Patent Publication Number: US-2016232858-A1

Title: Creep resistant reflective structure in mems display

Description:
TECHNICAL FIELD 
     This disclosure relates to microelectromechanical systems (MEMS) display devices and more particularly to annealed multilayer thin film stacks in movable reflective structures to improve creep resistance in MEMS display devices. 
     DESCRIPTION OF THE RELATED TECHNOLOGY 
     Electromechanical systems (EMS) include devices having electrical and mechanical elements, actuators, transducers, sensors, optical components such as mirrors and optical films, and electronics. EMS devices or elements can be manufactured at a variety of scales including, but not limited to, microscales and nanoscales. For example, microelectromechanical systems (MEMS) devices can include structures having sizes ranging from about a micron to hundreds of microns or more. Nanoelectromechanical systems (NEMS) devices can include structures having sizes smaller than a micron including, for example, sizes smaller than several hundred nanometers. Electromechanical elements may be created using deposition, etching, lithography, 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 EMS device is called an interferometric modulator (IMOD). The term IMOD or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In some implementations, an IMOD display element may include a pair of conductive plates, one or both of which may be transparent and/or reflective, wholly or in part, and capable of relative motion upon application of an appropriate electrical signal. For example, one plate may include a stationary layer deposited over, on or supported by a substrate and the other plate may include a reflective membrane separated from the stationary layer by an air gap. The position of one plate in relation to another can change the optical interference of light incident on the IMOD display element. IMOD-based display devices have a wide range of applications, and are anticipated to be used in improving existing products and creating new products, especially those with display capabilities. 
     SUMMARY 
     The systems, methods and devices of this disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. 
     One innovative aspect of the subject matter described in this disclosure can be implemented in a MEMS display device including a substrate, a movable reflective structure over the substrate, where the movable reflective structure includes an annealed thin film stack, and one or more support structures over the substrate and connected to the movable reflective structure to support the movable reflective structure. The annealed thin film stack includes a first layer including an aluminum or aluminum alloy, a second layer including aluminum or aluminum alloy and over the first layer, and a third layer between the first layer and the second layer. The third layer is in contact with at least one of the first layer and the second layer, where the third layer includes a transition metal, the transition metal including at least one of: zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium. 
     In some implementations, one or both of the first layer and the second layer is doped with about 0.1 atomic % to about 10 atomic % of the transition metal. In some implementations, a stress of the movable reflective structure is less than about 200 MPa. In some implementations, the third layer has a thickness of less than about 5 nm, and the first layer and the second layer each have a thickness of equal to or greater than about 20 nm. In some implementations, a reflectance of the movable reflective structure is greater than about 80%. In some implementations, each of the first layer and the second layer is doped to include between about 1 atomic % and about 20 atomic % of one or both of oxygen and nitrogen. In some implementations, the first layer and the second layer are substantially identical in composition and thickness. 
     Another innovative aspect of the subject matter described in this disclosure can be implemented in a method of manufacturing a MEMS display device. The method includes providing a substrate, forming a support structure over the substrate, forming a movable reflective structure over the substrate and connected to the support structure, and annealing the movable reflective structure. The movable reflective structure includes a first layer including aluminum or aluminum alloy, a second layer including aluminum or aluminum alloy and over the first layer, and a third layer between the first layer and the second layer. The third layer is in contact with at least one of the first layer and the second layer, where the third layer includes a transition metal, the transition metal including at least one of: zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium. 
     In some implementations, forming the movable reflective structure includes depositing the first layer over the substrate, depositing the third layer on the first layer, and depositing the second layer on the third layer. In some implementations, depositing the first layer includes doping the first layer with one or both of oxygen and nitrogen, and depositing the second layer includes doping the second layer with one or both of oxygen and nitrogen. In some implementations, the method further includes depositing a fourth layer between the substrate and the first layer, where the fourth layer is substantially identical in thickness and composition with the third layer. In some implementations, the method further includes depositing a fifth layer between the substrate and the fourth layer, where the fifth layer is substantially identical in thickness and composition with the first layer and the second layer. 
     Details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Although the examples provided in this disclosure are primarily described in terms of EMS and MEMS-based displays the concepts provided herein may apply to other types of displays such as liquid crystal displays (LCDs), organic light-emitting diode (“OLED”) displays, and field emission displays. Other features, aspects, and advantages will become apparent from the description, the drawings and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view illustration depicting two adjacent interferometric modulator (IMOD) display elements in a series or array of display elements of an IMOD display device. 
         FIG. 2  is a system block diagram illustrating an electronic device incorporating an IMOD-based display including a three element by three element array of IMOD display elements. 
         FIGS. 3A-3E  are cross-sectional illustrations of varying implementations of IMOD display elements. 
         FIG. 4  is a flow diagram illustrating a manufacturing process for an IMOD display or display element. 
         FIGS. 5A-5E  are cross-sectional illustrations of various stages in a process of making an IMOD display or display element. 
         FIG. 6  shows a perspective view of an example MEMS display device having a movable reflective structure spaced apart from a stationary electrode by a gap. 
         FIG. 7A  shows a cross-sectional side view of an example thin film stack for a movable reflective structure with a transition metal layer sandwiched between two reflective layers. 
         FIG. 7B  shows a cross-sectional side view of an example movable reflective structure including a thin film stack over an optical layer. 
         FIG. 7C  shows a cross-sectional side view of an example thin film stack for a movable reflective structure with an additional transition metal layer. 
         FIG. 7D  shows a cross-sectional side view of an example thin film stack for a movable reflective structure with an additional transition metal layer and an additional reflective layer. 
         FIG. 8A  shows a graph illustrating stress of a movable reflective structure as a function of the zirconium deposition times, where the movable reflective structure includes an aluminum alloy layer sandwiched between two zirconium layers. 
         FIG. 8B  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8A  as a function of the zirconium deposition times. 
         FIG. 8C  shows a graph illustrating stress of a movable reflective structure as a function of zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two aluminum alloy layers. 
         FIG. 8D  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8C  as a function of the zirconium deposition times. 
         FIG. 8E  shows a graph illustrating stress of a movable reflective layer as a function of the zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two aluminum alloy layers. 
         FIG. 8F  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8E  as a function of zirconium deposition times. 
         FIG. 8G  shows a graph illustrating stress of a movable reflective structure as a function of zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two thick aluminum alloy layers. 
         FIG. 8H  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8G  as a function of zirconium deposition times. 
         FIG. 8I  shows a graph illustrating stress of a movable reflective structure as a function of zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two thick aluminum alloy layers. 
         FIG. 8J  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8I  as a function of zirconium deposition times. 
         FIG. 9  is a flow diagram of an example method of manufacturing a MEMS display device. 
         FIGS. 10A and 10B  are system block diagrams illustrating a display device that includes a plurality of IMOD display elements. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The following description is directed to certain implementations for the purposes of describing the innovative aspects of this disclosure. However, a person having ordinary skill in the art will readily recognize that the teachings herein can be applied in a multitude of different ways. The described implementations may be implemented in any device, apparatus, or system that can be configured to display an image, whether in motion (such as video) or stationary (such as still images), and whether textual, graphical or pictorial. More particularly, it is contemplated that the described implementations may be included in or associated with a variety of electronic devices such as, but not limited to: mobile telephones, multimedia Internet enabled cellular telephones, mobile television receivers, wireless devices, smartphones, Bluetooth® devices, personal data assistants (PDAs), wireless electronic mail receivers, hand-held or portable computers, netbooks, notebooks, smartbooks, tablets, printers, copiers, scanners, facsimile devices, global positioning system (GPS) receivers/navigators, cameras, digital media players (such as MP3 players), camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, electronic reading devices (e.g., e-readers), computer monitors, auto displays (including odometer and speedometer displays, etc.), cockpit controls and/or displays, camera view displays (such as the display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, microwaves, refrigerators, stereo systems, cassette recorders or players, DVD players, CD players, VCRs, radios, portable memory chips, washers, dryers, washer/dryers, parking meters, packaging (such as in electromechanical systems (EMS) applications including microelectromechanical systems (MEMS) applications, as well as non-EMS applications), aesthetic structures (such as display of images on a piece of jewelry or clothing) and a variety of EMS devices. The teachings herein also can be used in non-display applications such as, but not limited to, electronic switching devices, radio frequency filters, sensors, accelerometers, gyroscopes, motion-sensing devices, magnetometers, inertial components for consumer electronics, parts of consumer electronics products, varactors, liquid crystal devices, electrophoretic devices, drive schemes, manufacturing processes and electronic test equipment. Thus, the teachings are not intended to be limited to the implementations depicted solely in the Figures, but instead have wide applicability as will be readily apparent to one having ordinary skill in the art. 
     Some implementations described herein relate to a MEMS display device having improved creep resistance and stress control. The MEMS display device can include a substrate, a movable reflective structure (e.g., mirror) over the substrate, and one or more support structures over the substrate and connected to the movable reflective structure to support the movable reflective structure. The movable reflective structure can include a multilayer thin film stack, where the multilayer thin film stack can include at least a thin transition metal layer sandwiched between two aluminum or aluminum alloy layers. The multilayer thin film stack can be annealed to permit the aluminum or aluminum alloy layers to be doped with the transition metal. The microstructures of the multilayer thin film stack can be altered or controlled through the annealing processes to control the stress in the movable reflective structure, improve the creep resistance of the MEMS display device, and minimize compromising the reflectance and electrical conductivity of the MEMS display device. In some implementations, the aluminum or aluminum alloy layers can be doped with oxygen and/or nitrogen to further enhance mechanical properties. 
     Particular implementations of the subject matter described in this disclosure can be implemented to realize one or more of the following potential advantages. The annealed multilayer thin film stack with a transition metal layer sandwiched between two aluminum or aluminum alloy layers provides greater resistance against creep. Accordingly, the MEMS display device can be more resistant to thermal cycles and mechanical stresses so that the lifetime of the MEMS display device can be increased. The annealed multilayer thin film stack can also increase the mechanical strength of the MEMS display device by reducing the stress in the movable reflective structure. The annealed multilayer thin film stack can also improve the optical characteristics of the MEMS display device by increasing the reflectance of the movable reflective structure. Furthermore, the incorporation of a transition metal layer between two aluminum or aluminum alloy layers can provide greater control over the mechanical, optical, and electrical properties of the movable reflective structure. In some implementations, such properties may be tuned by the thickness of the transition metal layer and/or the annealing conditions of the multilayer thin film stack. 
     An example of a suitable MEMS display device or apparatus, to which the described implementations may apply, is a reflective display device. Reflective display devices can incorporate interferometric modulator (IMOD) display elements that can be implemented to selectively absorb and/or reflect light incident thereon using principles of optical interference. IMOD display elements can include a partial optical absorber, a reflector that is movable with respect to the absorber, and an optical resonant cavity defined between the absorber and the reflector. In some implementations, the reflector can be moved to two or more different positions, which can change the size of the optical resonant cavity and thereby affect the reflectance of the IMOD. The reflectance spectra of IMOD display elements can create fairly broad spectral bands that can be shifted across the visible wavelengths to generate different colors. The position of the spectral band can be adjusted by changing the thickness of the optical resonant cavity. One way of changing the optical resonant cavity is by changing the position of the reflector with respect to the absorber. 
       FIG. 1  is an isometric view illustration depicting two adjacent interferometric modulator (IMOD) display elements in a series or array of display elements of an IMOD display device. The IMOD display device includes one or more interferometric EMS, such as MEMS, display elements. In these devices, the interferometric MEMS display elements can be configured in either a bright or dark state. In the bright (“relaxed,” “open” or “on,” etc.) state, the display element reflects a large portion of incident visible light. Conversely, in the dark (“actuated,” “closed” or “off,” etc.) state, the display element reflects little incident visible light. MEMS display elements can be configured to reflect predominantly at particular wavelengths of light allowing for a color display in addition to black and white. In some implementations, by using multiple display elements, different intensities of color primaries and shades of gray can be achieved. 
     The IMOD display device can include an array of IMOD display elements which may be arranged in rows and columns. Each display element in the array can include at least a pair of reflective and semi-reflective layers, such as a movable reflective layer (i.e., a movable layer, also referred to as a mechanical layer) and a fixed partially reflective layer (i.e., a stationary layer), positioned at a variable and controllable distance from each other to form an air gap (also referred to as an optical gap, cavity or optical resonant cavity). The movable reflective layer may be moved between at least two positions. For example, in a first position, i.e., a relaxed position, the movable reflective layer can be positioned at a distance from the fixed partially reflective layer. In a second position, i.e., an actuated position, the movable reflective layer can be positioned more closely to the partially reflective layer. Incident light that reflects from the two layers can interfere constructively and/or destructively depending on the position of the movable reflective layer and the wavelength(s) of the incident light, producing either an overall reflective or non-reflective state for each display element. In some implementations, the display element may be in a reflective state when unactuated, reflecting light within the visible spectrum, and may be in a dark state when actuated, absorbing and/or destructively interfering light within the visible range. In some other implementations, however, an IMOD display element may be in a dark state when unactuated, and in a reflective state when actuated. In some implementations, the introduction of an applied voltage can drive the display elements to change states. In some other implementations, an applied charge can drive the display elements to change states. 
     The depicted portion of the array in  FIG. 1  includes two adjacent interferometric MEMS display elements in the form of IMOD display elements  12 . In the display element  12  on the right (as illustrated), the movable reflective layer  14  is illustrated in an actuated position near, adjacent or touching the optical stack  16 . The voltage V bias  applied across the display element  12  on the right is sufficient to move and also maintain the movable reflective layer  14  in the actuated position. In the display element  12  on the left (as illustrated), a movable reflective layer  14  is illustrated in a relaxed position at a distance (which may be predetermined based on design parameters) from an optical stack  16 , which includes a partially reflective layer. The voltage V 0  applied across the display element  12  on the left is insufficient to cause actuation of the movable reflective layer  14  to an actuated position such as that of the display element  12  on the right. 
     In  FIG. 1 , the reflective properties of IMOD display elements  12  are generally illustrated with arrows indicating light  13  incident upon the IMOD display elements  12 , and light  15  reflecting from the display element  12  on the left. Most of the light  13  incident upon the display elements  12  may be transmitted through the transparent substrate  20 , toward the optical stack  16 . A portion of the light incident upon the optical stack  16  may be transmitted through the partially reflective layer of the optical stack  16 , and a portion will be reflected back through the transparent substrate  20 . The portion of light  13  that is transmitted through the optical stack  16  may be reflected from the movable reflective layer  14 , back toward (and through) the transparent substrate  20 . Interference (constructive and/or destructive) between the light reflected from the partially reflective layer of the optical stack  16  and the light reflected from the movable reflective layer  14  will determine in part the intensity of wavelength(s) of light  15  reflected from the display element  12  on the viewing or substrate side of the device. In some implementations, the transparent substrate  20  can be a glass substrate (sometimes referred to as a glass plate or panel). The glass substrate may be or include, for example, a borosilicate glass, a soda lime glass, quartz, Pyrex, or other suitable glass material. In some implementations, the glass substrate may have a thickness of 0.3, 0.5 or 0.7 millimeters, although in some implementations the glass substrate can be thicker (such as tens of millimeters) or thinner (such as less than 0.3 millimeters). In some implementations, a non-glass substrate can be used, such as a polycarbonate, acrylic, polyethylene terephthalate (PET) or polyether ether ketone (PEEK) substrate. In such an implementation, the non-glass substrate will likely have a thickness of less than 0.7 millimeters, although the substrate may be thicker depending on the design considerations. In some implementations, a non-transparent substrate, such as a metal foil or stainless steel-based substrate can be used. For example, a reverse-IMOD-based display, which includes a fixed reflective layer and a movable layer which is partially transmissive and partially reflective, may be configured to be viewed from the opposite side of a substrate as the display elements  12  of  FIG. 1  and may be supported by a non-transparent substrate. 
     The optical stack  16  can include a single layer or several layers. The layer(s) can include one or more of an electrode layer, a partially reflective and partially transmissive layer, and a transparent dielectric layer. In some implementations, the optical stack  16  is electrically conductive, partially transparent and partially reflective, and may be fabricated, for example, by depositing one or more of the above layers onto a transparent substrate  20 . The electrode layer can be formed from a variety of materials, such as various metals, for example indium tin oxide (ITO). The partially reflective layer can be formed from a variety of materials that are partially reflective, such as various metals (e.g., chromium and/or molybdenum), 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 implementations, certain portions of the optical stack  16  can include a single semi-transparent thickness of metal or semiconductor which serves as both a partial optical absorber and electrical conductor, while different, electrically more conductive layers or portions (e.g., of the optical stack  16  or of other structures of the display element) can serve to bus signals between IMOD display elements. The optical stack  16  also can include one or more insulating or dielectric layers covering one or more conductive layers or an electrically conductive/partially absorptive layer. 
     In some implementations, at least some of the layer(s) of the optical stack  16  can be patterned into parallel strips, and may form row electrodes in a display device as described further below. As will be understood by one having ordinary skill in the art, the term “patterned” is used herein to refer to masking as well as etching processes. In some implementations, a highly conductive and reflective material, such as aluminum (Al), may be used for the movable reflective layer  14 , and these strips may form column electrodes in a display device. The movable reflective layer  14  may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of the optical stack  16 ) to form columns deposited on top of supports, such as the illustrated posts  18 , and an intervening sacrificial material located between the posts  18 . When the sacrificial material is etched away, a defined gap  19 , or optical cavity, can be formed between the movable reflective layer  14  and the optical stack  16 . In some implementations, the spacing between posts  18  may be approximately 1-1000 μm, while the gap  19  may be approximately less than 10,000 Angstroms (Å). 
     In some implementations, each IMOD display element, whether in the actuated or relaxed state, can be considered as a capacitor formed by the fixed and moving reflective layers. When no voltage is applied, the movable reflective layer  14  remains in a mechanically relaxed state, as illustrated by the display element  12  on the left in  FIG. 1 , with the gap  19  between the movable reflective layer  14  and optical stack  16 . However, when a potential difference, i.e., a voltage, is applied to at least one of a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding display element becomes charged, and electrostatic forces pull the electrodes together. If the applied voltage exceeds a threshold, the movable reflective layer  14  can deform and move near or against the optical stack  16 . A dielectric layer (not shown) within the optical stack  16  may prevent shorting and control the separation distance between the layers  14  and  16 , as illustrated by the actuated display element  12  on the right in  FIG. 1 . The behavior can be the same regardless of the polarity of the applied potential difference. Though a series of display elements in an array may be referred to in some instances as “rows” or “columns,” a person having ordinary skill in the art will readily understand that referring to one direction as a “row” and another as a “column” is arbitrary. Restated, in some orientations, the rows can be considered columns, and the columns considered to be rows. In some implementations, the rows may be referred to as “common” lines and the columns may be referred to as “segment” lines, or vice versa. Furthermore, the display elements may be evenly arranged in orthogonal rows and columns (an “array”), or arranged in non-linear configurations, for example, having certain positional offsets with respect to one another (a “mosaic”). The terms “array” and “mosaic” may refer to either configuration. Thus, although the display is referred to as including an “array” or “mosaic,” the elements themselves need not be arranged orthogonally to one another, or disposed in an even distribution, in any instance, but may include arrangements having asymmetric shapes and unevenly distributed elements. 
       FIG. 2  is a system block diagram illustrating an electronic device incorporating an IMOD-based display including a three element by three element array of IMOD display elements. The electronic device includes a processor  21  that may be configured to execute one or more software modules. In addition to executing an operating system, the processor  21  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. 
     The processor  21  can be configured to communicate with an array driver  22 . The array driver  22  can include a row driver circuit  24  and a column driver circuit  26  that provide signals to, for example a display array or panel  30 . The cross section of the IMOD display device illustrated in  FIG. 1  is shown by the lines  1 - 1  in  FIG. 2 . Although  FIG. 2  illustrates a 3×3 array of IMOD display elements for the sake of clarity, the display array  30  may contain a very large number of IMOD display elements, and may have a different number of IMOD display elements in rows than in columns, and vice versa. 
     The details of the structure of IMOD displays and display elements may vary widely.  FIGS. 3A-3E  are cross-sectional illustrations of varying implementations of IMOD display elements.  FIG. 3A  is a cross-sectional illustration of an IMOD display element, where a strip of metal material is deposited on supports  18  extending generally orthogonally from the substrate  20  forming the movable reflective layer  14 . In  FIG. 3B , the movable reflective layer  14  of each IMOD display element is generally square or rectangular in shape and attached to supports at or near the corners, on tethers  32 . In FIG.  3 C, the movable reflective layer  14  is generally square or rectangular in shape and suspended from a deformable layer  34 , which may include a flexible metal. The deformable layer  34  can connect, directly or indirectly, to the substrate  20  around the perimeter of the movable reflective layer  14 . These connections are herein referred to as implementations of “integrated” supports or support posts  18 . The implementation shown in  FIG. 3C  has additional benefits deriving from the decoupling of the optical functions of the movable reflective layer  14  from its mechanical functions, the latter of which are carried out by the deformable layer  34 . This decoupling allows the structural design and materials used for the movable reflective layer  14  and those used for the deformable layer  34  to be optimized independently of one another. 
       FIG. 3D  is another cross-sectional illustration of an IMOD display element, where the movable reflective layer  14  includes a reflective sub-layer  14   a . The movable reflective layer  14  rests on a support structure, such as support posts  18 . The support posts  18  provide separation of the movable reflective layer  14  from the lower stationary electrode, which can be part of the optical stack  16  in the illustrated IMOD display element. For example, a gap  19  is formed between the movable reflective layer  14  and the optical stack  16 , when the movable reflective layer  14  is in a relaxed position. The movable reflective layer  14  also can include a conductive layer  14   c , which may be configured to serve as an electrode, and a support layer  14   b . In this example, the conductive layer  14   c  is disposed on one side of the support layer  14   b , distal from the substrate  20 , and the reflective sub-layer  14   a  is disposed on the other side of the support layer  14   b , proximal to the substrate  20 . In some implementations, the reflective sub-layer  14   a  can be conductive and can be disposed between the support layer  14   b  and the optical stack  16 . The support layer  14   b  can include one or more layers of a dielectric material, for example, silicon oxynitride (SiON) or silicon dioxide (SiO 2 ). In some implementations, the support layer  14   b  can be a stack of layers, such as, for example, a SiO 2 /SiON/SiO 2  tri-layer stack. Either or both of the reflective sub-layer  14   a  and the conductive layer  14   c  can include, for example, an aluminum (Al) alloy with about 0.5% copper (Cu), or another reflective metallic material. Employing conductive layers  14   a  and  14   c  above and below the dielectric support layer  14   b  can balance stresses and provide enhanced conduction. In some implementations, the reflective sub-layer  14   a  and the conductive layer  14   c  can be formed of different materials for a variety of design purposes, such as achieving specific stress profiles within the movable reflective layer  14 . 
     As illustrated in  FIG. 3D , some implementations also can include a black mask structure  23 , or dark film layers. The black mask structure  23  can be formed in optically inactive regions (such as between display elements or under the support posts  18 ) to absorb ambient or stray light. The black mask structure  23  also can improve the optical properties of a display device by inhibiting light from being reflected from or transmitted through inactive portions of the display, thereby increasing the contrast ratio. Additionally, at least some portions of the black mask structure  23  can be conductive and be configured to function as an electrical bussing layer. In some implementations, the row electrodes can be connected to the black mask structure  23  to reduce the resistance of the connected row electrode. The black mask structure  23  can be formed using a variety of methods, including deposition and patterning techniques. The black mask structure  23  can include one or more layers. In some implementations, the black mask structure  23  can be an etalon or interferometric stack structure. For example, in some implementations, the interferometric stack black mask structure  23  includes a molybdenum-chromium (MoCr) layer that serves as an optical absorber, an SiO 2  layer, and an aluminum alloy that serves as a reflector and a bussing layer, with a thickness in the range of about 30-80 Å, 500-1000 Å, and 500-6000 Å, respectively. The one or more layers can be patterned using a variety of techniques, including photolithography and dry etching, including, for example, tetrafluoromethane (or carbon tetrafluoride, CFO and/or oxygen (O 2 ) for the MoCr and SiO 2  layers and chlorine (Cl 2 ) and/or boron trichloride (BCl 3 ) for the aluminum alloy layer. In such interferometric stack black mask structures  23 , the conductive absorbers can be used to transmit or bus signals between lower, stationary electrodes in the optical stack  16  of each row or column. In some implementations, a spacer layer  35  can serve to generally electrically isolate electrodes (or conductors) in the optical stack  16  (such as the absorber layer  16   a ) from the conductive layers in the black mask structure  23 . 
       FIG. 3E  is another cross-sectional illustration of an IMOD display element, where the movable reflective layer  14  is self-supporting. While  FIG. 3D  illustrates support posts  18  that are structurally and/or materially distinct from the movable reflective layer  14 , the implementation of  FIG. 3E  includes support posts that are integrated with the movable reflective layer  14 . In such an implementation, the movable reflective layer  14  contacts the underlying optical stack  16  at multiple locations, and the curvature of the movable reflective layer  14  provides sufficient support that the movable reflective layer  14  returns to the unactuated position of  FIG. 3E  when the voltage across the IMOD display element is insufficient to cause actuation. In this way, the portion of the movable reflective layer  14  that curves or bends down to contact the substrate or optical stack  16  may be considered an “integrated” support post. One implementation of the optical stack  16 , which may contain a plurality of several different layers, is shown here for clarity including an optical absorber  16   a , and a dielectric  16   b . In some implementations, the optical absorber  16   a  may serve both as a stationary electrode and as a partially reflective layer. In some implementations, the optical absorber  16   a  can be an order of magnitude thinner than the movable reflective layer  14 . In some implementations, the optical absorber  16   a  is thinner than the reflective sub-layer  14   a.    
     In implementations such as those shown in  FIGS. 3A-3E , the IMOD display elements form a part of a direct-view device, in which images can be viewed from the front side of the transparent substrate  20 , which in this example is the side opposite to that upon which the IMOD display elements are formed. In these implementations, the back portions of the device (that is, any portion of the display device behind the movable reflective layer  14 , including, for example, the deformable layer  34  illustrated in  FIG. 3C ) can be configured and operated upon without impacting or negatively affecting the image quality of the display device, because the reflective layer  14  optically shields those portions of the device. For example, in some implementations a bus structure (not illustrated) can be included behind the movable reflective layer  14  that provides the ability to separate the optical properties of the modulator from the electromechanical properties of the modulator, such as voltage addressing and the movements that result from such addressing. 
       FIG. 4  is a flow diagram illustrating a manufacturing process  80  for an IMOD display or display element.  FIGS. 5A-5E  are cross-sectional illustrations of various stages in the manufacturing process  80  for making an IMOD display or display element. In some implementations, the manufacturing process  80  can be implemented to manufacture one or more EMS devices, such as IMOD displays or display elements. The manufacture of such an EMS device also can include other blocks not shown in  FIG. 4 . The process  80  begins at block  82  with the formation of the optical stack  16  over the substrate  20 .  FIG. 5A  illustrates such an optical stack  16  formed over the substrate  20 . The substrate  20  may be a transparent substrate such as glass or plastic such as the materials discussed above with respect to  FIG. 1 . The substrate  20  may be flexible or relatively stiff and unbending, and may have been subjected to prior preparation processes, such as cleaning, to facilitate efficient formation of the optical stack  16 . As discussed above, the optical stack  16  can be electrically conductive, partially transparent, partially reflective, and partially absorptive, and may be fabricated, for example, by depositing one or more layers having the desired properties onto the transparent substrate  20 . 
     In  FIG. 5A , the optical stack  16  includes a multilayer structure having sub-layers  16   a  and  16   b , although more or fewer sub-layers may be included in some other implementations. In some implementations, one of the sub-layers  16   a  and  16   b  can be configured with both optically absorptive and electrically conductive properties, such as the combined conductor/absorber sub-layer  16   a . In some implementations, one of the sub-layers  16   a  and  16   b  can include molybdenum-chromium (molychrome or MoCr), or other materials with a suitable complex refractive index. Additionally, one or more of the sub-layers  16   a  and  16   b  can be patterned into parallel strips, and may form row electrodes in a display device. Such patterning can be performed by a masking and etching process or another suitable process known in the art. In some implementations, one of the sub-layers  16   a  and  16   b  can be an insulating or dielectric layer, such as an upper sub-layer  16   b  that is deposited over one or more underlying metal and/or oxide layers (such as one or more reflective and/or conductive layers). In addition, the optical stack  16  can be patterned into individual and parallel strips that form the rows of the display. In some implementations, at least one of the sub-layers of the optical stack, such as the optically absorptive layer, may be quite thin (e.g., relative to other layers depicted in this disclosure), even though the sub-layers  16   a  and  16   b  are shown somewhat thick in  FIGS. 5A-5E . 
     The process  80  continues at block  84  with the formation of a sacrificial layer  25  over the optical stack  16 . Because the sacrificial layer  25  is later removed (see block  90 ) to form the cavity  19 , the sacrificial layer  25  is not shown in the resulting IMOD display elements.  FIG. 5B  illustrates a partially fabricated device including a sacrificial layer  25  formed over the optical stack  16 . The formation of the sacrificial layer  25  over the optical stack  16  may include deposition of a xenon difluoride (XeF 2 )-etchable material such as molybdenum (Mo) or amorphous silicon (Si), in a thickness selected to provide, after subsequent removal, a gap or cavity  19  (see also  FIG. 5E ) having a desired design size. Deposition of the sacrificial material may be carried out using deposition techniques such as physical vapor deposition (PVD, which includes many different techniques, such as sputtering), plasma-enhanced chemical vapor deposition (PECVD), thermal chemical vapor deposition (thermal CVD), or spin-coating. 
     The process  80  continues at block  86  with the formation of a support structure such as a support post  18 . The formation of the support post  18  may include patterning the sacrificial layer  25  to form a support structure aperture, then depositing a material (such as a polymer or an inorganic material, like silicon oxide) into the aperture to form the support post  18 , using a deposition method such as PVD, PECVD, thermal CVD, or spin-coating. In some implementations, the support structure aperture formed in the sacrificial layer can extend through both the sacrificial layer  25  and the optical stack  16  to the underlying substrate  20 , so that the lower end of the support post  18  contacts the substrate  20 . Alternatively, as depicted in  FIG. 5C , the aperture formed in the sacrificial layer  25  can extend through the sacrificial layer  25 , but not through the optical stack  16 . For example,  FIG. 5E  illustrates the lower ends of the support posts  18  in contact with an upper surface of the optical stack  16 . The support post  18 , or other support structures, may be formed by depositing a layer of support structure material over the sacrificial layer  25  and patterning portions of the support structure material located away from apertures in the sacrificial layer  25 . The support structures may be located within the apertures, as illustrated in  FIG. 5C , but also can extend at least partially over a portion of the sacrificial layer  25 . As noted above, the patterning of the sacrificial layer  25  and/or the support posts  18  can be performed by a masking and etching process, but also may be performed by alternative patterning methods. 
     The process  80  continues at block  88  with the formation of a movable reflective layer or membrane such as the movable reflective layer  14  illustrated in  FIG. 5D . The movable reflective layer  14  may be formed by employing one or more deposition steps, including, for example, reflective layer (such as aluminum, aluminum alloy, or other reflective materials) deposition, along with one or more patterning, masking and/or etching steps. The movable reflective layer  14  can be patterned into individual and parallel strips that form, for example, the columns of the display. The movable reflective layer  14  can be electrically conductive, and referred to as an electrically conductive layer. In some implementations, the movable reflective layer  14  may include a plurality of sub-layers  14   a ,  14   b  and  14   c  as shown in  FIG. 5D . In some implementations, one or more of the sub-layers, such as sub-layers  14   a  and  14   c , may include highly reflective sub-layers selected for their optical properties, and another sub-layer  14   b  may include a mechanical sub-layer selected for its mechanical properties. In some implementations, the mechanical sub-layer may include a dielectric material. Since the sacrificial layer  25  is still present in the partially fabricated IMOD display element formed at block  88 , the movable reflective layer  14  is typically not movable at this stage. A partially fabricated IMOD display element that contains a sacrificial layer  25  also may be referred to herein as an “unreleased” IMOD. 
     The process  80  continues at block  90  with the formation of a cavity  19 . The cavity  19  may be formed by exposing the sacrificial material  25  (deposited at block  84 ) to an etchant. For example, an etchable sacrificial material such as Mo or amorphous Si may be removed by dry chemical etching by exposing the sacrificial layer  25  to a gaseous or vaporous etchant, such as vapors derived from solid XeF 2  for a period of time that is effective to remove the desired amount of material. The sacrificial material is typically selectively removed relative to the structures surrounding the cavity  19 . Other etching methods, such as wet etching and/or plasma etching, also may be used. Since the sacrificial layer  25  is removed during block  90 , the movable reflective layer  14  is typically movable after this stage. After removal of the sacrificial material  25 , the resulting fully or partially fabricated IMOD display element may be referred to herein as a “released” IMOD. 
     Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. Creep is a time-dependent deformation mechanism that may or may not constitute a failure mode. MEMS devices may include one or more movable parts that may be subject to creep. MEMS devices can undergo deformation as a result of creep because of mechanical stresses and thermal energy. Such mechanical stresses may be below the yield strength of a material in the MEMS device, but over time and over elevated temperatures, the material will deform. Accordingly, mechanical actuation and thermal heating can cause atoms to diffuse around some of the layers of the MEMS device over time, eventually causing the MEMS device to fail. Creep resistance of the MEMS device can be increased by one or more of the following: reducing the operating temperature, reducing the applied stress levels, and changing the material. 
     In changing the composition of the material, the microstructure of the material may be controlled to limit the effects of creep. Deformation as a result of creep can be attributed to diffusional effects, such as the diffusion of atoms across grain boundaries. When the operating temperature increases, the thermal energy causes increased diffusion, thereby resulting in greater creep deformation. Solutions for limiting the mechanism of diffusional creep can include, for example, reducing the grain size of the material and making the material more mechanically robust. 
     Typically, some metallic layers and structures, especially those with low melting points or soft metal such as aluminum or gold, may be vulnerable to creep deformation due in part to its thermal and mechanical properties. Moreover, MEMS devices include movable parts that can undergo greater amounts of mechanical strain. Under elevated temperatures, the diffusion of atoms may increase, especially in regions exposed to more mechanical strain. Over time, this can lead to creep deformation that can result in device failure. Where movable parts include metallic layers and structures, these movable parts that are vulnerable to creep deformation may be strengthened to be more creep-resistant and more mechanically robust. 
     An example of a MEMS device can be a MEMS display device or MEMS display device element (e.g., pixel). In some implementations, the MEMS display device can include an IMOD, which is described with reference to  FIGS. 1-5E .  FIG. 6  shows a perspective view of an example MEMS display device having a movable reflective structure spaced apart from a stationary electrode by a gap. The MEMS display device  100  can include a substrate  200  and a stationary electrode  160  on the substrate  200 . The MEMS display device  100  can further include a movable reflective structure  140  over the stationary electrode  160 , where the movable reflective structure  140  and the stationary electrode  160  define a gap therebetween. The movable reflective structure  140  may be supported by one or more support structures  130 , where the one or more support structures  130  may be between the substrate  200  and the movable reflective structure  140 . In some implementations, the one or more support structures  130  may be symmetrically disposed around the movable reflective structure  140 . In some implementations, the MEMS display device  100  can be an IMOD, such as a bistable IMOD, multi-state IMOD, or analog IMOD. 
     In some implementations, each of the one or more support structures  130  can include a tether or hinge  150  connected to a support post  180 . The movable reflective structure  140  may be connected to each of the support posts  180  via tethers  150 . In some implementations, the tethers  150  may be tangential to the movable reflective structure  140  and can reduce the residual stress in the MEMS display device  100 . Other configurations for tethers  150 , including straight, curved, or folded, are also possible. In some implementations, the tethers  150  may be made of metals, such as aluminum, aluminum-titanium, or aluminum-zirconium, or other materials such as amorphous or polycrystalline silicon, oxides, nitrides, and oxynitrides. In some implementations, the tethers  150  may include the same or substantially the same material as the movable reflective structure  140 . 
     The substrate  200  can be made of any suitable substrate materials, including a substantially transparent material, such as glass or plastic. Substantial transparency as used herein may be defined as transmittance of visible light of about 70% or more, such as about 80% or more or about 90% or more. Glass substrates (sometimes referred to as glass plates or panels) may be or include a borosilicate glass, a soda lime glass, photoglass, quartz, Pyrex or other suitable glass material. A non-glass substrate can be used, such as a polycarbonate, acrylic, polyimide, polyethylene terephthalate (PET) or polyether ether ketone (PEEK) substrate. In some implementations, the substrate  200  can have dimensions of a few microns to hundreds of microns. For example, the substrate  200  can have a thickness between about 10 microns and about 1100 microns. 
     The stationary electrode  160  on the substrate  200  may be electrically conductive or include an electrically conductive layer. In some implementations, the stationary electrode  160  may be part of an optical stack. The optical stack can be at least partially absorbing of visible light and can include an optically absorbing material. An optically absorbing material can include, for example, a molybdenum-chromium compound having a thickness between about 20 Å and about 100 Å. The optical stack can include a plurality of sub-layers and can be configured similarly to the optical stack  16  in  FIGS. 3A-3E . 
     The movable reflective structure  140  can be referred to as a movable electrode or a mirror. The movable reflective structure  140  can be electrically conductive or include an electrically conductive layer. The movable reflective structure  140  can include a plurality of layers (not shown), including but not limited to a reflective layer and an optical layer. The reflective layer can include a material that is configured to substantially reflect visible light, where the reflectance can be about 70% or more, such as about 80% or more or about 90% or more. In some implementations, the reflective layer can provide a mirror for interferometrically modulating light with the optically absorbing material in the stationary electrode  160 . 
     In some implementations, the movable reflective structure  140  can include a reflective layer and a deformable layer, where the optical properties of the movable reflective structure  140  can be decoupled from its mechanical properties. The reflective layer can include a plurality of sub-layers, including but not limited to a reflective sub-layer, a dielectric sub-layer, and a metal sub-layer. The reflective sub-layer can have a thickness between about 100 Å and about 500 Å and can include aluminum or aluminum alloy. If a dielectric sub-layer is included in the reflective layer, the dielectric sub-layer can have a thickness between about 4000 Å and about 40000 Å to provide structural rigidity to the movable reflective structure  140 . The dielectric sub-layer can include any suitable dielectric material such as nitrous oxide, silicon dioxide, silicon oxynitride, and silicon nitride. If a metal sub-layer is included in the reflective layer, the metal sub-layer can have a thickness between about 100 Å and about 1000 Å and can include aluminum, copper, aluminum-copper alloy, aluminum-titanium alloy, aluminum-zirconium alloy, or other electrically conductive material. 
     The movable reflective structure  140  may be configured to electrostatically actuate towards the stationary electrode  160  when a voltage is applied. The one or more support structures  130  may bend and cause the movable reflective structure  140  to deflect towards the stationary electrode  160 . In some implementations, the movable reflective structure  140  can remain parallel or substantially parallel to the stationary electrode  160  during actuation. As the movable reflective structure  140  moves or otherwise deflects towards the stationary electrode  160 , a gap distance between the movable reflective structure  140  and the stationary electrode  160  can influence the reflective properties of the MEMS display device  100 . For example, different gap distances can reflect different wavelengths of light through the substrate  200 , which gives the appearance of different colors. 
     The repeated movement of the movable reflective structure  140  and the one or more support structures  130  can cause the MEMS display device  100  to undergo several cycles of stress. The movable reflective structure  140  and the one or more support structures  130  can be made of metallic materials. In addition, during the operating lifetime of the MEMS display device  100 , the MEMS display device  100  can be exposed to elevated temperatures. Thus, the MEMS display device  100  may be vulnerable to creep deformation and potential mechanical failure. 
     In some implementations, the one or more support structures  130  may be deformable. The one or more support structures  130  may permit the movable reflective structure  140  to actuate in the MEMS display device  100 . Over time, the creep behavior in the movable reflective structure  140  can cause the one or more support structures  130  to become unable to restore to its original position after actuation. This can lead to poor image retention and device failure in the MEMS display device  100 . For example, device failure can occur at connection points between the tethers  150  and the movable reflective structure  140  as a result of creep. 
     To increase the creep resistance of movable parts in the MEMS display device  100 , the materials of the movable parts can be changed to provide greater creep resistance and mechanical strength. The movable reflective structure  140  can include aluminum or aluminum alloy. Examples of creep-resistant aluminum alloys can include but is not limited to aluminum-copper, aluminum-scandium, and aluminum-zirconium. The aluminum or aluminum alloy can serve as a reflective material for the movable reflective structure  140 , where the reflectance of the movable reflective structure  140  can reflect greater than about 80% of visible light. The aluminum or aluminum alloy can serve as an electrically conductive material for the movable reflective structure  140 , where the sheet resistance of the movable reflective structure  140  can be less than about 1, 10, or 100 ohms per square. 
     The aluminum or aluminum alloy may provide some resistance against creep, but stress control in the movable reflective layer  140  may be difficult with the aluminum or aluminum alloy. For example, when the movable reflective layer  140  in the MEMS display device  100  is subject to subsequent deposition and annealing, the movable reflective layer  140  with aluminum or aluminum alloy can end up with increased tensile stress. For example, the movable reflective layer  140  with aluminum or aluminum alloy can have a tensile stress of greater than about 500 MPa after annealing at 350° C. 
       FIGS. 7A-7D  show cross-sectional side views of example thin film stacks for movable reflective structures. However, it is understood that such thin film stacks may be incorporated not only in movable reflective structures, but in other movable parts, such as support structures (e.g., hinges). A more creep-resistant and mechanically robust aluminum or aluminum alloy can be formed by doping a transition metal into the aluminum or aluminum alloy. How the transition metal is doped into the aluminum or aluminum alloy can be achieved by providing a thin film stack as shown in  FIGS. 7A-7D  and annealing the thin film stack. As a result, the aluminum or aluminum alloy can be doped with the transition metal upon annealing. 
       FIG. 7A  shows a cross-sectional side view of an example thin film stack for a movable reflective structure with a transition metal layer sandwiched between two reflective layers. The thin film stack  700  can include a bottom layer  710   a , a top layer  710   b  over the bottom layer  710   a , and a middle layer  720  between the bottom layer  710   a  and the top layer  710   a . The top and bottom layers  710   a ,  710   b  can constitute reflective layers and the middle layer  720  can constitute a transition metal layer. In some implementations, the reflective layers  710   a ,  710   b  may be substantially identical in composition and thickness with each other. The transition metal layer  720  may be in contact with at least one of the reflective layers  710   a ,  710   b . In some implementations, the transition metal layer  720  may be in contact with both of the reflective layers  710   a ,  710   b.    
     The thin film stack  700  may also be referred to as an annealed multilayer thin film stack. The thin film stack  700  can have a total thickness of less than about 5 microns, or between about 50 nm and about 3 microns. The transition metal layer  720  can have a thickness of less than about 20% of the total thickness of the thin film stack  700 , or less than about 10% of the total thickness of the thin film stack  700 . For example, the reflective layers  710   a ,  710   b  can each have a thickness equal to or greater than about 20 nm, and the transition metal layer  720  can have a thickness of less than about 5 nm. The reflective layers  710   a ,  710   b  can each include aluminum or aluminum alloy, and the transition metal layer  720  can include at least one of: zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium. For example, the transition metal can include zirconium. 
     The transition metal layer  720  upon doping into the reflective layers  710   a ,  710   b , can improve creep resistance and reduce the stress of the reflective layers  710   a ,  710   b  while minimizing degradation of the optical and electrical properties of the reflective layers  710   a ,  710   b . A movable reflective structure incorporating the thin film stack  700  can have a relatively low stress value, such as a stress value of less than about 200 MPa, or less than 100 MPa. Furthermore, the movable reflective structure incorporating the thin film stack  700  can have a sufficiently high reflectance of visible light, such as a reflectance of greater than about 80% or greater than about 90%. The reduced stress value and the sufficiently high reflectance value can be achieved even after annealing the thin film stack  700 . In some implementations, the movable reflective structure incorporating the thin film stack  700  can have a relatively low sheet resistance value, such as less than about 1, 10 or 100 ohms per square. 
     Incorporating a transition metal layer  720  between two reflective layers  710   a ,  710   b  permits diffusion of transition metal atoms to dope the reflective layers  710   a ,  710   b  with the transition metal. Upon annealing, atoms of the transition metal may diffuse into the two reflective layers  710   a ,  710   b  from both sides of the transition metal layer  720 . The thickness of the transition metal layer  720  and the annealing conditions can influence how the reflective layers  710   a ,  710   b  are doped. Each of the reflective layers  710   a ,  710   b  can be doped with about 0.1 atomic % to about 10 atomic % of the transition metal, or with about 0.5 atomic % to about 5 atomic % of the transition metal. The doped thin film stack  700  can provide greater creep resistance and stress control. 
     In some implementations, the thin film stack  700  can be doped to further include less than 20 atomic % of one or both of oxygen and nitrogen, or between about 1 atomic % and about 20 atomic % of one or both of oxygen and nitrogen. The thin film stack  700  can be doped with oxygen and/or nitrogen to further improve creep resistance and reduce stress. The aluminum or aluminum alloy may react with oxygen and/or nitrogen to form aluminum oxide, aluminum nitride, or aluminum oxynitride. When doping with one or both of oxygen and nitrogen, oxygen and nitrogen atoms can be provided at the grain boundaries of the reflective layers  710   a ,  710   b  to limit grain growth and limit the diffusion of atoms across the grain boundaries. The doping can occur by flowing oxygen and nitrogen gas during the aluminum or aluminum alloy deposition (e.g., sputter deposition). For example, oxygen gas can be flowed at less than about 5 standard cubic centimeters per minute (sccm), and nitrogen gas can be flowed at less than about 10 sccm while the sputtering gas such as argon or krypton can be ranged from 50 sccm to 100 sccm or 200 sccm or more. 
     The movable reflective structure incorporating the thin film stack  700  can be part of a “thin” film reflective structure or a “thick” film reflective structure. In implementations for a “thin” film reflective structure, the reflective layers  710   a ,  710   b  can be relatively thin, such as each having a thickness between about 10 nm and about 50 nm. However, the movable reflective structure in such implementations can include dielectric layers, where the dielectric layers can provide structural rigidity to the movable reflective structure and can each have a thickness between about 400 nm and about 4000 nm. In implementations for a “thick” film reflective structure, the reflective layers  710   a ,  710   b  can be relatively thick, such as each having a thickness between about 100 nm and about 1000 nm. The movable reflective structure in such implementations may or may not include dielectric layers to provide structural rigidity. 
       FIG. 7B  shows a cross-sectional side view of an example movable reflective structure including a thin film stack over an optical layer. As discussed earlier herein, the movable reflective structure can include a plurality of layers. Examples of such layers can include reflective layers, deformable layers, metal layers, optical layers, and dielectric layers. The thin film stack  700  may be incorporated in a movable reflective structure having one or more of the aforementioned layers. The thin film stack  700  includes the middle layer  720  sandwiched between the bottom layer  710   a  and the top layer  710   b . As shown in the example in  FIG. 7B , the thin film stack  700  is formed, positioned, or placed on an optical layer  730 . In some implementations, the optical layer  730  can include titanium oxide or zirconium oxide and can have a thickness between about 100 nm and about 500 nm. 
       FIG. 7C  shows a cross-sectional side view of an example thin film stack for a movable reflective structure with an additional transition metal layer.  FIG. 7D  shows a cross-sectional side view of an example thin film stack for a movable reflective structure with an additional transition metal layer and an additional reflective layer. Additional layers in the thin film stack  700  may provide greater control and tunability of mechanical, optical, and electrical properties of the movable reflective structure incorporating the thin film stack  700 . In some implementations, additional layers may provide a more balanced thin film stack  700  in terms of stress. 
     In  FIG. 7C , the thin film stack  700  includes the middle layer  720  sandwiched between the bottom layer  710   a  and the top layer  710   b . The thin film stack  700  can further include a first layer  740 , where the first layer  740  is below the bottom layer  710   a . In some implementations, the first layer  740  can be substantially identical in composition and thickness with the middle layer  720 . Hence, the first layer  740  can further dope the aluminum or aluminum alloy in the bottom layer  710   a  with a transition metal to further control the mechanical properties of the movable reflective structure incorporating the thin film stack  700 . 
     In  FIG. 7D , the thin film stack  700  includes the middle layer  720  sandwiched between the bottom layer  710   a  and the top layer  710   b , and further includes the first layer  740  below the bottom layer  710   a  and a second layer  750  below the first layer  740 . In some implementations, the second layer  750  can be substantially identical in composition and thickness with one or both of the bottom layer  710   a  and the top layer  710   b . In alternative implementations, the second layer  750  can be different in composition and/or thickness with one or both of the bottom layer  710   a  and the top layer  710   b . The transition metal can further dope the aluminum or aluminum alloy in the bottom layer  710   a , the top layer  710   b , and the second layer  750 . The additional layers in the thin film stack  700  are not limited to the configurations shown in  FIGS. 7C and 7D , but may be arranged in different configurations and may include even more or fewer layers. 
     Different configurations of the thin film stack, different thicknesses of the transition metal layer, and different annealing conditions can influence the mechanical, optical, and electrical properties of the movable reflective structure, as shown in  FIGS. 8A-8J . In  FIGS. 8A-8J , data is provided measuring the stress and reflectance of various thin film stack configurations. Some thin film stacks are exposed to different annealing conditions. The annealing can cause the transition metal atoms to dope the aluminum or aluminum alloy through a thermal diffusion process. Stress and reflectance values are measured with respect to varying deposition times for zirconium. Zirconium can provide increased creep resistance and control stress. Without being limited by theory, zirconium atoms may readily precipitate in the grain boundaries to limit diffusion of atoms as well as limit grain growth. 
       FIG. 8A  shows a graph illustrating stress of a movable reflective structure as a function of the zirconium deposition times, where the movable reflective structure includes an aluminum-scandium layer sandwiched between two zirconium layers.  FIG. 8B  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8A  as a function of the zirconium deposition times. The movable reflective structure includes a bottom layer of zirconium over the optical layer and having a thickness corresponding to 8 seconds of deposition time, a middle layer of aluminum-scandium over the bottom layer and having a thickness of 4000 nm, and a top layer of zirconium having a thickness corresponding to variable deposition times. The movable reflective structure is subjected to a thermal anneal at about 350° C. for 250 seconds in 1 standard liters per minute (slm) of nitrogen gas. As the zirconium deposition times increase (i.e., increasing zirconium thickness), the stress in the movable reflective structure decreases. For as-deposited films, the stress does not exhibit a significant decrease. This can show that the zirconium atoms do not diffuse very much into the aluminum-scandium layer unless the movable reflective structure is annealed. For annealed films, the stress can change dramatically and even cross over from tensile to compressive stress. This may indicate the tunability of the stress by thermal annealing. Furthermore, the increasing zirconium thickness corresponds to decreasing reflectance. In fact, the reflectance is less than about 80% after 8 seconds of zirconium deposition, and less than 60% after 40 seconds of zirconium deposition. Such a thin film stack configuration for a display may not provide sufficient image quality. 
       FIG. 8C  shows a graph illustrating stress of a movable reflective structure as a function of zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two aluminum alloy layers.  FIG. 8D  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8C  as a function of the zirconium deposition times. The movable reflective structure includes a bottom layer of aluminum-scandium over the optical layer and having a thickness of 20 nm, a middle layer of zirconium having a thickness corresponding to variable deposition times, and a top layer of aluminum-scandium having a thickness of 20 nm. The movable reflective structure is subjected to a thermal anneal at about 350° C. for 250 seconds in 1 slm of nitrogen gas. As the zirconium deposition time increases, the stress decreases for both as-deposited conditions and after annealing. The stress may be relatively low and similar under both conditions. This can mean that the grain structure does not significantly change even after annealing, meaning that the microstructure may be thermally stable. However, the reflectance decreases after anneal and decreases with increasing zirconium thickness, where the reflectance can drop below 85%. 
       FIG. 8E  shows a graph illustrating stress of a movable reflective layer as a function of the zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two aluminum alloy layers.  FIG. 8F  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8E  as a function of zirconium deposition times. The movable reflective structure includes a bottom layer of aluminum-scandium over the optical layer and having a thickness of 20 nm, a middle layer of zirconium having a thickness corresponding to variable deposition times, and a top layer of aluminum-scandium having a thickness of 20 nm. The movable reflective structure is subjected to different annealing conditions, where the movable reflective structure is thermally annealed for 2 hours at about 350° C. in nitrogen gas. Under such annealing conditions with a longer anneal time, the stress in the movable reflective structure becomes considerably high. The stress in the movable reflective structure without annealing decreases slightly. This can suggest a change in grain structure under such annealing conditions. The reflectance also decreases considerably after anneal and decreases with increasing zirconium thickness. Though not shown, the sheet resistance of the movable reflective structure increases from 1.5 ohms per square to 13.5 ohms per square as the zirconium deposition times increase from 8 seconds to 45 seconds. Overall, this shows that increasing the zirconium thickness can degrade the reflectance and electrical properties of the movable reflective structure, and incorporating zirconium in such a thin film stack configuration does not improve the mechanical properties with a longer anneal time. 
       FIG. 8G  shows a graph illustrating stress of a movable reflective structure as a function of zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two thick aluminum alloy layers.  FIG. 8H  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8G  as a function of zirconium deposition times. Here, the movable reflective structure includes a bottom layer of aluminum-scandium over the optical layer and having a thickness of 250 nm, a middle layer of zirconium having a thickness corresponding to variable deposition times, and a top layer of aluminum-scandium having a thickness of 250 nm. The movable reflective structure is subjected to a thermal anneal at about 350° C. for 250 seconds in 1 slm of nitrogen gas. With thicker aluminum-scandium layers, the reflectance of the movable reflective structure does not degrade, but remains above 90% reflectance of visible light even after annealing. However, while the stress remains relatively the same under both as-deposited conditions and after annealing, the stress after annealing is greater than about 350 MPa. 
       FIG. 8I  shows a graph illustrating stress of a movable reflective structure as a function of zirconium deposition times, where the movable reflective structure includes zirconium sandwiched between two thick aluminum alloy layers.  FIG. 8J  shows a graph illustrating reflectance of the movable reflective structure from  FIG. 8I  as a function of zirconium deposition times. The movable reflective structure includes a bottom layer of aluminum-scandium over the optical layer and having a thickness of 250 nm, a middle layer of zirconium having a thickness corresponding to variable deposition times, and a top layer of aluminum-scandium having a thickness of 250 nm. The movable reflective structure is subjected to an own anneal at about 350° C. for 2 hours in nitrogen gas. As the zirconium deposition times increased, the stress decreased in both as-deposited conditions and after annealing, where the stress was below 200 MPa after a zirconium deposition time of 200 seconds. In fact, the stress does not change much between as-deposited conditions and after annealing, which can indicate that not much structural changes occur irrespective of longer annealing times. The reflectance of the movable reflective structure is greater than 89% even after annealing. Though not shown, the sheet resistance of the movable reflective structure is measured to be about 0.052 ohms per square, showing that the electrical properties do not degrade irrespective of longer anneal times. 
     Data in  FIGS. 8A-8J  show that the incorporation of a layer of zirconium between two layers of aluminum-scandium can control stress in the movable reflective structure. Some configurations of the movable reflective structure show stress even less than 150 MPa after annealing at 350° C. for 2 hours. Furthermore, some configurations of the movable reflective structure provided a reflectance greater than 85% and some configurations even greater than 90%, showing that such configurations can meet desired optical standards. Some configurations of the movable reflective structure provided a sheet resistance of less than 1 or less than 10 ohms per square, indicating that such configurations can maintain desired electrical properties even after annealing. 
     In some implementations, doping the aluminum or aluminum alloy layers with oxygen and/or nitrogen can further improve the mechanical robustness of the movable reflective structure. Table 1 shows stress data for a movable reflective structure including an aluminum alloy mirror, where the stress values vary depending on the amount of nitrogen or oxygen gas introduced with the aluminum alloy. The stress values are taken before annealing and after annealing at about 350° C. for 3 hours. Oxygen and/or nitrogen gases are flowed in low amounts during the deposition of the aluminum alloy. For example, the oxygen and nitrogen gases can be introduced during sputtering of the aluminum alloy to form precipitates of aluminum oxide, aluminum nitride, or aluminum oxynitride. An inert carrier gas such as argon is also be flowed simultaneously. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Oxygen 
                 Stress (MPa) 
               
            
           
           
               
               
               
               
               
            
               
                 Argon (sccm) 
                 Nitrogen (sccm) 
                 (sccm) 
                 As-deposited 
                 Anneal 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 33 
                 0 
                 0 
                 45 
                 710 
               
               
                 33 
                 0 
                 3 
                 −168.8 
                 551 
               
               
                 33 
                 0 
                 6 
                 −328.1 
                 573 
               
               
                 33 
                 0 
                 10 
                 −195.1 
                 476 
               
               
                 33 
                 10 
                 0 
                 64.2 
                 427 
               
               
                 33 
                 20 
                 0 
                 168 
                 181 
               
               
                 33 
                 30 
                 0 
                 246 
                 196 
               
               
                 33 
                 40 
                 0 
                 204 
                 164 
               
               
                 33 
                 10 
                 3 
                 0.8 
                 146 
               
               
                 33 
                 10 
                 10 
                 −18.4 
                 47 
               
               
                 33 
                 30 
                 3 
                 −32.6 
                 −10 
               
               
                 33 
                 30 
                 10 
                 −13.7 
                 27 
               
               
                 33 
                 40 
                 3 
                 −21.1 
                 −77 
               
               
                   
               
            
           
         
       
     
     The flow rates of nitrogen and oxygen gas can be adjusted to control the stress of the movable reflective structure having an aluminum alloy mirror. The flow rate of oxygen gas can be less than about 5 sccm and the flow rate of nitrogen gas can be less than about 10 sccm. In some implementations, the flow rate of oxygen gas can be less than about 4 sccm, and the flow rate of nitrogen gas can be less than about 5 sccm. As shown in Table 1, introduction of oxygen and nitrogen can reduce the stress of the movable reflective structure to be less than 100 MPa. Additionally, the sheet resistance of the movable reflective structure doped with oxygen and nitrogen is about 1.2 ohms per square for a movable reflective structure having a thickness of 40 nm. The movable reflective structure doped with oxygen and nitrogen can improve the stress of creep-resistant films while maintaining a relatively low sheet resistance. 
       FIG. 9  is a flow diagram of an example method of manufacturing a MEMS display device. The process  900  may be performed in a different order or with different, fewer or additional operations. 
     At block  910 , a substrate is provided. The substrate can include any suitable substrate material, such as glass or plastic. In some implementations, the substrate material can be substantially transparent to visible light. One or more display elements may be formed on the substrate for the MEMS display device. In some implementations, the one or more display elements can include an active matrix OLED, shutter-based light modulator, or IMOD. Each of the display elements can be part of a pixel in the display device. In some implementations, the process  900  can further include forming a stationary electrode or optical stack over the substrate, where the stationary electrode or optical stack can include an electrically conductive material. 
     At block  920 , a support structure is formed over the substrate. In some implementations, the support structure can include a plurality of tethers or hinges symmetrically disposed around the edges of a movable reflective structure. The support structure can be formed on the substrate and configured to support a movable reflective structure. The support structure can be formed of a metal, such as aluminum or titanium, or other materials, such as oxides, nitrides, and oxynitrides. In some implementations, the support structure can include materials identical or substantially identical with the materials of the movable reflective structure. Thus, the support structure can include aluminum or aluminum alloy. The support structure may be bendable to permit the actuation of the movable reflective structure towards the substrate. In some implementations, the support structure can include a support post and a tether connected to the support post, where tether is connected to the movable reflective structure. The tether can be made of a flexible material while the support posts can be made of a relatively rigid material. 
     At block  930 , a movable reflective structure is formed over the substrate and connected to the support structure, where the movable reflective structure includes a first layer including aluminum or aluminum alloy, a second layer including aluminum or aluminum alloy and over the first layer, and a third layer between the first layer and the second layer where the third layer is in contact with at least one of the first layer and the second layer. The third layer includes a transition metal, where the transition metal includes at least one of zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium. 
     The movable reflective structure can constitute a movable electrode or mirror that is separated from the stationary electrode by a gap. The movable reflective structure can be configured to move across the gap towards the stationary electrode by electrostatic force. The movable reflective structure can be part of a pixel of the MEMS display device. Pixels of the MEMS display device can be arranged as an array to form a display. A gap distance between the movable reflective structure and the stationary electrode can reflect a certain wavelength of light to give the appearance of a particular color. 
     In some implementations, forming the movable reflective structure can include depositing the first layer over the substrate, depositing the third layer on the first layer, and depositing the second layer on the third layer. The movable reflective structure can form a thin film stack having a transition metal layer sandwiched between two aluminum or aluminum alloy layers. Each of the deposition steps may be subsequently followed by masking, patterning, etching, or planarization steps. Each of the layers can be deposited using deposition techniques known in the art, such as PVD, CVD, PECVD, ALD, and spin-coating. PVD processes can include pulsed laser deposition, sputter deposition, electron beam physical vapor deposition, and evaporative deposition. In some implementations, the movable reflective structure can further include an optical layer of titanium oxide, where the first layer is formed over the optical layer. 
     In some implementations, the first layer and the second layer of the movable reflective structure can be substantially identical in composition and thickness. The composition of the first layer and the second layer can include aluminum, such as pure aluminum, aluminum-scandium, aluminum-zirconium, or aluminum-copper. The thickness of the first layer and the second layer can each be equal to or greater than about 20 nm, such as between about 20 nm and about 400 nm. The thickness of the third layer can be less than about 5 nm. In some implementations, the thickness of the third layer can be about 10% or less than a total thickness of the movable reflective structure. 
     In some implementations, depositing the first layer can include doping the first layer with one or both of oxygen and nitrogen, and depositing the second layer can include doping the second layer with one or both of oxygen and nitrogen. Doping the first and second layers with oxygen and/or nitrogen can include flowing oxygen and/or nitrogen gas during reactive sputtering of aluminum or aluminum alloy. In some implementations, the flow rate of oxygen gas can be less than about 5 sccm and the flow rate of nitrogen gas can be less than about 10 sccm. 
     In some implementations, the process  900  further includes depositing a fourth layer between the substrate and the first layer, where the fourth layer is substantially identical in composition and thickness with the third layer. Therefore, another transition metal layer can be incorporated in the thin film stack of the movable reflective structure. In some implementations, the fourth layer can provide a more balanced structure in terms of stress. 
     In some implementations, the process  900  can further include depositing a fifth layer between the substrate and the fourth layer, where the fifth layer is substantially identical in thickness and composition with the first layer and the second layer. The fifth layer can include aluminum or aluminum alloy, and can have a thickness equal to or greater than about 20 nm. In some implementations, the fifth layer can provide a more balanced structure in terms of stress. 
     At block  940 , the movable reflective structure is annealed. By annealing the movable reflective structure, the first layer and the second layer can be doped with the transition metal. Upon annealing, the transition metal can diffuse from both sides so that the first layer can be doped and the second layer can be doped with the transition metal. In some implementations, the first and the second layers can each be doped with about 0.1 atomic % to about 10 atomic % of the transition metal, or with about 0.5 atomic % to about 5 atomic % of the transition metal. In some implementations, the temperature during anneal can be between about 100° C. and about 600° C. It may be desirable to control the stress so that the stress of the movable reflective structure under as-deposited conditions and after annealing may be relatively similar. That way, the microstructure of the movable reflective structure can be thermally stable. 
     Such an annealed thin film stack in the movable reflective structure can reduce stress in a creep-resistant movable reflective structure, where the stress can be less than about 200 MPa or less than about 100 MPa. The annealed thin film stack can strengthen the mechanical robustness of the movable reflective structure without considerably degrading the optical and electrical properties of the movable reflective structure. For example, the sheet resistance of the movable reflective structure can be less than about 1 or less than about 10 ohms per square and the reflectance of the movable reflective structure can be greater than about 80%. 
       FIGS. 10A and 10B  are system block diagrams illustrating a display device  40  that includes a plurality of MEMS display elements, such as the MEMS display device shown in  FIG. 6 . The display device  40  can be, for example, a smart phone, a cellular or mobile telephone. However, the same components of the display device  40  or slight variations thereof are also illustrative of various types of display devices such as televisions, computers, tablets, e-readers, hand-held devices and portable media devices. 
     The display device  40  includes a housing  41 , a display  30 , an antenna  43 , a speaker  45 , an input device  48  and a microphone  46 . The housing  41  can be formed from any of a variety of manufacturing processes, including injection molding, and vacuum forming. In addition, the housing  41  may be made from any of a variety of materials, including, but not limited to: plastic, metal, glass, rubber and ceramic, or a combination thereof. The housing  41  can include removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols. 
     The display  30  may be any of a variety of displays, including a bi-stable or analog display, as described herein. The display  30  also can be configured to include a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD, or a non-flat-panel display, such as a CRT or other tube device. In addition, the display  30  can include an IMOD-based display, as described herein. 
     The components of the display device  40  are schematically illustrated in  FIG. 10A . The display device  40  includes a housing  41  and can include additional components at least partially enclosed therein. For example, the display device  40  includes a network interface  27  that includes an antenna  43  which can be coupled to a transceiver  47 . The network interface  27  may be a source for image data that could be displayed on the display device  40 . Accordingly, the network interface  27  is one example of an image source module, but the processor  21  and the input device  48  also may serve as an image source module. The transceiver  47  is connected to a processor  21 , which is connected to conditioning hardware  52 . The conditioning hardware  52  may be configured to condition a signal (such as filter or otherwise manipulate a signal). The conditioning hardware  52  can be connected to a speaker  45  and a microphone  46 . The processor  21  also can be connected to an input device  48  and a driver controller  29 . The driver controller  29  can be coupled to a frame buffer  28 , and to an array driver  22 , which in turn can be coupled to a display array  30 . One or more elements in the display device  40 , including elements not specifically depicted in  FIG. 10A , can be configured to function as a memory device and be configured to communicate with the processor  21 . In some implementations, a power supply  50  can provide power to substantially all components in the particular display device  40  design. 
     The network interface  27  includes the antenna  43  and the transceiver  47  so that the display device  40  can communicate with one or more devices over a network. The network interface  27  also may have some processing capabilities to relieve, for example, data processing requirements of the processor  21 . The antenna  43  can transmit and receive signals. In some implementations, the antenna  43  transmits and receives RF signals according to the IEEE 16.11 standard, including IEEE 16.11(a), (b), or (g), or the IEEE 802.11 standard, including IEEE 802.11a, b, g, n, and further implementations thereof. In some other implementations, the antenna  43  transmits and receives RF signals according to the Bluetooth® standard. In the case of a cellular telephone, the antenna  43  can be designed to receive code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), Global System for Mobile communications (GSM), GSM/General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), Terrestrial Trunked Radio (TETRA), Wideband-CDMA (W-CDMA), Evolution Data Optimized (EV-DO), 1×EV-DO, EV-DO Rev A, EV-DO Rev B, High Speed Packet Access (HSPA), High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Evolved High Speed Packet Access (HSPA+), Long Term Evolution (LTE), AMPS, or other known signals that are used to communicate within a wireless network, such as a system utilizing 3G, 4G or 5G technology. The transceiver  47  can pre-process the signals received from the antenna  43  so that they may be received by and further manipulated by the processor  21 . The transceiver  47  also can process signals received from the processor  21  so that they may be transmitted from the display device  40  via the antenna  43 . 
     In some implementations, the transceiver  47  can be replaced by a receiver. In addition, in some implementations, the network interface  27  can be replaced by an image source, which can store or generate image data to be sent to the processor  21 . The processor  21  can control the overall operation of the display device  40 . The processor  21  receives data, such as compressed image data from the network interface  27  or an image source, and processes the data into raw image data or into a format that can be readily processed into raw image data. The processor  21  can send the processed data to the driver controller  29  or to the frame buffer  28  for storage. Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation and gray-scale level. 
     The processor  21  can include a microcontroller, CPU, or logic unit to control operation of the display device  40 . The conditioning hardware  52  may include amplifiers and filters for transmitting signals to the speaker  45 , and for receiving signals from the microphone  46 . The conditioning hardware  52  may be discrete components within the display device  40 , or may be incorporated within the processor  21  or other components. 
     The driver controller  29  can take the raw image data generated by the processor  21  either directly from the processor  21  or from the frame buffer  28  and can re-format the raw image data appropriately for high speed transmission to the array driver  22 . In some implementations, the driver controller  29  can re-format the raw image data into a data flow having a raster-like format, such that it has a time order suitable for scanning across the display array  30 . Then the driver controller  29  sends the formatted information to the array driver  22 . Although a driver controller  29 , such as an LCD controller, is often associated with the system processor  21  as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. For example, controllers may be embedded in the processor  21  as hardware, embedded in the processor  21  as software, or fully integrated in hardware with the array driver  22 . 
     The array driver  22  can receive the formatted information from the driver controller  29  and can re-format the video data into a parallel set of waveforms that are applied many times per second to the hundreds, and sometimes thousands (or more), of leads coming from the display&#39;s x-y matrix of display elements. 
     In some implementations, the driver controller  29 , the array driver  22 , and the display array  30  are appropriate for any of the types of displays described herein. For example, the driver controller  29  can be a conventional display controller or an active matrix display controller (such as an IMOD display element controller). Additionally, the array driver  22  can be a conventional driver or an active matrix display driver (such as an IMOD display element driver). Moreover, the display array  30  can be a conventional display array or a MEMS display array (such as a display including an array of IMOD display elements). In some implementations, the driver controller  29  can be integrated with the array driver  22 . Such an implementation can be useful in highly integrated systems, for example, mobile phones, portable-electronic devices, watches or small-area displays. 
     In some implementations, the input device  48  can be configured to allow, for example, a user to control the operation of the display device  40 . The input device  48  can include a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a rocker, a touch-sensitive screen, a touch-sensitive screen integrated with the display array  30 , or a pressure- or heat-sensitive membrane. The microphone  46  can be configured as an input device for the display device  40 . In some implementations, voice commands through the microphone  46  can be used for controlling operations of the display device  40 . 
     The power supply  50  can include a variety of energy storage devices. For example, the power supply  50  can be a rechargeable battery, such as a nickel-cadmium battery or a lithium-ion battery. In implementations using a rechargeable battery, the rechargeable battery may be chargeable using power coming from, for example, a wall socket or a photovoltaic device or array. Alternatively, the rechargeable battery can be wirelessly chargeable. The power supply  50  also can be a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell or solar-cell paint. The power supply  50  also can be configured to receive power from a wall outlet. 
     In some implementations, control programmability resides in the driver controller  29  which can be located in several places in the electronic display system. In some other implementations, control programmability resides in the array driver  22 . The above-described optimization may be implemented in any number of hardware and/or software components and in various configurations. 
     As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c. 
     The various illustrative logics, logical blocks, modules, circuits and algorithm steps described in connection with the implementations disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. The interchangeability of hardware and software has been described generally, in terms of functionality, and illustrated in the various illustrative components, blocks, modules, circuits and steps described above. Whether such functionality is implemented in hardware or software depends upon the particular application and design constraints imposed on the overall system. 
     The hardware and data processing apparatus used to implement the various illustrative logics, logical blocks, modules and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose single- or multi-chip processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, or, any conventional processor, controller, microcontroller, or state machine. A processor also may be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In some implementations, particular steps and methods may be performed by circuitry that is specific to a given function. 
     In one or more aspects, the functions described may be implemented in hardware, digital electronic circuitry, computer software, firmware, including the structures disclosed in this specification and their structural equivalents thereof, or in any combination thereof. Implementations of the subject matter described in this specification also can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on a computer storage media for execution by, or to control the operation of, data processing apparatus. 
     Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein. Additionally, a person having ordinary skill in the art will readily appreciate, the terms “upper” and “lower” are sometimes used for ease of describing the figures, and indicate relative positions corresponding to the orientation of the figure on a properly oriented page, and may not reflect the proper orientation of, e.g., an IMOD display element as implemented. 
     Certain features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, a person having ordinary skill in the art will readily recognize that such operations need not be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flow diagram. However, other operations that are not depicted can be incorporated in the example processes that are schematically illustrated. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the illustrated operations. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Additionally, other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results.