MEMS devices having support structures

Embodiments of MEMS devices comprise a conductive movable layer spaced apart from a conductive fixed layer by a gap, and supported by rigid support structures, or rivets, overlying depressions in the conductive movable layer, or by posts underlying depressions in the conductive movable layer. In certain embodiments, both rivets and posts may be used. In certain embodiments, these support structures are formed from rigid inorganic materials, such as metals or oxides. In certain embodiments, etch barriers may also be deposited to facilitate the use of materials in the formation of support structures which are not selectively etchable with respect to other components within the MEMS device.

BACKGROUND OF THE INVENTION

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 another embodiment, a method of fabricating a MEMS device is provided, including providing a substrate, depositing an electrode layer over the substrate, depositing a sacrificial layer over the electrode layer, patterning the sacrificial layer to form apertures, forming support structures over the sacrificial layer, wherein the support structures are formed at least partially within the apertures in the sacrificial material and wherein the support structures include a substantially horizontal wing portion extending over a substantially flat portion of the sacrificial material, and depositing a movable layer over the sacrificial layer and the support structures.

In another embodiment, a MEMS device is provided, including a substrate, an electrode layer located over the substrate, a movable layer located over the electrode layer, wherein the movable layer is generally spaced apart from the electrode layer by a gap, and support structures underlying at least a portion of the movable layer, wherein the support structures include a substantially horizontal wing portion, the substantially horizontal wing portion being spaced apart from the electrode layer by the gap.

In another embodiment, a MEMS device is provided, including first means for electrically conducting, second means for electrically conducting, and means for supporting the second conducting means over the first conducting means, wherein the second conducting means overlie the supporting means, and wherein the second conducting means is movable relative to the first conducting means in response to generating electrostatic potential between the first and second conducting means, wherein the supporting means include a substantially horizontal wing portion spaced apart from the first conducting means.

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.

Individual MEMS elements, such as interferometric modulator elements, may be provided with support structures both within and at the edges of individual elements. In certain embodiments, these support structures may include structures underlying a movable layer within the MEMS element. By forming these structures from rigid inorganic material such as metal or oxides, stability of the operation of the MEMS device can be improved as compared with structures formed from less rigid material. In addition, the use of rigid material alleviates problems with gradual degradation or deformation of the support structures over time, which can lead to a gradual shift in the color reflected by a given pixel. Further embodiments may include both overlying and underlying support structures. Etch barriers may also be deposited to facilitate the use of materials in the formation of support structures which are not selectively etchable with respect to other components within the MEMS device. Additional layers may also be disposed between the support structures and other layers so as to improve the adhesion of the various components of the MEMS device to one another.

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 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. The partially reflective layer can be formed from a variety of materials that are partially reflective such as various metals, semiconductors, and dielectrics. The partially reflective layer can be formed of one or more layers of materials, and each of the layers can be formed of a single material or a combination of materials.

In some embodiments, the layers of the optical stack16are 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 metal 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 gap19. 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 5Billustrate 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 II®, 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. 4,5A, and5B illustrate 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 −ΔV, 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 the 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 the 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 antenna43, which is coupled to a transceiver47. The transceiver47is connected to a 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 an 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, the 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 memory device such as 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, the array driver22, and the 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, a 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, or 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 embodiments, control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some embodiments, control programmability resides in the array driver22. Those of skill in the art will recognize that the above-described optimizations may be implemented in any number of hardware and/or software components and in various configurations.

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 supports18at 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 substrate20around the perimeter of the deformable layer34. These connections are herein referred to as support structures, which can take the form of isolated pillars or posts and/or continuous walls or rails. The embodiment illustrated inFIG. 7Dhas support structures18that include support 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 posts by filling holes between the deformable layer34and the optical stack16. Rather, the support posts18are formed of a planarization material, which is used to form the 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-7C, as 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 modulator is arranged. In these embodiments, the reflective layer14optically shields the portions of the interferometric modulator on the side of the reflective layer opposite the substrate20, including the deformable layer34. This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. Such shielding allows the bus structure44inFIG. 7E, which provides the ability to separate the optical properties of the modulator from the electromechanical properties of the modulator, such as addressing and the movements that result from that addressing. This separable modulator architecture allows the structural design and materials used for the electromechanical aspects and the optical aspects of the modulator to be selected and to function independently of each other. Moreover, the embodiments shown 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.

In certain embodiments, it may be desirable to provide additional support to a movable layer such as the movable reflective layer14illustrated inFIG. 7A, or the combination of mechanical layer34and movable reflective layer14ofFIGS. 7C-7E. The movable layer may comprise a reflective sublayer and a mechanical sublayer, as will be discussed in greater detail below. Such support may be provided by a series of support structures which may be located both along the edges of an individual modulator element and in the interior of such an element. In various embodiments, these support structures may be located either over or underneath a movable layer. In alternate embodiments, support structures may extend through an aperture formed in the mechanical layer, such that support is provided from both above and below the mechanical layer. As used herein, the term “rivet” generally refers to a patterned layer overlying a mechanical layer in a MEMS device, usually in a recess or depression in the post or support region, to lend mechanical support for the mechanical layer. Preferably, though not always, the rivet includes wings overlying an upper surface of the mechanical layer to add stability and predictability to the mechanical layer's movement. Similarly, support structures underlying a mechanical layer in a MEMS device to lend mechanical support for the mechanical layer are generally referred to herein as support “posts.” In many of the embodiments herein, the preferred materials are inorganic for stability relative to organic resist materials.

An exemplary layout of such support structures is shown inFIG. 8, which depicts an array of MEMS elements. In certain embodiments, the array may comprise an array of interferometric modulators, but in alternate embodiments, the MEMS elements may comprise any MEMS device having a movable layer. It can be seen that support structures62, which in the illustrated embodiment are overlying rivet structures62, are located both along the edges of a movable layer66and in the interior of a MEMS element, in this example an interferometric modulator element60. Certain support structures may comprise rail structures64, which extend across the gap65between two adjacent movable layers66. It can be seen that movable layer66comprises a strip of deformable material extending through multiple adjacent elements60within the same column. The support structures62serve to stiffen the movable layer66within the elements or pixels60.

Advantageously, these support structures62are made small relative to the surrounding area of the modulator element60. As the support posts constrain deflection of the movable layer66and may generally be opaque, the area underneath and immediately surrounding the support structures62is not usable as active area in a display, as the movable layer in those areas is not movable to a fully actuated position (e.g., one in which a portion of the lower surface of the movable layer14ofFIG. 7Ais in contact with the upper surface of the optical stack16). Because this may result in undesirable optical effects in the areas surrounding the post, a mask layer may advantageously be provided between the support structures and the viewer to avoid excessive reflection in these regions that may wash out the intended color.

In certain embodiments, these support structures may comprise a depression in the movable layer, along with a substantially rigid structure which helps to maintain the shape. While such support structures may be formed of a polymer material, an inorganic material having greater rigidity is preferably used, and provides advantages over similar structures comprising polymeric materials.

For instance, a polymeric support structure may not maintain a desired level of rigidity over a wide range of operating temperatures, and may be subject to gradual deformation or mechanical failure over the lifetime of a device. As such failures may affect the distance between the movable layer and the optical stack, and this distance at least partially determines the wavelengths reflected by the interferometric modulator element, such failures may lead to a shift in the reflected color due to wear over time or variance in operating temperatures. Other MEMS devices experience analogous degradation over time when supports are formed of polymeric material.

One process for forming an interferometric modulator element comprising overlying rivet support structures is described with respect toFIGS. 9A-9J. InFIG. 9A, it can be seen that a transparent substrate70is provided, which may comprise, for example, glass or a transparent polymeric material. A conductive layer72, which may comprise indium-tin-oxide (ITO) is then deposited over the transparent substrate, and a partially reflective layer74, which may comprise chromium, is deposited over the conductive layer72. Although in one embodiment conductive layer72may comprise ITO, and may be referred to as such at various points in the below specification, it will be understood that the layer72may comprise any suitable conductive material, and need not be transparent for non-optical MEMS structures. Similarly, although sometimes referred to as a chromium layer, partially reflective layer74may comprise any suitable partially reflective layer, and may be omitted for non-optical MEMS structures.

The conductive layer72and partially reflective layer74are then patterned and etched to form bottom electrodes, also referred to as row electrodes, which run perpendicular to the movable layer66ofFIG. 8. In certain embodiments, the conductive and partially reflective layers72and74may advantageously also be patterned and etched to remove the ITO and chromium underlying the areas where the support post structures will be located, forming apertures76as depicted inFIG. 9B. This patterning and etching is preferably done by the same process which forms the row electrodes. The removal of ITO and chromium (or other conductive materials) underlying the support structures helps to prevent shorting between the movable layer and the bottom electrode. Thus,FIG. 9Band the subsequent figures depict a cross-section of a continuous row electrode formed by layers72and74, in which apertures76have been etched, taken along a line extending through those apertures. In other embodiments in which the conductive layer72and partially reflective layer74are not etched to form apertures76, a dielectric layer, discussed below, may provide sufficient protection against shorting between the bottom electrode and the movable layer.

The conductive layer72and partially reflective layer74may be patterned via photolithography and etched via, for example, commercially available wet etches. Chromium wet etches include solutions of Acetic Acid (C2H4O2) and Cerium Ammonium Nitrate [Ce(NH4)2(NO3)6]. ITO wet etches include HCl, a mixture of HCl and HNO3, or a mixture of FeCl3/HCl/DI in a 75%/3%/22% ratio and H2O. Once the apertures76have been formed, a dielectric layer78is deposited over the conductive and partially reflective layers72and74, as seen inFIG. 9C, forming the optical stack16. In certain embodiments, the dielectric layer may comprise SiO2or SiNx, although a wide variety of suitable materials may be used.

The thickness and positioning of the layers forming the optical stack16determines the color reflected by the interferometric modulator element when the element is actuated (collapsed), bringing the movable layer66into contact with the optical stack. In certain embodiments, the optical stack is configured such that the interferometric modulator element reflects substantially no visible light (appears black) when the movable layer is in an actuated position. Typically, the thickness of the dielectric layer78is about 450 Å. While illustrated as planar (which can be achieved if the dielectric layer78is a spin-on glass), the dielectric layer78is typically conformal over the patterned lower electrode formed from layers72and74.

As seen inFIG. 9D, a layer82of sacrificial material is then deposited over the dielectric layer78. In certain embodiments, this sacrificial layer82is formed from a material which is etchable by XeF2. For example, the sacrificial layer82may be formed from molybdenum or amorphous silicon (a-Si). In other embodiments, the sacrificial layer may comprise tantalum or tungsten. Other materials which are usable as sacrificial materials include silicon nitride, certain oxides, and organic materials. The thickness of the deposited sacrificial layer82will determine the distance between the optical stack16and the movable layer66, thus defining the dimensions of the interferometric gap19(seeFIG. 7A). As the height of the gap19determines the color reflected by the interferometric modulator element when in an unactuated position, the thickness of the sacrificial layer82will vary depending on the desired characteristics of the interferometric modulator. For instance, in an embodiment in which a modulator element that reflects green in the unactuated position is formed, the thickness of the sacrificial layer82may be roughly 2000 Å. In further embodiments, the sacrificial layer may have multiple thicknesses across an array of MEMS devices, such as in a multicolor display system where different interferometric gap sizes are used to produce different colors.

InFIG. 9E, it can be seen that the sacrificial layer82has been patterned and etched to form tapered apertures86. The apertures86overlie the apertures76cut into the layers72and74of ITO and chromium. These apertures86may be formed by masking the sacrificial layer, using photolithography, and then performing either a wet or dry etch to remove portions of the sacrificial material. Suitable dry etches include, but are not limited to, SF6, CF4, Cl2, or any mixture of these gases with O2or a noble gas such as He or Ar. Wet etches suitable for etching Mo include a PAN etch, which may be a mix of phosphoric acid, acetic acid, nitric acid and deionized water in a 16:1:1:2 ratio. Amorphous silicon can be etched by wet etches including KOH and HF Nitrate. Preferably, however a dry etch is used to etch the sacrificial layer82, as dry etches permit more control over the shape of tapered apertures86.

InFIG. 9F, it can be seen that the components which will form the movable layer66(see, e.g., moveable reflective layer14inFIG. 7A) are then deposited over the etched sacrificial layer82, lining the tapered apertures86. In the embodiment ofFIG. 9F, a highly reflective layer90, also referred to as a mirror or mirror layer, is deposited first, followed by a mechanical layer92. The highly reflective layer90may be formed from aluminum or an aluminum alloy, due to their high reflectance over a wide spectrum of wavelengths. The mechanical layer92may comprise a metal such as Ni and Cr, and is preferably formed such that the mechanical layer92contains residual tensile stress. The residual tensile stress provides the mechanical force which pulls the movable layer66away from the optical stack16when the modulator is unactuated, or “relaxed.” For convenience, the combination of the highly reflective layer90and mechanical layer92may be collectively referred to as movable layer66, although it will be understood that the term movable layer, as used herein, also encompasses a partially separated mechanical and reflective layer, such as the mechanical layer34and the movable reflective layer14ofFIG. 7C, the fabrication of which in conjunction with support structures is discussed below with respect toFIGS. 35A-35Hand36A-36C.

In an embodiment in which the sacrificial layer is to be etched by a XeF2etch, both the reflective layer90and the mechanical layer92are preferably resistant to XeF2etching. If either of these layers is not resistant, an etch stop layer may be used to protect the non-resistant layer. It can also be seen that the taper of the tapered apertures86facilitates the conformal deposition of the reflective layer90and mechanical layer92, as they may comprise non-planarizing materials. Absent this taper, it may be difficult to deposit these layers such that the layers have substantially even thicknesses within the apertures86.

In an alternate embodiment, the movable layer66may comprise a single layer which is both highly reflective and has the desired mechanical characteristics. However, the deposition of two distinct layers permits the selection of a highly reflective material, which might otherwise be unsuitable if used as the sole material in a movable layer66, and similarly allows selection of a suitable mechanical layer without regard to its reflective properties. In yet further embodiments, the movable layer may comprise a reflective sublayer which is largely detached from the mechanical layer, such that the reflective layer may be translated vertically without bending (See, e.g.,FIGS. 7C-7Eand attendant description). One method of forming such an embodiment comprises the deposition of a reflective layer over the sacrificial layer, which is then patterned to form individual reflective sublayers. A second layer of sacrificial material is then deposited over the reflective layer and patterned to permit the connections to be made through the second sacrificial layer between the mechanical sublayer and the reflective sublayers, as well as to form tapered apertures for the support structures.

In other embodiments in which the MEMS devices being formed comprise non-optical MEMS devices (e.g., a MEMS switch), it will be understood that the movable layer66need not comprise a reflective material. For instance, in embodiments in which MEMS devices such as MEMS switches are being formed comprising the support structures discussed herein, the underside of the movable layer66need not be reflective, and may advantageously comprise a single layer, selected solely on the basis of its mechanical properties or other desirable properties.

InFIG. 9G, a rigid layer96, also referred to as a rivet layer, is deposited over the mechanical layer92. As the rivet layer96will form a structure which provides support to the underlying mechanical layer92but will not be substantially deformed during actuation of the modulator, the material forming the rivet layer96need not be as flexible as that forming the mechanical layer92. Suitable materials for use in the rivet layer96include, but are not limited to, aluminum, AlOx, silicon oxide, SiNx, nickel and chromium. Alternate materials which may be used to form the rivet structure include other metals, ceramics, and polymers. The thickness of the rivet layer96will vary according to the mechanical properties of the material used.

As discussed with respect to the mechanical and reflective layers, it may be desirable to select for the rivet layer96a material that is resistant to XeF2etching, which may be used to etch the sacrificial layer in certain embodiments. In addition, the rivet layer96is preferably selectively etchable with respect to the underlying mechanical layer92, so as to permit etching of the rivet layer96while leaving the mechanical layer92unaffected. However, if the rivet layer96is not selectively etchable relative to the mechanical layer92, an etch stop layer (not shown) may be provided between the rivet layer96and the mechanical layer92.

InFIG. 9H, the rivet layer96is patterned via photolithography and etched to remove portions of the rivet layer96located away from the apertures86, forming support structures62, also referred to as rivet structures. The etching of the rivet layer96may be performed by either a wet etch or a dry etch. In embodiments in which the rivet layer96comprises aluminum, suitable wet etches include phosphoric acid or bases such as KOH, TMAH, and NaOH, and a suitable dry etch uses Cl2. In other embodiments in which the rivet layer96comprises SiO2, a mixture of fluorine-bases gases and either O2or noble gases may be used as a dry etch, and HF or BOE are suitable wet etches.

Referring still toFIG. 9H, it can be seen that the support structures62may comprise a lip area98, where the support structure62extends out of the tapered aperture86over the upper surface of the mechanical layer92. Advantageously, the size of this lip can be minimized, as the lip constrains deflection of the underlying mechanical layer, reducing the active area of the interferometric modulator element. As can be seen in the illustrated embodiment, the support structures62may also comprise a sloped sidewall portion97and a substantially flat base area99.

Next, inFIG. 9I, it can be seen that photolithography is used to pattern the mechanical layer92, and etch the mechanical layer92and the reflective layer90to form etch holes100, which expose portions of the sacrificial layer82, in order to facilitate etching of the sacrificial layer. In certain embodiments, multiple etches are employed to expose the sacrificial layer. For example, if the mechanical layer92comprises nickel and the reflective layer90comprises aluminum, HNO3may be used to etch the mechanical layer92, and phosphoric acid or a base such as NH4OH, KOH, THAM, or NaOH may be used to etch the reflective layer90. This patterning and etching may also be used to define the strip electrodes seen inFIG. 8, by etching gaps65between strips of the movable layer66(seeFIG. 8), separating columns of MEMS devices from one another.

Finally, inFIG. 9J, it can be seen that a release etch is performed to remove the sacrificial layer, creating the interferometric gap19through which the movable layer66can move. In certain embodiments, a XeF2etch is used to remove the sacrificial layer82. Because XeF2etches the sacrificial materials well, and is extremely selective relative to other materials used in the processes discussed above, the use of a XeF2etch advantageously permits the removal of the sacrificial material with very little effect on the surrounding structures.

Thus,FIG. 9Jdepicts a portion of an interferometric modulator element such as one of the interferometric modulator elements60ofFIG. 8, shown along line9J-9J. In this embodiment, the movable layer66is supported throughout the gap19by support structures62formed over depressions86in the movable layer66. As discussed above, portions of the underlying optical stack16have advantageously been etched so as to prevent shorting between conductive portions of the optical stack16and conductive layers in the movable layer66, although this step need not be performed in all embodiments.

Although the thickness of the rivet layer96deposited inFIG. 9Gmay be determined based upon the mechanical characteristics of the material used, in alternate embodiments, the rivet layer96may be made much thicker than merely sufficient for the function of providing support for the mechanical layer.FIG. 10depicts a portion of an interferometric modulator in which the support structures62have been formed from a much thicker rivet layer. Such an embodiment enables the support structures62to perform other functions, such as supporting additional components of the modulator (seeFIG. 7Eand attendant description), providing spacers to protect the interferometric modulator element from damage due to mechanical interference with the movable layer66, or to support a protective backplate. In certain embodiments the thickness of the rivet layer may be between 300 Å and 1000 Å. In other embodiments, the thickness of the rivet layer may be between 1000 Å and 10 microns. In other embodiments, the thickness of the rivet layer may be 20 microns or higher. In certain embodiments, the thickness of the rivet layer may be between 0.1 and 0.6 times the thickness of the mechanical layer. In other embodiments, the thickness of the rivet layer may be between 0.6 and 1 times the thickness of the mechanical layer. In other embodiments, the thickness of the rivet layer may be between 1 and 200 times the thickness of the mechanical layer. It will be understood that in certain embodiments, thicknesses both within and outside of the above ranges may be appropriate.

In an embodiment in which the movable layer66comprises a conductive reflective layer90, the separate mechanical layer92can be omitted, and the rivet layer96may serve as the mechanical layer, while the conductive reflective layer90may provide the desired electrical connectivity across a MEMS array, serving as the electrodes. In a further embodiment, the conductive reflective layer90may be made thicker than is necessary to provide the desired optical characteristics in order to provide better conductive characteristics, such as by lowering the resistivity of the strip electrodes formed from the patterned conductive reflective layer90.

In another variation, a thick mechanical layer may be deposited after performing the steps described with respect toFIGS. 9A-9E. This thick mechanical layer may subsequently be polished down or otherwise etched back to achieve a desired thickness in those portions overlying the remaining sacrificial layer. However, as the mechanical layer is initially thicker than the desired final thickness in the areas overlying the sacrificial material, a thicker mechanical layer will remain in the apertures in the sacrificial layer, untouched by the polishing, providing support similar to that resulting from the support structures62(see, e.g.,FIG. 9H), as discussed above. Advantageously, the mechanical layer may be thick enough to totally fill the apertures in the sacrificial layer, although it will be understood that sufficient support may be provided with a thinner mechanical layer in certain embodiments.

In another embodiment, the support structures may take the form of inorganic posts underlying the movable layer. An exemplary process for fabricating an interferometric modulator comprising inorganic support posts is discussed with respect toFIGS. 11A-11G, the early steps of which process may correspond generally to the early steps in the process ofFIGS. 9A-9J. In various embodiments, as discussed above, fabricating an interferometric modulator comprises forming an optical stack on a substrate, which may be a light-transmissive substrate, and in further embodiments is a transparent substrate. The optical stack may comprise a conductive layer, which forms an electrode layer on or adjacent the substrate; a partially reflective layer, which reflects some incident light while permitting some light to reach the other components of the interferometric modulator element; and a dielectric layer, which insulates the underlying electrode layer from the other components of the interferometric modulator. InFIG. 11A, it can be seen that a transparent substrate70is provided, and that a conductive layer72and a partially reflective layer74are deposited over the substrate70. A dielectric layer78is then deposited over the partially reflective layer74.

As discussed above, in some embodiments, the conductive layer72is transparent and comprises ITO, the partially reflective layer74comprises a semireflective thickness of metal, such as chromium (Cr), and the dielectric layer78comprises silicon oxide (SiO2). At some point during this process, at least the conductive layer72is patterned (as shown inFIG. 9B) to form row electrodes which will be used to address a row of interferometric modulators. In one embodiment, this patterning takes place after the deposition of the conductive and partially reflective layers72and74, but prior to the deposition of the dielectric layer78. In a further embodiment, the conductive and partially reflective layers72and74are patterned so as to form gaps (not shown) underneath the support structures, so as to minimize the possibility of a short between the layers72and74and an overlying conductive layer forming part of or extending underneath the support structure.

The combination of the layers72,74, and78is referred to herein as the optical stack16, and may be indicated by a single layer in later figures, for convenience. It will be understood that the composition of the optical stack16may vary both in the number of layers and the components of those layers, and that the layers discussed above are merely exemplary.

A variety of methods can be used to perform the patterning and etching processes discussed with respect to the various embodiments disclosed herein. The etches used may be either a dry etch or a wet etch, and may be isotropic or anisotropic. Suitable dry etches include, but are not limited to: SF6/O2, CHF3/O2, SF2/O2, CF4O2, and NF3/O2. Generally, these etches are suitable for etching one or more of SiOx, SiNx, SiOxNy, spin-on glass, Nissan™ hard coat, and TaOx, but other materials may also be etched by this process. Materials which are resistant to one or more of these etches, and may thus be used as etch barrier layers, include but are not limited to Al, Cr, Ni, and Al2O3. In addition, wet etches including but not limited to PAD etches, BHF, KOH, and phosphoric acid may be utilized in the processes described herein, and may generally be used to etch metallic materials. Generally, these etches may be isotropic, but can be made anisotropic through the use of a reactive ion etch (RIE), by ionizing the etch chemicals and shooting the ions at the substrate. The patterning may comprise the deposition of a photoresist (PR) layer (either positive or negative photoresist), which is then used to form a mask. Alternately, a hard mask can be utilized. In some embodiments, the hard mask may comprise metal or SiNx, but it will be understood that the composition of the hard mask may depend on the underlying materials to be etched and the selectivity of the etch to be used. In The hard mask is typically patterned using a PR layer, which is then removed, and the hard mask is used as a mask to etch an underlying layer. The use of a hard mask may be particularly advantageous when a wet etch is being used, or whenever processing through a mask under conditions that a PR mask cannot handle (such as at high temperatures, or when using an oxygen-based etch). Alternate methods of removing layers may also be utilized, such as an ashing etch or lift-off processes.

InFIG. 11B, it can be seen that a layer82of sacrificial material is deposited over the optical stack16. InFIG. 11C, the sacrificial layer82has been patterned and etched to form tapered apertures86, which correspond to the locations of post or support regions. These apertures86are advantageously tapered in order to facilitate continuous and conformal deposition of overlying layers.

InFIG. 11D, a layer84of inorganic post material is deposited over the patterned sacrificial layer82, such that the inorganic post layer84also coats the side walls and the base of the tapered apertures86. In certain embodiments, the inorganic post layer84is thinner than the sacrificial layer82, and is conformal over the sacrificial layer82. In other embodiments, post layer84may have a thickness between 1000 Å and 5000 Å. It will be understood that depending on the embodiment and the materials being used, thicknesses both less than this range and greater than this range are usable. In certain embodiments, the inorganic post layer84may comprise silicon nitride (SiNx) or SiO2, although a wide variety of other materials may be used, some of which are discussed in greater detail below. InFIG. 11E, the inorganic post layer84is patterned and etched to form inorganic posts88. It can be seen inFIG. 11Ethat the edges of the inorganic posts88preferably taper which, like the tapered or sloped sidewalls of the apertures86, facilitate continuous and conformal deposition of overlying layers. It can be seen that the post structure88in the illustrated embodiment has a thickness which is thinner than that of the sacrificial layer82, and comprises a substantially flat base portion89, a sloped sidewall portion87, and a substantially horizontal wing portion85which extends over a portion of the sacrificial material. Thus, the post88advantageously provides a substantially flat surface at the edge of the post for supporting an overlying movable layer66(SeeFIG. 11G), minimizing stress and the resultant undesired deflection which might occur if the movable layer66were deposited over a less flat edge.

In one embodiment, the inorganic post layer84and resultant post88comprise diamond-like carbon (DLC). In addition to being extremely hard and stiff (roughly 10× harder than SiO2), the DLC inorganic post layer84can be etched with an O2dry etch. Advantageously, an O2dry etch is highly selective relative to a wide variety of sacrificial materials, including but not limited to Mo and a-Si sacrificial material, as well as other sacrificial materials discussed above. An inorganic post comprising DLC thus provides a very stiff post, lessening the likelihood and amount of downward flexure of the edges of the support post88when overlying moving or mechanical layers are pulled downward during MEMS operation, while permitting the use of an etch which is relatively benign to a wide variety of materials.

InFIG. 11F, a highly reflective layer90is deposited over the inorganic posts88and the exposed portions of the sacrificial layer82. A mechanical layer92is then deposited over the highly reflective layer90. For convenience, as noted above, the highly reflective layer90and the mechanical layer92may be referred to and depicted in subsequent figures as a movable layer66(seeFIG. 11G), or more particularly as a deformable reflective layer whenever the mechanical layer92is deposited directly over the highly reflective layer90. In alternate embodiments, the movable layer66may comprise a single layer which has the desired optical and mechanical properties. For example, mechanical or moving layers for MEMS mechanical switches need not include reflective layers. In still further embodiments, as already discussed, the movable layer may comprise a mechanical layer and a reflective layer which are substantially separated, such as layers14and34ofFIG. 7C. An exemplary process for forming such a MEMS device having partially separated mechanical and reflective layers is discussed in greater detail below with respect toFIGS. 35A-35Hand36A-36C. InFIG. 11G, a release etch is performed to selectively remove the sacrificial layer82, forming an interferometric modulator element60having an interferometric gap19through which the movable layer66can be moved in order to change the color reflected by the interferometric modulator element60. Prior to the release etch, the movable layer66is preferably patterned to form columns (not shown), and may advantageously be further patterned to form etch holes (see, e.g., etch holes100inFIG. 9J) which facilitate access to the sacrificial layer by the release etch.

In an alternate embodiment (as described below with respect toFIG. 17), the reflective layer may be deposited prior to the deposition and etching of the support layer84, such that the reflective layer will underlie the support structure88in the finished modulator element.

In yet another embodiment, support structures may be formed both above and below the movable layer66.FIGS. 12A-12Ddepict such an embodiment, which includes the steps ofFIGS. 11A-11F. InFIG. 12A, it can be seen that once the reflective layer90and the mechanical layer92have been deposited over the underlying support structure88, a rivet layer96is deposited over the mechanical layer92.

Subsequently, as seen inFIG. 12B, the rivet layer96is patterned and etched to form support structures62located above the mechanical layer92. In certain embodiments, the same mask used in the steps ofFIG. 11Eto pattern the underlying support structures88may be used to pattern the overlying support structures62.FIG. 12Cdepicts the patterning and etching of the mechanical layer92and the reflective layer90to form etch holes100in those layers, exposing the sacrificial layer82.

Finally, as shown inFIG. 12D, the sacrificial layer82is etched to remove the sacrificial material and release the interferometric modulator, permitting movement of movable layer66through the interferometric gap19. Thus, an interferometric modulator display element has been formed, wherein support structures62and88sandwich portions of the movable layer66in the depression originally defined by the aperture86(FIG. 11C), providing additional support and rigidity, and in certain embodiments, permitting the use of the upper support structures62for other purposes (e.g., seeFIG. 7Eand attendant description), as discussed above.

In other embodiments, it may be desirable to provide an underlying rigid support structure having a substantially flat upper surface. One process for fabricating one such embodiment of an interferometric modulator is described with respect toFIGS. 13A-13E. This process includes the steps ofFIGS. 11A-11D. InFIG. 13A, it can be seen that a layer of photoresist material134is deposited over the layer of rigid support material84in order to form a mask, which will be used to etch the support material84to form support structures88, as discussed above with respect toFIG. 11D. It can be seen that the deposited photoresist material134is thick enough to extend above the level of the rigid support layer84, completely filling depressions136in the support layer84corresponding to the underlying tapered apertures86(FIG. 11B).

InFIG. 13B, the photoresist material134has been patterned to form a mask140, and the mask has been used to etch the underlying rigid support layer84, forming support structures88. InFIG. 13C, the photoresist material of the mask has been etched back such that the remaining photoresist material134is located within the depressions136in the support structures88. InFIG. 13D, a reflective layer90and a mechanical layer92are deposited over the top of the support structures88, including the remaining photoresist material134, forming a movable layer66. As can be seen, the use of the remaining photoresist material134forms a substantially flat or planar surface on which the components of the movable layer66may be deposited, as compared to the embodiment shown inFIG. 11G. The rigidity of the support structures is also increased by the additional material within the depression. InFIG. 13E, etch holes100have been formed in the movable layer66, and a release etch has been performed to remove the sacrificial layer82, thereby releasing the interferometric modulator element60.

In alternate embodiments, the photoresist mask used to form the support structures88may be completely removed, and a filler material filling the cavities136of the support structures88may be deposited in a separate step, which may have the advantage of providing a stiffer rivet material, such as spin-on dielectric. In such an embodiment, any suitable material may be utilized, including but not limited to planarization materials discussed above. However, the process discussed with respect toFIGS. 13A-13Eadvantageously minimizes the steps required to fabricate such a modulator element by eliminating the separate deposition of an additional layer. In yet further embodiments, a rigid support structure similar to the rigid support structures62ofFIG. 9Jand other embodiments may additionally be formed over the movable layer66ofFIG. 13E, in order to provide additional support.

FIGS. 14A-14Cillustrate one set of alternative steps which may be performed to ensure that the reflective layer90will not underlie the base of the support structure. These steps may be performed, for example, after the steps ofFIG. 9A-9D. InFIG. 14A, it can be seen that a reflective layer90is deposited over the unetched sacrificial layer82. InFIG. 14B, it can be see that both the reflective layer90and the underlying sacrificial layer82have been patterned and etched to form tapered apertures116. InFIG. 14C, a mechanical layer92is deposited over the etched sacrificial and reflective layers82and90. Unlike the tapered apertures86ofFIG. 9E, it can be seen that the side walls of the tapered apertures116will not be coated with the reflective layer90(seeFIG. 9F), but are rather coated with the mechanical layer92, such that the mechanical layer92is in contact with the underlying dielectric layer78. It will be understood that an interferometric modulator element may be fabricated by, in one embodiment, subsequently performing the steps described with respect toFIGS. 9G-9J, including formation of a rivet structure.

FIGS. 15A-15Cillustrate another series of alternative steps which may be used to eliminate those portions of the reflective layer which will underlie the base of the support structure to be formed. These steps may be performed after the steps ofFIGS. 9A-9E. Once the sacrificial layer82has been patterned and etched to form tapered apertures86, a reflective layer90is deposited over the sacrificial layer82, as shown inFIG. 15A. InFIG. 15B, the reflective layer90is patterned and etched to remove at least the portions of the reflective layer that are in contact with the underlying dielectric layer78. In further embodiments, the portions of the reflective layer90in contact with the side walls of the tapered aperture86may also be removed. InFIG. 15C, it can be seen that a mechanical layer92is deposited over the etched sacrificial and reflective layers82and90. Subsequently, the steps described with respect toFIGS. 9G-9Jmay be performed in order to fabricate an interferometric modulator element including a rivet structure.

With reference toFIG. 16A, in certain embodiments comprising a post structure, an etch barrier layer130is provided which protects the sacrificial layer82during the etching of the inorganic post layer84(seeFIG. 11D) to form the inorganic posts88(SeeFIG. 16B). In the illustrated embodiment, the etch barrier layer130is deposited over the sacrificial layer82prior to patterning and etching to form the tapered apertures86(e.g., between the steps ofFIG. 9DandFIG. 9E). The etch barrier layer130is then patterned and etched either prior to or at the same time as the forming of the tapered apertures86(e.g., may be deposited and patterned in the same manner as the reflective layer ofFIGS. 14A-14C). As can be seen inFIG. 16A, the etch barrier layer130covers only the portion of the sacrificial layer82away from the tapered aperture86. Advantageously, patterning and etching the etch barrier layer130separately from (e.g., prior to) etching the sacrificial layer82permits greater control over the etching of the etch barrier130, preventing the barrier130from overhanging the aperture86due to the aperture etch undercutting the etch barrier130. Such an undercut would negatively affect the continuous and conformal deposition of the post layer84(seeFIG. 11D). Examples of suitable etch barriers include, but are not limited to, Al, Al2O3, Cr, and Ni. In certain embodiments, as discussed in greater detail below with respect toFIGS. 17A-17B, a reflective layer may advantageously serve as an etch barrier layer130.

The inorganic post layer is then deposited, and etched to form the inorganic posts88, as seen inFIG. 16B. As can be seen, the sacrificial layer82has not been exposed to the etching process which forms the posts, as the mask used to protect the inorganic post layer84and define the post structures88during the etching process protects the post layer overlying the tapered aperture86, and the etch barrier layer130, which now extends between the inorganic posts88, protect those portions of the sacrificial layer82. Because of the etch barrier130, an etch can be used to form the support post88which is nonselective between the inorganic post and the sacrificial layer. This is particularly advantageous with respect to dry etches, such as etches involving chemistries such as SF6/O2, CHF3/O2, CF4/O2, NF3/O2and all other fluorine-containing chemistry, but is also useful with respect to wet etches. As discussed in greater detail, below, in certain embodiments the etch barrier layer130may advantageously remain in the finished device.

With reference toFIGS. 17A and 17B, in an alternate embodiment, an etch barrier layer130is deposited after the sacrificial layer82has been patterned and etched to form the tapered apertures86, such that it coats the walls and base of the tapered apertures86. The inorganic post layer is then deposited above the etch barrier layer130and patterned and etched to form posts88, as depicted inFIG. 17A. As can be seen inFIGS. 17A and 17B, this etch barrier layer130underlies the entire inorganic post88, in addition to protecting the sacrificial layer82not covered by the inorganic post88.

As can be seen inFIG. 17B, which depicts the modulator section ofFIG. 17Aafter the release etch has been performed, the upper portion of the inorganic post88is protected by the mechanical layer92deposited over the inorganic post88. Thus, the inorganic post88is completely enclosed by the combination of the etch barrier layer130and the mechanical layer92during the release etch. Because it is completely enclosed, etch chemistries which are nonselective with respect to the inorganic post material and the sacrificial material may be used in both the inorganic post etch and the release etch. In a particular embodiment, the same material may be used as both the sacrificial material82and the inorganic post material which forms post88, due to the isolation of each layer from the etch performed on the other layer.

In the embodiment depicted inFIG. 17B, that portion of the etch barrier layer130which extends beyond the patterned inorganic post88may remain in the finished interferometric modulator, or may be removed at some point during the fabrication process, as described with respect toFIGS. 18 and 19, below. In one embodiment, the etch barrier layer130may comprise aluminum or another highly reflective material capable of serving as an etch barrier layer. In this embodiment, the etch barrier layer130may be left in the finished modulator to serve as the reflective surface in a deformable reflective layer. In such an embodiment, only the mechanical layer92need be deposited over the inorganic post88and the etch barrier layer130, as the reflective material comprising the etch barrier layer130will deform along with the mechanical layer92. In another embodiment, the etch barrier layer may comprise a substantially transparent material, such as a thin layer of Al2O3. In interferometric modulators or other optical MEMS elements of this type, an additional reflective layer (not shown), is preferably deposited prior to deposition of the mechanical layer92, in order to form a deformable reflective layer such as movable layer66ofFIG. 11G.

In one particular embodiment, the etch barrier layer130comprises Al, and is resistant to a fluorine-based etch. In another embodiment, which is particularly suitable for use when the sacrificial layer comprises a-Si, rather than Mo, the etch barrier layer comprises Al or Al2O3, and may alternately comprise Ti or W. Other suitable etch barrier materials include, but are not limited to, Cr and Ni. In one embodiment, the etch barrier layer is between 40 and 500 Angstroms, but may be either thicker or thinner, depending on the embodiment. In an embodiment in which the etch barrier layer130comprises a conductive material, removal of the conductive layers within the optical stack16in the area directly underlying the support structure88advantageously minimizes the risk of a short between the conductive etch barrier layer and the conductive layers within the optical stack16(see, e.g.,FIG. 9Band attendant description).

In an alternate embodiment, described with respect toFIG. 18, an etch barrier layer130may be deposited, and an overlying post structure88formed, as described with respect toFIG. 17A. After the overlying post structure88is formed, a patterning and etching process may be used to remove those portions of the etch barrier layer130located away from the post structure88, such that the remaining portions of the etch barrier layer130remain underneath the post structure88, protecting it from the subsequent release etch. Advantageously, because the portions of the etch barrier layer not underlying or very close to the support post have been removed, the optically active portions of the display are substantially unaffected by the etch barrier layer. Thus, the composition and thickness of the etch barrier layer may be selected purely on the basis of the desired level of protection from the release etch, without regard for the opacity of the etch barrier layer.

In a further refinement of the above process, described with respect toFIG. 19, it can be seen that depending on the composition of the post structure88, the exposed portions of the etch barrier layer130may be etched without the need for an additional patterning process, using the post structure88itself as a hard mask during the etching of the etch barrier layer130. Advantageously, the remaining portion of the etch barrier layer130is substantially flush with the edge of the post structure88, such that no more of the etch barrier layer130is left than is necessary to protect the post structure88from the release etch, even further minimizing optical effects of the etch stop130.

With respect toFIG. 20, in an embodiment in which a support structure is formed adjacent to a movable layer66, such as the illustrated rivet structure62overlying the movable layer66, it may be desirable to provide for additional adhesion to secure the support structure62to the movable layer. In particular, because the actuation of the interferometric modulator will tend to pull the movable layer66in a direction away from the overlying support structure62, improved adhesion between the movable layer66and the overlying support structure62will minimize the risk that the movable layer66will begin to pull away from the rivet62. In the illustrated embodiment, after the deposition of the mechanical layer92(seeFIG. 9F), an adhesion enhancement layer136may be deposited. As shown, the adhesion enhancement layer136has been deposited after deposition of the mechanical layer and prior to patterning of the rivet layer, which are simultaneously patterned to form the rivet structure62.

In another embodiment in which support structures such as post structures88ofFIG. 11Eare formed prior to deposition of the movable layer, an adhesion enhancement layer may be formed over the post layer84(seeFIG. 11D) prior to patterning the post layer84to form support posts88(seeFIG. 11E). However, it will be understood that the adhesion enhancement layer may alternately be deposited and patterned after the formation of support structure88, such that the adhesion enhancement layer overlies the tapered edges of the support post88, enhancing the efficacy of the adhesion enhancement layer but adding to the complexity of the process by adding separate mask and etch steps.

These adhesion enhancement layers may comprise any of a wide variety of materials based on the composition of the movable layer and the layers forming the support structures, as certain materials may provide different amounts of adhesion enhancement when in contact with different materials. One example of an adhesion enhancement material which is useful in conjunction with a wide variety of mechanical and rivet materials is Cr, but many other materials may be used as adhesion enhancement layers.

As discussed above, modifications may be made to a fabrication process in order to protect a deposited rivet structure from the release etch. Advantageously, this both permits the use of a wider range of materials in the rivet structure, as the sacrificial material need not be selectively etchable relative to the rivet material if the rivet material is not exposed to the release etch, and minimizes any damage which might be caused to the rivet structure if it was exposed to the release etch.

In one embodiment, described with respect toFIG. 21, it can be seen that a rivet structure62has been formed over a mechanical or moving layer, which in the illustrated embodiment is a reflective movable layer66, which extends over a patterned sacrificial layer82. The rivet62is then covered with a protective layer104, which will remain over the rivet62at least until the release etch has been performed, at which point it may or may not be removed. In one embodiment, the protective layer104comprises a layer of photoresist material. In another embodiment, a distinct layer of an alternate etch barrier material forms the protective layer104. The protective layer104may be any material sufficiently resistant to the release etch to provide the desired level of protection for the rivet. In one embodiment, for example, the rivet62may comprise SiNx, the release etch may be a XeF2etch, and the protective layer104may comprise a layer of photoresist material deposited after the rivet104has been formed.

In another embodiment, the stability of a rivet structure may be increased through the securing or anchoring of the rivet structure to structures underlying the mechanical layer or the deformable reflective layer. In one embodiment, depicted inFIG. 22, the movable layer66(which may comprise a mechanical layer92and a reflective layer90, seeFIG. 9J) is deposited over the patterned sacrificial layer82such that it takes the shape of the tapered apertures86. The movable layer66is then etched at at least a portion of the base of the tapered aperture86so as to expose an underlying layer, which in this case is the dielectric layer at the top of the optical stack16. The rivet layer is then deposited as discussed above and patterned to form the rivet structure62. As can be seen, the rivet structure62now extends through an aperture106extending through the substantially flat base portion99of the movable layer66, securing the rivet structure62to the underlying optical stack16, advantageously providing additional stability to the rivet structure, both because the adhesion of the rivet material to the underlying dielectric layer may be better than the adhesion to the mechanical layer92and because the rivet structure62no longer relies on the adhesion between the movable layer66and the optical stack16to hold the rivet structure62in place. It will also be understood that in alternate embodiments, the rivet structure62may be secured to a structure other than the upper surface of optical stack16. For instance, in an alternate embodiment (not shown) in which the rivet structure62and a post structure underlying the movable layer66sandwich a portion of the movable layer66, the rivet structure can be secured to the underlying post structure through an aperture in the movable layer66, or to any underlying layer with better adhesion, such as, in certain embodiments, the reflective layer90of the movable layer66.

In another process, described with respect toFIGS. 23A-23E, a plating process can be used to form inorganic post structures. This process includes the steps ofFIGS. 9A-9E. InFIG. 23A, it can be seen that a thin seed layer208is deposited over the patterned sacrificial layer82. In one embodiment, the seed layer208comprises a thin layer of copper and can be formed by sputtering or CVD. In another embodiment, the seed layer may comprise aluminum, and may serve as the reflective layer in an optical MEMS device by omitting the removal step described below with respect toFIG. 23E. InFIG. 23B, a mask202is formed over the seed layer208, having an aperture210which defines the shape of the post to be formed by the plating process. It can be seen that the edges of the illustrated aperture210have a reentrant profile or overhang (also referred to herein as a negative angle), such that the post structure to be formed will have a taper which corresponds to the tapered edges of the aperture210. InFIG. 23C, it can be seen that a plating process is used to form a layer212of post material. InFIG. 23D, the mask202is removed, leaving only the seed layer208and the post layer212. Next, inFIG. 23E, the portions of the seed layer208located away from the post layer212are etched away (e.g., using the post layer212as a mask for this etch), forming an inorganic post214comprising the remaining portions of the seed layer208and the post layer212. Subsequently, a mechanical or deformable reflective layer can be deposited over the post, which is facilitated by the tapered angle at edge of the post wings. As discussed above, in an embodiment in which the seed layer comprises aluminum or another reflective material, the removal step ofFIG. 23Emay be omitted from the process, and a mechanical layer may be deposited over the reflective seed layer.

Metal which has been anodized to form metal oxide can also be used to form support structures. In one embodiment, discussed with respect toFIGS. 24A-24B, anodized aluminum or Ta is utilized in the formation of an inorganic post. InFIG. 24A, it can be seen that a metallic layer254, which may be Al or Ta, is formed over a patterned sacrificial layer82. InFIG. 24B, the layer254has been patterned to form the shape of the inorganic posts, and has been anodized to form Al2O3or Ta2O5inorganic posts256. Advantageously, anodized Al2O3or Ta2O5forms a dielectric layer which is free from pinhole defects, greatly reducing the chance of a short between the mechanical layer deposited thereover and the optical stack16.

As discussed above, it is easier to consistently and conformally deposit rivet material over a tapered aperture. However, because of the tapered shape, certain rivet structures may be susceptible to downward deflection of the edges of the rivet structures, particularly in embodiments in which the rivet layer is thin relative to the mechanical layer. In certain embodiments, it may be desirable to provide additional underlying support for a rivet structure, in order to constrain such downward deflection of the edges of the rivet structure.FIGS. 25A-25HandFIG. 26illustrate embodiments in which additional support may be provided through modification of a support structure.

In one embodiment, described with respect toFIGS. 25A-25H, sacrificial material which is protected from the release etch may be utilized to provide additional support to the rivet structure. This process for fabricating an interferometric modulator element comprising such supports includes the steps described with respect toFIGS. 9A-9D. InFIG. 25A, the sacrificial layer82is patterned and etched to remove annular sections120of sacrificial material, leaving columns122of sacrificial material separated from the remainder of the sacrificial layer82.

InFIG. 25B, protective material124is deposited such that it fills annular sections120. As can be seen, the protective material preferably completely fills the annular sections120. Advantageously, the material comprising the sacrificial layer82is selectively etchable relative to the protective material124, which may be, for example, a polymeric material or a photoresist material. Advantageously, the protective material124may comprise a self-planarizing material, such as spin-on-dielectric, so as to facilitate filling the annular section120, and so as to provide a planar surface for the subsequent deposition of an overlying movable layer. However, depending on the size of the annular structure120and the method used to deposit the protective material124, a variety of materials may be suitable for use as the protective material124. InFIG. 25C, it can be seen that the protective material has been etched back to the level of the sacrificial layer82, such that the upper surface of the isolated columns122of sacrificial material is exposed.

InFIG. 25D, a second patterning and etching process is utilized to form tapered apertures126within the isolated columns122of sacrificial material. InFIG. 25E, a reflective layer90and a mechanical layer92are deposited over the sacrificial material, followed by the deposition of a rivet layer96over the mechanical layer. It will be understood that variations in the fabrication process as discussed above may advantageously be used to remove the portion of the reflective layer90which will underlie the support post, as depicted inFIG. 25E.

InFIG. 25F, the rivet layer96is etched to form support structures62, and the mechanical layer92and reflective layer90are subsequently patterned and etched to form etch holes100and optionally also to separate strips of the movable layer66, as shown inFIG. 8. Thus,FIG. 25Fshows an unreleased MEMS device. InFIG. 25G, a release etch is performed to remove those portions of the sacrificial layer82not enclosed by the annular sheaths of protective material124(e.g., the columns122). At this point, an interferometric modulator element60is formed having rivet structures62overlying the movable layer66, and columns122of unetched sacrificial material surrounded by sheaths of protective material124located underneath and around the depressions in the movable layer66. Optionally, the sheath of protective material124may be removed by a subsequent step, through, for example, an ashing or etching process, resulting in an interferometric modulator comprising posts of exposed, but unetched, sacrificial material, as seen inFIG. 25H.

In yet another embodiment, desired supplemental support for rivet support structures such as62may be provided through use of the same material used to form the rivet structures. In one embodiment, described with respect toFIGS. 26A-26E, an alternate support post and rivet structure is formed from a spin-on material. InFIG. 26A, it can be seen that a layer of sacrificial material82has been deposited and patterned to form tapered apertures86, and a movable layer66has been deposited over the patterned sacrificial material82. InFIG. 26B, holes140have been patterned in the movable layer66, and the sacrificial material82is etched to form vias142which extend, in this embodiment, from the holes140to the underlying optical stack16.FIG. 26Cdepicts an overhead view of this area at this point in the fabrication process, in which it can be seen that multiple vias142surround the depression corresponding to the tapered aperture86. Any number or shape of the vias may be utilized, and the tapered aperture86may take multiple possible shapes. InFIG. 26D, a layer146of spin-on material is deposited. The spin-on material, or other self-planarizing material, will flow to fill the vias142. In this embodiment, the spin-on material fills the tapered aperture86, and flows through the holes140to fill the vias142. Finally, inFIG. 26E, it can be seen that the spin-on material is cured and patterned to remove the spin-on material located away from the tapered aperture86and the vias142, forming a support structure150which comprises a rivet-like upper portion and post-like structures or legs152extending from the rivet-like portion through the movable layer66to the optical stack16. The sacrificial layer82(seeFIG. 26D) has also been removed by a release etch to form an interferometric gap19. Advantageously, the legs152lend stability to the support structure, such that the sloped portion of the mechanical layer is not so easily pulled down into the cavity beneath it during operation, and the adhesion between the rivet and the mechanical layer is thereby enhanced. The rivet structure150is also adhered to the underlying optical stack, anchoring the rivet structure in place.

It will be understood that variations can be made to the above process flow. In certain embodiments, the holes140may be formed in the portions of the movable layer66overlying the sidewalls of the tapered aperture86. In other embodiments, the cavities142need not be vertical cavities, as depicted inFIG. 26B, but may extend in a diagonal direction, or may not extend all the way through the sacrificial layer to the optical stack16. For example, the holes140may be formed in the movable layer66in the sidewalls of the apertures86, and the cavities142may extend in a diagonal direction down to the optical stack16.FIG. 27illustrates such an embodiment, in which overlying support structures150comprise legs152which extend at an angle through holes in the tapered portion of the movable layer66. Such an angled etch may be performed, in one embodiment, through the use of a reactive ion etch (RIE), although other suitable techniques may also be used. In certain embodiments, the support structures150may comprise discrete legs152, as shown inFIGS. 26E and 27, or may comprise a continuous annular support structure.

Various other methods may be used to form support structures and other components of the interferometric modulator. In certain embodiments, a plating process can be utilized to form component of an interferometric modulator such as rivet and post support structures.FIGS. 28A-28Billustrate a portion of a process for utilizing a plating process to form a rivet structure160. This process includes the steps ofFIGS. 9A-9F. InFIG. 28A, it can be seen that a mask162, which may be a photoresist mask in certain embodiments, is deposited over the movable layer66, and patterned to form an aperture164which will define the shape of the desired rivet structure. InFIG. 28B, it can be seen that a plating process has been used to form a rivet structure160within the aperture164. In one embodiment, the plating process is an electroplating process. In various embodiments, the rivet160may comprise materials included, but not limited to, nickel, copper, and gold, but any material that can be plated and is preferably not susceptible to the release etch may be used.

In addition to forming the various components of the interferometric modulators, the layers deposited in the fabrication processes discussed herein can also be used to form other components within or connected to an array of interferometric modulator elements.FIG. 29depicts a portion of an interferometric modulator element in which a movable layer66forms a strip electrode170, and a conductive layer such as a conductive layer72within the optical stack16(seeFIG. 9A) forms a second strip electrode172which runs beneath and perpendicular to the first strip electrode170. It can also be seen that multiple support structures may be provided across the length of the strip electrode170, such as rivet structures62. The first, or upper, strip electrode170is electrically connected to a conductive interconnect or lead174, which may in turn be electrically connected to a landing pad or connection point176, at which an electrical connection may be made with an external component, such as a bump. Similarly, the second, or lower, strip electrode172is electrically connected to a lead178and a connection point180. The first strip electrode170, which may also be referred to as a column electrode (although it will be understood that the designation of the upper electrode as the column electrode is arbitrary and depends simply on the orientation of the MEMS array), is generally spaced apart from the substrate by an air gap or interferometric cavity within the array, although it will be understood that at various locations within the array (e.g., at the support regions), no air gap may exist between the column electrode170and the substrate. The second strip electrode172, which may also be referred to as a row electrode, is generally fabricated either directly on the substrate, or if there are intervening layers, such that no interferometric gap exists between the second strip electrode172and the substrate).

In certain embodiments in which the lead178and the connection point180are formed from ITO with no overlying layers, a connection may be made directly between an external device and the connection point180. However, the high resistivity and contact resistance with ITO may make such an embodiment undesirable. In another embodiment, a layer of conductive material, such as the material which forms the movable layer66, may be deposited over the ITO for most of the length of the connection point180and lead178, in order to reduce the resistance of that portion of the structure. However, in certain embodiments in which the mechanical layer comprises a deformable reflective layer formed from two layers (e.g., a mechanical layer92and reflective layer90, as can be seen inFIG. 9F), contact resistance between certain of those layers may have an undesirable effect on the resistance of the lead178, particularly when one of those layers is aluminum, which has poor contact resistance in contact with an ITO layer.

Advantageously, a conductive material may be deposited over the ITO layer which has desirable contact resistance in contact with the ITO layer.FIGS. 30A and 30Bdepict steps in such a fabrication process, showing cross-sections taken along line30-30ofFIG. 29. InFIG. 30A, it can be seen that at a stage in the fabrication process prior to the deposition of the mechanical layer (e.g., a stage corresponding toFIG. 9Eor earlier), only a layer72of ITO has been deposited at this area (or any overlying layers, such as partially reflective layer74ofFIG. 9A, have been selectively removed). InFIG. 30B, however, the mechanical layer92has been deposited not only over the layers in the area where the interferometric modulator element is to be formed, but also over the connection point180(not shown) and the lead178, and thus directly overlies the ITO layer72. It can also be see that the reflective layer90(seeFIG. 9E) has either not been deposited over the ITO layer72, or has been selectively removed after deposition and prior to the deposition of mechanical layer92. In one embodiment, the reflective layer90(seeFIG. 9E) is deposited over the lead178and connection point180, but patterned and etched to remove those portions of the reflective layer prior to the deposition of the mechanical layer92. In one embodiment, the mechanical layer comprises Ni, which has favorable contact resistance in contact with ITO. The mechanical layer92is then patterned and etched to remove the portions of the layer not overlying the lead178or connection point180, as seen inFIG. 30B. Also, as can be seen with respect toFIG. 29, the mechanical layer (shown as shaded) is preferably also removed at the edge of the lead close to the array, to avoid shorting the strip electrodes170and172to one another.

Thus, in one embodiment, the mechanical layer is utilized as a conductive layer in contact with the ITO leads and connection points. In another embodiment in which the rivet material comprises a conductive material, the rivet material can instead be deposited over the ITO and used to form the conductive layer over the ITO, in place of the mechanical layer92ofFIGS. 30A-30B. In a particular embodiment, the rivet layer comprises Ni. Advantageously, this embodiment does not involve patterning and etching one portion of a deformable reflective layer (the reflective layer) separately from the other portion (the mechanical layer).

In a particular embodiment, the mechanical layer92comprises Ni, which has desirable resistance and contact resistance properties, but a wide variety of mechanical layer materials may be used. In another embodiment, the ITO layer need not extend all the way through the lead178to the connection point180. Rather, the deposited mechanical layer92may alone form the connection point180and a large portion of the lead178. In addition to lowering the resistance and contact resistance of these components, the deposition of the mechanical layer92also advantageously increases the height of these components, facilitating connections between external components.

Similarly, the mechanical layer92may form the lead174and the connection point176. In one embodiment, there is no need for the lead174or connection point176, which are in connection with column electrode170, to comprise any ITO, and the mechanical layer92may extend the entire length of the lead174in order to form a connection between the lead174and the strip electrode170. This is because the column electrode170is separated from the substrate, unlike the row electrode172which is formed on the substrate (e.g., a patterned strip of ITO).

Because the row and column leads would otherwise be exposed, and thus vulnerable to shorting and other damage which may occur due to environmental or mechanical interference, it may be desirable to deposit a passivation layer over the exposed row and column leads174and178. In a particular embodiment, the same material which is used to form the rivet structure62can be utilized to passivate the leads174,178, protecting them from external electrical or mechanical interference. Such an embodiment is described with respect toFIGS. 31A-31D. InFIG. 31A, which is a cross section of a partially fabricated lead174ofFIG. 29taken along the line31-31in accordance with a different embodiment, it can be seen that the mechanical layer92has been deposited, but not yet etched. InFIG. 31B, it can be seen that the layer96of rivet material has been deposited (as seen in, for example,FIG. 9G), and this layer of rivet material has also been deposited over the mechanical layer92located outside the array of interferometric modulators. InFIG. 31C, it can be seen that the layer of rivet material has been patterned (as seen inFIG. 9H), and that the layer of rivet material has simultaneously been patterned to form a strip182which will overlie the lead304. Finally, inFIG. 31D, the mechanical layer has been patterned to separate the strip electrode170from the surrounding electrodes (and to form any necessary etch holes in the strip electrode), and is simultaneously patterned to form the lead174. In an alternate embodiment, the mechanical layer may be patterned and etched at the same time as the rivet layer. It will be understood that this rivet layer is either not deposited over the connection point180, or is etched to remove the portion of the rivet layer which covers the connection point180, in order to permit a connection to be made with an external component. It will also be understood that if the lead178(seeFIG. 19) in connection with the row electrode is passivated in accordance with the above process, the resultant lead178may comprise a layer of ITO72underlying the mechanical layer92.

In yet another embodiment, the mechanical layer92may be patterned prior to the deposition of the rivet layer, forming the lead174and separating the strip electrode170from the neighboring strip electrodes. Thus, the rivet layer may be subsequently patterned so as to cover not only the upper portion of the lead174, but also to protect the sides, as can be seen inFIG. 32, which depicts a lead174fabricated by this process viewed along the line31-31ofFIG. 19. Advantageously, this further protects the lead174. In other embodiments, passivation material may be deposited over the leads in a process distinct from the deposition of the support structure layer. In such a process, any suitable dielectic layer may be used to passivate the lead, and need not be suitable for use as a support structure layer. For example, any suitable dielectric layer used in the fabrication of the MEMS device, such as for example the dielectric layer within the optical stack or a dielectic layer used as an etch stop layer, may be used to passivate a lead.

In certain embodiments, it may be desirable to provide a movable layer66having varying stiffness over different parts of an MEMS element, or to more easily provide an array of MEMS elements wherein adjacent elements comprise movable layers having differing stiffness. For example, a modulator element in which the movable layer has different actuation voltages in different areas can be used to create grayscale, as differing amounts of the modulator elements can be actuated by modifying the applied voltage, as the actuation voltage will vary across the array of modulator elements. In other embodiments, additional stiffness may be desirable in areas which have less support, such as around the edges of the modulator element. One method of fabricating an interferometric modulator element having such a varying stiffness is described with respect toFIGS. 23A-23Band comprises the steps ofFIGS. 9A-9G.

InFIG. 33A, it can be seen that the rivet layer, which in this embodiment may comprise silicon oxide, has been etched to form support structures62, as described with respect toFIG. 9H. However, the embodiment ofFIG. 33Adiffers from that of9H, in that additional patches, or ribs,190of the rivet material have been left unetched. InFIG. 33B, it can be seen that the fabrication process is completed as discussed with respect toFIGS. 9I and 9J, resulting in an interferometric modulator element60(viewed along the line33B-33B ofFIG. 34) having support structures62and ribs190overlying portions of the movable layer66. A top view of the interferometric modulator element60ofFIG. 33Bhaving residual ribs190comprising rivet material is depicted inFIG. 24.

As discussed above, these residual ribs190may inhibit deformation of the movable layer66in the area surrounding the patch, such that that section of the movable layer66will require a higher actuation voltage. They may also be used to provide additional support to the mechanical area near the edges of the movable layer. In certain embodiments, the movable layer66may be susceptible to undesired curling or flexure. This may be particularly problematic at those areas of the movable layer66close to the gaps65between the strip electrodes of the movable layer66. The placement of such ribs190may control this undesired flexure, so as to ensure that the height of the interferometric gap19(see Figure23B) remains more constant across an interferometric modulator. In addition, as the positioning of these ribs structures190affects the stiffness of the movable layer66in the surrounding area, these rib structures190may be used to modify the actuation voltage required to move the MEMS device into an actuated state. This may be done, for instance, to normalize the actuation voltage across the element, or alternately to provide a differing actuation voltage across the MEMS element, such as to provide grayscale, as discussed above.

In a further embodiment, the rivet layer which is etched to form support structures62and residual rib structures190may comprise an electroactive material, such as a piezoelectric material. Through the application of electroactive material to the upper surface of the mechanical layer92in the form of ribs190, the behavior of the movable layer66can be further controlled. The application of electroactive material can, for instance, be used to modify the voltage applied at a given location in the modulator element.

As discussed above, the methods and structures discussed above may be used in conjunction with an optical MEMS device having a movable layer comprising a reflective layer which is partially detached from a mechanical layer.FIGS. 35A-35Hillustrate an exemplary process for forming support posts underlying a portion of the movable layer in such a MEMS device, which in the illustrated embodiment is an interferometric modulator. This process may include, for example, the steps described with respect toFIGS. 9A-9D, in which an optical stack is deposited, and a sacrificial layer is deposited over the optical stack.

InFIG. 35A, it can be seen that a reflective layer90is deposited over the sacrificial layer82. In certain embodiments, the reflective layer90may comprise a single layer of reflective material. In other embodiments, the reflective layer90may comprise a thin layer of reflective material with a layer of more rigid material (not shown) overlying the thin layer of sacrificial material. As the reflective layer of this embodiment will be partially detached from an overlying mechanical layer, the reflective layer90preferably has sufficient rigidity to remain in a substantially flat position relative to the optical stack16even when partially detached, and the inclusion of a stiffening layer on the side of the reflective layer located away from the optical stack can be used to provide the desired rigidity.

InFIG. 35B, the reflective layer90ofFIG. 35Ais patterned to form a patterned mirror layer220. In one embodiment, the patterned mirror layer220comprises a contiguous layer in which apertures corresponding to the locations of (but wider or narrower than) support structures have been formed. In another embodiment, the patterned mirror layer220may comprise multiple reflective sections detached from one another.

InFIG. 35C, a second sacrificial layer226is deposited over the patterned mirror layer220. Preferably, the second sacrificial layer226is formed from the same material as the first sacrificial layer82, or is etchable selectively with respect to surrounding materials by the same etch as the first sacrificial layer82. InFIG. 35D, tapered apertures86are formed which extend through both the second sacrificial layer226and the first sacrificial layer82.

InFIG. 35E, a layer of post material84has been deposited over the patterned sacrificial layers92and226, such that it coats the sides of the apertures86, as described with respect toFIG. 11D. InFIG. 35F, the layer of post material has been patterned to form post structures88, as described with respect toFIG. 11E. The patterned post structures88may overlap with the edges of the mirror layer220. It can also be seen inFIG. 35Ethat an aperture228has been formed in a portion of the second sacrificial layer196overlying the patterned mirror layer220, exposing at least a portion of the patterned mirror layer220.

InFIG. 35G, a mechanical layer92is deposited over the posts88and the exposed portions of the second sacrificial layer226and the patterned mirror layer220. IN particular, it can be seen that the mechanical layer92at least partially fills the aperture198(seeFIG. 35F), such that a connector portion222connecting the mechanical layer92and the patterned mirror layer220is formed.

InFIG. 35H, a release etch is performed which removes both the first sacrificial layer82and the second sacrificial layer226, thereby forming an interferometric gap19between the patterned mirror layer220and the optical stack. Thus, an optical MEMS device is formed, which includes a movable layer66comprising a mechanical layer92from which a patterned mirror layer220is suspended, where the patterned mirror layer220is partially detached from the mechanical layer92. This optical MEMS device, may be, for example, an interferometric modulator such as that described with respect toFIG. 7Cand elsewhere throughout the application. In non-optical MEMS, the suspended upper electrode need not be reflective.

It will be understood that the above process may be modified to include any of the methods and structures discussed above. In particular, it will be seen that the above process may be modified to include the formation of a rivet structure, either instead of or in conjunction with the formation of a post structure. In particular, in an embodiment in which only rivet structures are formed, the above process may be further simplified by forming the tapered apertures at the same time as the aperture overlying a portion of the mirror layer in which the connecting portion will be formed. In another embodiment in which a rivet layer is deposited, only a very thin layer of conductive material may be deposited in a step equivalent to that ofFIG. 35G, and a later deposited rivet layer (which can be dielectric) may be patterned and etched to serve the mechanical function of the mechanical layer, with the thin layer of conductive material serving the conductive function.

In a further embodiment, the same material which forms the post structures may be used to form stiffening portions on the upper surface of a detached mirror layer200.FIG. 36A-36Cillustrate such an embodiment, which includes the steps ofFIGS. 35A-35C. InFIG. 36A, it can be seen that at the same time as the tapered apertures86are formed, additional apertures230have been formed over the patterned mirror layer220, exposing portions of the patterned mirror layer220. In certain embodiments, these apertures230may advantageously take the form of grooves extending near edges of the patterned movable layer220, however a wide variety of shapes, including annular or substantially annular shapes, may be suitable.

InFIG. 36B, it can be seen that a layer of post material84has been deposited such that it not only coats the edges of the tapered apertures86, but also is deposited over the exposed portions of the patterned mirror layer220within the additional apertures230. InFIG. 36C, it can be seen that the fabrication process has proceeded in a similar fashion to that described with respect toFIGS. 35F-35H, and that a released interferometric modulator has been formed. In particular, it can be seen that the patterned mirror layer220comprises stiffening structures232(e.g., annular rings) on the upper surface of the patterned mirror layer220, formed from the same material as the posts88. It can also be seen that the portion of the mechanical layer92overlying the stiffening structures232has been removed to form apertures234. It will be understood that because the mirror layer220has been partially detached from the mechanical layer92, the mechanical layer92need not comprise a continuous layer of material, but may instead comprise, for instance, strips of mechanical material extending between connector portions222and support structures such as posts88. Thus, portions of the mechanical layer may be removed by the same patterning step that forms mechanical strips (seeFIG. 8), as depicted inFIG. 36C, in order to ensure that no connection remains between the stiffening structures232and the overlying mechanical layer92.

It will be understood that various combinations of the above embodiments are possible. For instance, in certain embodiments, certain of the support structures disclosed herein may be used in conjunction with other support structures disclosed herein, as well as other suitable support structures not discussed in this application. Various combinations of the support structures discussed above are contemplated and are within the scope of the invention. In addition, it will be understood that support structures formed by any of the methods above may be utilized in combination with other methods of forming support structures, in order to improve the rigidity and durability of those support structures.

It will also be recognized that the order of layers and the materials forming those layers in the above embodiments are merely exemplary. Moreover, in some embodiments, other layers, not shown, may be deposited and processed to form portions of an interferometric modulator element or to form other structures on the substrate. In other embodiments, these layers may be formed using alternative deposition, patterning, and etching materials and processes, may be deposited in a different order, or composed of different materials, as would be known to one of skill in the art.

It is also to be recognized that, depending on the embodiment, the acts or events of any methods described herein can be performed in other sequences, may be added, merged, or left out altogether (e.g., not all acts or events are necessary for the practice of the methods), unless the text specifically and clearly states otherwise.

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 of 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.