Patent Publication Number: US-2007115415-A1

Title: Light absorbers and methods

Description:
BACKGROUND  
      Digital projectors often include micro-displays that include arrays of pixels. Each pixel may include a liquid crystal on silicon (LCOS) device, an interference-based modulator, etc. A micro-display is used with a light source and projection lens of the digital projector, where the projection lens images and magnifies the micro-display. The micro-display receives light from the light source. When the pixels of the micro-display are ON, the pixels direct the light to the projection lens. When the pixels are OFF, they produce a “black” state. The quality of black state determines a projector&#39;s black/white contrast ratio that is often defined as the ratio of the light imaged by the projection lens when all of the pixels in the micro-display are ON to the light imaged by the projection lens when all of the pixels are OFF and is a measure of the “blackness” of the projector&#39;s black state.  
      Some interference-based modulators, such as Fabry-Perot modulators, include a total reflector and a partial reflector separated by a gap, such as an air-containing gap, that can be adjusted by moving the total and partial reflectors relative to each other. The black state is produced when the air gap is adjusted to produce constructive interference of light beams passing through the absorptive partial reflector. The intensity of the light can vary greatly within different materials due to absorption and interference effects. One such interference effect that can occur within a thin film stack is referred to as electric field enhancement. It occurs when phase shifts from reflections within the stack add linearly to increase the electric field amplitude and thus increase the localized intensity in the layer. This yields maximum absorbance of the incident light and thus optimal black state. In the light state, the phase shifts are not constructive in the partial reflecting layer thus more energy escapes the cavity. Residual reflections may still occur because of design and material limitations, with the amount of residual reflection depending on the wavelength of the light incident on the modulator. This can cause problems, especially for multi-colored modulators, where the wavelength of incident light varies according to its color.  
      The absorption of incident radiation (or alternatively extinction of the electric field) by the partial reflector determines the maximum allowable thickness of the layer. Effectively the greater the absorption, the less light enters and escapes the SFX device and thus the modulator acts more like a mirror than a tunable modulator. At high thicknesses (greater than skin depth), the radiation is unaffected by the Fabry Perot cavity (air gap), and the reflected spectra is the native reflectance of the partial reflector. At low thicknesses, (i.e. less than skin depth) the device tunes color states well, but a poor black state results. At proper thicknesses, the device maintains wavelength tunability with the ability to absorb the bulk of the incident light in the black state. 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view of a portion of an embodiment of a micro-display display with compensation, according to an embodiment of the invention.  
       FIG. 2  is a cross-sectional view of an embodiment of a filter, according to another embodiment of the invention.  
       FIG. 3  presents results of a computer simulation of an exemplary embodiment of the invention.  
       FIG. 4  is a cross-sectional view of another embodiment of a filter, according to another embodiment of the invention.  
       FIG. 5  is a cross-sectional view of another embodiment of a filter, according to another embodiment of the invention.  
       FIG. 6  is a cross-sectional view of another embodiment of a micro-display, according to another embodiment of the invention.  
       FIGS. 7A-7C  are reflection diagrams (of prior art?) without compensation.  
       FIGS. 8A-8C  are reflection diagrams with compensation, according to another embodiment of the invention.  
       FIGS. 9A-9B  are reflection diagrams, according to another embodiment of the invention.  
       FIG. 10  is a cross-sectional view of a portion of an embodiment of a micro-display display without compensation.  
    
    
     DETAILED DESCRIPTION  
      In the following detailed description of the present embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice disclosed subject matter, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the claimed subject matter. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the claimed subject matter is defined only by the appended claims and equivalents thereof.  
       FIG. 1  is a cross-sectional view of a portion of a micro-display  100 , e.g., as a portion of a digital projector, according to an embodiment. For one embodiment, the micro-display is a modulator, such as an interference-based modulator, of the digital projector.  
      Micro display  100  includes a total reflector (or micro-mirror)  102  that may be formed overlying a semiconductor substrate, e.g., of silicon or the like. Total reflector  102  may be directly mounted on the substrate or be movable with respect to the substrate. For one embodiment, total reflector  102  is a pixel of a pixel array of micro-display  100 . A gap  106 , e.g., filled with a gas, such as air or an inert gas (argon, etc.), separates total reflector  102  from a partially reflective layer  108 , e.g., a tantalum aluminum (TaAl) layer. Alternatively, gap  106  may contain a vacuum. A compensator  109  is formed overlying partially reflective layer  108 . For one embodiment, compensator  109  includes a compensator layer  110 , e.g., a dielectric layer, such as an oxide layer (e.g., a silicon dioxide (SiO 2 ) layer) formed on partially reflective layer  108 . Compensator  109  also includes a compensator layer  112 , e.g., a dielectric layer, such as a nitride (e.g., a silicon nitride (SiN) layer) or a carbide layer formed on the compensator layer  110 . For a further embodiment, compensator layer  112  may be a partially reflective layer, such as a partially reflecting metal, e.g., of tantalum aluminum (TaAl). For one embodiment, compensator layer  112  is a high-index-of-refraction layer and compensator layer  110  a low-index-of-refraction layer. For example, compensator layer  110  may have an index of refraction of about 1.46, whereas compensator layer  112  may have an index of refraction of about 2.02. For another embodiment, partially reflective layer  108  has a non-zero extinction coefficient, for example a complex index of refraction of about 2.96-2.65i. For some embodiments, a transparent stiffening layer  114 , e.g., of TEOS (tetraethylorthosilicate) oxide, silicon oxide, etc., is formed on compensation layer  112 . For one embodiment, transparent stiffening layer  114  has substantially the same index of refraction as compensator layer  110 .  
      For one embodiment, total reflector  102  is movable relative to partially reflective layer  108  (e.g., may be mounted on flexures as is known in the art) for adjusting the size of gap  106 . Alternatively, for another embodiment, the size of gap  106  may be adjusted by moving transparent stiffening layer  114  and the layers attached thereto while total reflector  102  is stationary. In another embodiment, the partially reflecting layer  108  is mounted on a transparent substrate (not shown) that is illuminated from one side. The partially reflective layer  108  and total reflector  102  are defined on the opposite side of the transparent substrate. Gap  106  is adjusted by moving the total reflector  102  relative to partially reflective layer  108 .  
      The arrows of  FIG. 1  illustrate light paths, according to an embodiment, in response to micro-display  100  receiving incident light  150  from a light source located exteriorly of micro-display  100 , such as a laser, light emitting diode (LED), a high-pressure mercury light source, etc., and such light may pass through a multi-colored color wheel. Incident light  150  passes through transparent stiffening layer (or incidence layer)  114 , is refracted at an interface  151  between transparent stiffening layer  114  and compensator layer  112 , and passes through compensator layer  112 . A portion  152  of the refracted light is reflected off an interface  153  between compensator layer  112  and compensator layer  110 , passes back through compensator layer  112 , is refracted at interface  151 , and passes through transparent stiffening layer  114 . A portion  154  of the refracted light is refracted at interface  153  and passes through compensator layer  110 . A portion  156  of refracted light portion  154  is reflected off an interface  155  between compensator layer  110  and partially reflective layer  108 , passes back through compensator layer  110 , is refracted at interface  153 , passes through compensator layer  112 , is refracted at interface  151 , and passes through transparent stiffening layer  114 . A portion  158  of refracted light portion  154  is refracted at interface  155  and passes through partially reflective layer  108 .  
      Note that a portion of each reflection from total reflective layer  102  to partially reflective layer  108  is reflected to produce multiple reflections between total reflective layer  102  and partially reflective layer  108  as just described above. Another portion of each reflection from total reflective layer  102  to partially reflective layer  108  is transmitted through partially reflective layer  108 , compensator layer  110 , compensator layer  112 , and transparent stiffening layer  114 , as just described above.  
       FIG. 2  is a cross-sectional view of a light-absorbing, anti reflective stack (or filter)  200 , used for instance as a shadow mask or hide layer to absorb unwanted incident light  150  on micro display  100 , according to another embodiment used for instance as a shadow mask or hide layer to absorb unwanted incident light  150  on micro display  100 . Common reference numbers in  FIGS. 1 and 2  denote similar (or analogous) elements. Note that a dielectric layer  220 , such as silicon dioxide, replaces gap  106  of  FIG. 1 . A comparison of  FIGS. 1 and 2  indicates that the light paths through micro display  100  and light-absorbing, anti reflective stack  200  in response to light  150  are similar. More specifically, gap  106  of  FIG. 1 , containing a dielectric material, e.g., air, and dielectric layer  220  of  FIG. 2  are analogous. Therefore, compensation layers  110  and  112  of light-absorbing, anti reflective stack  200  have substantially the same compensating effect as in the structure of  FIG. 1 . That is, the reflectance of light-absorbing, anti reflective stack  200  is substantially independent of the wavelength of incident light  150  and that compensation layers  110  and  112  can be selected to compensate for different thicknesses of partially reflective layer  108 , as discussed below.  
       FIG. 3  presents the results of a computer simulation of an exemplary embodiment. Plot  300  shows the reflectance for a micro-display  1000  of  FIG. 10 . Common numbering in  FIGS. 1 and 10  denotes similar elements. Note that Micro-display  1000  does not include compensator  109 . Plot  350  shows the reflectance for micro-display  100  of  FIG. 1 . Therefore,  FIG. 3  compares the effect of compensator  109  on the reflectance. The results of  FIG. 3  correspond to micro-displays  100  and  1000  being in an OFF state or black state, obtained by adjusting gap  106 . Plot  300  shows the reflectance for a total reflector, e.g., that corresponds to a total reflector  102  of  FIG. 10 , a partially reflective layer of 79 angstroms, e.g., that corresponds to partially reflective layer  108  of  FIG. 10 , and an air gap of 1010 angstroms, e.g., that corresponds to gap  106  of  FIG. 10  without compensator  109 , interposed between the total reflector and the partially reflective layer. Plot  350  shows the reflectance for a total reflector, e.g., that corresponds to a total reflector  102  of  FIG. 1 , a partially reflective layer of 94 angstroms, e.g., that corresponds to partially reflective layer  108  of  FIG. 1 , an air gap of 960 angstroms, e.g., that corresponds to gap  106  of  FIG. 1 , interposed between the total reflector and the partially reflective layer, a silicon dioxide (SiO 2 ) layer of 300 angstroms and an index of refraction of about 1.46, e.g., that corresponds to compensator layer  110  of  FIG. 1 , on the partially reflective layer, and a silicon nitride (SiN) of 126 angstroms and an index of refraction of about 2.00, e.g., that corresponds to compensator layer  112  of  FIG. 1 , on the silicon dioxide layer.  
      In  FIG. 3 , note that, for plot  300 , the reflectance is the reflectance at an upper surface  1055  of partially reflective layer  108  ( FIG. 10 ), whereas for plot  350  the reflectance is the reflectance at interface  151  of  FIG. 1  or at an upper surface of compensator layer  112 . Therefore, a comparison of plots  300  and  350  illustrates the effect of compensator layers  110  and  112 , and thus compensator  109 , on the reflectance in the black state.  
      In  FIG. 3 , note that for plot  350 , the presence of the silicon dioxide layer (compensator layer  110 ) and the silicon nitride layer (compensator layer  112 ) for this exemplary embodiment acts to reduce the dependence of the reflectance on the wavelength of the incident light, e.g., corresponding to incident light  150  on micro-display  100 , so that it is essentially independent of the wavelength of the incident light. This means that compensator layers  110  and  112  compensate for the effect of wavelength of incident light on the reflectance (or the black state). Therefore, the black state is essentially independent of the color of the incident light on display  100 .  
      At wavelengths between about 5300 to about 5600 angstroms ( FIG. 3 ), the reflectance at interface  1055  ( FIG. 10 ) is substantially the same as at interface  151  ( FIG. 1 ). Note that partially reflective layer  108  for plot  300  is 79 angstroms and is 94 angstroms for plot  350 . From a manufacturing standpoint, if a design (or desired) thickness of partially reflective layer  108  is 79 angstroms and partially reflective layer  108  is manufactured to have a thickness (an actual thickness) of 94 angstroms, it is clear that the reflectance at the upper surface of the 94-angstrom layer will be different than the desired reflectance at the upper surface of the 79-angstrom layer. Therefore, plot  350  shows that compensation layer  109  can be adjusted, by adjusting the thicknesses of compensator layers  110  and/or  112 , to compensate for the difference in reflectance due to the error in the thickness of partially reflective layer  108  between the desired and actual thickness. Therefore, during manufacturing, partially reflective layer  108  can be measured after it is formed and compensator layers  110  and/or  112  can be adjusted to give a desired reflectance. A comparison of  FIGS. 1 and 2  reveals that the compensation layers  110  and  112  of light-absorbing, anti reflective stack  200  can be selected to compensate for different thicknesses of partially reflective layer  108  of light-absorbing, anti reflective stack  200 .  
       FIG. 4  is a cross-sectional view of a light-absorbing, anti reflective stack (or filter)  400 , such as a hide layer, that may be a portion of micro-display  100 , according to another embodiment. Common reference numbers in  FIGS. 2 and 4  denote analogous elements. For one embodiment, light-absorbing, anti reflective stack  400  includes light-absorbing, anti reflective stack  200  and an light-absorbing, anti reflective stack  410  that is formed below light-absorbing, anti reflective stack  200 . For one embodiment, light-absorbing, anti reflective stack  410  includes dielectric layer  220   2  formed on total reflective layer  102  and partial reflecting layer  108   2  formed on dielectric layer  220   2 . For another embodiment, transparent stiffening layer (or incidence layer)  114   2  may be formed on partial reflecting layer  108   2 . Light-absorbing, anti reflective stack  200  performs as described above in conjunction with  FIG. 2  in response to receiving light  150  at transparent stiffening layer  114   1 . Light-absorbing, anti reflective stack  410 , receives light  450 , e.g., reflected light, such as from interior components of a micro-display, from below. Light-absorbing, anti reflective stack  410  acts to reduce or prevent light  450  from being reflected off total reflective layer  102  that would otherwise occur in the absence of light-absorbing, anti reflective stack  410 . Therefore, light-absorbing, anti reflective stack  400  acts to produce black states from above and below. This is discussed further below.  
       FIG. 5  is a cross-sectional view of a light-absorbing, anti reflective stack (or filter)  500 , such as a hide layer, that may be a portion of micro-display  100 , according to another embodiment. Common reference numbers in  FIGS. 2, 4 , and  5  denote analogous elements. For one embodiment, light-absorbing, anti reflective stack  500  includes light-absorbing, anti reflective stack  200  and a light-absorbing, anti reflective stack  510  that is formed below light-absorbing, anti reflective stack  200 . For one embodiment, light-absorbing, anti reflective stack  510  includes dielectric layer  220   2  formed on total reflective layer  102  and partial reflecting layer  108   2  formed on dielectric layer  220   2 . Compensator  109   2  is formed underlying partial reflecting layer  108   2 , and includes compensator layer  110   2  formed on partial reflecting layer  108   2  and compensator layer  112   2  formed on compensator layer  110   2 . Note that compensators  109  are disposed symmetrically about total reflective layer  102  for one embodiment. For another embodiment, transparent stiffening layer (or incidence layer)  114   2  may be formed on compensator layer  112   2 . Light-absorbing, anti reflective stack  200  performs as described above in conjunction with  FIG. 2  in response to receiving light  150  at transparent stiffening layer  114   1 . Light-absorbing, anti reflective stack  510 , receives light  450 . Light-absorbing, anti reflective stack  510  acts to reduce or prevent light  450  from being reflected off total reflective layer  102  that would otherwise occur in the absence of light-absorbing, anti reflective stack  510 . Therefore, light-absorbing, anti reflective stack  500  acts to produce black states from above and below. This is discussed further below. Also note that light-absorbing, anti reflective stack  510  together with total reflective layer  102  performs as described above in conjunction with light-absorbing, anti reflective stack  200 . Other combinations of opposed hide layers with and without compensator layers are also possible and considered disclosed herein.  
       FIG. 6  is a cross-sectional view of a micro-display  600 , e.g., as a portion of a digital projector, according to another embodiment. For one embodiment, micro-display  600  functions as a light modulator of the digital projector. For another embodiment, micro-display  600  includes a device  601  and a driver  603 . For some embodiments, device  601  includes one or more micro-electromechanical system (MEMS) devices  620 , such as micro-mirrors, liquid crystal on silicon (LCOS) devices, interference-based modulators, etc., that correspond to pixels.  
      For one embodiment, device  601  includes pixel plates  602  as a portion of the MEMS devices  620 . Each of pixel plates  602  is analogous to total reflector (or micro-mirror)  102  of  FIG. 1 . For one embodiment, each of pixel plates  602  is suspended by flexures as is known in the art. Each of gaps  606  is analogous to gap  102  of  FIG. 1  and separates a respective one of pixel plates  602  from a stack  611  having a partially reflecting layer  608  analogous to partially reflecting layer  108  of  FIG. 1 . Stack  611  includes a compensator  609  that is analogous to compensator  109  of  FIG. 1  and is formed overlying partially reflective layer  608 . For one embodiment, compensator  609  includes a compensator layer  610  that is formed on partially reflective layer  608  and that is analogous to compensator layer  110  of  FIG. 1 . Compensator  609  also includes a compensator layer  612  that is formed on compensator layer  610  and that is analogous to compensator layer  112  of  FIG. 1 . A transparent stiffening layer  614  that is analogous to transparent stiffening layer  114  of  FIG. 1  is formed on compensator layer  612  of each of the stacks  611 .  
      For one embodiment, driver  603  is a Complementary Metal Oxide Semiconductor (CMOS) substrate. Driver  603  can be formed using semiconductor-processing methods known to those skilled in the art. Driver  603  includes driver circuits adapted to respectively control the positions of pixel plates  602 , and thus the corresponding gaps  606 , to turn pixels corresponding to pixel plates  602  ON or OFF.  
      Note that pixel plate  602 , the corresponding gap  606 , partially reflecting layer  608 , compensator  609 , and transparent stiffening layer  614  form a structure analogous to the portion of micro-display  100  of  FIG. 1 . Therefore, the structure of  FIG. 6  performs substantially the same way as described above for the analogous structure of  FIG. 1 . That is, the black state produced when the pixels of micro-display  600  are OFF is essentially independent of the color of the incident light on micro-display  600 . Moreover, compensation layers  610  and  612  can be selected to compensate for different thicknesses of partially reflective layer  608 .  
      For one embodiment, light-absorbing, anti reflective stacks  650  are formed directly above gaps  652  that separate adjacent pixel plates  602  and portions of adjacent pixel plates  602  that are adjacent to a gap  652 . For another embodiment, light-absorbing, anti reflective stacks  650  are formed on a portion of stiffening layer  614  located between adjacent stacks  611 . Note for other embodiments, another portion of stiffening layer  614  overlies light-absorbing, anti reflective stacks  650 . For another embodiment, light-absorbing, anti reflective stacks  650  are analogous to light-absorbing, anti reflective stacks  200 ,  400 , or  500 , respectively of  FIGS. 2, 4 , and  5 . When analogous to absorbing stacks  200 , light-absorbing, anti reflective stacks  650  act to reduce reflections due to incoming incident light  150 , as described in conjunction with  FIG. 2 , and thus act to produce a black state from above. In some instances, there may be internal reflections off pixel plates  602 , e.g., corresponding to light  450  of  FIGS. 5 and 6 , that may be reflected back to the pixel plates  602  when using light-absorbing, anti reflective stacks  650  analogous to light-absorbing, anti reflective stack  200 , e.g., off total reflective layer  102  ( FIG. 2 ), that may pass through gaps  652  and into driver  603 . Therefore, it is advantageous, for some embodiments, to use a light-absorbing, anti reflective stacks  650  analogous to light-absorbing, anti reflective stacks  400  or  500  that act to produce black states above and below and that act to reduce light from being reflected back to the pixel plates  602 . For another embodiment, posts may be formed between successive pixel plates or groups of pixel plates as is known in the art. For these embodiments, a light-absorbing, anti reflective stack  650  may be placed over each of the posts.  
      Note that micro-display  600  need not have gaps  606 , such as a Fabry-Perot micro-display for the light-absorbing, anti reflective stacks  650  analogous to light-absorbing, anti reflective stacks  200 ,  400  or  500  to be effective and beneficial. Rather, anti reflective stacks  650  can be used with any micro-display having a plurality of pixels that modify color, output directionality, polarity or other characteristic of incoming light. For example, each pixel may include a liquid crystal on silicon (LCOS) device.  
      Electric field enhancement caused by phase shifts upon reflection from partially reflective layer  108  of  FIG. 1  and total reflector  102  and proper sizing of gap  106  contribute to the achievement of the black state. The black state occurs when these phase shifts add constructively to yield maximum field amplitude in the absorbing partially reflective layer  108 . Because partially reflective layer  108  absorbs proportional to the intensity, it absorbs the majority of the power in gap  106  yielding little light escaping from the device. In the light ON state the phase shifts do not add constructively (because the size of gap  106 ) and less total light is absorbed in partially reflective layer  108 , allowing light to escape from the device.  
       FIGS. 7A-7C  are reflection diagrams, e.g., for micro-display  1000  of  FIG. 10  respectively at different wavelengths, e.g. substantially spanning visible spectrum of about 380 nm to about 700 nm, of incident light  150 .  FIGS. 7A-7C  have common vertical axes that correspond to the imaginary part of the amplitude reflection coefficient as the film is grown, as shown in  FIG. 7A , and horizontal axes that correspond to the real part of the amplitude reflection coefficient as the film is grown.  FIGS. 8A-8C  are reflection diagrams, according to another embodiment, e.g., for micro-display  100  of  FIG. 1  respectively at different wavelengths of incident light  150 .  FIGS. 8A-8C  have common vertical axes that correspond to the imaginary part of the amplitude reflection coefficient as the film is grown, as shown in  FIG. 8A , and horizontal axes that correspond to the real part of the amplitude reflection coefficient as the film is grown.  
      In  FIGS. 7A-7C , point  710  corresponds to the surface of total reflector  102 , and point  720  corresponds to a lower surface  157  of partially reflective layer  108  adjacent an interface between gap  106  and partially reflective layer  108  ( FIG. 10 ). Point  730  corresponds to upper surface  1055  partially reflective layer  108  ( FIG. 10 ) and denotes the end of the stack to which  FIGS. 7A-7C  correspond. The point of no reflection (i.e., the ideal black state) is located at the origin ( 0 , 0 ) of the respective diagrams of  FIGS. 7A-7C . The intensity of reflection at points  710 ,  720 , and  730  is given by the complex electric field (E) times its complex conjugate (E*), which is respectively represented by the distance between  710 ,  720 , and  730  and the origin. Therefore, the reflection (or reflectance) at the end of the stack is the magnitude of the vector  740  between the origin and point  730 . Note that the reflection is substantially zero at a wavelength of incident light  150  of about 550 nanometers. However, at a wavelength of incident light  150  of about 370 nanometers and about 700 nanometers the reflections are different from each other and from the substantially zero reflection at about 550 nanometers. This is in agreement with the behavior of plot  300  of  FIG. 3  that illustrates that the reflection depends on the wavelength of the incident light.  
      In  FIGS. 8A-8C , point  802  corresponds to the surface of total reflector  102  of micro-display  100  of  FIG. 1 , and point  804  corresponds to lower surface  157  of partially reflective layer  108  adjacent an interface between gap  106  and partially reflective layer  108  ( FIG. 1 ). Point  806  corresponds to interface  155  between compensator layer  110  and partially reflective layer  108  ( FIG. 1 ). Point  810  corresponds to interface  153  between compensator layer  112  and compensator layer  110  ( FIG. 1 ), and point  820  corresponds to interface  151  between transparent stiffening layer  114  and compensator layer  112  ( FIG. 1 ) and denotes the end of the stack for which  FIGS. 8A-8C  correspond. Note that the curves between point  806  and point  820  represent the effect of compensator  109 . It is seen that compensator  109  compensates for the effect of wavelength of incident light on the reflectance (or the black state) in that the reflection at point  820  is substantially zero at each of the wavelengths incident light  150 , as the distance between point  820  and the origin at each of the wavelengths is substantially zero. Therefore, the black state is essentially independent of the color of the incident light, and compensator  109  acts improve the broadband black state performance of a device across the visible spectrum (e.g., roughly 380 nm to 700 nm).  
       FIGS. 8A-8C  also show that the reflection (or reflectance) is fairly uniform between points  802  and  804  within gap  106  of  FIG. 1 . The reflection is reduced between points  804  and  806  within partially reflective layer  108 . Between points  806  and  820 , compensator  109  of  FIG. 1  reduces the reflection to substantially zero at point  820  across the visible spectrum. That is, compensator  109  acts to substantially extinguish the reflection across the visible spectrum. Note that similar behavior occurs for light-absorbing, anti reflective stack  200  of  FIG. 2 , where dielectric layer  220  replaces gap  106 .  
      The absorption of incident radiation (or alternatively extinction of the electric field) by partially reflective layer  108  determines an allowable thickness, such as the maximum allowable thickness, of partially reflective layer  108 . Effectively the greater the absorption, the less light enters and escapes the device, and thus the modulator acts more like a mirror than a tunable modulator. At high thicknesses of partially reflective layer  108  (e.g., greater than skin depth), the radiation is unaffected by gap  106  (e.g., Fabry Perot cavity), and the reflected spectra is the native reflectance of partially reflective layer  108 . At low thicknesses of partially reflective layer  108  (e.g., less than skin depth), the device tunes color states well, but a poor black state results. At proper thicknesses of partially reflective layer  108 , the device maintains wavelength tunability with the ability to absorb the bulk of the incident light in the black state.  
      The behavior described above regarding performance as a function of the thickness of partially reflective layer  108  is modified by the addition of compensator  109  in the thin film stack. Compensator  109  allows for increased film variability by decreasing performance sensitivity to phase; e.g., to account for manufacturing variability. This effect is illustrated in  FIGS. 9A and 9B , according to another embodiment.  
       FIGS. 9A and 9B  are reflection diagrams and are similar in construction to  FIGS. 8A-8C . The intensity of reflection is represented by the magnitude of a vector  840  between the origin and point  820  in  FIGS. 9A and 9B . In  FIG. 9A , vector  840  corresponds to the reflection for a device with an error in the thickness of partially reflective layer  108  ( FIG. 1 ). In  FIG. 9B  vector  840  corresponds to the reflection for a device with the error in the thickness of partially reflective layer  108  corrected by compensator  109  ( FIG. 1 ) to account for the error. Compensator  109  decreases the magnitude of vector  740 , thereby accounting for the manufacturing error and thus improving the black state performance.  
      Note that the effect of compensator  109  on the performance of light-absorbing, anti reflective stack  200  of  FIG. 2  is similar to that described above in conjunction with  FIGS. 8A-8C  and  9 A- 9 B for the structure of  FIG. 1 .  
      Compensator  109  acts to improve the broadband black state performance of the device, as well as decreasing the sensitivity to manufacturing variation. This makes the device more practical to fabricate. Compensator  109  adjusts for the broadband admittance mismatch that would have occurred in it&#39;s absence at the dielectric/metal interface  104  to  108  using combination of high-index (e.g., an index of refraction of about 2.02) and low index (e.g., an index of refraction of about 1.46) materials or dielectric and non-dielectric (absorbing) materials. Compensator  109  improves manufacturability by decreasing effect of slight errors in deposition thickness of partially reflective layer  108 . Compensator  109  relies upon combination of dielectric and non-dielectric (metal) layers for performance. Exemplary material sets include but are not limited to: SiC, SiO 2 , TaAl, and air; SiN, SiO 2 , TaAl, and air.  
     CONCLUSION  
      Although specific embodiments have been illustrated and described herein it is manifestly intended that the scope of the claimed subject matter be limited only by the following claims and equivalents thereof.