Aluminum fluoride films for microelectromechanical system applications

A microelectromechanical systems (MEMS) device utilizing an aluminum fluoride layer as an etch stop is disclosed. In one embodiment, a MEMS device includes a first electrode having a first surface; and a second electrode having a second surface facing the first surface and defining a gap therebetween. The second electrode is movable in the gap between a first position and a second position. At least one of the electrodes includes an aluminum fluoride layer facing the other of the electrodes. During fabrication of the MEMS device, a sacrificial layer is formed between the first and second electrodes and is released to define the gap. The aluminum fluoride layer serves as an etch stop to protect the first or second electrode during the release of the sacrificial layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to microelectromechanical systems devices and methods for making the same.

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. One type of MEMS device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In certain embodiments, an interferometric modulator may comprise a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed.

SUMMARY OF THE INVENTION

In one aspect, a microelectromechanical systems (MEMS) device is provided. The MEMS device includes a first electrode having a first surface and a second electrode having a second surface facing the first surface and defining a gap therebetween The second electrode is movable in the gap between a first position and a second position. The first position is a first distance from the first electrode. The second position is a second distance from the first electrode. The second distance is greater than the first distance. At least one of the electrodes comprises an aluminum fluoride layer facing the other of the electrodes.

In another aspect, an interferometric modulator is provided. The interferometric modulator includes transmissive means for at least partially transmitting incident light. The transmissive means has a first surface. The interferometric modulator also includes reflective means for substantially reflecting incident light. The reflective means has a second surface facing the first surface. Moving means is provided for moving the reflective means relative to the transmissive means between a driven position and an undriven position. The driven position is closer to the transmissive means than is the undriven position. At least one of the transmissive and reflective means comprises a layer formed of aluminum fluoride. The layer faces the other of the transmissive and reflective means.

In still another aspect, a method of making a microelectromechanical systems (MEMS) device is provided. The method includes forming a lower electrode. A sacrificial layer is formed over the lower electrode. An upper electrode is formed over the sacrificial layer. An aluminum fluoride layer is formed between forming the lower electrode and forming the upper electrode. The aluminum fluoride layer has a thickness of less than about 200 Å.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

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.

In fabricating a MEMS device, a sacrificial layer is formed between a moving electrode and a fixed electrode, and is released to form a cavity or gap therebetween. An etch stop is used to protect the moving or fixed electrodes during the release of the sacrificial layer. In embodiments of the invention, an interferometric modulator, which is an optical MEMS device, employs an aluminum fluoride (AlF3) layer as an etch stop for a moving or fixed electrode. The AlF3layer may also serve to prevent electrical shorts or to minimize stiction between the moving and fixed electrodes during operation.

One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated inFIG. 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.

The depicted portion of the pixel array inFIG. 1includes two adjacent interferometric modulators12aand12b. In the interferometric modulator12aon the left, a movable reflective layer14ais illustrated in a relaxed position at a predetermined distance from an optical stack16a, which includes a partially reflective layer. In the interferometric modulator12bon the right, the movable reflective layer14bis illustrated in an actuated position adjacent to the optical stack16b.

The optical stacks16aand16b(collectively referred to as optical stack16), as referenced herein, typically comprise of several fused layers, which can include an electrode layer, such as indium tin oxide (ITO), a partially reflective layer, such as chromium, and a transparent dielectric. The optical stack16is 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 substrate20. In some embodiments, the layers are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective layers14a,14bmay be formed as a series of parallel strips of a deposited metallic layer or layers (orthogonal to the row electrodes of16a,16b) deposited on top of posts18and an intervening sacrificial material deposited between the posts18. When the sacrificial material is etched away, the movable reflective layers14a,14bare separated from the optical stacks16a,16bby a defined gap or cavity19. A highly conductive and reflective material such as aluminum may be used for the reflective layers14, and these strips may form column electrodes in a display device.

With no applied voltage, the cavity19remains between the movable reflective layer14aand optical stack16a, with the movable reflective layer14ain a mechanically relaxed state, as illustrated by the pixel12ainFIG. 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 layer14is deformed and is forced against the optical stack16. A dielectric layer (not illustrated in this Figure) within the optical stack16may prevent shorting and control the separation distance between layers14and16, as illustrated by pixel12bon the right inFIG. 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 5illustrate one exemplary process and system for using an array of interferometric modulators in a display application.

FIG. 2is 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 processor21which 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 processor21may 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 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 row2in 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 5illustrate one possible actuation protocol for creating a display frame on the 3×3 array ofFIG. 2.FIG. 4illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves ofFIG. 3. In theFIG. 4embodiment, actuating a pixel involves setting the appropriate column to −Vbias, and the appropriate row to +ΔV, which may correspond to −5 volts and +5 volts respectively Relaxing the pixel is accomplished by setting the appropriate column to +Vbias, and the appropriate row to the same +ΔV, producing a zero volt potential difference across the pixel. In those rows where the row voltage is held at zero volts, the pixels are stable in whatever state they were originally in, regardless of whether the column is at +Vbias, or −Vbias. As is also illustrated inFIG. 4, it will be appreciated that voltages of opposite polarity than those described above can be used, e.g., actuating a pixel can involve setting the appropriate column to +Vbias, and the appropriate row to −ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −Vbias, and the appropriate row to the same Δ−V, producing a zero volt potential difference across the pixel.

In theFIG. 5Aframe, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) are actuated. To accomplish this, during a “line time” for row1, columns1and2are set to −5 volts, and column3is 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. Row1is 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 row2as desired, column2is set to −5 volts, and columns1and3are set to +5 volts. The same strobe applied to row2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again, no other pixels of the array are affected. Row3is similarly set by setting columns2and3to −5 volts, and column1to +5 volts. The row 3 strobe sets the row 3 pixels as shown inFIG. 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 ofFIG. 5A. It will be appreciated that the same procedure can be employed for arrays of dozens or hundreds of rows and columns. It will also be appreciated that the timing, sequence, and levels of voltages used to perform row and column actuation can be varied widely within the general principles outlined above, and the above example is exemplary only, and any actuation voltage method can be used with the systems and methods described herein.

FIGS. 6A and 6Bare system block diagrams illustrating an embodiment of a display device40. The display device40can be, for example, a cellular or mobile telephone. However, the same components of display device40or slight variations thereof are also illustrative of various types of display devices such as televisions and portable media players.

The display device40includes a housing41, a display30, an antenna43, a speaker45, an input device48, and a microphone46. The housing41is 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 housing41may 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 housing41includes removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols.

The display30of exemplary display device40may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display30includes 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 display30includes an interferometric modulator display, as described herein.

The components of one embodiment of exemplary display device40are schematically illustrated inFIG. 6B. The illustrated exemplary display device40includes a housing41and can include additional components at least partially enclosed therein. For example, in one embodiment, the exemplary display device40includes a network interface27that includes an antenna43which is coupled to a transceiver47. The transceiver47is connected to the processor21, which is connected to conditioning hardware52. The conditioning hardware52may be configured to condition a signal (e.g. filter a signal). The conditioning hardware52is connected to a speaker45and a microphone46. The processor21is also connected to an input device48and a driver controller29. The driver controller29is coupled to a frame buffer28and to the array driver22, which in turn is coupled to a display array30. A power supply50provides power to all components as required by the particular exemplary display device40design.

The network interface27includes the antenna43and the transceiver47so that the exemplary display device40can communicate with one or more devices over a network. In one embodiment the network interface27may also have some processing capabilities to relieve requirements of the processor21. The antenna43is any antenna known to those of skill in the art for transmitting and receiving signals. In one embodiment, the antenna transmits and receives RF signals according to the IEEE 802.11 standard, including IEEE 802.11(a), (b), or (g). In another embodiment, the antenna transmits and receives RF signals according to the BLUETOOTH standard. In the case of a cellular telephone, the antenna is designed to receive CDMA, GSM, AMPS or other known signals that are used to communicate within a wireless cell phone network. The transceiver47pre-processes the signals received from the antenna43so that they may be received by and further manipulated by the processor21. The transceiver47also processes signals received from the processor21so that they may be transmitted from the exemplary display device40via the antenna43.

In an alternative embodiment, the transceiver47can be replaced by a receiver. In yet another alternative embodiment, network interface27can be replaced by an image source, which can store or generate image data to be sent to the processor21. 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.

The processor21generally controls the overall operation of the exemplary display device40. The processor21receives data, such as compressed image data from the network interface27or an image source, and processes the data into raw image data or into a format that is readily processed into raw image data. The processor21then sends the processed data to the driver controller29or to the frame buffer28for storage. Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation, and gray-scale level.

In one embodiment, the processor21includes a microcontroller, CPU, or logic unit to control operation of the exemplary display device40. The conditioning hardware52generally includes amplifiers and filters for transmitting signals to the speaker45, and for receiving signals from the microphone46. The conditioning hardware52may be discrete components within the exemplary display device40, or may be incorporated within the processor21or other components.

The driver controller29takes the raw image data generated by the processor21either directly from the processor21or from the frame buffer28and reformats the raw image data appropriately for high speed transmission to the array driver22. Specifically, the driver controller29reformats 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 array30. Then the driver controller29sends the formatted information to the array driver22. Although a driver controller29, such as a LCD controller, is often associated with the system processor21as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. They may be embedded in the processor21as hardware, embedded in the processor21as software, or fully integrated in hardware with the array driver22.

Typically, the array driver22receives the formatted information from the driver controller29and reformats the video data into a parallel set of waveforms that are applied many times per second to the hundreds and sometimes thousands of leads coming from the display's x-y matrix of pixels.

In one embodiment, the driver controller29, array driver22, and display array30are appropriate for any of the types of displays described herein. For example, in one embodiment, the driver controller29is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, the array driver22is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, the driver controller29is integrated with the array driver22. Such an embodiment is common in highly integrated systems such as cellular phones, watches, and other small area displays. In yet another embodiment, the display array30is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators).

The input device48allows a user to control the operation of the exemplary display device40. In one embodiment, the input device48includes a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a touch-sensitive screen, a pressure- or heat-sensitive membrane. In one embodiment, the microphone46is an input device for the exemplary display device40. When the microphone46is used to input data to the device, voice commands may be provided by a user for controlling operations of the exemplary display device40.

The power supply50can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, the power supply50is a rechargeable battery, such as a nickel-cadmium battery or a lithium ion battery. In another embodiment, the power supply50is a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell, and solar-cell paint. In another embodiment, the power supply50is configured to receive power from a wall outlet.

In some implementations control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some cases control programmability resides in the array driver22. Those of skill in the art will recognize that the above-described optimization may be implemented in any number of hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,FIGS. 7A-7Eillustrate five different embodiments of the movable reflective layer14and its supporting structures.FIG. 7Ais a cross section of the embodiment ofFIG. 1, where a strip of metal material14is deposited on orthogonally extending supports18. InFIG. 7B, the movable reflective layer14is attached to supports at the corners only, on tethers32. InFIG. 7C, the movable reflective layer14is suspended from a deformable layer34, which may comprise a flexible metal. The deformable layer34connects, directly or indirectly, to the substrate20at various locations. The connections are herein referred to as support structures or posts18. The embodiment illustrated inFIG. 7Dhas support structures18including support post plugs42upon which the deformable layer34rests. The movable reflective layer14remains suspended over the cavity, as inFIGS. 7A-7C, but the deformable layer34does not form the support posts18by filling holes between the deformable layer34and the optical stack16. Rather, the support posts18are formed of a planarization material, which is used to form support post plugs42. The embodiment illustrated inFIG. 7Eis based on the embodiment shown inFIG. 7D, but may also be adapted to work with any of the embodiments illustrated inFIGS. 7A-7Cas well as additional embodiments not shown. In the embodiment shown inFIG. 7E, an extra layer of metal or other conductive material has been used to form a bus structure44. 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 substrate20.

In embodiments such as those shown inFIG. 7, the interferometric modulators function as direct-view devices, in which images are viewed from the front side of the transparent substrate20, the side opposite to that upon which the moving electrode is arranged. In these embodiments, the reflective layer14optically shields some portions of the interferometric modulator on the side of the reflective layer opposite the substrate20, including the deformable layer34and the bus structure44. This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. This separable modulator architecture allows the structural design and materials used for the electromechanical aspects and the optical aspects of the modulator to be selected and to function independently of each other. Moreover, the embodiments shown inFIGS. 7C-7Ehave additional benefits deriving from the decoupling of the optical properties of the reflective layer14from its mechanical properties, which are carried out by the deformable layer34. This allows the structural design and materials used for the reflective layer14to be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer34to be optimized with respect to desired mechanical properties.

Use of Aluminum Fluoride Layer

A microelectromechanical systems (MEMS) device includes a fixed (lower) electrode and a moving (upper) electrode. The fixed and moving electrodes face each other with a gap or cavity therebetween. The moving electrode is movable in the gap between a first (driven) position and a second (undriven) position. The first position is a first distance from the fixed electrode. The second position is a second distance from the fixed electrode. The second distance is greater than the first distance.

In one embodiment, a MEMS device is fabricated as follows. First, a fixed electrode is formed over a substrate. The fixed electrode typically includes a conductive layer(s) and a dielectric layer(s) stacked over the conductive layer. Subsequently, a sacrificial layer is formed over the dielectric layer. Then, support structures such as posts are formed through the sacrificial layer. In addition, a moving electrode is formed over the sacrificial layer and the posts. Then, the sacrificial layer is released or etched away to define the gap or cavity between the fixed and moving electrodes. In other embodiments, the sequence of forming the layers and posts may vary widely depending on the structure of the MEMS device.

In the process described above, an etch stop layer is preferably used to protect the fixed and/or moving electrodes during a release etch of the sacrificial layer. In one embodiment, an etch stop layer may be formed between the fixed electrode and the sacrificial layer. In another embodiment, an etch stop layer may be formed between the moving electrode and the sacrificial layer. In yet another embodiment, etch stop layers may be interposed between the fixed electrode and the sacrificial layer and between the moving electrode and the sacrificial layer.

In some embodiments, after the release of the sacrificial layer, the etch stop layer remains in the MEMS device, forming part of the final device structure. The etch stop layer may be formed of an insulating material, thereby preventing electrical shorts between the fixed and moving electrodes during operation. In certain embodiments, it may serve to minimize stiction between surfaces of the fixed and moving electrodes during operation. “Stiction,” as used herein, refers to a tendency of a moving electrode in an actuated position to stick to a fixed electrode in a MEMS device.

In one embodiment, an aluminum fluoride (AlF3) layer is used as an etch stop during a release etch of a sacrificial layer. The sacrificial layer may be formed of molybdenum, silicon or tungsten. A fluorine-based etchant may be used for the release etch of the sacrificial layer. The aluminum fluoride layer may also serve to prevent electrical shorts or to minimize stiction between fixed and moving electrodes during operation of a MEMS device.

While embodiments in this disclosure are illustrated in the context of optical MEMS devices, particularly interferometric modulators, the skilled artisan will appreciate that the configurations of the AlF3layer described below may apply to other MEMS devices, such as electromechanical capacitive switches.

FIG. 8is a schematic illustration of an interferometric modulator80including an AlF3layer according to an embodiment. The interferometric modulator80has a fixed electrode81(preferably at least partially transparent for the illustrated embodiment) and a moving electrode82(preferably reflective for the illustrated embodiment) which is supported by support posts84. The interferometric modulator80has a cavity85between the fixed and moving electrodes81,82. The fixed electrode81includes an aluminum fluoride (AlF3) layer83having a surface exposed to the cavity85and facing the moving electrode82.

In the illustrated embodiment, the moving electrode82of the interferometric modulator80is in a relaxed position. In the relaxed position, the moving electrode82is at a relative large distance (e.g., 100 nm to 600 nm) from the fixed electrode81. The distance between the electrodes81and82depends on desired color. The moving electrode82can move down to an actuated position (seeFIG. 1, modulator12b). In the actuated position, the moving electrode82is positioned more closely adjacent to the fixed electrode81, and may be in contact with a top surface83aof the AlF3layer83.

The illustrated fixed electrode81overlies a transparent substrate20, and includes a transparent conductor such as the illustrated indium tin oxide (ITO) layer16coverlying the substrate20, a metallic absorber layer16doverlying the ITO layer16c, and a dielectric layer16eoverlying the absorber layer16d. The absorber layer16dis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the absorber layer16dmay be formed of a semiconductor layer. The semiconductor layer is preferably formed of germanium. The dielectric layer16eis preferably formed of silicon dioxide and serves to prevent the two electrodes from electrically shorting during operation. In certain embodiments, the dielectric layer16emay be omitted.

In one embodiment, the fixed electrode81further includes the aluminum fluoride (AlF3) layer83overlying the dielectric layer16e. The AlF3layer83can serve as an etch stop layer during a “release” etch of a sacrificial layer that defines the cavity85between the electrodes81,82, as will be better appreciated from the description ofFIG. 10below. In addition, the AlF3layer83may serve as a dielectric layer to prevent the two electrodes81,82from shorting during operation. The AlF3layer may also serve to minimize stiction between the fixed and moving electrodes81,82during operation. In certain embodiments, the aluminum fluoride layer may replace the dielectric layer16e, directly overlying the absorber layer16d.

In one embodiment, the ITO layer16cmay have a thickness between about 100 Å and about 800 Å. The absorber layer16dmay have a thickness between about 1 Å and about 50 Å, preferably between about 10 Å and about 40 Å. In certain embodiments, the absorber layer may be omitted. The dielectric layer16emay have a thickness between about 100 Å and about 1,600 Å. The AlF3layer83may have a thickness between about 50 Å and about 200 Å, preferably between about 70 Å and about 150 Å. Together, the layers define an optical stack or fixed electrode81.

In the illustrated embodiment, the moving electrode82includes a reflective layer or mirror82aand a mechanical or deformable layer82b. In the illustrated embodiment, the reflective layer82ais attached or fused to the deformable layer82b; in other arrangements, the reflective layer or mirror may be suspended from the deformable layer (see, e.g.,FIGS. 7C-7E). The reflective layer82ais preferably formed of a reflective metal, preferably, Al, Au, Ag, or an alloy of the foregoing, and is thick enough to reflect light incident upon the substrate for interferometric effect. The deformable layer82bis preferably formed of nickel. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the moving electrode may include a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing. The deformable layer82bpreferably has a thickness that is sufficient to provide mechanical support while being sufficiently thin and flexible to allow the moving electrode82to move toward the fixed electrode81. The deformable layer82bmay have a thickness on the order of several thousand angstroms. In an exemplary embodiment, the reflective layer82ahas a thickness of about 300 Å, and the deformable layer82bhas a thickness of about 1000 Å. The thicknesses of the layers82aand82bcan be different in other embodiments.

The support posts84are configured to support the moving electrode82. The posts84can be made of a number of materials, but in the illustrated embodiment are formed of an inorganic dielectric material, such as silicon nitride, silicon dioxide or aluminum oxide. The reflective layer82ais configured to cover top surfaces of the support posts84, as shown inFIG. 8. In other arrangements, the support posts can include a “rivet” formed in the depression above the deformable layer82b. In certain embodiments, the reflective layer may be tethered to the support posts, as shown inFIG. 7B.

FIG. 9illustrates an interferometric modulator90including an AlF3layer according to another embodiment. The interferometric modulator90has a fixed electrode91and a moving electrode92supported by support posts94. The interferometric modulator90has a cavity95between the fixed and moving electrodes91,92. The moving electrode92includes an AlF3layer93having a surface exposed to the cavity95and facing the fixed electrode91.

InFIG. 9, the moving electrode92of the interferometric modulator90is in a relaxed position. In the relaxed position, the moving electrode92is at a relative large distance from the fixed electrode91. The moving electrode92can move down to an actuated position (not shown). In the actuated position, the moving electrode92is positioned more closely adjacent to the fixed electrode91, and may be in contact with a top surface91aof the fixed electrode91.

The fixed electrode91overlies a transparent substrate20, and includes an indium tin oxide (ITO) layer16coverlying the substrate20, a metallic absorber layer16doverlying the ITO layer16c, a first dielectric layer16eoverlying the absorber layer16d, and a second dielectric layer16foverlying the first dielectric layer16e. The absorber layer16dis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the absorber layer16dmay be formed of a semiconductor layer. The semiconductor layer is preferably formed of germanium. The first dielectric layer16emay be formed of silicon dioxide. The second dielectric layer16fmay be formed of aluminum oxide and may serve as an etch stop during a release etch of a sacrificial layer, as will be better appreciated from the description ofFIG. 11below. In certain embodiments, either or both of the dielectric layers16eand16fmay be omitted. In one embodiment, the ITO layer16cmay have a thickness between about 100 Å and about 800 Å. The absorber layer16dmay have a semitransparent thickness, preferably between about 1 Å and about 50 Å, more preferably between about 10 Å and about 40 Å. The overall thickness of the first and second dielectric layers16eand16fmay be between about 100 Å and about 1,600 Å. in other embodiments, the thicknesses of the dielectric layers16eand16fmay be adjusted such that the fixed electrode91is a color filter.

The moving electrode92may include a reflective layer92aand a deformable layer92b. In the illustrated embodiment, the reflective layer92ais preferably formed of a reflective metal, such as Al, Au, Ag, or an alloy of the foregoing. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the moving electrode92may be formed of a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing. The deformable layer92bis preferably formed of nickel. The layers92aand92bcan have thicknesses as described above with respect to the layers82aand82bofFIG. 8.

In the illustrated embodiment, the moving electrode92further includes an AlF3layer having a surface exposed to the cavity95. The AlF3layer93forms part of the moving electrode92and may serve to prevent the moving and fixed electrodes91,92from electrically shorting during operation. In addition, the AlF3layer93may serve to minimize stiction between the electrodes during operation. The AlF3layer93may have a thickness between about 50 Å and about 200 Å, preferably between about 70 Å and about 150 Å.

The support posts94are configured to support the moving electrode92, and are preferably formed of a dielectric material. The support posts94can be as described above with respect to the support post84ofFIG. 8. The deformable layer92b, which is preferably formed of nickel, covers a top surface of the reflective layer92a, as shown inFIG. 9. In other embodiments, the reflective layer may be suspended from the deformable layer, as shown inFIGS. 7C-7E. In certain embodiments, the reflective layer may be tethered to the support posts, as shown inFIG. 7B.

In an unpictured embodiment, an interferometric modulator has a moving electrode and a fixed electrode, both of which have an AlF3layer. Each AlF3layer is exposed to a cavity between the electrodes, facing the other electrode. The configurations of the electrodes and the AlF3layers can be as described above with respect to those of the interferometric modulators ofFIGS. 8 and 9.

The interferometric modulators of the above embodiments are described by way of example. The AlF3layers in the embodiments may generally apply to microelectromechanical devices which have electrodes different from those shown. A skilled artisan will appreciate that electrode structure and configuration may be varied depending on the design of a given microelectromechanical device.

Method of Making an Interferometric Modulator

FIGS. 10A-10Fillustrate a method of making the interferometric modulator ofFIG. 8according to an embodiment. In the method, an AlF3layer is formed on a fixed electrode, facing a moving electrode.

InFIG. 10A, an optical stack is provided over a transparent substrate100. In the illustrated embodiment, the optical stack has a transparent conductor in the form of an ITO layer101aoverlying the substrate100, an absorber layer101boverlying the ITO layer101a, and a dielectric layer101coverlying the absorber layer101b. The absorber layer101bis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the absorber layer101bmay be formed of a semiconductor layer. The semiconductor layer is preferably formed of germanium. The dielectric layer101cmay be formed of silicon dioxide. The layers101a-101cmay have thicknesses as described above with respect to the layers16c-16eofFIG. 8. In a process not shown here, the ITO layer101aand the absorber layer101bare patterned and etched to form electrode lines or other useful shapes as called for by the display design. By convention, the lower electrodes defined by the optical stack will be referred to as row electrodes.

An aluminum fluoride (AlF3) layer102is formed on the dielectric layer101c, as shown inFIG. 10B. In the illustrated embodiment, the AlF3layer102has a thickness between about 50 Å and about 200 Å, preferably between about 70 Å and about 150 Å. The AlF3layer102will serve as an etch stop during a release step which will be described later. The AlF3layer102may also serve as a dielectric layer together with the dielectric layer101cduring operation of the interferometric modulator. In certain embodiments, the AlF3layer102may replace the dielectric layer101c. In such embodiments, the AlF3layer may have a thickness between about 50 Å and about 150 Å.

In one embodiment, the AlF3layer102may be formed by a physical vapor deposition (PVD) process. An exemplary PVD process is a sputtering deposition process. In the sputtering deposition process, the substrate100having the layers101a-101cformed thereon is placed in a PVD chamber. A solid target formed of AlF3is also placed in the chamber. During sputtering deposition, an ion beam is irradiated onto the target. Upon irradiation of the ion beam, AlF3is sputtered from the target onto a top surface of the dielectric layer101c. In one embodiment, the sputtering process is performed in an argon (Ar) atmosphere.

Subsequently, a sacrificial layer104is formed over the AlF3layer102, as shown inFIG. 10C. The sacrificial layer104is preferably formed of a material capable of selective removal without harm to other materials that define the cavity. In the illustrated embodiment, the sacrificial layer104is formed of molybdenum. Other examples of sacrificial materials susceptible to selective removal by fluorine-based etchants include silicon and tungsten.

Next, steps for forming support posts and a moving electrode are performed. A lithographic process, preferably a photolithographic process, is performed to pattern the sacrificial layer104to provide recesses for support posts105. A photoresist is provided and patterned over the sacrificial layer104. Then, the sacrificial layer104is etched using a dry or wet etch process, preferably using a fluorine-based dry etchant such as SF6/O2, CF4/O2, or NF3, or a chlorine-based etchant such as Cl2/BCl3. The photoresist is then stripped. Then, a material for the posts105, preferably an inorganic dielectric material such as silicon dioxide, is deposited over exposed surfaces, including surfaces of the sacrificial layer104. Subsequently, the material for the posts105is etched back or patterned to form posts, using a suitable etch process, including a wet or dry etch process, as shown inFIG. 10D. While illustrated schematically as merely filling the opening in the sacrificial layer104, it will be understood that the upper end of the posts may be wider than the opening, extending over an upper surface of the sacrificial layer104.

A reflective layer106is then deposited over the sacrificial layer104and the support posts105, as shown inFIG. 10E. The reflective layer106is preferably formed of a specular metal such as Al, Au, Ag, or an alloy of the foregoing. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the layer106may be formed of a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing.

Then, a material for a mechanical or deformable layer107is deposited over the reflective layer106, as shown inFIG. 10E. The material for the deform able layer107is preferably nickel. Then, the aluminum and nickel layers106and107are patterned and etched to define arrays of MEMS devices. In certain embodiments, the deformable layer107and the reflective layer106are etched to provide through-holes (not shown) in the middle. The etch process can be either a wet or dry etch process. The holes serve to permit etchant to enter and etch by-product to exit at a release step which will be later described. In addition, the holes provide an exit for air when the reflective layer106moves between the relaxed and actuated positions.FIG. 10Eillustrates a cross-section of a completed “unreleased” interferometric modulator structure with the sacrificial layer104in place.

In an unpictured embodiment, after a sacrificial layer is formed on an optical stack, a reflective layer is formed and patterned thereon. Subsequently, another sacrificial layer is deposited over the reflective layer. Then, the sacrificial layers are patterned to provide recesses for support posts, and the support posts are formed. Subsequently, a deformable layer is formed over the second sacrificial layer and the support posts. This process provides a deformable layer from which the reflective layer can be suspended, as described above with reference toFIGS. 7C-7E. Although not illustrated, a skilled artisan will appreciate that different steps may be performed to form electrode structures having options such as a tethered moving electrode, as shown inFIG. 7B.

Finally, the sacrificial layer104is selectively removed, leaving a cavity or gap109between the reflective layer106and the AlF3layer102, as shown inFIG. 10F. This step is referred to as a “release” or “sacrificial etch” step. The illustrated sacrificial layer104which is formed of molybdenum is preferably etched using a fluorine-based etchant, for example, a XeF2-based etchant, which selectively etches molybdenum without attacking other exposed materials (AlF3, SiO2, Al, etc.) that define the cavity109. A resulting “released” MEMS device, particularly interferometric modulator, is shown inFIG. 10F. Although not illustrated, a skilled artisan will appreciate that different steps may be performed to form electrode structures having options such as tethered or suspended moving electrode, as shown inFIGS. 7B-7E.

FIGS. 11A-11Eillustrate a method of making the interferometric modulator ofFIG. 9according to another embodiment. In the method, an AlF3layer is formed on a moving electrode beneath a reflective layer, facing a fixed electrode.

InFIG. 11A, an optical stack is provided over a transparent substrate110. In the illustrated embodiment, the optical stack has a transparent conductor in the form of an ITO layer111aoverlying the substrate110, a metallic absorber layer111boverlying the ITO layer111a, a first dielectric layer111coverlying the absorber layer111, and a second dielectric layer111doverlying the first dielectric layer111c. The absorber layer111bis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the absorber layer111bmay be formed of a semiconductor layer. The semiconductor layer is preferably formed of germanium. The first dielectric layer111cmay be formed of silicon dioxide. The second dielectric layer111dmay be formed of aluminum oxide and may serve as an etch stop layer during a release etch of a sacrificial layer overlying the second dielectric layer111d. The layers111a-111dmay have thicknesses as described above with respect to the layers16c-16fofFIG. 9. The use of the second dielectric layer111dis optional, such that, in certain embodiments, the optical stack may have only one dielectric layer, depending on materials, etchants and selectivity of a release etch which will be described later.

Subsequently, a sacrificial layer114is formed on the second dielectric layer111d, as shown inFIG. 11A. In the illustrated embodiment, the sacrificial layer114is formed of molybdenum. Other examples of sacrificial materials include silicon and tungsten. A thickness of the sacrificial layer114corresponds to a height of a relaxed MEMS device cavity (the cavity is typically larger than the thickness of the sacrificial layer114due to the mechanical layer's launch angle from the supports and inherent tension), and thus determines color displayed by the MEMS device during operation. In other arrangements, multiple thicknesses of sacrificial material are provided across the array for forming interferometric modulators for different colors, as will be better understood from the description below ofFIGS. 12A-12H.

Next, a step for forming support posts115is performed. A photolithographic process is performed to pattern the sacrificial layer114for forming recesses for the support posts115. While illustrated as having vertical sidewalls, in reality the recesses may have sloped sidewalls. A photoresist is provided and patterned over the sacrificial layer114. Then, the sacrificial layer114is etched using a dry or wet etch process, preferably using a fluorine-based dry etchant such as SF6/O2, CF4/O2, or NF3, or a chlorine-based etchant such as Cl2/BCl3. The photoresist is then stripped. Then, a material for the posts115, preferably an inorganic dielectric material such as silicon dioxide, is deposited over exposed surfaces, including surfaces of the sacrificial layer114. Subsequently, the material for the posts115is etched back or patterned to form the posts115, using a suitable etch process, including a wet or dry etch process. While illustrated schematically as being etched back to fill only the support recesses, instead a photoresist mask can be used to pattern the posts115with a wider upper portion or wings overlying the top surface of the sacrificial layer114.

Next, an AlF3layer112is deposited on the sacrificial layer114and the posts115. The AlF3layer112preferably has a thickness of between about 50 Å and about 200 Å, preferably between about 70 Å and about 150 Å. AlF3may be deposited using a PVD process as described above with respect to the AlF3layer ofFIG. 10B.

Subsequently, a reflective layer116is deposited over the AlF3layer112, as shown inFIG. 11D. The reflective layer116is preferably formed of a specular metal such as Al, Au, Ag, or an alloy of the foregoing. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the layer116may be formed of a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing.

Next, a material for a mechanical or deformable layer117is deposited over the reflective layer116, as shown inFIG. 11D. The material is preferably nickel. In certain embodiments, the deformable layer116and the reflective layer115are etched to provide through-holes in the middle. The etch process can be either a wet or dry etch process. The holes serve to permit etchant to enter and etch by-product to exit at a release step which will be later described. In addition, the holes provide an exit for air when the reflective layer moves between the relaxed and actuated positions.FIG. 11Dillustrates a cross-section of a completed “unreleased” interferometric modulator structure with the sacrificial layer in place.

In an unpictured embodiment, after a sacrificial layer is formed on an optical stack, a reflective layer is formed and patterned thereon. Subsequently, another sacrificial layer is deposited over the reflective layer. Then, the sacrificial layers are patterned to provide recesses for support posts, and the support posts are formed. Subsequently, a deformable layer is formed over the second sacrificial layer and the support posts. This process provides a deformable layer from which the reflective layer can be suspended, as described above with reference toFIGS. 7C-7E. Although not illustrated, a skilled artisan will appreciate that different steps may be performed to form electrode structures having options such as a tethered moving electrode, as shown inFIG. 7B.

Finally, the sacrificial layer114is selectively removed, leaving a cavity or gap119between the second dielectric layer111dand the AlF3layer112, as shown inFIG. 11E. The illustrated sacrificial layer114which is formed of molybdenum is preferably etched using a fluorine-based etchant such as a XeF2-based etchant. The AlF3layer112protects the critical mirror surface of the reflective layer116from etch damage while the sacrificial layer114is removed. A resulting MEMS device, particularly a released interferometric modulator, is shown inFIG. 11E. In addition, in the resulting MEMS device, the AlF3layer112may serve to minimize stiction between the moving and fixed electrodes.

In an unpictured embodiment, an interferometric modulator is formed to have a moving electrode and a fixed electrode, both of which have an AlF3layer. Each AlF3layer is exposed to a cavity between the electrodes, facing the other electrode. First, an optical stack is provided over a transparent substrate. Then, an AlF3layer is formed on the optical stack. Subsequently, a sacrificial layer is formed on the AlF3layer. Then, a step for forming support posts is performed. Another AlF3layer is formed on the sacrificial layer. Then, a reflective layer is deposited on the other AlF3layer. Next, a material for a mechanical or deformable layer is deposited on the reflective layer. Finally, the sacrificial layer is released, forming a cavity between the AlF3layers. Details about forming the layers are as described above with reference toFIGS. 10 and 11.

FIGS. 12A-12Hare cross-sectional views illustrating a method of making an array of interferometric modulators according to another embodiment. The method utilizes a lower and upper etch stop layers stacked over each other to protect a fixed electrode or optical stack from exposure to multiple etch steps. In one embodiment, the lower etch stop layer is preferably formed of aluminum fluoride. Generally, the upper etch stop layer can be used to protect the lower etch stop layer from etchants used to pattern layers formed above the upper etch stop layer. In one embodiment, the layers above the upper etch stop layer to be patterned are multiple sacrificial material layers used to form various gap heights of an interferometric modulator device. This upper etch stop layer is eventually at least partially removed in a release etch process, during which the lower etch stop layer then serves to protect underlying layers from the etch release process.

InFIGS. 12A-12H, the formation of an array of three interferometric modulators including a red subpixel200, a green subpixel210, and a blue subpixel220(FIG. 12H) will be illustrated. Each of the interferometric modulators200,210,220has a different distance in the relaxed state between a lower electrode121a,121band an upper metal mirror layer126a,126b,126c, as indicated inFIG. 12H, which shows final configurations. Color displays may be formed by using three (or more) modulator elements to form each pixel in the resulting image. The dimensions of each interferometric modulator cavity (e.g., the cavities129a,129b,129cinFIG. 12H) determine the nature of the interference and the resulting color.

One method of forming color pixels is to construct arrays of interferometric modulators, each having cavities of differing sizes, e.g., three different sizes corresponding to red, green and blue as shown in this embodiment. The interference properties of the cavities are directly affected by their dimensions. In order to create these varying cavity dimensions, multiple sacrificial layers may be fabricated and patterned as described below so that the resulting pixels reflect light corresponding to each of the three primary colors. Other color combinations are also possible, as well as the use of black and white pixels.

FIG. 12Aillustrates an optical stack150similar to those previously discussed (e.g., the optical stack101a-101cand102ofFIGS. 10A-10F) formed by first creating an electrode layer by depositing an indium tin oxide (ITO) electrode layer121aon a transparent substrate120, then depositing a metallic absorber layer121bon the indium tin oxide electrode layer121a, forming a composite layer which will be referred to as the lower electrode layer121a,121b. In the illustrated embodiment, the absorber layer121bcomprises chromium. Other metals, such as molybdenum and titanium, or semiconductors such as germanium, may also be used to form the absorber layer121b.

The viewing surface120aof the transparent substrate120is on the opposite side of the substrate120from the lower electrode layer121a,121b. In a process not shown here, the lower electrode layer121a,121bis patterned and etched to form electrode lines (e.g., row electrodes) or other useful shapes as required by the display design.

As indicated inFIG. 12A, the optical stack150also includes a dielectric layer121c, which may comprise, for example, silicon oxide or a charge trapping layer, such as silicon nitride, over the lower electrode layer121a,121b. The dielectric layer121cis typically formed after the lower electrode layer121a,121bhas been patterned and etched. In addition, the optical stack150includes a first etch stop or barrier layer122over the dielectric or charge trapping layer121c. In one embodiment, the first etch stop layer122preferably comprises aluminum fluoride (AlF3). A second etch stop or barrier layer123is deposited over the first etch stop layer122. In various embodiments, the second etch stop layer123comprises silicon oxide, silicon nitride, molybdenum, titanium, or amorphous silicon, depending on the choice of the overlying sacrificial layers.

FIG. 12Afurther illustrates a first pixel sacrificial layer124aformed by depositing molybdenum (in the illustrated embodiment) over the optical stack150(and thus over the first and second etch stop layers122,123, the dielectric layer121c, and the lower electrode layer121a,121b). In other arrangements, the sacrificial material can be, e.g., titanium or amorphous silicon, but in any event is selected to be different from and selectively etchable relative to the second etch stop layer123directly below. The molybdenum of the illustrated embodiment is etched to form the first pixel sacrificial layer124a, thereby exposing a portion123aof the second etch stop layer123, which overlies a corresponding portion of the first etch stop layer122that will ultimately be included in the resulting green and blue interferometric modulators210,220(FIG. 12H). The thickness of the first sacrificial layer124a(along with the thicknesses of subsequently deposited layers as described below) influences the size of the corresponding cavity119a(FIG. 12H) in the resulting interferometric modulator200. The etchant used to remove a portion of first sacrificial layer124ais preferably chosen so as to not etch the second etch stop layer123, or to etch it at a much lower rate than the sacrificial layer124a. Thus, although the portion123aof the second etch stop layer123is exposed, it is preferably as unaffected by these etchants as is possible. An exemplary etchant is a phosphoric/acetic/nitric acid or “PAN” etchant, which selectively removes molybdenum relative to the material of the second etch stop layer123(e.g., silicon oxide, silicon nitride, titanium or amorphous silicon).

FIGS. 12B-12Cillustrate forming a second pixel sacrificial layer124bby deposition, masking and patterning over the exposed portion123aof the second etch stop layer123and the first pixel sacrificial layer124a. The second pixel sacrificial layer124bpreferably comprises the same sacrificial material as the first pixel sacrificial layer124a(molybdenum in this embodiment). Accordingly, the same selective etch chemistry can be employed. The second pixel sacrificial layer124bis patterned and etched as illustrated inFIG. 12Cto expose a portion123bof the second etch stop layer123which overlies a corresponding portion of the first etch stop layer122that will ultimately be included in the resulting blue interferometric modulator220(FIG. 12H).

A third pixel sacrificial layer124cis then deposited over the exposed portion123bof the second etch stop layer123and the second pixel sacrificial layer124bas illustrated inFIG. 12D. The third pixel sacrificial layer124cneed not be patterned or etched in this embodiment, since its thickness will influence the sizes of all three cavities119a,119b,119cin the resulting interferometric modulators200,210,220(FIG. 12H). The three deposited pixel sacrificial layers124a,124b,124cdo not necessarily have the same thickness.

FIG. 12Eillustrates forming a reflective layer126by depositing a layer of aluminum-containing metal over the third pixel sacrificial layer124c. In the illustrated embodiment, the reflective layer126also serves as an electrode. Although the foregoing description refers to certain exemplary materials for the fabrication of the various layers illustrated inFIG. 12, it will be understood that other materials may also be used, e.g., as described elsewhere in this application.

FIG. 12Fillustrates an intermediate stage of the fabrication process, wherein the reflective layer126has been etched to form mirror portions126a,126b,126c, and an additional layer124dof sacrificial material has been deposited above the mirror portions126a,126b,126c. The mirror portions126a,126b,126care preferably formed of a specular metal such as Al, Au, Ag, or an alloy of the foregoing. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the mirror portions126a,126b,126cmay be formed of a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing. Pockets of sacrificial material124a,124b,124c,124dexist between and around the optical stack150and the mirror portions126a,126b,126c. These pockets are separated by support posts125a,125b,125c,125d.

In addition, connectors128are formed through the additional layer124dof sacrificial material. In one embodiment, the connectors128may be simultaneously formed with the support posts125a,125b,125c,125d. In other embodiments, the connectors128may be formed prior to or subsequent to forming the support posts125a,125b,125c,125d.

Subsequently, a mechanical layer127is formed over the support posts125a,125b,125c,125d, the connectors128, and the additional layer124dof sacrificial material. The mechanical layer is preferably formed of nickel.FIG. 12Fillustrates a patterned mechanical layer127which forms multiple column electrodes. In certain embodiments, the mechanical layer127is patterned along with the columns to provide through-holes (not shown). The patterning process can include either a wet or dry etch process. The holes serve to permit etchant to enter and etch by-product to exit at a release step which will be described below. In addition, the holes provide an exit for air when the mechanical layer127moves between the relaxed and actuated positions.FIG. 12Fillustrates a cross-section of completed “unreleased” interferometric modulators with the sacrificial layers in place.

FIG. 12Gillustrates removing the sacrificial layers124a,124b,124c,124dto form the cavities129a,129b,129c, thereby exposing the second etch stop layer123underlying the mirror portions126a,126b,126c. In the illustrated embodiment, gaseous or vaporous XeF2is used as an etchant to remove the molybdenum sacrificial layers124a,124b,124c,124d. It will be understood that XeF2may serve as a source of fluorine-containing gases, and F2or HF (or other fluorine sources) may be used in place of or in addition to XeF2as an etchant for the preferred sacrificial materials.

The exposed portions123of the second etch stop layer123and the sacrificial layers124a,124b,124c,124dwill typically be at least partially removed by the release etch. For example, a very thin SiO2etch stop layer such as123may be removed by a XeF2etchant used to remove a molybdenum sacrificial layer. The same is true of silicon nitride, titanium, and amorphous silicon. Typically, all of the second etch stop layer123is removed from over the first etch stop layer122in the cavity regions129a,129b,129c, as shown inFIG. 12H. The second etch stop layer123located outside of the cavities, underneath the support posts125a,125b,125c,125dhas not been removed by the etch, as can be seen inFIG. 12H. However, some of the second etch stop layer123may remain even in the cavity areas after the release etching process (not shown inFIG. 12H). Any remaining second etch stop layer123is transparent and so thin as to not affect optical properties of the device. Additionally, any remaining second etch stop layer123will typically have a non-uniform thickness, due to differential exposure to the etchants during removal of differential thicknesses of sacrificial material. In a further embodiment, a second etchant is used to remove the second etch stop layer123.

A comparison ofFIGS. 12H and 12Eillustrates that the size of the red pixel cavity129a(FIG. 12H) corresponds to the combined thicknesses of the three sacrificial layers124a,124b,124c. Likewise, the size of the green pixel cavity129bcorresponds to the combined thicknesses of the two sacrificial layers124b,124c, and the size of the blue pixel cavity129ccorresponds to the thickness of the third sacrificial layer124c. Thus, the dimensions of the cavities129a,129b,129cvary according to the various combined thicknesses of the three layers124a,124b,124c, resulting in an array of interferometric modulators200,210,220capable of displaying three different colors such as red, green and blue. In certain embodiments, additional etch stops may be interposed between two of the sacrificial layers124a,124b,124c.

As discussed above, portions of the second etch stop layer123will be exposed to a greater amount of etchant than other portions of the second etch stop layer123. This is due to the repeated deposition and etching of bulk sacrificial layer, as discussed above and depicted inFIGS. 12A-12E. While the etchant used in the pattern etching of sacrificial layers124aand124bis preferably selected to have as minimal an effect as possible on the second etch stop layer123, the etchant may have some undesirable effects on the layer123. Thus, by the stage of the process depicted inFIG. 12G, immediately prior to removal of the bulk sacrificial material through an etching process, the second etch stop layer123may have varying properties or height in different locations as a result of variations in etchant exposure. However, because the second etch stop layer123is thin and transparent or completely removed from the cavities during the subsequent release etch, these variations will have a minimal effect on the optical or electromechanical behavior of the finished MEMS device. Because of the protection provided by this second etch stop layer123, the first etch stop layer122formed of AlF3, which in certain embodiments is intended to form a part of the finished MEMS device, will be exposed to only a single etching process (the release etch), which typically is highly selective and will not attack AlF3, and variations in the properties of layer122can be minimized.

Importantly, the first etch stop layer122protects the underlying dielectric (e.g., SiO2) or charge trapping layer (e.g., Si3N4) during the release etch. The release etch is a prolonged and harmful etch, whose by-products take a long time to diffuse out of the cavities129a,129b,129c. Accordingly, underlying functional layers in the optical stack150are protected by the AlF3etch stop layer122.

It should be noted that advantages of the embodiments described above are applicable to an interferometric modulator structure viewed from the opposite side, compared to that shown inFIG. 1. Such a configuration has a reflective electrode closer to the substrate (which need not be transparent) and a semitransparent electrode farther from the substrate. Either or both electrodes could be made movable. In addition, although not shown, it should be noted that the embodiments ofFIGS. 8-12may be combined with options of the embodiments described above with reference toFIGS. 1-7.

The above-described modifications can lead to a more robust design and fabrication. Additionally, while the above aspects have been described in terms of selected embodiments of the interferometric modulator, one of skill in the art will appreciate that many different embodiments of interferometric modulators may benefit from the above aspects. Of course, as will be appreciated by one of skill in the art, additional alternative embodiments of the interferometric modulator can also be employed. The various layers of interferometric modulators can be made from a wide variety of conductive and non-conductive materials that are generally well known in the art of semi-conductor and electro-mechanical device fabrication.

In addition, the embodiments, although described with respect to an interferometric modulator, are applicable more generally to other MEMS devices, particularly electrostatic MEMS with electrodes capable of relative movement, and the described AlF3layers can minimize stiction between the electrodes in an actuated or collapsed position.

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.