Patent Publication Number: US-7221497-B2

Title: Optical interference pixel display

Description:
RELATED APPLICATIONS 
   The present patent application is a continuation of the previously filed patent application, entitled “Optical interference pixel display with charge control,” filed on Apr. 30, 2003, assigned Ser. No. 10/428,261, and now U.S. Pat. No. 7,072,093, which is hereby incorporated by reference. 

   BACKGROUND 
   Nearly all conventional displays are active in nature. This means that power must continually be supplied to the displays for them to maintain the images they are displaying. Such conventional displays include direct view and projection cathode-ray tube (CRT) displays, direct view and projection liquid crystal displays (LCD&#39;s), direct view plasma displays, projection digital light processing (DLP) displays, and direct view electroluminescent (EL) displays, among others. 
   Since power must continually be supplied to these types of displays, they can be a significant cause of power usage in devices where supplied power is at a premium, such as portable devices like laptop and notebook computers, personal digital assistant (PDA) devices, wireless phones, as well as other types of portable devices. As a result, designers of such devices usually choose to increase the size of the battery size contained in such devices, increasing weight and cost, or choose to reduce the running time of the devices between battery charges. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings referenced herein form a part of the specification. Features shown in the drawing are meant as illustrative of only some embodiments of the invention, and not of all embodiments of the invention, unless otherwise explicitly indicated, and implications to the contrary are otherwise not to be made. 
       FIG. 1A  is a diagram of an electronic device for at least partially displaying a pixel of a displayable image, according to an embodiment of the invention. 
       FIGS. 1B ,  1 C, and  1 D are diagrams showing different approaches to control the charge stored on the electronic device of  FIG. 1A , according to varying embodiments of the invention. 
       FIGS. 2A and 2B  are graphs of representative spectral responses of the electronic device of  FIG. 1A , according to varying embodiments of the invention. 
       FIG. 3A  is a diagram of an array of passive pixel mechanisms, according to an embodiment of the invention. 
       FIG. 3B  is a cross-sectional diagram of a display device, according to an embodiment of the invention. 
       FIG. 4  is a method of use, according to an embodiment of the invention. 
       FIG. 5  is a diagram of an electronic device that is more specific than but consistent with the electronic device of  FIG. 1A , according to an embodiment of the invention. 
       FIG. 6  is a method of manufacture, according to an embodiment of the invention. 
       FIGS. 7A ,  7 B, and  7 C are diagrams of electronic devices that are more specific than but consistent with the electronic device of  FIG. 1A , according to varying embodiments of the invention. 
       FIGS. 8A and 8B  are diagrams of electronic devices that are more specific than but consistent with the electronic device of  FIG. 1A , and which include lenses, according to varying embodiments of the invention. 
       FIGS. 9A ,  9 B, and  9 C are diagrams illustratively depicting how anti-stiction bumps can be fabricated within the electronic device of  FIG. 1A , according to an embodiment of the invention. 
       FIGS. 10A ,  10 B, and  10 C are diagrams illustratively depicting how anti-stiction bumps can be fabricated within the electronic device of  FIG. 1A , according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
   Overview 
     FIG. 1A  shows an electronic device  100  for at least partially displaying a pixel of a displayable image, according to an embodiment of the invention. The device  100  includes a top reflector  102  and a bottom reflector  104 , as well as a flexure  110  and a spring mechanism  112 . A resonant optical cavity  106  is defined by the reflectors  102  and  104 , which has a variable thickness, or width,  108 . The top reflector  102  is in one embodiment highly reflective, such as completely reflective. The bottom reflector  104  is in one embodiment semi-transparent; that is, the bottom reflector  104  is in one embodiment semi-reflective. The spring mechanism  112  may be a flexible material, such as a polymer, in one embodiment of the invention, that has linear or non-linear spring functionality. 
   The optical cavity  106  is variably selective of a visible wavelength at an intensity, by optical interference. Depending on the desired configuration of the electronic device  100 , the optical cavity  106  may either reflect or transmit the wavelength at the intensity. That is, the cavity  106  may be reflective or transmissive in nature. No light is generated by the optical cavity  106 , such that the device  100  relies on ambient light or light provided by the device  100  that is reflected or transmitted by the cavity  106 . The visible wavelength selected by the optical cavity  106 , and its intensity selected by the optical cavity  106 , are dependent on the thickness  108  of the cavity  106 . That is, the optical cavity  106  can be tuned to a desired wavelength at a desired intensity by controlling its thickness  108 . 
   The flexure  110  and the spring mechanism  112  allow the thickness  108  of the cavity  106  to vary, by allowing the bottom reflector  104  to move. More generally, the flexure  110  and the spring mechanism  112  constitute a mechanism that allows variation of the optical properties of the optical cavity  106  to variably select a visible wavelength at an intensity. The optical properties include the optical index of the cavity  106 , and/or the optical thickness of the cavity  106 . A voltage applied between the reflectors  102  and  104 , or electrical charge stored on the reflectors  102  and  104 , causes the thickness  108  of the cavity  106  to change, because the flexure  110  and the spring mechanism  112  allow the reflector  104  to move. Thus, the flexure  110  has a stiffness, and the spring mechanism  112  has a spring restoring force, such that the voltage applied to the reflectors  102  and  104  or the charge stored on the reflectors  102  and  104  causes the flexure  110  and the spring mechanism  112  to yield and allow the reflector  104  to move, achieving the desired thickness  108 . No power is dissipated in maintaining a given thickness  108 . 
   In one embodiment, the bottom reflector  104  is maintained at a fixed voltage, and the top reflector  102  is set to a voltage depending on the desired visible wavelength and the desired intensity, as calibrated to the stiffness of the flexure  110 . Whereas the flexure  110  is shown in the embodiment of  FIG. 1A  as positioned under the bottom reflector  104 , in another embodiment it may be positioned over the bottom reflector  104 . In other embodiments, the flexure  110  may be positioned over or under the top reflector  102  as well, such that the bottom reflector  104  is movable, instead of the top reflector  102 , to adjust the thickness  108  of the optical cavity  106 . Furthermore, in another embodiment, there may be more than one optical cavity, such that the optical cavity  106  is inclusive of more than one such cavity. 
   In one embodiment, the bottom reflector  104  and the top reflector  102  can be considered the plates of a capacitor, where the optical cavity  106  represents the dielectric therebetween. A potential applied between the bottom reflector  104  and the top reflector  102  moves the bottom reflector  104 , due to the flexure  110  and the spring mechanism  112 , but also causes a charge to be stored in the capacitor. It is this electrostatic charge that then allows maintenance of the given thickness  108  without any further voltage application over the bottom reflector  104  and the top reflector  102 . 
   The wavelength and the intensity selected by the optical cavity  106  correspond to a pixel of a displayable image. Thus, the electronic device  100  at least partially displays the pixel of the image. The electronic device  100  can operate in either an analog or a digital manner. As an analog device, the electronic device  100  selects a visible wavelength of light and an intensity corresponding to the color and the intensity of the color of the pixel. In an alternative embodiment, the electronic device  100  may be used to display the pixel in an analog manner in black-and-white, or in gray scale, in lieu of color. 
   As a digital device, the electronic device  100  is responsible for either the red, green, or blue color component of the pixel. The device  100  maintains a static visible wavelength, either red, green, or blue, and varies the intensity of this wavelength corresponding to the red, green, or blue color component of the pixel. Therefore, three of the device  100  are needed to display the pixel digitally, where one device  100  selects a red wavelength, another device  100  selects a green wavelength, and a third device  100  selects a blue wavelength. More generally, there is a device  100  for each color component of the pixel, or portion, of the image. Furthermore, in an alternative embodiment, the electronic device  100  may be used to display the pixel in a digital manner in black-and-white, or in gray scale, in lieu of color. 
   Optical Interference to Variably Select Wavelength and Intensity 
   The optical cavity  106  of the electronic device  100  utilizes optical interference to transmissively or reflectively select a wavelength at an intensity. The optical cavity  106  in one embodiment is a thin film having a light path length equal to the thickness  108 . Light is reflected from the boundaries of the reflectors  102  and  104  on either side of the cavity  106 , interfering with itself. The phase difference between the incoming beam and its reflected image is k(2d), where d is the thickness  108 , because the reflected beam travels the distance 2d within the cavity  106 . Since 
             k   =       2   ⁢   π     λ       ,         
then when
 
             d   =     λ   2       ,         
the phase difference between the incoming and the reflected waves is k2d=2π, giving constructive interference. All multiples of π/2, which are the modes of the optical cavity  106 , are transmitted. As a result of optical interference, then, the optical cavity  106  passes the most light at integer multiples of λ/2 and the least amount of light at odd integer multiples of λ/4. Although the above calculations capture the primary mechanism for interference-based light modulation, more rigorous electromagnetic simulations may be desired to more accurately describe actual device performance.
 
   In one embodiment, the top reflector  102  includes a thin, partially transmitting metallic film, where n−ik=2.5−2.5i titanium, where n represents the real optical index of the cavity  106 , and k represents the imaginary optical index of the cavity  106 . In this embodiment, both absorption and interference play roles in modulating the color and intensity of the output. The optical cavity  106  is an adjustable spacer, and the bottom reflector  104  is a high-reflectance metallic substrate, like aluminum. In one embodiment, where the device  100  is digital, the optical cavity  106  may select a red wavelength of 6100 angstrom (Å), a green wavelength of 5500 Å, or a blue wavelength of 4500 Å, at an intensity depending on the corresponding color component of the pixel to be displayed. Furthermore, the optical cavity  106  can achieve low reflection or transmission. In this latter state, the optical cavity  106  is a so-called “dark mirror” that can be optimized for less than five percent reflection or transmission. 
   For example, in this embodiment, the film stack sequence of the bottom reflector  104 , the optical cavity  106 , and the top reflector  102  can achieve a red wavelength of 6100 Å, with an incident n of 1.5 at the bottom reflector  104  and a substrate n of 1.52 at the top reflector  102  in accordance with the following table: 
   
     
       
         
             
             
             
             
             
             
           
             
                 
             
             
                 
               Real 
                 
                 
               Target 
               Number of 
             
             
                 
               index 
               Imaginary 
               Thickness 
               wavelength 
               waves at 
             
             
               Layers 
               (n) 
               index (k) 
               (Å) 
               intensity 
               target 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
             
          
             
               Bottom 
               0.2 
               5 
               6250 
               5000 
               0.25 
             
             
               reflector 
             
             
               104 (silver) 
             
             
               Optical cavity 
               1 
               0 
               2750 
               5000 
               0.55 
             
             
               106 
             
             
               Top reflector 
               2.5 
               2.5 
               200 
               5000 
               0.1 
             
             
               102 (titanium) 
             
             
                 
             
          
         
       
     
   
   Similarly, this film stack sequence can achieve a green wavelength of 5500 Å with an incident n of 1.5 at the top reflector  102  and a substrate n of 1.52 at the bottom reflector  104  in accordance with the following table: 
   
     
       
         
             
             
             
             
             
             
           
             
                 
             
             
                 
               Real 
                 
                 
               Target 
               Number of 
             
             
                 
               index 
               Imaginary 
               Thickness 
               wavelength 
               waves at 
             
             
               Layers 
               (n) 
               index (k) 
               (Å) 
               intensity 
               target 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
             
          
             
               Bottom 
               0.2 
               5 
               6250 
               5000 
               0.25 
             
             
               reflector 
             
             
               104 (silver) 
             
             
               Optical cavity 
               1 
               0 
               2500 
               5000 
               0.5 
             
             
               106 
             
             
               Top reflector 
               2.5 
               2.5 
               200 
               5000 
               0.1 
             
             
               102 (titanium) 
             
             
                 
             
          
         
       
     
   
   The film stack sequence can also achieve a blue wavelength of 4500 Å with an incident n of 1.5 at the top reflector  102  and a substrate n of 1.52 at the bottom reflector  104  in accordance with the following table: 
                                                   Real           Target   Number of           index   Imaginary   Thickness   wavelength   waves at       Layers   (n)   index (k)   (Å)   intensity   target                                                        Bottom   0.2   5   6250   5000   0.25       reflector       104 (silver)       Optical cavity   1   0   2000   5000   0.5       106       Top reflector   2.5   2.5   200   5000   0.1       102 (titanium)                    
Thus, the film stack sequence achieves a red wavelength of 6100 Å, a green wavelength of 5500 Å, or a blue wavelength of 4500 Å, depending on whether the thickness of the optical cavity  106  is 2750 Å, 2500 Å, or 2000 Å, respectively.
 
   Finally, the film stack sequence can achieve a low reflection or a low transmission with an incident n of 1.5 at the top reflector  102  and a substrate n of 1.52 at the bottom reflector  104  in accordance with the following table: 
                                                   Real           Target   Number of           index   Imaginary   Thickness   wavelength   waves at       Layers   (n)   index (k)   (Å)   intensity   target                                                        Bottom   0.2   5   6250   5000   0.25       reflector       104 (silver)       Optical cavity   1   0   400   5000   0.08       106       Top reflector   2.5   2.5   200   5000   0.1       102 (titanium)                    
This results in dark gray, nearly black output, where the thickness of the optical cavity  106  is 400 Å. By ratioing the amount of time that a pixel remains in the colored or black states, a large range of average hues and intensities can be obtained.
 
Controlling Thickness of Optical Cavity
 
   As has been indicated, the flexure  110  and the spring mechanism  112  allow the thickness  108  of the optical cavity  106  to vary when an appropriate voltage has been applied across the reflectors  102  and  104 , such that a desired wavelength at a desired intensity is selected. This voltage is determined in accordance with the following equation, which is the force of attraction between the reflectors  102  and  104  acting as plates of a parallel plate capacitor, and which does not take into account fringing fields: 
                   F   =         ɛ   0     ⁢     V   2     ⁢   A       2   ⁢     d   2           ,           (   1   )               
where ε 0  is the permittivity of free space, V is the voltage across the reflectors  102  and  104 , A is the area of each of the reflectors  102  and  104 , and d is the thickness  108 . Thus, a one volt potential applied across a 100 micron square pixel, with a thickness  108  of 0.25 micron, yields an electrostatic force of 7×10 −7  Newton (N).
 
   Therefore, a small voltage between the reflectors  102  and  104  provides sufficient force to move the bottom reflector  104 , and hold it against gravity and shocks. Once the voltage has been applied, the electrostatic charge stored in the capacitor created by the reflectors  102  and  104 , and defining the cavity  106 , is sufficient to hold the bottom reflector  104  in place without additional power. Charge leakage may require occasional refreshing of the charge, however. 
   The force defined in equation (1) is balanced with the linear spring force provided by the spring mechanism  112 :
 
 F=k ( d   0   −d ),  (2)
 
where k is the linear spring constant, and d 0  is the initial value of the thickness  108 . The range in which the forces of equations (1) and (2) are in stable equilibrium occurs when the value (d 0 −d) is between zero and d 0 /3. At
 
                 d   0     -   d     &gt;       d   0     3       ,         
the electrostatic force of attraction of equation (1) overcomes the spring force of equation (2), such that the reflector  104  snaps to the reflector  102 , which is undesirable. This occurs because when the reflector  104  is beyond the d 0 /3 position, excess charge is drawn onto the reflectors  102  and  104  due to increased capacitance, which in turn increases the attractive force of equation (1) between the reflectors  102  and  104 , causing the reflector  104  to pull towards the reflector  102 .
 
   To overcome this limitation, the force between the reflectors  102  and  104  of equation (1) can instead be written as a function of charge: 
                   F   =       -     Q   2         2   ⁢   ɛ   ⁢           ⁢   A         ,           (   3   )               
where Q is the charge on the capacitor. Thus, the force F is now not a function of the distance d, and stability of the reflector  104  can exist over the entire range of 0 to d 0 . By limiting the amount of charge on the reflectors  102  and  104 , in other words, the position of the reflector  104  can be set over the entire range of travel.
 
   Although the description of the preceding paragraphs is with respect to an ideal parallel-plate capacitor and an ideal linear spring restoring force, those of ordinary skill within the art can appreciate that the principle described can be adapted to other configurations, such as non-linear springs and other types of capacitors. Eliminating or reducing the range of operation where snap down of the reflector  104  against the reflector  102  occurs enables more practical analog operation, or non-contact discrete operation, without limiting the number of colors as may otherwise occur when snap down occurs. That is, because the usable range is increased, more colors, saturation levels, and intensities can be achieved. 
   In addition, in one embodiment, the range within which non-contact operation can occur without snap down may be increased by constructing the flexure  110  in a particular manner. The particular manner is such that the restoring force of the spring mechanism  112  is a non-linear function of the displacement of the flexure  110 , and increases at a faster rate than the displacement. This can be achieved by increasing the thickness of the flexure  110 , or by using a flexure that is first bent and then stretched, which is known as a “bend and stretch” design. 
   Furthermore, the device  100  can be operated at smaller values of the thickness  108 , allowing a black state to be achieved without any portion of the reflectors  102  and  104  coming into contact with one another. This prevents stiction and the accompanying hysteresis that occurs when the reflectors  102  and  104  contact one another. Even if the reflectors  102  and  104  are allowed to contact one another, the voltage difference between the reflectors  102  and  104  will be less where the amount of charge on the reflectors  102  and  104  is specifically controlled (that is, where a predetermined amount of fixed charge is controlled), as opposed to where the voltage between the reflectors  102  and  104  is specifically controlled. This advantageously reduces electrostatic breakdown in the dielectric separating the reflectors  102  and  104  that defines the optical cavity  106 , as well as reducing the electrostatic force between the reflectors  102  and  104  that would otherwise increase stiction, and the wear on any anti-stiction standoffs employed to reduce the surface area between the reflectors  102  and  104 . 
   Controlling Charge on Reflectors 
     FIGS. 1B ,  1 C, and  1 D show different approaches to control the amount of charge on the reflectors  102  and  104  of the electronic device  100 , as opposed to specifically controlling the voltage between the reflectors  102 , and  104 , according to varying embodiments of the invention. As has been described in the preceding section of the detailed description, the thickness  108  between the reflectors  102  and  104  can be regulated by controlling the charge stored on the reflectors  102  and  104 . The reflectors  102  and  104  thus act as the plates of a parallel plate capacitor. 
   In  FIG. 1B , a controlled, or predetermined, amount of charge is injected onto the reflectors  102  and  104  by integrating a known current for a known time, utilizing the current integration mechanism  120  electrically coupled to the reflectors  102  and  104 . The current, I, the time, t, or both the current and the time can thus be manipulated to yield the desired amount of charge. The mechanism  120  may include a current source, a digital-to-analog current source, and/or time division circuitry to create the desired level of charge. 
   In  FIG. 1C , the charge available to the reflectors  102  and  104  is limited to prevent snap down of the reflectors  102  and  104  together. This is specifically accomplished in one embodiment of the invention by utilizing a voltage divider circuit  129 . The circuit  129  includes a voltage source  130  placed in series with a capacitor  134 . A switch  132  controls the on-off operation of the circuit  129 . A switch  136 , placed in parallel with the voltage source  130  and the capacitor  134 , acts as a reset switch, which may be utilized to avoid voltage or charge drift over time, due to charge leakage. The reset is desirably performed more quickly than the mechanical response time of the circuit  129 . 
   Where the flexure  110  is linear, the range of stable travel can be extended through the entire initial thickness  108  of the optical cavity  106  if 
             C   &lt;       C   init   ′     2       ,         
where C is the capacitance of the capacitor  134 , and C′ init  is the initial capacitance of the variable capacitor formed by the reflectors  102  and  104 , and the optical cavity  106 . As the voltage of the voltage source  130  increases, the resulting charge is shared between the variable capacitor and the capacitor  134  to at least substantially eliminate snap down. As can be appreciated by those of ordinary skill within the art, this principle can be applied to other configurations than a parallel plate capacitor and a linear spring restoring force, such as non-linear springs, and capacitors other than parallel plate capacitors.
 
   In  FIG. 1D , the charge on the reflectors  102  and  104  is controlled by using an approach referred to as fill-and-spill, utilizing a fill-and-spill circuit  131 . The switch  136  is closed and opened to discharge the variable capacitor formed by the reflectors  102  and  104 , and the optical cavity  106 . The switch  138  of the circuit  131  is then opened and the switch  132  is closed, to charge the fixed capacitor  134 . That is, the capacitor  134  is “filled.” Next, the switch  132  is opened and the switch  138  is closed, so that the capacitor  134  shares its charge with the variable capacitor. That is, the capacitor  134  “spills” its charge. The charge on the reflectors  102  and  104  reaches a stable value, even though it depends on the thickness  108  of the optical cavity  106 . The voltage source  130  has thus provided a controlled charge to maintain the desired thickness  108 . 
   Higher-order Gaps 
   The optical interference as described in the preceding sections of the detailed description to transmissively or reflectively select wavelengths at desired intensities relies upon first-order gaps in one embodiment of the invention. That is, the gap of the optical cavity  106 , which is the thickness  108  of the optical cavity  106 , is regulated so as to control the interference first-order wavelengths of light. However, as the thickness  108  of the optical cavity  106  increases, reflectance peaks shift to longer wavelengths, and additional, higher order, peaks move into the spectral region. 
   The spectral bandwidth of the electronic device  100  is determined by the optical constants of the films utilized for the reflectors  102  and  104 , their thicknesses, and the thickness  108  of the optical cavity  106  between the reflectors  102  and  104 . In such instances, the electronic device  100  functions as a so-called Fabry-Perot-based light modulator. The spectral purity, or saturation, of the reflected light is determined by the spectral bandwidth of the device  100 , and tradeoffs may have to be made between peak reflectance, spectral bandwidth, black state reflectance, and optical efficiency of the white state. 
   Peak reflectance occurs for reflective Fabry-Perot modulators when:
 
2nd=mλ,  (4)
 
where, as before, n is the gap index, d is the thickness  108  of the optical cavity  106 , m is a non-negative integer specifying the interference order, and λ is the wavelength of light. Equation (4) thus specifies a simple model of interference. It is noted that the actual reflectance spectra may be more accurately modeled by performing rigorous electromagnetic simulations, involving all material constants and interfaces within the device  100 , as can be appreciated by those of ordinary skill within the art of optical thin films.
 
   The higher-order peaks exhibit a narrower spectral bandwidth and thus increased saturation. The spectral bandwidth of the green state is particularly significant in determining saturation, since the wavelengths in and around the green wavelengths overlap the blue and red sensitivity curves of the human eye. The red and blue saturation may be improved by shifting the peak spectral wavelength away from the adjacent color-response curves and into the relatively insensitive portion of the spectrum, which is not possible with green. Narrowing the spectral bandwidth to increase the green saturation therefore has the problem of limiting the brightness of the display, since the peak sensitivity of the human eye is in the green region, leading to a reduced white level and lower overall contrast. 
   To overcome this limitation, the thickness  108  may be increased to produce second-order, or more generally higher-order, color, rather than first-order color.  FIG. 2A  shows a graph  220  of a representative first-order green spectral response  226  and a representative green second-order spectral response  228 , according to an embodiment of the invention. The y-axis  224  denotes reflectance as a function of wavelength on the x-axis  222 . The second-order response  228  has a narrower spectral bandwidth and improved color saturation. Thus, the second-order response  228  can be utilized in one embodiment of the invention in lieu of the first-order response  226  for increased saturation and color component. In another embodiment, the second-order response  228  is utilized for increased saturation, whereas the first-order response  226  is utilized for increased brightness and white level. 
   Color saturation is typically improved for second-order responses for blue through green.  FIG. 2B  shows a graph  240  of a second-order blue spectral response  242 , according to an embodiment of the invention. The graph  240  has the y-axis  224  denoting reflectance as a function of wavelength on the x-axis  222 , as before. The second-order blue response  242  provides for increased saturation, as compared to using a first-order blue spectral response. However, the second-order red spectral response  244  is less useful, because the third-order blue spectral response  246  begins to enter the visible spectral range. 
   Display Device and Method of Use Thereof 
     FIG. 3A  shows an array of passive pixel mechanisms  200 , according to an embodiment of the invention. The passive pixel mechanisms  200  include the mechanisms  200 A,  200 B, . . . ,  200 N, organized into columns  202  and rows  204 . Each of the pixel mechanisms  200  is able to variably select a visible wavelength at an intensity by optical interference and absorption, in correspondence with a displayable image. The pixel mechanisms  200  can be considered the apparatus for performing this functionality in one embodiment of the invention. The mechanisms  200  are passive in that they do not generate light by themselves, but rather reflect or transmit ambient and/or supplemental light. 
   In one embodiment, each of the passive pixel mechanisms  200  includes one or more of the electronic device  100 . Thus, a pixel may include one or more of the device  100 . Where the passive pixel mechanisms  200  display their corresponding pixels of the displayable image in an analog manner, each of the mechanisms  200  may include only one electronic device  100 , because the single device  100  is able to display substantially any color at any intensity. Where the mechanisms  200  display their corresponding pixels in a digital manner, each of the mechanisms  200  may include three of the electronic devices  100 , one for each of the red color component, the green color component, and the blue color component. 
     FIG. 3B  shows a cross-sectional profile of a display device  300 , according to an embodiment of the invention, which incorporates the array of passive pixel mechanisms  200 . An optional supplemental light source  304  outputs light for reflection by the mechanisms  200 . Where the light source  304  is present, the mechanisms  200  reflect both the light provided by the source  304 , as well as any ambient light. Where the light source  304  is absent, the mechanisms  200  reflect ambient light. The light source  304  is indicated in the embodiment of  FIG. 3B  such that it outputs light for reflection by the mechanisms  200 . In another embodiment, the light source  304  may be behind the mechanisms  200 , such that the mechanisms  200  transmit light output by the source  304 . 
   A controller  302  controls the pixel mechanisms  200 , effectively providing a pixilated displayable image to the pixel mechanisms  200 . That is, in the embodiment where the mechanisms  200  each include one or more of the electronic device  100 , the controller  302  changes the thickness  108  of the cavity  106  of each device  100 , so that the image is properly rendered by the pixel mechanisms  200 , for display to a user  308 . The controller  302  thus electrically or otherwise adjusts the thickness  108  of the optical cavity  106 , where, once adjusted, the thickness  108  is maintained by the flexure  110 . 
   The controller  302  may receive the displayable image from an image source  306  in a pixilated or a non-pixilated manner. If non-pixilated, or if pixilated in a manner that does not correspond on a one-to-one basis to the array of passive pixel mechanisms  200 , the controller  302  itself divides the image into pixels corresponding to the array of passive pixel mechanisms  200 . The image source  306  itself may be external to the display device  300 , as in the embodiment of  FIG. 3B , or internal thereto. The image source  306  may thus be a desktop computer external to the display device  300 , or may be a laptop or notebook computer, personal digital assistant (PDA) device, wireless phone, or other device of which the display device  300  is a part. 
     FIG. 4  shows a method of use  400 , according to an embodiment of the invention, for a display device, such as the display device  300  of  FIG. 3B . First, a displayable image is divided into pixels ( 402 ), resulting in a pixilated displayable image. Light is optionally provided ( 404 ), to supplement any ambient light. For each pixel of the image, a corresponding visible wavelength is selected, at a corresponding intensity, by optical interference and absorption ( 406 ), as has been described. The corresponding wavelength at the corresponding intensity may be selected in a digital or an analog manner, as has also been described. 
   Specific Electronic Device and Method of Manufacture Thereof 
     FIG. 5  shows a pair of electronic devices  500 A and  500 B for at least partially displaying a corresponding pair of pixels of a displayable image, according to an embodiment of the invention. Each of the electronic devices  500 A and  500 B is a specific embodiment of the electronic device  100  of  FIG. 1A , and thus the description of  FIG. 1A  is equally applicable to  FIG. 5  as well. Furthermore, the electronic devices  500 A and  500 B can each be used to realize each of the passive pixel mechanisms  200  of  FIG. 3A , in one embodiment of the invention. The following description of  FIG. 5  is made with specific reference to the electronic device  500 A, but is identically applicable to the electronic device  500 B. Furthermore,  FIG. 5  is not drawn to scale, for illustrative clarity. 
   The bottom reflector  104  is positioned over a silicon substrate  502 , and more generally is a conductive reflective layer. A thin dielectric  504  is present over the bottom reflector  104  to prevent shorting of the reflector  102 . The optical cavity  106  is defined between the top reflector  102  and the bottom reflector  104 , where the top reflector  102  is also more generally a conductive reflective layer. The flexure  110 , positioned over the top reflector  102 , is also referred to as a flexure layer, and acts as a flexible electrode for the top reflector  102 , as well as maintains tension on the top reflector  102  and allows the reflector  102  to move. The spacing of the optical cavity  106  can be controlled by calibrating voltage to the stiffness of the flexure  110  in an analog mode, or by providing stops of varying thickness for red, green, and blue pixels in a digital mode. 
   A dielectric pixel plate  506 , which may be oxide, partially covers the flexure  110  and the top reflector  102 . In one embodiment, the dielectric pixel plate  506  may have a width  508  of between  40  and  100  micron, and can have a height  510  of between three and five micron. An air cavity  514  surrounds the dielectric pixel plate  506 , and is larger than the coherence length of the optical cavity  106  to prevent additional interference effects. The air cavity  514  in one embodiment may have a height  520  of between three and five micron. The oxide  512  and  518  represent an additional layer used to define the air cavity  514 , where in one embodiment the oxide  518  may also have a height  522  of between three and five micron. 
   The via hole  516  is used to allow removal of material from the air cavity  514  and the optical cavity  106 . For instance, polysilicon or another filler material may be deposited to reserve space for the air cavity  514  and the optical cavity.  106 , but then is removed to actually form the cavities  514  and  106 . A protective layer  524  covers the oxide  518 , and an anti-reflective coating (ARC)  526  covers the protective layer  524 . The ARC  526  is desirable to avoid unwanted coherent interactions within the optical cavity  106  itself. 
     FIG. 6  shows a method  600  for manufacturing an electronic device, such as the electronic device  500 A or  500 B of  FIG. 5 , or a display device having a number of such electronic devices, according to an embodiment of the invention. First, a bottom metal reflector layer is provided on a silicon substrate layer ( 602 ). This may include depositing and patterning the bottom metal reflector layer. In  FIG. 5 , the bottom metal reflector layer is the bottom reflector  104 . Next, an oxide dielectric layer is deposited ( 604 ), which in  FIG. 5  is the thin dielectric  504 . 
   Polysilicon or a different filler material is deposited and patterned ( 604 ). The polysilicon acts as a placeholder for the resonant optical cavity to be formed. In  FIG. 5 , the polysilicon thus occupies the space of the optical cavity  106 . A flexure layer and a top metal reflector layer are then provided on the polysilicon ( 608 ). This can include depositing the flexure layer first and then the top metal reflector layer, or vice-versa, and patterning the flexure layer and the top metal reflector layer. In  FIG. 5 , the flexure layer is the flexure  110 , whereas the top metal reflector layer is the top reflector  102 . 
   An oxide pixel plate layer is provided on the flexure layer and the top metal reflector layer ( 610 ). This can include depositing the oxide and patterning the oxide. In  FIG. 5 , the oxide pixel plate layer is the dielectric pixel plate  506 . Additional polysilicon or additional filler material is then deposited on the oxide pixel plate layer and patterned ( 612 ), to act as a placeholder for an air cavity to be formed. In  FIG. 5 , the polysilicon thus occupies the space of the air cavity  514 . An oxide layer is deposited on this polysilicon ( 614 ), which in  FIG. 5  is the oxide  518  and  512 . 
   Next, a via hole is defined through the polysilicon ( 616 ), which is represented in  FIG. 5  as the via hole  616 . The polysilicon that has been previously deposited is then removed to define the resonant optical cavity and the air cavity ( 618 ). For instance, the removal can be conducted by performing isotropic polysilicon cleanout etching. In  FIG. 5 , this results in formation of the optical cavity  106  and the air cavity  514 . Finally, a protective layer is provided over the oxide layer ( 620 ), and an anti-reflective coating is provided over the protective layer ( 622 ). In  FIG. 5 , the protective layer is the protective layer  524 , and the anti-reflective coating is the anti-reflective coating  526 . 
   Additional Specific Electronic Devices 
     FIGS. 7A and 7B  shows the electronic device  100  of  FIG. 1A , according to a specific embodiment of the invention. The description of  FIG. 1A  is thus applicable to  FIGS. 7A and 7B  as well. The electronic device  100  of the embodiment of  FIGS. 7A and 7B  is more generally a Fabry-Perot-based device. The sawing and packaging of optical micro-electrical mechanical system (MEMS) devices, such as micro-mirrors, Fabry-Perot devices, and diffraction-based devices, can be difficult because of the fragility of the MEMS components, and the need for a transparent package. MEMS are generally semiconductor chips that have a top layer of mechanical devices, such as mirrors, fluid sensors, and so on. Wafer sawing is a wet process that can damage and/or contaminate the delicate devices upon release. Releasing the devices from sacrificial layers after sawing is difficult and costly if performed on a die-by-die basis. Packaging of such devices usually includes bonding a glass window to a package on a ceramic or other substrate, which can be costly, difficult to perform, and may add considerable size to the device. The electronic device  100  of the embodiment of  FIGS. 7A and 7B  overcomes these problems. 
   Referring first to  FIG. 7A , a sacrificial material  704  is deposited over the movable components of the device  100 , including the flexure  110 , the reflective layers  102  and  104  that define the optical cavity  106 , and the spring mechanism  112  that have been described. A layer  702  is deposited over and makes contact with this substrate at the locations indicated by the reference number  708 . Openings  706  are patterned and etched in the layer  702 . The device  100  is released by isotropically etching away the sacrificial material  704 , using selective release chemistries known within the art, which may be dry or wet processes. 
   Referring next to  FIG. 7B , a material  710  is then deposited into the openings, or vias,  706 , to provide a sealed environment for the device  100 . The layer  702  and the material  710  can be transparent dielectrics, or multi-layer films. The material  710  can perform a dual role as both an anti-reflective coating, and a sealing layer. Where techniques such as physical vapor deposition (PVD) or chemical vapor deposition (CVD) are utilized, a vacuum or hermetic environment can be achieved. Utilizing CVD at higher pressures can be employed where a higher-pressure environment is utilized. 
   The material  710  is optional, however, if a hermetic seal is not desired. Even without the material  710 , some protection for the device  100  is achieved, as non-hermetic seals also help to protect the device  100  from water, contaminants, and particulates. If the material  710  is used to seal the openings  706 , but is not desired over the entire surface, it may be patterned and etched away using lithographic techniques known within the art. 
   Furthermore, the process described in relation to  FIGS. 7A and 7B  enables encapsulation within a clean-room environment without conventional packaging, such that the process may be described as self-packaging. Because the process is preferably performed in a clean-room environment, and the release operation occurs inside a protective cavity, increased yields can result. Once the cavities are sealed, the die can be sawed off, as known within the art, without damaging the device  100 . 
     FIG. 7C  shows the electronic device  100  of  FIG. 1A , according to another specific embodiment of the invention. The description of  FIG. 1A  is thus applicable to  FIG. 7C  as well. It is noted that the ratio of the active light modulator area to the non-active area is referred to as the aperture ratio. The non-active area includes the space between pixels, support posts, the flexure area, and so on. Light reflected from the non-active area can increase the black state reflectance, reducing overall system contrast. The electronic device  100  of the embodiment of  FIG. 7C  reduces this effect by including an absorbing layer, or border mask,  722  to cover such non-active areas. The self-packaging material  710  that has been described in conjunction with  FIG. 7B  provides a substrate for the border mask  722 . Other like-numbered components of  FIG. 7C  relative to  FIGS. 7A and 7B  are identical to their counterparts of  FIGS. 7A and 7B , and are not re-described in relation to  FIG. 7C . 
   The border mask  722  may be composed of a variety of different materials, including absorptive polymers, photo-imageable absorptive polymers, metal and/or dielectric composites, and/or interference-based induced absorbers. Absorptive polymers are typically spun on and imaged with a photoresist mask and develop process. Photo-imageable polymers can be patterned directly with lithographic techniques known within the art. Metal and/or dielectric composites known as cermets are other materials that can be used, and have typically been developed for use as solar absorbers. Such materials include black molybdenum, black tungsten, and black chrome, and have very high absorbance. Further, they can be deposited with sputtering or evaporation techniques known within the art. Induced absorbers maximize the absorbance within a dissipating layer, by tuning layer thickness. Induced absorbers are relatively thin, such as less than 1000 Å. 
   The electronic device  100  of the embodiment of  FIG. 7C  lends itself to a three-state operation having dedicated pixel types. For instance, there may be a type-one three-state pixel, having the color states red, green, and black, or there may be a type-two three-state pixel, having the color states red, blue, and black. There may also be a type-three three-state pixel, having the color states green, blue, and black. Thus, the configuration of this operation includes groups of three-state pixels. Different pixels in the group are designed to operate with different states. The different color states are controlled by the thickness of the sacrificial material  704 . Such a configuration can be operated in a digital mode, with one pixel plate, or reflector, state in a non-contact position, and the other two states in contact with either the top or bottom capacitor plates, or reflectors. This has the advantage over a single-gap, two-state, configuration by allowing a color to be produced by two of the three pixels, instead of one of the three pixels, leading to brighter colors. 
   The electronic device  100  of the embodiment of  FIG. 7C  also lends itself a dual-gap, dual-capacitor pixel design, which is characterized by the reflector  102  moving forming two variable capacitors, as is now described. A layer  720  is a partial reflector on the underside of the layer  702 , and is over the reflector  102 . The layer  720  acts as both a partial reflector and as a capacitor plate. The reflector  102  may be driven up towards the layer  720 , or down towards reflector, or capacitor plate,  104  electrostatically. The spring mechanism  112  thus is deflected in two directions, and needs to travel only about half as far from its equilibrium position to cover the same total travel as when deflected in just one direction. This increased travel range enables modes of operation where pixels can produce multiple colors, multiple saturations, and black. The cavity made by removing the sacrificial material  106  serves as one gap, and the optical cavity  704  serves as another gap in this design. 
   Such a design can function in at least two different modes of operation. For example, in one mode of operation, individual pixels are capable of creating multiple colors and intensities as needed for color displays. The pixels operate in contact mode at one or both of the gap extremes, and otherwise operate in on-contact mode. As another example, in another mode of operation, multiple hues and intensities can be achieved without operating in contact mode. 
   Furthermore, the electronic device  100  of any of the embodiments of  FIGS. 7A ,  7 B, and  7 C lends itself to single-gap, dual-mode (or, multi-level) operation, where the modes include contact between the reflectors  102  and  104 , and non-contact between the reflectors  102  and  104 . Each pixel is capable of creating multiple colors and intensities as needed for color displays. The pixels operate in a contact mode at one gap extreme, and in a non-contact mode for the remaining states. 
   When pixels are dedicated to specific hues, such as red, green, and blue, optical efficiency may be reduced, since pixels of the wrong color cannot be used to generate the desired color. Therefore, it is advantageous to control the pixel gap, which is the thickness  108  of  FIG. 1A  that has been described, in a non-contact mode, such as an analog mode, a multi-level digital mode, or a combination analog and digital mode. The device  100  may need the thickness  108  to be less than 1000 Å to create black, about 1800 Å to create blue, and about 2800 Å to create red. To provide such different thicknesses, a single-gap, voltage control mode of operation that can be utilized is to operate in a non-contact mode between red and blue, and then allow the pixel to snap to the black state in a digital mode. 
     FIGS. 8A and 8B  show a pair of electronic devices  800 A and  800 B for at least partially displaying a corresponding pair of pixels of a displayable image, according to varying embodiment of the invention. Each of the electronic devices  800 A and  800 B is a specific embodiment of the electronic device  100  of  FIG. 1A , and thus the description of  FIG. 1A  is equally applicable to  FIGS. 8A and 8B  as well. It is noted that as pixel size is reduced, a smaller aperture ratio usually results. Like-numbered components of  FIGS. 8A and 8B  relative to FIGS.  1 A and  7 A- 7 C are identical, and are not otherwise described with respect to  FIGS. 8A and 8B . Further, for illustrative clarity only, not all components of FIGS.  1 A and  7 A- 7 C are shown in  FIGS. 8A and 8B . 
   In  FIG. 8A , the disadvantage of reduced aperture ratio is overcome by the electronic devices  800 A and  800 B by employing integral lenses  804 A and  804 B applied directly to the monolithic MEMS devices  800 A and  800 B, using coating or depositional techniques. The self-packaging layer  702  provides a substrate for these micro-lenses  804 A and  804 B, after an initial layer  802  has been deposited. The lenses  804 A and  804 B can be formed by patterning photoresist or other photo-imageable polymer using known lithographic techniques, and then partially flowing the patterns to the desired lens profile with heat treatment. The polymer may remain as the final lenses, or can be used as a mask to transfer the lens pattern to the underlying layer  802  with plasma or reactive-ion etching. The lenses  804 A and  804 B can be made more efficient by matching the shape thereof to the underlying pixels. 
   In  FIG. 8B , the self-packaging layer  702  is itself used as a simple form of a micro-lens. Such a technique relies on the coverage of the deposition over the reflector  102  to form a lensing action over the non-active region of the pixel where needed. For the layer  702  to effectively act as a lens, deposition thickness, pixel gap spacing, and pixel plate, or reflector, thickness and profile are desirably optimized. The advantage to the approach of  FIG. 8B  is that no additional lens is needed, and the lensing action is present only where it is needed, around the non-active region of the pixels. 
   Anti-stiction Bumps 
   When two surfaces come into contact, they are frequently attracted to one another by a variety of different forces, such as Van Der Waals attractive forces, chemical bonding forces, capillary forces, and Casimir forces. These forces often lead to surfaces that cannot be separated once they come into contact. Therefore, to prevent the reflectors  102  and  104  of the electronic device  100  from coming into contact with one another, in one embodiment of the invention anti-stiction bumps are placed on the bottom reflector  104  prior to fabrication of the top reflector  102 . 
     FIGS. 9A ,  9 B, and  9 C illustratively depict the manner by which anti-stiction bumps can be fabricated on the bottom reflector  104 , according to one embodiment of the invention. In  FIG. 9A , the flexure  110  and the bottom reflector  104  of the electronic device  100  are already present. A sacrificial material  902  is deposited, and then, in  FIG. 9B , is patterned and partially etched to yield recesses  904 . Subsequent layers, such as the layer  906  in  FIG. 9C , are then subsequently deposited into the recessions  904  to yield bumps  908  within the recessions  904 . 
     FIGS. 10A ,  10 B, and  10 C illustratively depict the manner by which anti-stiction bumps can be fabricated on the bottom reflector  104 , according to another embodiment of the invention. In  FIG. 10A , the flexure  110  and the bottom reflector  104  of the electronic device  100  are already present, as before. A first sacrificial material  910  is deposited that has the same thickness of the desired anti-stiction bump height. The material  910  is patterned and etched to yield the recesses  912 . In  FIG. 10B , a second sacrificial material  914  is deposited to achieve the total sacrificial layer thickness. Finally, in  FIG. 10C , subsequent layers, such as the layer  916 , are deposited into the recessions  912  to yield bumps  918  within the recessions  912 . 
   Conclusion 
   It is noted that, although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. For example, whereas embodiments of the invention have primarily been described as relating to a direct display device, other embodiments are applicable to a projection display device, such that the terminology of displaying a pixel references both of these, as well as additional, such display scenarios. For instance, in projection applications, the pixel size may be on the order of ten-to-twenty microns. Therefore, it is manifestly intended that this invention be limited only by the claims and equivalents thereof.