Patent Publication Number: US-7719747-B2

Title: Method and post structures for interferometric modulation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/052,004, filed Feb. 4, 2005, which claims priority benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 60/613,471, filed Sep. 27, 2004, which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates generally to optical modulator devices and methods for making optical modulators, and more particularly to interferometric modulators used in displays. 
     2. Description of the Related Art 
     A variety of different types of spatial light modulators can be used for imaging applications. One type of a spatial light modulator is an interferometric modulator. Interferometric modulator devices can be arranged in an array configuration to provide a display assembly having advantageous operational and performance characteristics. Interferometric modulator devices operate by employing optical interference to provide displays having rich color characteristics as well as low power consumption. Interferometric modulators have at least two states and are configured such that, in at least one of the states, incident light is reflected and interferes to provide corresponding different appearances to a viewer. In one state, a relatively narrow band reflection may present a distinct color to the viewer, such as a red, green, or blue color. In another state, the interferometric modulator can act on the incident light so as to present a reflected dark or black appearance to a viewer. 
     In order to provide increased contrast in the various viewing states, it is desirable that an array of interferometric devices presents a more uniformly dark or black appearance in the dark state. Similarly, it is desirable in the colored viewing states to present colors which are richer and more vibrant. 
     SUMMARY 
     The system, method, and devices described herein each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of Preferred Embodiments” one will understand how the features of the systems, methods and devices described herein provide advantages that include, for example, improved device performance. 
     An embodiment provides a spatial light modulator (e.g., an interferometric modulator) that includes a post structure, wherein the post structure contains an optical element. In a preferred embodiment, the optical element in the post structure is a reflective element, e.g., a mirror. In another embodiment, the optical element in the post structure is an etalon, e.g., a dark etalon. In another embodiment, the post structure comprises a reflective element and a dark etalon. 
     The optical element in the post structure may be configured in various ways. For example, in certain embodiments, the optical element in the post structure decreases the amount of light that would otherwise be retroreflected from the post structure. In other embodiments, the optical element in the post structure increases the backlighting of the interferometric modulator. In various embodiments, the optical element in the post structure increases the brightness of the interferometric modulator by redirecting light into the interferometric cavity. 
     Another embodiment provides a method for making an interferometric modulator. In this method, a reflective layer is deposited onto a substrate to form a first mirror. A sacrificial layer is deposited over the first mirror. Apertures are formed in the sacrificial layer and post material is deposited into the apertures. An optical element is formed over the post material and a moveable second mirror is formed over the sacrificial layer and the optical element. The sacrificial layer is removed to thereby form an interferometric cavity. 
     These and other embodiments are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be readily apparent from the following description and from the appended drawings (not to scale), which are meant to illustrate and not to limit the invention, and wherein: 
         FIG. 1  is an isometric view depicting a portion of one embodiment of an interferometric modulator display in which a movable reflective layer of a first interferometric modulator is in a released position and a movable reflective layer of a second interferometric modulator is in an actuated position. 
         FIG. 2  is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display. 
         FIG. 3  is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . 
         FIG. 4  is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display. 
         FIGS. 5A and 5B  illustrate one exemplary timing diagram for row and column signals that may be used to write a frame of display data to the 3×3 interferometric modulator display of  FIG. 2 . 
         FIG. 6A  is a cross section of the device of  FIG. 1 . 
         FIG. 6B  is a cross section of an alternative embodiment of an interferometric modulator. 
         FIG. 6C  is a cross section of another alternative embodiment of an interferometric modulator. 
         FIG. 7A  shows a cross section of an interferometric modulator that schematically illustrates a post structure that is transparent.  FIG. 7B  shows a cross section of an interferometric modulator that schematically illustrates a post structure that is reflective. 
         FIG. 8A  is a reproduction of a photomicrograph of an array of interferometric modulators having metal posts.  FIG. 8B  is a reproduction of a photomicrograph of an array of interferometric modulators having silicon oxide posts. 
         FIG. 9  shows a cross-section of an interferometric modulator that schematically illustrates post structures that comprise reflective elements, some of which form etalons. 
         FIG. 10  shows a cross-section of a backlit interferometric modulator schematically illustrating post structures that comprise reflective elements configured to direct light into the optical cavity of the interferometric modulator. 
         FIG. 11  shows a cross-section of a front lit interferometric modulator schematically illustrating post structures that comprise reflective elements configured to increase brightness of the interferometric modulator. 
         FIG. 12  shows a flow chart illustrating a process for making an interferometric modulator having a post structure that contains an optical element. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As described more fully below, reflective optical elements may be included in post structures in interferometric modulators. In certain embodiments, these reflective optical elements may form etalons that reflect a particular color or wavelength range. Such etalons, which may comprise, for example, dark etalons, may cause the post structure to appear dark as seen by a viewer. These optical elements may increase the contrast of a display by providing a more uniformly dark or black appearance (reducing the area of the display that is relatively bright) when the interferometric modulator is in a dark state. Similarly, brighter more vibrant colors that are not “washed out” by bright regions are provided when the interferometric modulator is in a bright state. Reflective elements may also be included that direct illumination, either backlighting or front lighting, into the optical cavity of the interferometric modulator. 
     As will be apparent from the following description, the structures described herein may be implemented in any device that is configured to display an image, whether in motion (e.g., video) or stationary (e.g., still image), and whether textual or pictorial. More particularly, it is contemplated that the structures and methods may be implemented in or associated with a variety of electronic devices such as, but not limited to, mobile telephones, wireless devices, personal data assistants (PDAs), hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, computer monitors, auto displays (e.g., odometer display, etc.), cockpit controls and/or displays, display of camera views (e.g., display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures (e.g., tile layouts), packaging, and aesthetic structures (e.g., display of images on a piece of jewelry). More generally, the structures and methods described herein may be implemented in electronic switching devices, their manufacture, and use. 
     The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. 
     One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated in  FIG. 1 . In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“off” or “closed”) state, the display element reflects little incident visible light to the user. Depending on the embodiment, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at selected colors, allowing for a color display in addition to black and white. 
       FIG. 1  is an isometric view depicting two adjacent pixels in a series of pixels of a visual display, wherein each pixel comprises a MEMS interferometric modulator. In some embodiments, an interferometric modulator display comprises a row/column array of these interferometric modulators. Each interferometric modulator includes a pair of reflective layers positioned at a variable and controllable distance from each other to form a resonant optical cavity with at least one variable dimension. In one embodiment, one of the reflective layers may be moved between two positions. In the first position, referred to herein as the released state, the movable layer is positioned at a relatively large distance from a fixed partially reflective layer. In the second position, the movable layer is positioned more closely adjacent to the partially reflective layer. Incident light that reflects from the two layers interferes constructively or destructively depending on the position of the movable reflective layer, producing either an overall reflective or non-reflective state for each pixel. 
     The depicted portion of the pixel array in  FIG. 1  includes two adjacent interferometric modulators  12   a  and  12   b . In the interferometric modulator  12   a  on the left, a movable and highly reflective layer  14   a  is illustrated in a released position at a predetermined distance from a fixed partially reflective layer  16   a . In the interferometric modulator  12   b  on the right, the movable highly reflective layer  14   b  is illustrated in an actuated position adjacent to the fixed partially reflective layer  16   b.    
     The fixed layers  16   a ,  16   b  are electrically conductive, partially transparent and partially reflective, and may be fabricated, for example, by depositing one or more layers each of chromium and indium-tin-oxide onto a transparent substrate  20 . The layers are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable layers  14   a ,  14   b  may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes  16   a ,  16   b ) deposited on top of posts  18  and an intervening sacrificial material deposited between the posts  18 . When the sacrificial material is etched away, the deformable metal layers are separated from the fixed metal layers by a defined air gap  19 . A highly conductive and reflective material such as aluminum may be used for the deformable layers, and these strips may form column electrodes in a display device. 
     With no applied voltage, the cavity  19  remains between the layers  14   a ,  16   a  and the deformable layer is in a mechanically relaxed state as illustrated by the pixel  12   a  in  FIG. 1 . However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel becomes charged, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable layer is deformed and is forced against the fixed layer (a dielectric material which is not illustrated in this Figure may be deposited on the fixed layer to prevent shorting and control the separation distance) as illustrated by the pixel  12   b  on the right in  FIG. 1 . The behavior is the same regardless of the polarity of the applied potential difference. In this way, row/column actuation that can control the reflective vs. non-reflective pixel states is analogous in many ways to that used in conventional LCD and other display technologies. 
       FIGS. 2 through 5  illustrate one exemplary process and system for using an array of interferometric modulators in a display application.  FIG. 2  is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor  21  which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor  21  may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application. 
     In one embodiment, the processor  21  is also configured to communicate with an array controller  22 . In one embodiment, the array controller  22  includes a row driver circuit  24  and a column driver circuit  26  that provide signals to a pixel array  30 . The cross section of the array illustrated in  FIG. 1  is shown by the lines  1 - 1  in  FIG. 2 . For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices illustrated in  FIG. 3 . It may require, for example, a 10 volt potential difference to cause a movable layer to deform from the released state to the actuated state. However, when the voltage is reduced from that value, the movable layer maintains its state as the voltage drops back below 10 volts. In the exemplary embodiment of  FIG. 3 , the movable layer does not release completely until the voltage drops below 2 volts. There is thus a range of voltage, about 3 to 7 V in the example illustrated in  FIG. 3 , where there exists a window of applied voltage within which the device is stable in either the released or actuated state. This is referred to herein as the “hysteresis window” or “stability window.” For a display array having the hysteresis characteristics of  FIG. 3 , the row/column actuation protocol can be designed such that during row strobing, pixels in the strobed row that are to be actuated are exposed to a voltage difference of about 10 volts, and pixels that are to be released are exposed to a voltage difference of close to zero volts. After the strobe, the pixels are exposed to a steady state voltage difference of about 5 volts such that they remain in whatever state the row strobe put them in. After being written, each pixel sees a potential difference within the “stability window” of 3-7 volts in this example. This feature makes the pixel design illustrated in  FIG. 1  stable under the same applied voltage conditions in either an actuated or released pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or released state, is essentially a capacitor formed by the fixed and moving reflective layers, this stable state can be held at a voltage within the hysteresis window with almost no power dissipation. Essentially no current flows into the pixel if the applied potential is fixed. 
     In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row 1 electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row 2 electrode, actuating the appropriate pixels in row 2 in accordance with the asserted column electrodes. The row 1 pixels are unaffected by the row 2 pulse, and remain in the state they were set to during the row 1 pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used in conjunction with the present invention. 
       FIGS. 4 and 5  illustrate one possible actuation protocol for creating a display frame on the 3×3 array of  FIG. 2 .  FIG. 4  illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves of  FIG. 3 . In the  FIG. 4  embodiment, actuating a pixel involves setting the appropriate column to −V bias , and the appropriate row to +ΔV, which may correspond to −5 volts and +5 volts respectively Releasing the pixel is accomplished by setting the appropriate column to +V bias , and the appropriate row to the same +ΔV, producing a zero volt potential difference across the pixel. In those rows where the row voltage is held at zero volts, the pixels are stable in whatever state they were originally in, regardless of whether the column is at +V bias , or −V bias . 
       FIG. 5B  is a timing diagram showing a series of row and column signals applied to the 3×3 array of  FIG. 2  which will result in the display arrangement illustrated in  FIG. 5A , where actuated pixels are non-reflective. Prior to writing the frame illustrated in  FIG. 5A , the pixels can be in any state, and in this example, all the rows are at 0 volts, and all the columns are at +5 volts. With these applied voltages, all pixels are stable in their existing actuated or released states. 
     In the  FIG. 5A  frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) are actuated. To accomplish this, during a “line time” for row 1, columns 1 and 2 are set to −5 volts, and column 3 is set to +5 volts. This does not change the state of any pixels, because all the pixels remain in the 3-7 volt stability window. Row 1 is then strobed with a pulse that goes from 0, up to 5 volts, and back to zero. This actuates the (1,1) and (1,2) pixels and releases the (1,3) pixel. No other pixels in the array are affected. To set row 2 as desired, column 2 is set to −5 volts, and columns 1 and 3 are set to +5 volts. The same strobe applied to row 2 will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again, no other pixels of the array are affected. Row 3 is similarly set by setting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3 strobe sets the row 3 pixels as shown in  FIG. 5A . After writing the frame, the row potentials are zero, and the column potentials can remain at either +5 or −5 volts, and the display is then stable in the arrangement of  FIG. 5A . It will be appreciated that the same procedure can be employed for arrays of dozens or hundreds of rows and columns. It will also be appreciated that the timing, sequence, and levels of voltages used to perform row and column actuation can be varied widely within the general principles outlined above, and the above example is exemplary only, and any actuation voltage method can be used with the present invention. 
     The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,  FIGS. 6A-6C  illustrate three different embodiments of the moving mirror structure.  FIG. 6A  is a cross section of the embodiment of  FIG. 1 , where a strip of metal material  14  is deposited on orthogonally extending supports  18 . In  FIG. 6B , the moveable reflective material  14  is attached to supports at the corners only, on tethers  32 . In  FIG. 6C , the moveable reflective material  14  is suspended from a deformable layer  34 . This embodiment has benefits because the structural design and materials used for the reflective material  14  can be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer  34  can be optimized with respect to desired mechanical properties. The production of various types of interferometric devices is described in a variety of published documents, including, for example, U.S. Published Application 2004/0051929. A wide variety of well known techniques may be used to produce the above described structures involving a series of material deposition, patterning, and etching steps. 
     Interferometric modulators of the general design discussed above comprise at least one post structure. (See, e.g., post  18  in  FIGS. 1 and 6 ). A “post” or “post structure” is a structure located at the side or corner of an interferometric cavity (or between adjoining interferometric cavities) that supports an electrode (and/or mirror) and/or maintains or helps to maintain a desired distance between electrodes (and/or mirrors). Accordingly, the post structure may be adjacent an open region and may be an orthogonally extending support (e.g., support  18 ). As described above, the electrode or mirror supported by the post structure may comprise a flexible member that flexes in the open region in the interferometric modulator with application of an electric field in the cavity. Post structures typically have a width of about 3 to about 15 microns, although the width of the post structure may be outside this range. The shape of the post structure may vary as well. The post structure may have a height that corresponds approximately to the cavity height (e.g., the spacing between the upper and lower mirrors). The height of the post structure, however, may be larger or smaller. For example, the post structure may be formed on a layer of material or on a pedestal and thus be raised to a level above the lower mirror. Similarly, the post structure may extend to a height higher than the upper mirror or to a depth lower than the lower mirror. See, e.g.,  FIG. 6C  which shows the upper mirror attached to an electrode supported by the post structure. 
     Post structures may be formed from various materials (e.g., metals, silicon oxide, metal oxides, polymers, etc.) having optical properties that may differ significantly from one another. It has been found that the configuration of the post structure and the material(s) from which it is constructed may significantly affect the performance of the interferometric modulator. For example,  FIG. 7A  shows a cross-section of an interferometric modulator that schematically illustrates a transparent post structure  305  and an interferometric cavity  310 . Light entering through the substrate  315  and entering the post structure  305  may reflect from the upper mirror structure  320  and exit back through the substrate  315  as depicted by the series of arrows  325  in  FIG. 7A . The optical characteristics of the post are largely uncontrolled and unlikely to be beneficial when the configuration of the post  305  and the material(s) from which it is made are selected for reasons unrelated to the optical characteristics of the reflected light depicted by the series of arrows  325 . 
     As another example,  FIG. 7B  shows a cross-section of an interferometric modulator that schematically illustrates a reflective post structure  350  and an interferometric cavity  355 . Light entering through the substrate  360  reflects from the base of the post  350  and exits back through the substrate  360  as depicted by the series of arrows  365  in  FIG. 7B . As in  FIG. 7A , the optical characteristics of the reflected light depicted by the series of arrows  365  are uncontrolled and unlikely to be beneficial. 
     As another example,  FIG. 8A  shows a photomicrograph of an array of interferometric modulators having metal posts. The bright spots  405  result from light reflecting from the bases of the metal posts.  FIG. 8B  shows a similar photomicrograph in which the dark spots  410  result from light passing through transparent silicon oxide posts. The color of the dark spots  410  is generally different from the light that exits from the interferometric modulators. 
     It has now been found that optical elements may be incorporated into the post structures of interferometric modulators in order to provide various desirable optical effects. In one embodiment, the optical element is a reflector. This reflector may have various configurations. For example,  FIG. 9  shows a cross-section of an interferometric modulator that schematically illustrates post structures that comprise optical elements. In  FIG. 9 , a reflector  550  is fabricated within a transparent post structure  555 . This reflector  550  has a reflective surface that reflects incoming light incident thereon. This reflector  550  may comprise a reflective material such as metal. The reflector  550  may be formed using semiconductor fabrication techniques, for example, by depositing silicon oxide as a lower post section  555 A, then forming the reflector  550  comprising, e.g., metal, then depositing additional silicon oxide as an upper post section  555 B. The optical element within the post structure may also be a reflector that is a component of an etalon (Fabry-Perot interferometer), e.g., an etalon  565  formed by an upper metal reflector  570  and the reflector  575 . The etalon  565  forms an optical cavity (a Fabry-Perot cavity) that contains the post structure material(s), e.g., silicon oxide. The color of reflected light  580  exiting the etalon  565  may be controlled by controlling the vertical position of the reflector  570  within the post structure  585  (as well as the material between the reflectors  570  and  575 ). The color of the reflected light  580  may be adjusted to produce a black appearance by adjusting the vertical position of the reflector so that substantially all of the incident light or visible incident light is absorbed or is not reflected back toward a viewer, thereby producing a dark etalon. An etalon  588  (e.g., a dark or colored etalon) may also be incorporated into a post structure by incorporating two metal reflector layers  590 ,  592  into the post structure as illustrated in  FIG. 9 . The color of the etalon  588  may be adjusted by controlling the thickness of the post material  591  between the reflector layers. 
     The reflectors  550 ,  570 ,  590  and  592  are depicted in  FIG. 9  as having reflective surfaces that are substantially parallel to the reflector  575 . Those skilled in the art will understand that optical elements such as reflectors may be oriented at various angles and configured in various shapes.  FIG. 10  shows additional examples of optical elements that may be incorporated into the post structures of interferometric modulators. In  FIG. 10 , reflectors  605 ,  610 ,  615 ,  620  have been fabricated within post structures  606 ,  611 ,  616 ,  621  using semiconductor fabrication techniques. The surfaces of the reflectors  605 ,  610 ,  615 ,  620  are inclined at various angles. Optical elements having angled surfaces may be fabricated using various techniques known to those skilled in the art. Such techniques include, for example, tapered angle etching (see, e.g., U.S. Pat. No. 5,473,710). On the left side of  FIG. 10 , the upper mirror  625  is in a raised position. Light from a backlight source  630  is reflected from the inclined surfaces of the reflectors  605 ,  610  to the upper mirror  625  as indicated by the series of arrows  635 . Thus, the reflectors  605 ,  610  are examples of optical elements configured to direct light into the interferometric cavity  623 . The directed light also reflects from the upper mirror  625  and exits as indicated by the arrows  626 , thereby increasing the brightness of the interferometric modulator. 
     On the right side of  FIG. 10 , the upper mirror  650  is in a lowered position. Light from the backlight source  660  is reflected from the inclined surfaces of the reflectors  615 ,  620  generally back towards the source  660  (instead of through the post structure) as indicated by the series of arrows  665 .  FIG. 10  also illustrates a dark etalon  673  formed by incorporating two metal reflectors  671 ,  672  into the post structure  621  that includes reflector  620 . Thus,  FIG. 10  illustrates the use of optical elements within post structures to control backlighting and the incorporation of multiple optical elements into the same post structure. Those skilled in the art will appreciate that  FIG. 10  illustrates that the performance of an optical element in a post structure may vary depending on the state (e.g., driven or undriven) of a nearby interferometric modulator. 
       FIG. 11  illustrates the use of an optical element in a post structure to increase the brightness of an interferometric modulator by redirecting light coming from the front of the interferometric modulator into the interferometric cavity. On the left side of  FIG. 11 , the upper mirror  705  is in a raised position. A series of arrows  710  represents light coming from a front source  702  that enters a post structure  715 , reflects from an angled reflector  720  to the upper mirror  705 , and then exits the interferometric cavity  725  back in the general direction of the source  702 , thereby increasing brightness. On the right side of  FIG. 11 , the upper mirror  750  is in a lowered position, and a similar configuration is used to increase black level by redirecting light (represented by a series of arrows  755 ) away from the source  703 . Thus light from the front source  703  reflects from the angled surface of the reflector  760  and exits the back side of the interferometric cavity  765  in a direction generally away from the source  703 . 
     Those skilled in the art will understand that various optical elements  780  having a variety of configurations may be incorporated into post structures. Non-limiting examples of such optical elements  780  include reflectors  720  and  760  etalons, light scattering elements (such as microscopic glass particles), light diffracting elements, total internal reflection (TIR) elements, and refractive elements. Lenses  786   a ,  786   b ,  786   c  and prisms  784  are possible, as schematically illustrated in the exploded inset  782  of  FIG. 11 . The surfaces of the optical elements  780 , such as the surfaces of the reflectors, may be curved (e.g., spherical or parabolic) or flat, and inclined or declined at various angles. Likewise, lenses  786   a ,  786   b ,  786   c  may be configured in various ways, e.g., convex as illustrated by the lenses  786   a ,  786   b , concave as illustrated by the lens  786   c , etc., and also may be inclined or declined at various angles within the post structure. Asymmetrical as well as symmetrical shapes and configurations are possible. These surfaces of the optical elements may be smooth or rough. Reflection may be specular or diffuse. The optical elements may be located in different positions in the post structure. The optical element may be located at different heights and may be off-center within the post. The optical element may have different orientations and may be tilted. Different posts structures may have optical elements with different characteristics. 
     Another embodiment provides a method for making an interferometric modulator. The steps in such a method  800  ( FIG. 12 ) may be carried out using techniques known to those skilled in the art. The process begins at step  805  by depositing a reflective layer onto a substrate to form a first mirror. The deposition of the reflective layer may be conducted by, e.g., chemical vapor deposition of a semi-reflective material such as a metal (e.g., indium tin oxide and/or chrome). The formation of the first mirror at step  805  may further comprise depositing a dielectric material (e.g., silicon oxide) over the metal layer. Those skilled in the art will understand that the first mirror may be an optical stack, and thus formation of the first mirror at step  805  may involve depositing multiple metal layers, e.g., chrome and indium tin oxide. 
     The process continues at step  810  by depositing a sacrificial layer over the first mirror. The deposition of the sacrificial layer may be conducted by, e.g., chemical vapor deposition of a material that may be selectively removed in a later etching step. Examples of such sacrificial materials include molybdenum and silicon. The process continues at step  815  by forming apertures in the sacrificial layer using masking and etching techniques known to those skilled in the art. The process continues at step  820  by depositing post material into the apertures. Post material that is optically transmissive to visible light may be used. Examples of suitable post materials include silicon dioxide and photoresists, which may be deposited by, e.g., known spin-on and chemical vapor deposition techniques. In an embodiment, the deposited post material partially fills the apertures, depending on the desired vertical position of the optical element within the post. A layer of material for forming the optical element is deposited over the post material within the aperture in a subsequent step  825 . Various known deposition and/or patterning methods (such as tapered etching for angled surfaces) may be used to form the optical element. Optionally, additional post material may be deposited over the optical element within the aperture. 
     The process continues at step  830  by forming a moveable second mirror over the sacrificial layer and the optical element. The formation of the moveable second mirror may be conducted in various ways, depending on the desired mirror configuration as discussed above. The process continues at step  835  by removing the sacrificial layer to thereby form an interferometric cavity. Various etching methods may be used to efficiently remove the sacrificial layer, e.g., by exposing the sacrificial layer to an etchant such as XeF 2  that selectively removes sacrificial materials such as molybdenum and silicon. Those skilled in the art will understand that the process illustrated in  FIG. 12  may be modified as needed to produce interferometric modulators having a variety of configurations. 
     Those skilled in the art will appreciate that a post structure may contain a plurality of optical elements. For examples, two or more reflectors may be fabricated into a post structures at various heights using minor modifications of the fabrication methods described above. Those skilled in the art will also appreciate that the use of such a plurality of optical elements in a post structure may be used to provide various optical benefits or combinations thereof, e.g., a wider variety of colors than the use of a single optical element. 
     While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others.