Microelectromechanical device and method utilizing a porous surface

A microelectromechanical device (MEMS) utilizing a porous electrode surface for reducing stiction is disclosed. In one embodiment, a microelectromechanical device is an interferometric modulator that includes a transparent electrode having a first surface; and a movable reflective electrode with a second surface facing the first surface. The movable reflective electrode is movable between a relaxed and actuated (collapsed) position. An aluminum layer is provided on either the first or second surface. The aluminum layer is then anodized to provide an aluminum oxide layer which has a porous surface. The porous surface, in the actuated position, decreases contact area between the electrodes, thus reducing stiction.

CROSS-REFERENCE TO RELATED APPLICATIONS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to microelectromechanical devices and methods for making the same. More particularly, this invention relates to engineering surfaces of moving and stationary electrode assemblies on either side of collapsing gap.

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. The second electrode is movable in a gap between a first position and a second position, where 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 being greater than the first distance. At least one of the electrodes comprises a porous layer having a porous surface facing the other of the electrodes.

The MEMS device may serve as an interferometric modulator. In the MEMS device, the porous layer may comprise an anodized layer, particularly anodized aluminum oxide (alumina or Al2O3). The porous layer may have a hexagonal array structure. In one embodiment, the porous layer is formed on the first, stationary electrode either on a dielectric or directly on a conductor. In another embodiment, the porous layer is formed under the second or moving electrode, preferably in contact with a reflective layer.

In another aspect, a display system is provided. The display device includes: the MEMS device described above; a display; a processor that is in electrical communication with the display, the processor being configured to process image data; and a memory device in electrical communication with the processor.

In yet another aspect, an interferometric modulator is provided. The interferometric modulator includes transmissive means for at least partially transmitting incident light, the transmissive means having a first surface. Reflective means for substantially reflecting incident light has a second surface facing the first surface. Moving means are provided for moving the reflective means relative to the transmissive means between a driven position and an undriven position, the driven position being closer to the transmissive means than is the undriven position. At least one of the transmissive and reflective means includes a porous surface facing the other of the transmissive and reflective means.

In still another aspect, a method of making an interferometric modulator is provided. The method includes providing transparent and reflective electrodes facing each other across a collapsible gap. A metallic layer is provided on at least one of facing surfaces. The metallic layer is anodized to form an anodized layer.

In another aspect, a method of making an electrostatic microelectromechanical systems device is provided. The method includes providing transparent and reflective electrodes facing each other across a cavity. A porous layer is provided on at least one of the electrodes, where the porous layer faces the other of the electrodes.

In another aspect, a method of making a microelectromechanical systems device is provided. The method includes forming a lower electrode. A sacrificial layer is formed over the lower electrode. An upper electrode porous layer is formed between forming the lower electrode and forming the upper electrode.

In another aspect, an interferometric modulator made by the method described above is provided.

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.

Stiction can be one of the most important reliability issues in microelectromechanical systems in general and interferometric modulator in particular. “Stiction,” as used herein, refers to a tendency of a movable layer in an actuated position to stick to a stationary layer in a microelectromechanical system. In embodiments of the invention, an interferometric modulator, which is an optical MEMS device, employs an anodized porous layer facing the MEMS cavity on either a movable or stationary layer.

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 row1electrode, 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 row2electrode, actuating the appropriate pixels in row2in accordance with the asserted column electrodes. The row1pixels are unaffected by the row2pulse, and remain in the state they were set to during the row1pulse. 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 +?V, 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 +?V, 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 −?V. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −Vbias, and the appropriate row to the same −?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 row3strobe sets the row3pixels 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 ore 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 moveable reflective layer14is attached to supports at the corners only, on tethers32. InFIG. 7C, the moveable 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 movable 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 Porous Layer

Stiction can be one of the most important reliability issues in microelectromechanical systems in general and interferometric modulator in particular. “Stiction,” as used herein, refers to a tendency of a movable layer in an actuated position to stick to a stationary layer in a microelectromechanical system.

Stiction occurs when the total of adhesion forces between two layers is greater than a restoring force. Adhesion forces become more significant when decreasing device dimensions. Restoring forces, however, decrease with decreasing device sizes. Thus, stiction is an inherent reliability concern for microelectromechanical systems of small dimensions. Accordingly, there is a need to provide a solution to the stiction problem in microelectromechanical systems.

Adhesion forces may arise from several mechanisms such as, capillary forces, van der Waals interactions, chemical bonds, solid bridging, etc. Adhesion forces, including short range and long range adhesion forces, depend on contact area and surface separation between two layers. Short range adhesion forces may be decreased by decreasing contact area between contacting surfaces, e.g., by increasing an effective hardness and/or roughening the surfaces. Long-range adhesion forces may be decreased by increasing an average surface separation between two layers in the actuated or collapsed condition of the MEMS.

Creep is another source of increasing stiction in a microelectromechanical device. “Creep,” used herein, refers to time-dependent material deformation which occurs as a result of exposure to high stress and/or high temperature. Deformation resulting from creep brings about an increase in contact area and a decrease in surface separation, thus increasing stiction.

In the illustrated embodiments, a surface on a MEMS electrode that faces the collapsing gap or cavity is defined by a porous layer. Accordingly, when actuated, the contact area between the surfaces that meet is reduced and stiction is thereby alleviated.

In one embodiment, an interferometric modulator has a fixed electrode having a porous layer. The porous layer is configured to include a porous surface facing a movable electrode. The porous layer is a layer of aluminum oxide (alumina or Al2O3) formed by anodizing aluminum. The porous surface reduces contact area between the electrodes, thereby reducing stiction. In addition, because the fixed electrode surface has pores recessed into the electrode, an average surface separation between the fixed and movable electrodes is increased. Thus, both short and long range adhesion forces can be effectively reduced, thereby decreasing stiction between the electrodes.

In another embodiment, an interferometric modulator has a movable reflective electrode having a porous layer. The porous layer is configured to include a porous surface facing a fixed electrode. This configuration decreases contact area. In addition, the porous layer increases an effective hardness of the movable electrode, and thus effectively reduces contact area between the electrodes by reducing the layers' ability to conform to one another in the collapsed or actuated state. In addition, the porous layer may prevent creep of the movable electrode and thus can prevent stiction arising from creep.

In yet another embodiment, an interferometric modulator has a movable electrode and a fixed electrode, both of which have a porous layer. Each porous layer is configured to include a porous surface facing the other electrode. This configuration decreases contact area similarly to the above embodiments. In addition, the porous layer of the moving electrode increases an effective hardness of the electrode. In addition, the porous layer may prevent creep of the movable electrode and thus can prevent stiction arising from creep.

While illustrated in the context of optical MEMS devices, particularly interferometric modulators, the skilled artisan will appreciate that the reduced stiction between collapsed parts is advantageous for other MEMS devices, such as electromechanical capacitive switches.

FIG. 8illustrates an interferometric modulator80according to an embodiment. The interferometric modulator80has a fixed electrode81(preferably at least partially transparent for the illustrated embodiment) and a movable electrode82(preferably reflective for the illustrated embodiment) which is supported by support posts84. The fixed electrode81is configured to have a porous top surface83awhich faces the movable electrode82. The porous surface83areduces contact area between the electrodes81and82, and increases surface separation between the electrodes81and82, thereby reducing stiction between them.

In the illustrated embodiment, the movable electrode82of the interferometric modulator80is in a relaxed position. In the relaxed position, the movable electrode82is at a relative large distance (e.g., 100 nm to 600 nm) from the fixed electrode. The distance between the electrodes81and82depends on desired color. The movable electrode82can move down to an actuated position (seeFIG. 1, modulator12). In the actuated position, the movable electrode82is positioned more closely adjacent to the fixed electrode81, and may be in contact with the top surface83aof the fixed electrode81.

The illustrated fixed electrode81overlies a transparent substrate20, and includes a transparent conductor such as the illustrated indium tin oxide (ITO) layer16coverlying the substrate20, and a metallic semitransparent layer16doverlying the ITO layer16c. The metallic layer16dis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the metallic layer16dmay be replaced with a semiconductor layer. The semiconductor layer is preferably formed of germanium. In one embodiment, the ITO layer16cmay have a thickness between about 100 Å and about 800 Å. The metallic 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 metallic layer may be omitted. In other embodiments, the fixed electrode81may further include a dielectric layer which will be described later in detail. Together, the layers define an optical stack or fixed electrode81.

In the illustrated embodiment, the movable electrode82includes a reflective layer82aand a mechanical or deformable layer82b. In the illustrated embodiment, the reflective layer82ais attached or fused to the deformable layer82b; in other arrangements, the reflector 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 movable electrode82may be formed of 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 movable 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 movable 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 deformable layer82b, which is preferably formed of nickel, is configured to cover top surfaces of the support posts84and the reflective layer82a, as shown inFIG. 8. In other arrangements, the support posts can include a “rivet” formed in the depression above the deformable layer. In certain embodiments, the reflective layer may be fused or tethered to the support posts, as shown inFIGS. 7A and 7B.

In the illustrated embodiment, the fixed electrode81includes a porous layer83. The porous layer83has the porous surface83afacing the movable electrode82. Preferably, the porous layer83is formed by anodizing an aluminum layer and is formed of aluminum oxide (Al2O3). Preferably, the porous layer83has a pore density of between about 1012m−2and about 1015m−2. The porous layer may have a thickness of between about 300 Å and about 1,500 Å. The pore density may be controlled to optimally reduce stiction while minimizing interference with optical properties of the interferometric modulator.

FIG. 9Aillustrates an enlarged cross-section of the porous layer83and the underlying metallic layer16d. The porous layer83includes vertical walls83cand pores83b, as shown inFIG. 9A. The illustrated pores83bpenetrate the porous layer83down to the metallic layer16d. In certain embodiments, there may be unanodized residual metal under the porous layer83. The unanodized metal may replace the metallic layer16dserving as an absorber. This configuration may be obtained by a partial anodizing process which will be described later in detail. The remaining aluminum layer, which would intervene between the porous layer83and the ITO layer16c, may have a thickness selected to produce a reflectance of between about 33% and 37%, for example between about 30 Å and about 50 Å.

FIG. 9Bis a partial top plan view of the porous layer83. The porous layer83has a hexagonal array structure, as shown inFIG. 9B. The hexagonal array structure has pores83bof substantially the same size which are uniformly distributed throughout the porous surface83a. Stiction between the electrodes in the actuated position may be decreased by increasing the average diameter of the pores83band/or by increasing pore density. However, optical performance of the interferometric modulator80may be negatively affected by the pores83b. For examples, light passing through the pores83bmay traverse a different optical path compared to light passing through the walls83c. However, this drawback may be reduced by controlling the thickness of the porous anodized alumina layer and pore dimension and density. The pore size and pore density of the porous layer83may be interdependently adjusted to optimally reduce stiction while minimizing interference with optical properties of the interferometric modulator. In one embodiment, the pores83bhave an average width or diameter between about 50Å and about 3,000 Å, and the pore density is between 1012m−2and about 1015m−2. More preferably, the pores may have an average diameter between about 100 Å and about 1,500 Å, and the pore density is between 1013m−2and about 1014m−2.

The above pore diameter and thickness of the porous layer83have been chosen to prevent full penetration into the pores by a sacrificial material such as molybdenum when forming a sacrificial layer over the porous layer, as will be better understood from the description ofFIGS. 12A-12Cbelow. Because the sacrificial material does not fully penetrate into the pores, it can be easily removed at a release step which will be later described.

In the actuated position, application of a voltage causes electrostatic attraction between the electrodes81,82, and the movable electrode82is positioned more closely adjacent to the porous surface83aof the fixed electrode81. A bottom surface82cof the movable electrode82is close to and typically in contact with the porous surface83a. Because the porous surface83aof the fixed electrode81has pores83b, contact area between the surfaces of the fixed and movable electrodes81and82is reduced by the total area of the pores83b. Thus, short range adhesion forces between the contacting surfaces of the electrodes decrease. In addition, because of the pores83b, the average surface separation between the electrodes81and82increases compared with that of an unmodified interferometric modulator. Thus, long range forces are also reduced. These effects in combination significantly reduce stiction between the electrodes.

In addition, an optical constant of the fixed electrode81may be controlled by adjusting the porosity of the porous layer83. For example, a refractive index of the fixed electrode81may be controlled by changing the porosity of the porous layer83. The refractive index of the porous layer83may be represented by Equation 1 below:
Reflective Index (n)=(nAl2O3−1)X+1, (0<X<1)   Equation 1

In Equation 1, nAl2O3is the refractive index of Al2O3, and X is a porosity of the porous layer (“1” indicates no pores while “0” indicates air). In the equation, the refractive index may be decreased by increasing the porosity of the porous layer83.

In addition, dielectric properties, e.g., a dielectric constant, of the fixed electrode81may be tailored by controlling the porosity of the porous layer83. The porous layer83has the vertical walls83cof aluminum oxide and the pores83bfilled with air. Because both aluminum oxide and air are dielectric materials, the capacitance of the porous layer83can be controlled by adjusting the area ratio of the pores to the vertical walls, i.e., the porosity of the layer83. The capacitance of the porous layer may be decreased by increasing the porosity of the layer. A lower capacitance is advantageous in that the interferometric modulator can consume less power. In addition, a lower capacitance reduces electrical response time, which equals to electrical resistance multiplied by capacitance. However, a low capacitance may negatively affect the hysteresis characteristics of the interferometric modulator. The porosity should therefore be optimally adjusted to achieve low power consumption while not negatively affecting the hysteresis properties. In the illustrated embodiments, the porous layer83, because it is dielectric, replaces a continuous dielectric layer which would serve to prevent electrical shorting between the fixed and movable electrodes in the actuated position.

FIG. 10Aillustrates an interferometric modulator100according to another embodiment. The interferometric modulator100has a fixed electrode101and a movable electrode102supported by support posts104. In the illustrated embodiment, a fixed electrode101overlies a transparent substrate20, and includes a transparent conductor, such as the illustrated indium tin oxide (ITO) layer16coverlying the substrate20, a metallic layer16doverlying the ITO layer16c, and a dielectric layer16eoverlying the metallic layer16d. The metallic layer16dis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the metallic layer16dmay be replaced with a semiconductor layer. The semiconductor layer is preferably formed of germanium. The dielectric layer16eis preferably formed of silicon dioxide and/or aluminum oxide and serves to prevent the two electrodes from shorting during operation. In one embodiment, the dielectric layer16emay have a two-layered structure, including an upper layer and a lower layer (not shown). The upper layer may be formed of aluminum oxide (see16fofFIG. 11and attendant description) which can serve as an etch stop layer during a “release” etch of the sacrificial layer that defines the cavity between electrodes, as will be better appreciated from the description ofFIGS. 12 and 13below. The lower layer may be formed of silicon dioxide. The dielectric layer16emay have a thickness between about 100 Å and about 1,600 Å. Together, the layers define an optical stack16. The movable electrode102and the support posts104can have a layer structure and material as described above with respect to those ofFIG. 8.

A porous layer103is formed over the dielectric layer16ein the illustrated embodiment.FIG. 10Bis an enlarged partial cross-section of the porous layer103and the dielectric layer16e. The porous layer103has vertical walls103cand pores103b, as shown inFIG. 10B. The illustrated pores103bpenetrate the porous layer103down to the dielectric layer16e. In the illustrated embodiment, the porous layer103may have a thickness between about 30 Å and about 200 Å.

In the actuated position (see e.g.,FIG. 1, modulator12b), a bottom surface102cof the movable electrode102is close to and typically in contact with the porous surface103aof the fixed electrode101. Because the layer103produces a porous surface, contact area between the surfaces of the fixed and movable electrodes101and102is reduced, and surface separation between them is increased, thereby reducing stiction between them.

FIG. 11illustrates an interferometric modulator110according to another embodiment. The interferometric modulator110has a fixed electrode111and a movable electrode112supported by support posts114. The movable electrode112includes a porous bottom surface113awhich faces the fixed electrode111. The porous surface113areduces contact area between the electrodes111and112. The pores also provide larger surface separation. In addition, the illustrated porous surface113areduces creep because the porous surface is formed of a hard and creep-resistant material such as aluminum oxide. These effects in combination may significantly reduce stiction between the electrodes.

InFIG. 11, the movable electrode112of the interferometric modulator110is in a relaxed position. In the relaxed position, the movable electrode112is at a relative large distance from the fixed electrode111. The movable electrode112can move down to an actuated position (not shown). In the actuated position, the movable electrode112is close to and typically in contact with a top surface111aof the fixed electrode111.

The fixed electrode111overlies a transparent substrate20, and includes an indium tin oxide (ITO) layer16coverlying the substrate20, a metallic layer16doverlying the ITO layer16c, a first dielectric layer16eoverlying the metallic layer16d, and a second dielectric layer16foverlying the first dielectric layer16e. The metallic layer16dis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the metallic layer16dmay be replaced with 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 stopper during the release etch. 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 metallic 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 layers may be adjusted such that the optical stack16is a color filter.

The movable electrode112may include a reflective layer112aand a deform able layer112b. In the illustrated embodiment, the reflective layer112ais preferably formed of a reflective metal, preferably, Al, Au, Ag, or an alloy of the foregoing. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the movable electrode112may be formed of a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing. The deformable layer112bis preferably formed of nickel. The layers112aand112bcan have thicknesses as described above with respect to the layers82aand82bofFIG. 8.

The support posts114are configured to support the movable electrode112, and is preferably formed of a dielectric material. The support posts114can be as described above with respect to the support post84ofFIG. 8. The deformable layer112b, which is preferably formed of nickel, covers top surfaces of the post114and the reflective layer112a, as shown inFIG. 11. 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 fused or tethered to the support posts, as shown inFIGS. 7A and 7B.

In the illustrated embodiment, the movable electrode112has a porous layer113. The porous layer113has a porous surface113afacing the fixed electrode111. The porous layer113is preferably formed of aluminum oxide (Al2O3) which has been formed by anodizing aluminum. Preferably, the porous layer113has a pore density of between about 1012m−2and about 1015m−2. Preferably, the porous layer113has a thickness of between about 50 Å and about 1,500 Å.

The porous layer113has a hexagonal array structure similar to the one described above with reference toFIG. 9B. The porous layer113has pores uniformly distributed throughout the porous surface113a. The pores may have an average diameter between about 50 Å and about 3,000 Å.

The above pore diameter and thickness of the porous layer113have been chosen to prevent full penetration into the pores by deposited electrode material when forming the overlying aluminum layer112a, as will be better understood from the description ofFIGS. 13A-13Cbelow. Thus, there remain some air cavities in the pores at the bottom of the porous layer113. Because the porous layer material (Al2O3) and air are dielectric, the porous layer113can replace a dielectric layer of the fixed electrode111.

In the actuated position (seeFIG. 1, modulator12b), the porous surface113aof the movable electrode112is closer, typically in contact with the top surface111aof the fixed electrode111. Because of the porous surface113a, contact area between the surfaces of the fixed and movable electrodes111and112is reduced, thereby reducing stiction.

In an embodiment where the reflective layer112ais formed of aluminum, because aluminum oxide has a higher hardness than aluminum, the porous aluminum oxide layer113increases an effective hardness of the movable electrode112, relative to the aluminum reflective layer112a, and thus reduces contact area of the aluminum layer112a. This effect also alleviates the increase in contact area that accompanies creep, and thus reduces an increase in stiction over time.

In an unpictured embodiment, an interferometric modulator has a movable electrode and a fixed electrode, both of which have a porous layer. Each porous layer is configured to include a porous surface facing the other electrode. The structures and materials of the electrodes and the porous layers can be as described above with reference toFIGS. 8-11.

The interferometric modulators of the above embodiments are described by way of examples. The porous layers in the embodiments may generally apply to microelectromechanical devices which have electrodes different from those of the embodiments. 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. 12A-12Eillustrate a method of making the interferometric modulator ofFIG. 8according to an embodiment. In the method, a porous surface is formed on a fixed electrode surface facing a movable electrode.

InFIG. 12A, an optical stack121is provided over a transparent substrate120. In the illustrated embodiment, the optical stack121has a transparent conductor in the form of an ITO layer121aoverlying the substrate120, a metallic layer121boverlying the ITO layer121a, a first dielectric layer121coverlying the metallic layer121b, and a second dielectric layer121doverlying the first dielectric layer121c. The metallic layer121bis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the metallic layer121bmay be replaced with a semiconductor layer. The semiconductor layer is preferably formed of germanium. The first dielectric layer121cmay be formed of silicon dioxide. The second dielectric layer121dmay be formed of aluminum oxide and may serve as an etch stop layer. The layers121a-121dmay have a thickness as described above with respect to the layers16c-16fofFIG. 11. In certain embodiments, the optical stack may have only one dielectric layer or none, depending on materials and selectivity of a release etch which will be described later. In another embodiment, the optical stack may have an unanodized aluminum layer replacing the chromium layer121b, and an anodized porous layer replacing the dielectric layers121cand121d, as will be described later in detail.

An aluminum layer122is provided over the second dielectric layer121d, as shown inFIG. 12A. In the illustrated embodiment, the aluminum layer122has a thickness between about 20 Å and about 140 Å. In certain embodiments where the optical stack includes no dielectric layer, the aluminum layer may have a thickness between about 300 Å and about 1,500 Å.

Next, as shown inFIG. 12B, the aluminum layer122is anodized to form a porous aluminum oxide layer123. In anodizing the aluminum layer122, desired pore spacing and diameter may be obtained by selecting an appropriate anodizing voltage and an anodizing electrolyte. Pore spacing and diameter tend to be proportional to the anodizing voltage with proportionality constants of 2.5 nmV−1for the pore spacing and 1.29 nmV−1for the pore diameter. In the illustrated embodiment, the anodizing voltage is preferably between about 5 V and about 300 V. In addition, examples of the anodizing electrolytes include, but are not limited to, sulfuric, phosphoric, oxalic, chromic, and citric acid. In the illustrated embodiment, a concentration of the anodizing electrolyte is preferably between about 0.1 M and about 1 M. The anodizing step is preferably performed for about 10 min. and about 100 min. at a temperature between about 0° C. and about 40° C.

The pores resulting from the above step have a width or diameter between about 50 Å and about 3,000 Å. In addition, after the anodizing step, the porous aluminum oxide layer123becomes about 1.2 to 1.7 times thicker than the aluminum layer122. In the illustrated embodiment, the porous layer123has a thickness between about 30 Å and about 200 Å. In other embodiments where the optical stack includes no dielectric layer, the porous layer may have a thickness between about 300 Å and about 1,500 Å.

In the illustrated embodiment, the aluminum layer122has been fully anodized into the porous aluminum oxide layer123. The pores of the porous layer123extend completely down to the second dielectric layer121d. In certain embodiments where the fixed electrode includes neither a chromium layer nor a dielectric layer, the aluminum layer may be partially anodized, leaving a non-anodized residual layer of aluminum between an anodized porous layer and an underlying ITO layer. The residual aluminum layer serves as an absorber instead of the chromium layer.

Subsequently, a sacrificial layer124is provided over the porous layer123, as shown inFIG. 12C. The sacrificial layer124is preferably formed of a material capable of selective removal without harm to other materials that define the cavity. In the illustrated embodiment, the sacrificial layer124is formed of molybdenum. Other examples of sacrificial materials include silicon and tungsten. Because the diameters of the pores are very small, the sacrificial layer124does not fill the pores and thus can be completely removed by an etchant which will be described later. In addition, a thickness of the porous layer can be chosen to avoid filling the pores. A suitable deposition method, e.g., sputtering deposition, can also avoid filling the pores.

Next, steps for forming a movable electrode and support posts are performed. A reflective layer125is first deposited over the sacrificial layer124, as shown inFIG. 12C. The reflective layer125is preferably formed of Al, Au, Ag, or an alloy of the foregoing. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the layer125may be formed of a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing. In the illustrated optical MEMS embodiment, the reflective layer125is formed of aluminum (Al). The reflective layer125is then patterned using a lithographic process, preferably a photolithographic process. Subsequently, another lithographic process, preferably a photolithographic process, is performed to pattern the sacrificial layer124to provide recesses for support posts127. Then, a material for posts127, preferably silicon dioxide, is deposited and patterned over exposed surfaces, including surfaces of the sacrificial layer124and the reflective layer125.

Then, a material for a deformable layer126is deposited over the posts127and the reflective layer125, as shown inFIG. 12D. The material for the deformable layer126is preferably nickel. Then, the nickel and aluminum layers125and126are patterned and etched to define arrays of MEMS devices and provide through-holes128in the movable electrode layers. The holes128serve to permit etchant to enter and etch byproduct to exit at a release step which will be later described. In addition, the holes128provide an exit for air when the reflective layer moves between the relaxed and actuated positions.FIG. 12Dillustrates a cross-section of a completed “unreleased” interferometric modulator structure with the sacrificial layer in place.

In an unpictured embodiment, another sacrificial layer is deposited over the aluminum reflective layer after patterning the reflective layer and before patterning the sacrificial 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.

Finally, the sacrificial layer124is selectively removed, leaving a cavity or gap129between the reflective layer125and the porous layer123, as shown inFIG. 12E. This step is referred to as a “release” or “sacrificial etch” step. The illustrated sacrificial layer124which 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 (SiO2, Al2O3, Al, etc.) that define the cavity129. A resulting “released” MEMS device, particularly interferometric modulator, is shown inFIG. 12E. 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 movable electrode, as shown inFIGS. 7B-7E.

FIGS. 13A-13Lillustrate a method of making the interferometric modulator ofFIG. 11according to another embodiment. In the method, a porous surface is formed on a reflective layer132surface facing a fixed electrode.

InFIG. 13A, an optical stack is provided over a transparent substrate130. In the illustrated embodiment, the optical stack131has a transparent conductor in the form of an ITO layer131aoverlying the substrate130, a metallic layer131boverlying the ITO layer131a, a first dielectric layer131coverlying the metallic layer131b, and a second dielectric layer131doverlying the first dielectric layer131c. The metallic layer131bis preferably formed of chromium. In another embodiment for a broad-band white interferometric modulator, the metallic layer131bmay be replaced with a semiconductor layer. The semiconductor layer is preferably formed of germanium. The first dielectric layer131cmay be formed of silicon dioxide. The second dielectric layer131dmay be formed of aluminum oxide and may serve as an etch stop layer. The layers131a-131dmay have a thickness as described above with respect to the layers16c-16fofFIG. 11. In certain embodiments, the optical stack may have only one dielectric layer or none, depending on materials and selectivity of a release etch which will be described later.

Subsequently, a sacrificial layer134is provided over the second dielectric layer131d, as shown inFIG. 13A. In the illustrated embodiment, the sacrificial layer134is formed of molybdenum. Other examples of sacrificial materials include silicon and tungsten. A thickness of the sacrificial layer134is equal to a size of a relaxed MEMS device cavity. It also determines color displayed by the MEMS device during operation. Next, an aluminum layer132is deposited on the sacrificial layer134. The aluminum layer132preferably has a thickness of between about 30 Å and about 1,000 Å.

Next, as shown inFIG. 13B, the aluminum layer132is anodized to form a porous aluminum oxide layer133. As in the anodizing step described above with reference toFIG. 12, desired pore spacing and pore diameter may be obtained by selecting an appropriate anodizing voltage and an anodizing electrolyte. In the illustrated embodiment, the anodizing voltage is preferably between about 5 V and about 300 V. In addition, the anodizing electrolyte may be selected from sulfuric, phosphoric, oxalic, chromic, and citric acid. In the illustrated embodiment, a concentration of the anodizing electrolyte is preferably between about 0.1 M and about 1 M. The anodizing step is preferably performed for about 10 min. and about 100 min. at a temperature between about 0° C. and about 40° C.

Resulting pores have a diameter between about 50 Å and about 3,000 Å. In addition, after the anodizing step, the porous aluminum oxide layer133becomes about 1.5 times thicker than the aluminum layer132. In the illustrated embodiment, the porous layer133has a thickness between about 50 Å and about 1,500 Å. In the embodiment, the aluminum layer132has been fully transformed into the porous aluminum oxide layer133. The pores of the porous layer133have been etched completely down to the sacrificial layer134.

Next, steps for forming a movable electrode and support posts are performed. A reflective layer135is first deposited over the porous layer133, as shown inFIG. 13C. The reflective layer135is preferably formed of Al, Au, Ag, or an alloy of the foregoing. In certain embodiments where the MEMS device is used as an electromechanical capacitive switch, the layer135may be formed of a conductor such as Cu, Pt, Ni, Au, Al, or an alloy of the foregoing. In the illustrated optical MEMS embodiment, the reflective layer135is formed of aluminum (Al). Because the pore diameter and thickness of the porous layer133have been chosen to avoid full penetration of an aluminum layer135through the pores down to the sacrificial layer134, there remain some air cavities at the bottom surface of the anodized layer133.

The reflective layer135and the porous layer133are then patterned using a lithographic process, preferably a photolithographic process. A photoresist140ais provided over the reflective layer135and is patterned to provide a mask for etching the underlying reflective layer135and porous layer133. Then, the porous and reflective layers133and135are etched through openings of the photoresist140a, as shown inFIG. 13D. This etching step can be performed using any suitable etch process, including a dry or wet etch process. In certain embodiments, the etching step may include two etch processes for the reflective layer135and the porous layer133, respectively, using the same mask. Then, the photoresist140ais stripped, exposing portions of the sacrificial layer surface, as shown inFIG. 13E.

Subsequently, another photolithographic process is performed to pattern the sacrificial layer134for forming support posts. As illustrated inFIG. 13F, a photoresist140bis provided and patterned over the sacrificial layer134and the reflective layer135. Then, the sacrificial layer134is etched using a dry etch process, as shown inFIG. 13G, preferably using a fluorine-based etchant such as SF6/O2, CF4/O2, or NF3, or a chlorine-based etchant such as Cl2/BCl3. The photoresist140bis then stripped, as shown inFIG. 13H.

Then, a material for posts137, preferably an inorganic dielectric material such as silicon dioxide, is deposited over exposed surfaces, including surfaces of the sacrificial layer134and the reflective layer135. Subsequently, the silicon dioxide layer137is patterned to form posts, using a suitable etch process, including a wet or dry etch process. When a dry etch is used, the aluminum reflective layer135may serve as an etch stopper. A resulting layer structure is illustrated inFIG. 13I.

Next, a material for a mechanical or deformable layer136is deposited over the support posts137and the reflective layer135as shown inFIG. 13J. The material is preferably nickel. Then, the deformable layer136, the reflective layer135, and the porous layer133are etched to provide through-holes138in the middle, as shown inFIG. 13K. The etch process can be either a wet or dry etch process. The holes138serve to permit etchant to enter and etch byproduct to exit at a release step which will be later described. In addition, the holes138provide an exit for air when the reflective layer moves between the relaxed and actuated positions.FIG. 13Killustrates a cross-section of a completed “unreleased” interferometric modulator structure with the sacrificial layer in place.

In an unpictured embodiment, another sacrificial layer is deposited over the aluminum reflective layer after patterning the reflective layer and before patterning the sacrificial 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 movable electrode, as shown inFIG. 7B.

Finally, the sacrificial layer134is selectively removed, leaving a cavity or gap139between the dielectric layer131dand the porous layer133, as shown inFIG. 13L. The illustrated sacrificial layer134which is formed of molybdenum is preferably etched using a fluorine-based etchant such as a XeF2-based etchant. A resulting MEMS device, particularly a released interferometric modulator, is shown inFIG. 13L.

In an unpictured embodiment, a partially anodized layer can be formed on a movable electrode. First, an optical stack is provided over a transparent substrate. The optical stack can have a layer structure and material as described above with respect to the optical stack ofFIG. 13. Subsequently, a sacrificial layer, preferably formed of molybdenum, is provided over the optical stack. Next, a reflective layer, preferably formed of aluminum, is formed over the sacrificial layer. Then, the reflective layer and the sacrificial layer are patterned to provide recesses for support posts. Then, the support posts are formed in the recesses. Then, a material for a deformable layer is deposited over the support posts and the reflective layer. Then, the reflective and deformable layers are etched to provide through-holes in the middle. Next, the sacrificial layer is removed, leaving a cavity or gap between the reflective layer and the optical stack. Details of each step are as described above with reference toFIG. 12. After this step, the aluminum reflective layer is anodized through the hole and the cavity. At this anodizing step, the aluminum reflective layer is partially anodized from the lower surface up to a desired depth, leaving a layer of aluminum to serve as a reflective layer between the mechanical layer and the porous alumina.

In another unpictured embodiment, porous layers are formed prior to providing a sacrificial layer and after providing the sacrificial layer. A resulting interferometric modulator is configured to have a movable electrode and a fixed electrode, both of which have a porous layer. First, an optical stack is provided over a transparent substrate. The optical stack can have a layer structure and material as described above with respect to the optical stack ofFIG. 13. Subsequently, an aluminum layer is provided over the optical stack and is anodized. Then, a sacrificial layer, preferably formed of molybdenum, is provided over the anodized alumina layer. Next, another aluminum layer is provided over the sacrificial layer and is anodized. Subsequently, a reflective layer, preferably formed of aluminum, is formed over the anodized alumina layer. Then, the reflective layer, the porous layer, and the sacrificial layer are patterned to provide recesses for support posts. Then, the support posts are formed in the recesses. Then, a material for a deformable layer is deposited over the support posts and the reflective layer. Then, the anodized layer, the reflective layer, and the deformable layer are patterned and etched. Next, the sacrificial layer is removed, leaving a cavity or gap between the two anodized alumina layers.

It should be noted that 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. 8-13may 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 can prevent stiction 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.