduce costs. The thin film 820 can reduce hermetic requirements to allow for not only elimination of a backplane but also allows any additional moisture barrier requirements to be incorporated into the module level packaging. It is generally desirable to keep the package structure as thin as possible and the package structure 800 shown in FIG. 7 provides for a thin structure.
The elimination of the need for a desiccant also allows the package structure 800 to be even thinner. 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 display will fail as sufficient moisture enters the package structure to cause damage to the interferometric modulator. The theoretical maximum lifetime of the device is determined by the water vapor flux into the package as well as the amount and type of desiccant. In this package structure 800, the interferometric modulator 830 will not fail due to a consumed desiccant as the package structure 800 of this embodiment does not contain any desiccant.
In another embodiment, the thin film 820 is not hermetic and may be permeable to xenon diflouride (XeF2) or another removal gas, which reacts with the sacrificial layer 850 to remove it, leaving a cavity 840 between the interferometric modulator 830 and the thin film 820. According to this embodiment, some suitable materials for the thin film 820 include, but are not limited to porous alumina and certain aerogels. In this embodiment, it is not necessary for the thin film 820 to be formed with any openings so long as it is permeable to xenon diflouride (XeF2) or another removal gas. Preferably, after removal of the sacrificial layer 850, a hermetic overcoat layer 860 is deposited over the thin film 820 to hermetically seal the package structure 800. In these embodiments, the overcoat layer 860 is preferably formed of a metal.
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

1. A display device comprising an array of movable mirrors configured to interferometrically modulate light, the display device comprising:
a transparent substrate;
an interferometric modulator comprising the array of movable mirrors, wherein said interfereometric modulator is configured to modulate light transmitted through said transparent substrate, and
a thin film backplane sealing said array of movable mirrors within a package between said transparent substrate and said thin film backplane, wherein a gap exists between said array of movable mirrors and said thin film backplane.

2. The display device of claim 1, wherein said gap is formed from removal of a sacrificial layer located between said array of movable mirrors and said thin film backplane.

3. The display device of claim 1, wherein the thin film backplane comprises a hermetic material.

4. The display device of claim 1, wherein the thin film is nickel.

5. The display device of claim 1, wherein the thin film is aluminum.

6. A method of manufacturing a display comprising an array of movable mirrors configured to interferometrically modulate light, the method comprising:
providing a transparent substrate;
forming an interferometric modulator on the transparent substrate, and wherein said interferometric modulator comprises the array of movable mirrors; and
depositing a thin film backplane over the array of movable mirrors and the transparent substrate to seal said array of movable mirrors between said transparent substrate and said thin film backplane, wherein a gap exist between said array of movable mirrors and said thin film backplane.

7. The method of claim 6, further comprising:
depositing a sacrificial layer on said interferometric modulator prior to depositing said thin film backplane; and
removing said sacrificial layer after depositing said thin film backplane in order to provide a gap between said interferometric modulator and said thin film backplane.

8. The method of claim 7, further comprising patterning said thin film backplane to create at least one opening in said thin film backplane.

9. The method of claim 7, further comprising patterning said thin film backplane to expose a portion of said sacrificial layer.

10. The method of claim 6, wherein the thin film backplane is formed of aluminum.

11. The method of claim 6, wherein the thin film backplane is formed of nickel.

12. The method of claim 6, wherein the thin film backplane is formed of spin-on glass.

13. The method of claim 6, wherein the thin film backplane is formed of a hermetic material.

14. The method of claim 7, wherein the sacrificial layer is formed of spin-on glass.

15. A microelectromechanical systems display device comprising an array of movable mirrors configured to interferometrically modulate light, the microelectromechanical systems display device comprising:
a transparent substrate;
an interferometric modulator formed on the transparent substrate, the interferometric modulators comprising the array of movable mirrors; and
a thin film backplane sealed to the transparent substrate to encapsulate the array of movable mirrors between the transparent substrate and the thin film backplane, wherein a cavity exists between the array of movable mirrors and the thin film backplane.

16. The microelectromechanical systems display device of claim 15, wherein the cavity is created by removing a sacrificial layer between the array of movable mirrors and the thin film backplane.

17. The microelectromechanical systems display device of claim 15, wherein the cavity allows one or more movable mirrors in the array of movable mirrors to move.

18. The microelectromechanical systems display device of claim 15, wherein the thin film backplane comprises a hermetic material.

19. The microelectromechanical systems display device of claim 15, further comprising an overcoat layer deposited over the thin film backplane.

20. The microelectromechanical systems display device of claim 19, wherein the overcoat layer comprises a vapor barrier material.

21. The microelectromechanical systems display device of claim 19, wherein the overcoat layer comprises a polymer.

22. The microelectromechanical systems display device of claim 15, wherein the thin film backplane comprises a metal.

23. The microelectromechanical systems display device of claim 15, wherein the thin film backplane comprises a polymer.

24. A display device comprising an array of moveable mirrors configured to interferometrically modulate light, the display device comprising:
a transparent substrate;
an interferometric modulator comprising the array of movable mirrors, wherein said interferometric modulator is configured to modulate light transmitted through the transparent substrate, and wherein said interferometric modulator is formed on the transparent substrate,
a thin film backplane deposited over the array of movable mirrors, wherein the thin film backplane seals the array of movable mirrors within a package between the transparent substrate and the thin film backplane; and
a cavity between the array of moveable mirrors and the thin film backplane, wherein the cavity is formed by removing a sacrificial material.

25. The display device of claim 24, wherein the thin film backplane is hermetic.

26. The display device of claim 24, wherein the thin film backplane is a metal.

27. The display device of claim 24, wherein the thin film backplane is a polymer.

28. A display device comprising an array of movable mirrors configured to interferometrically modulate light, the display device comprising:
a transmitting means for transmitting light therethrough;
a modulating means configured to modulating light transmitted through the transmitting means, wherein the modulating means comprises an interferometric modulator, wherein said interferometric modulator comprises the array of movable mirrors; and
a sealing means for sealing the array of movable mirrors within a package between the transmitting means and the sealing means, wherein the sealing means comprises a thin film, and wherein a cavity exists between the array of movable mirrors and the sealing means.

29. The display device of claim 28, wherein the sealing means comprises a hermetic material.

30. The display device of claim 28, wherein the thin film is permeable to xenon diflouride and the sealing means further comprises a hermetic material formed over the thin film.

31. The method of claim 6 further comprising depositing a sacrificial layer over the array of movable mirrors prior to depositing said thin film backplane.

32. The method of claim 31, wherein depositing a sacrificial layer over the array of movable mirrors comprises depositing the thin film backplane over at least a portion of the sacrificial layer.

33. The display device of claim 1, wherein the interferometric modulator is organized into an array.