WO2003029896A1

WO2003029896A1 - Reflective display with optical tuning

Info

Publication number: WO2003029896A1
Application number: PCT/US2002/030339
Authority: WO
Inventors: Salman Saeed; Matthias Thomas Peter Pfeiffer
Original assignee: Three Five Systems, Inc.
Priority date: 2001-09-28
Filing date: 2002-09-27
Publication date: 2003-04-10

Abstract

The present invention provides a method and structure for increasing contrast in reflective display devices. A high-quality, high-contrast image in a pixel is provided with a dielectric stack on pixel mirror that is tuned not only to enhance the reflectance of a bright state, but also to minimize the reflectance of a dark state. By maximizing the ratio between the reflectance of the bright state and the reflectance of the dark state, the contrast of the display can be increased. The dark state reflectance within the operational color band is computed for every combination of dielectric layer thicknesses. Likewise, the bright state reflectance within the operational color bands is computed for a sufficiently large sample of combination of dielectric layer thicknesses. The dark state reflectance and bright state reflectance for the various dielectric layer thicknesses can then be compared to determine what combination of dielectric layer thicknesses results in the largest ratio between bright state and dark state reflectance within the operational color band, while providing a predefined level of bright state reflection. That combination of dielectric layer thicknesses can be used to form LCD displays that have increased contrast, resulting in improved image quality.

Description

REFLECTIVE DISPLAY WITH OPTICAL TUNING

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 60/325,605, filed September 28, 2001.

TECHNICAL FIELD

[0002] This invention relates generally to liquid crystal displays (LCDs), and more particularly to a high-contrast reflective display including a reflector having a tuned dielectric stack thereon.

BACKGROUND OF THE INVENTION

[0003] For many decades, the cathode ray tube (CRT) was the dominant display device creating an image by scanning a beam of electrons across a phosphor-coated screen causing the phosphors to emit visible light. The beam is generated by an electron gun and is passed through a deflection system that causes the beam to rapidly scan left-to-right and top-to- bottom. A magnetic lens focuses the beam to create a small moving dot on the phosphor screen. This rapidly moving spot of light paints an image on the surface of the viewing screen.

[0004] Light emitting diodes (LEDs) have also found a multitude of uses in the field of optoelectronics. An LED is a solid-state device capable of converting a flow of electrons into light. By combining two types of semiconductive material, LEDs emit light when electricity is passed through them. Displays comprised of LEDs may be used to display a number of digits each having seven segments. Each segment consists of a group of LEDs, which in combination can form alphanumeric images. They are commonly used in, for example, digital watch displays, pager displays, cellular handset displays, etc., and due to their excellent brightness, LEDs are often used in outdoor signs. Generally speaking, however, they have been used primarily in connection with non-graphic, low-information- content alphanumeric displays. In addition, in a low-power CMOS digital system, the dissipation of LEDs or other comparable display technology can dominate the total system's power requirements, which could substantially negate the low-power dissipation advantage of CMOS technology. [0005] Liquid crystal displays (LCDs) were developed in the 1970s in response to the inherent limitations in the then existing display technologies (e.g. CRTs, LED displays, etc.) such as excessive size, limited useful life, excessive power consumption, and limited information content. LCD displays comprise a matrix of pixels that are arranged in rows and columns that can be selectively energized to form letters or pictures in black and white or in a wide range of color combinations. An LCD modifies light that passes through it or is reflected from it as opposed to emitting light, as does an LED or CRT. An LCD generally comprises a layer of liquid crystalline material suspended between two glass plates or between a glass plate and a substrate. A principle advantage of an LCD over other display technologies is the ability to include thousands or even millions of pixels in a single display paving the way for much greater information content.

[0006] With the shift from segmented, very low information content displays to more information-rich digital products, reflective displays such as LCDs now appear in products throughout the communications, office automation, and industrial, medical, and commercial electronics industries. Power consumption and integration into silicon led to reflective displays with internal reflective electrodes. For high brightness and high contrast the reflective electrode in these displays needs to be optimized for its optical properties. For example, the pixel mirrors commonly used as reflective electrodes should be characterized by high reflectance, and aluminum, which has a reflectance of approximately 90%, is a typical choice for an economical and semiconductor-compatible metal reflector. Dielectric layers on the aluminum mirrors have been employed to increase the reflectance of the mirror and therefore enhance bright states. See for example "A reflective PDLC LightNalve Display Technology" by Ph. Cacharelis et al. Proceedings of the 27^th European Solid State Device Research Conference 1997. However, their effectiveness to improve contrast has been limited because they have only examined the bright state performance of the display. Because contrast is especially important in reflective displays, the performance of these displays has been limited.

[0007] In view of the foregoing, it should be appreciated that it would desirable to provide a high-contrast reflective display wherein the reflectance of the pixel mirrors are tuned to improve contrast in the display. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention will hereinafter be described in conjunction with the accompanying drawings wherein like reference numerals denote like elements, in which:

[0009] FIG. 1 is a schematic diagram of a single analog pixel cell;

[0010] FIG. 2 is a simplified functional diagram illustrating how pixel circuitry interacts with pixel mirrors and the remainder of an LCD micro display;

[0011] FIG. 3 is a simple cross-sectional view showing major components of an LCD micro display;

[0012] FIG. 4 is a graph of a simulated reflectance spectrum of a bright state for a pixel cell not equipped with a dielectric stack;

[0013] FIG. 5 is a graph of a simulated reflectance spectrum of a dark state for a pixel cell not equipped with a dielectric stack;

[0014] FIG. 6 is a graph of a simulated reflectance spectrum of a bright state for a pixel cell equipped with a dielectric stack;

[0015] FIG. 7 is a simulated reflectance spectrum of a dark state for a pixel cell equipped with a dielectric stack;

[0016] FIG. 8 is a graph of a light leakage spectrum for a tuned dielectric stack for a green imager;

[0017] FIG. 9 is a graph of the envelope curves for a spectral response of an imager that includes a dielectric stack, and an imager with no dielectric stack;

[0018] FIG. 10 is a cross-sectional view of a display cell having a dielectric stack;

[0019] FIG. 11 is a cross-sectional view of the dielectric stack shown in FIG. 10;

[0020] FIG. 12 is a cross-sectional view of an imager which has been tuned in accordance with the teachings of the present invention; and

[0021] FIG. 13 is a block diagram illustrating a method for designing a reflective display with a tuned dielectric stack. DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENT

[0022] The following detailed description of a preferred embodiment is mainly exemplary in nature and is not intended to limit the invention or the application or use of the invention. The present invention recognizes that to produce a high-quality, high-contrast image of a video signal, the dielectric stack on an aluminum reflector or pixel mirror should be tuned not only to enhance the reflectance of a bright state, but also to minimize the reflectance of a dark state. By maximizing the ratio between the reflectance of the bright state and the reflectance of the dark state, the contrast of the display can be increased.

[0023] In the preferred embodiments, the dark state reflectance within the operational color band is computed for a sufficiently large number of combinations of dielectric layer thicknesses. Likewise, the bright state reflectance within the operational color bands is computed for a sufficiently large number of combinations of dielectric layer thicknesses. The dark state reflectance and bright state reflectance for the various dielectric layer thicknesses can then be compared to determine what combination of dielectric layer thicknesses results in the largest ratio between bright state and dark state reflectance within the operational color band, while providing a predefined level of bright state reflection. That combination of dielectric layer thicknesses can be used to form LCD displays that have increased contrast, resulting in improved image quality.

[0024] FIG. 1 is a schematic diagram of an individual pixel 20 coupled to a row line 22 and a column line 24. Of course it should be understood, that an actual LCD micro display would include a large matrix of row lines 22, column lines 24, and pixels 20. It should also be understood that this is just one example of how a LCD micro display can be implemented, and the present invention applies equally to different implementations. Each pixel includes an access n-channel field-effect-transistor 26, which has a gate coupled to row line 22 and a drain coupled to column line 24. The source of access transistor 26 is coupled to a first terminal of pixel capacitor 28 and to pixel mirror 30, the function of which will be described more fully in connection with FIG. 2. The other terminal of capacitor 28 is coupled to a source of potential; e.g. ground.

[0025] FIG. 2 is a simplified functional diagram illustrating how each pixel 20 interacts with an associated mirror 30 to create a liquid crystal image. FIG. 3 is a simplified cross- sectional view of a liquid crystal display that likewise will be useful in explaining the operation of a liquid crystal micro display. In both cases, like reference numerals denote like elements. Again, FIGS. 2 and 3 are only examples of display implementations, and those skilled in the art will recognize that the invention can be implanted using different structures and connections. Referring to both FIG. 1 and FIG. 2, pixel 20, described in connection with FIG. 1, is again shown coupled to mirror 30, a plurality of which reside on the surface of a semiconductor substrate (e.g. silicon) 32 as is shown in FIG. 3. Mirrors 30 may be metallic (e.g. aluminum) and have a thickness of, for example, 2000 angstroms, and each has a reflective surface 34 that may or may not have enhanced reflective properties. When row line 22 is asserted, transistor 26 becomes conductive, thus permitting the video signal (e.g. an analog video signal) appearing on column line 24 to charge pixel capacitor 28. Thus, the voltage on mirror 30 will vary in accordance with the voltage across pixel capacitor 28. Located within region 38 is a liquid crystal material, the molecules of which orient themselves in a relationship that depends on the voltage applied. A glass seal 46 is provided under which a layer of indium-tin-oxide (ITO) 40 is provided which is a transparent conductive material to which a potential N_com is applied as is shown at 42. N_Co_m may, for example, be approximately 7 volts. The voltage stored across pixel capacitor 28 and therefore the voltage on mirror 30 may approach a much higher voltage (e.g. 17 - 18 volts) or a much lower voltage (e.g. 1-2 volts) thus placing a significant potential difference between mirror 30 and ITO layer 40 and causing the molecules of the liquid crystal material in region 38 to assume a first orientation corresponding to black. Alternatively, if the voltage stored across pixel capacitor 28 is similar to Ncom, thus reducing the potential difference between mirror 30 and ITO layer 40, the molecules of the liquid crystal material in region 38 will assume a different orientation (e.g. corresponding to white). That is a high voltage on mirror 30 may cause the molecules of the liquid crystal material to substantially leave the polarization state of incoming light 60 unchanged, while a lower voltage on mirror 30 will cause light reflected from the display to have a changed polarization state. If the display is viewed in conjunction with polarizing means, the change in polarization state causes a contrast. That is if for example a polarizing beam splitter is used, light with unchanged polarization state will travel back to the source while light with changed polarization state will exit the polarizing beam splitter at its exit face.

[0026] Mirrors 30 (e.g. aluminum) reside on the surface of a semiconductor substrate 32 (e.g. silicon), which has deposited therein or formed thereon all the active regions (e.g. pixel capacitors, access transistors, etc.) required to produce a working device. Semiconductor die 32 is supported by a substrate 50 (e.g. ceramic) which may have a flexible printed circuit board 52 disposed thereon for the purpose of making external connection to semiconductor die 32 and ITO layer 40 by, for example, wire bond 54 and conductive epoxy crossover 56. Finally, a perimeter seal 58 is provided between the surface of semiconductor dye 32 and the surface of ITO layer 40 to seal the liquid crystal material within region 38.

[0027] Typically liquid crystal microdisplays are used in conjunction with an optical engine that as a minimum generally provides the following functions; 1) a light source, 2) polarization of source light, 3) if necessary color separation, 4) modification of polarization, 5) recombination of colors if necessary, 6) analyzing polarization state, and 7) directing light to observer. The polarization analyzing means can be combined, for example, in a polarization beam splitter where polarized light gets directed to the display and light reflected from the display gets directed back to the light source if polarization state remains unchanged after reflection from the display. If polarization state is changed after reflection from the display, light gets directed through the PBS to the observer. For further reference reading refer to patent 5,327,270 by Miyatke. Returning to FIG. 2, in operation polarized ambient or generated light (indicated by arrows 60) impinges upon and passes through transparent glass layer 46 and ITO layer 40. If the potential difference between mirror 30 and ITO layer 42 is high light reflected from surface 34 of mirror 30 will remain unchanged in its polarization state and therefore that portion of the video image created by pixel 20 will when viewed through proper polarization means approach black. If, on the other hand, the potential difference between mirror 30 and ITO layer 42 is very low, virtually all of the light striking surface 34 will be reflected with its polarization state rotated by 90 degrees and that portion of the video image to be created by pixel 20 will approach white when viewed through the same polarization means. It should be clear that between these two extremes, there are a multiple of shades extending from white to black, which may be displayed depending on the magnitude of the video voltage stored on pixel capacitor 28 and applied to mirror 30. The operation and structure of liquid crystal micro displays is well known and well documented in technical literature. For example, see U.S. Patent No. 3,862,360 entitled "Liquid Crystal Display System With Integrated Signal Display Storage Circuitry" issued January 21, 1975 and assigned to Hughes Aircraft Company, the teachings of which are hereby incorporated by reference.

[0028] As stated previously, to enhance the quality and contrast of a video image produced by liquid crystal display, a dielectric stack on each pixel mirror should be tuned to enhance both the bright states and the dark states. This can be accomplished by altering the thickness, materials, and/or the locations of the individual dielectric layers in the dielectric stack. Liquid crystal microdisplays can be used in such a way that one and the same display reflects light of any color either simultaneously or sequentially. Liquid crystal microdisplays can also be used such that three displays reflect light from the red, green and blue part of the spectrum. Liquid crystal microdisplays that are optimized and intended for use with a specific color of light are further referred to as red, green, or blue imagers.

[0029] FIG. 4 is a graph of a simulated reflectance spectrum of a bright state for a pixel cell without a dielectric stack disposed on an aluminum mirror. In the case of a green imager, the wavelength of interest would be approximately 500 nm to 575 mn. As can be seen, the maximum reflectance in the green band is approaching 0.9. FIG. 5 is a graph of a simulated reflectance spectrum of a dark state for an aluminum pixel mirror without a dielectric stack disposed thereon. In this case, the waveform in the band of interest consists of a plurality of deep valleys and high peaks ranging from a reflectance of 0 to a reflectance of approximately 0.0056, substantially lower than the reflectance of the light state shown in FIG. 4 for a given wavelength.

[0030] Referring now to FIG. 6, there is shown a graph of a simulated reflectance spectrum of a bright state for pixel cell equipped with a dielectric stack. In this case, the reflectance approaches 0.95 in the wavelength band of interest (i.e. 500 - 575 nm for a green imager). Comparing this with the reflectance spectrum in FIG. 4, it can be seen that as a result of the incorporation of a dielectric stack on the aluminum mirror, there has been an improvement of approximately 10% in the reflectance of the bright state. Referring to FIG. 7, there is shown the graph of a simulated reflectance spectrum of a dark state for a pixel cell including a dielectric stack. Comparing this with FIG. 5, it can be seen that there is little improvement (i.e. reduction in reflectance) in the green band. However, it can also be seen that the reflectance has decreased dramatically in the range of between approximately 610 - 680 nm. Therefore, it should be appreciated that if this portion of the spectrum could be shifted into the band of interest, it would represent a significant reduction in reflectance in the dark state thus achieving the desired result and corresponding improvement in contrast between the light and dark states. This situation is shown in FIG. 8 which illustrates the spectrum for a tuned dielectric stack for a green imager. [0031] Referring to FIG. 9, a graph illustrates the envelope curves for the spectral response of an imager that includes a dielectric stack, and an imager with no dielectric stack. The envelope curve for the spectral response of an imager with a dielectric stack is illustrated with the solid line. The envelope curve for the spectral response of an imager without a dielectric stack is illustrated with the dashed line. As can be seen, the imager with the dielectric stack has a number of valleys in its spectral envelope. The methods described herein provide the ability to reposition one of the valleys in the color band of interest. This reduces dark state leakage and thereby increases the contrast of the display.

[0032] Stated another way, the addition of the dielectric stack introduces a secondary periodicity which is visible in the envelope curve illustrated in FIG. 9. Tuning the dielectric stack allows the valley created by the secondary periodicity to be moved into the color band of interest. In FIG. 9, it can be clearly seen that the minima of the envelop curve for the cell with the dielectric stack is below the curve for the cell without the stack. Therefore, proper tuning of the dielectric stack can be used as a tool to minimize light leakage and increase contrast.

[0033] FIG. 10 is a cross-sectional view of a display cell equipped with a dielectric stack. A rear substrate 80 includes a silicon substrate 82 having an aluminum mirror 84 disposed thereon. Located on top of aluminum mirror 84 is a dielectric stack 86 of the type previously described which can be tuned to enhance both bright and dark states. An alignment layer 88 (e.g. polyimide) is deposited on dielectric stack 86 and forms the bottom of a chamber 90 which contains the liquid crystal material. Chamber 90 is completed by frame 92 and front substrate 94 which comprises an alignment layer (e.g. polyimide) 96, an ITO electrode 98, a glass layer 100, and an anti -reflective coating 102.

[0034] The dielectric stack of FIG. 10 is shown in more detail in FIG. 11 wherein like reference numerals denote like layers. For example, dielectric stack 86 may comprise a first layer 104 of silicon dioxide, a second layer 106 of silicon nitride, a third layer 108 of silicon dioxide, and a fourth layer 110 of silicon nitride. If the index of refraction of layer 106 is greater than that of layer 104, then the index of refraction of layer 108 must be less than the index of refraction of layer 106. Furthermore, the index of refraction of layer 110 must be greater than that of layer 108. Stated another way, the index of refraction of successive layers must be alternately increasing and decreasing or alternately decreasing and increasing with respect to the preceding layer. [0035] As stated previously, the dielectric stack can be tuned to enhance both the bright states and the dark states. To do this via simulation, it is first desirable to calculate the director configuration; that is, the orientation of the liquid crystal molecules within the display for a specific applied voltage. The director is one factor in determining how the liquid crystal material will affect the characteristics of light passing through it. The liquid crystal molecules will then align along the electric field as the applied voltage is varied resulting in a varying reflectivity. Much has been written about determining the director configuration and further discussion at this point is not deemed necessary. However, the interested reader is directed to "Numerical Modeling Of Twisted Nematic Devices ", D. W. Berreman, Philosophical transactions of the Royal Society Of London, released June 3, 1983, the teachings of which are hereby incorporated by reference. For further reading, see A. Killian and S. Hess, On the Simulation of the Director Field Of A Nematic Liquid Crystal, Liquid Crystal 8, 465 (1990) and 2). Commercially available software such as DIMOS from Autronic Melchers GmbH can perform such simulation calculations.

[0036] Next, the director configuration and other characteristics of the liquid crystal display are applied to a 4 x 4 Berreman matrix. Using a Berreman matrix, a dielectric stack may be simulated with simulated light impinging upon it to determine what impact dielectric layer thickness, choice of material, and order of material has on reflectance of the light and dark states. Tuning for both light and dark states will enhance image contrast. Commercially available software like for example DIMOS from Autronic Melchers GmbH can be used to perform Berreman 4x4 matrix calculations. The Berreman 4 x 4 technique is widely known in the field of optics, and the interested reader is directed to "4 x 4 and 2 x 2 Matrix Formulations for the Optics in Stratified and Biaxial Media", A. Lein, C.-J. Chen, I. Nathan, Journal Of The Optical Society Of America, Vol. 14, No. 11, November, 1997, the teachings of which are hereby incorporated by reference. For further reading, see: D. W. Berreman: "Optics in Stratified and Anisotropic Media: 4 x 4 Matrix Formulation", Journal Of The Optical Society Of America 62, 4 (1972) 502.

[0037] FIG. 12 illustrates one example of an embodiment where the various layers of an imager have been tuned using a director configuration calculation and 4 x 4 Berreman matrix. An aluminum mirror 124 having a thickness of 2000 angstroms is deposited on a glass substrate 122. A dielectric stack comprised of silicon dioxide layer 126, a silicon nitride layer 128, silicon dioxide layer 130 and silicon nitride layer 132. Dielectric layers

126, 128, 130, and 132 are 750 angstroms, 640 angstroms, 840 angstroms, and 1500 angstroms in thickness respectively and have an index of refraction of 1.46, 2.0, 1.46, and 2.0 respectively. A polyimide layer 134 is deposited on the dielectric stack and has a thickness of 250 angstroms and an index of refraction of 1.59. Region 136 comprises a liquid crystal region having a thickness of 3.7 microns and an index of refraction which is different for different imagers. A second polyimide layer 138 overlies the liquid crystal material and has a thickness of 250 angstroms and an index of refraction of 1.59. Polyimide layer 138 is followed by an ITO layer 140 having a thickness of 210 angstroms and an index of refraction of 1.85, a silicon dioxide layer 142 having a thickness of 380 angstroms and an index of refraction of 1.46, an ITO layer 144 having a thickness of 250 angstroms and an index of refraction of 1.85, a silicon dioxide layer 146 having a thickness of 400 angstroms and an index of refraction of 1.46, and an ITO layer having a thickness of 190 angstroms and an index of refraction of 1.85. Disposed on ITO layer 148 is a glass layer 150 having an index of refraction of 1.52 followed by one or more anti-reflective coatings 152.

[0038] FIG. 13 illustrates a method 300 for designing a liquid crystal display. Method 300 can be used to design LCDs that have improved contrast, and hence improved display quality. Method 300 recognizes that to produce a high-quality, high-contrast image of a video signal, the dielectric stack on an aluminum reflector or pixel mirror should be tuned not only to enhance the reflectance of a bright state, but also to minimize the reflectance of a dark state. By maximizing the ratio between the reflectance of the bright state and the reflectance of the dark state, the contrast of the display can be increased.

[0039] In method 300, the dark state reflectance is computed for a sufficiently large number of combinations of dielectric layer thicknesses. Likewise, the bright state reflectance is computed for a sufficiently large sample of combinations of dielectric layer thicknesses. The dark state reflectance and bright state reflectance for the various dielectric layer thicknesses can then be compared to determine what combination of dielectric layer thicknesses results in the largest ratio between bright state and dark state reflectance within the operational color band. That combination of dielectric layer thicknesses can be used to form LCD displays that have increased contrast, resulting in improved image quality.

[0040] The first step 302 in method 300 is to determine the optical behavior of the liquid crystal for both the bright and dark states of the LCD. As described above, determining the director configuration is the first step in modeling the optical characteristics of liquid crystal. The director configuration method models the liquid crystal by pretending the liquid crystal is made up of many different separate layers. Typically, this method uses a high number of layers, such as forty layers, to most accurately model the director configuration. The orientation of the molecules within these layers is then computed using any suitable technique, such as the methods described in article by Killian and Hess referenced above. Thus, by determining the director configuration of the LCD in both dark and bright states and computing the orientation of the molecules in these states, the optical behavior of the liquid in the LCD can be accounted for when determining reflectance.

[0041] The next step 304 is to select the number of dielectric layers TF₁ to TF_N that will be formed on the reflective surface. Generally, it is desirable to select a number of layers that will provide sufficient reflectance without introducing excessive process complexity. Additionally, it is generally desirable to select a number of dielectric layers that is compatible with the fabrication processed used to form the LCD. For these reasons it is typically suitable to use between two and six dielectric layers, with four being preferred in many cases. Of course, in some applications it may be desirable to use more than six layers. As described above, the dielectric layers will be selected to alternate between increasing and decreasing indices of refraction. As such, the dielectric layers can typically comprise a first silicon dioxide layer, a first silicon nitride layer, a second silicon dioxide layer, a second silicon nitride layer, and so forth. Of course, other suitable dielectric materials that have the desired indices of refraction can be used instead.

[0042] The next step 306 is to select the thickness range for the dielectric layers TFi to TF_N. The thickness range for each dielectric layer determines what dielectric layer thicknesses will be evaluated for bright and dark state reflectance. Generally, the thickness range will be limited to those thicknesses that are compatible with the fabrication process used to form the LCD. This allows the entire range of practical options to be evaluated while still minimizing computing time.

[0043] The next step 308 is to compute the dark state reflection band for a sufficiently large combination of dielectric layer thicknesses to be representative. The dark state reflection can be computed using any suitable optical modeling technique, but the Berreman matrix method described above is generally preferred. In the Berreman matrix method, the optical characteristics of each element in the LCD is entered into the matrix, including the director configuration for the dark state determined in step 302, the dielectric layers, the front glass layer, any coatings used, and the dark state reflectance of the element is then computed. In step 308 this computation is preformed for each possible combination of dielectric layer thicknesses within the selected ranges. This can be done by iterating through the dielectric layer thickness with a step equal to the thickness precision that can be provided by the fabrication system.

[0044] The next step 310 is to compute the bright state reflection band for a sufficiently large combination of dielectric layer thicknesses. Again, the bright state reflection can be computed using any suitable modeling technique, but the Berreman matrix method described above is generally preferred. In this step the director configuration for the bright state, determined in step 302, is used in the calculations.

[0045] The next step 312 is to determine the dielectric layer combination that maximizes the dark state to bright state differential within the operational color band, while maintaining a predetermined level of bright state reflectance. By computing both the dark state and bright state reflectance for each possible combination of dielectric layer thicknesses, those combinations that will result in the greatest contrast at the operational color band can be recognized. The goal is to maximize the contrast between the dark and bright states, while assuring sufficient bright state reflectance. In some cases, it may not be desirable to choose a solution that maximizes the contrast if the bright state reflectance drops below a certain level. The minimum amount of bright state reflectance that would be acceptable would depend upon the application for which the LCD display is being used. Generally, it is desirable to assure that the bright state reflectance does not drop below the level obtained without the dielectric layers being present.

[0046] Each imager in an LCD projection display generally is for one of three colors. The operational color range for each of these colors are generally 500 nm to 575 nm for green, 430 nm to 498 nm for blue, and 600 nm to 670 nm for red. Thus, the bright and dark state reflectance at issue for green imagers is that which occurs to light within the 500 to 575 nm range. Thus, in step 312, the combination of dielectric layers that have the greatest contrast between the bright state reflectance and the dark state reflectance for the operational color bands are determined. Each different color imager will use a different combination of dielectric layer thicknesses, with each being selected to maximize the ratio within its operational color band. Thus, the method facilitates the design of a LCD, that when formed, will have increased contrast for each color imager while maintaining bright state reflectance, resulting in a high contrast, high quality image. [0047] The present invention thus recognizes that to produce a high-quality, high-contrast image of a video signal, the dielectric stack on an aluminum reflector or pixel mirror should be tuned not only to enhance the reflectance of a bright state, but also to minimize the reflectance of a dark state. By maximizing the ratio between the reflectance of the bright state and the reflectance of the dark state, the contrast of the display can be increased.

[0048] From the foregoing description, it should be appreciated that there has been provided a mechanism for tuning a dielectric stack on a pixel mirror so as to enhance both the bright states and the dark states thereby improving the contrast on an image of an LCD. While the preferred exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations in the embodiments exist. It should also be appreciated that this preferred embodiment is only an example and not intended to limit the scope, applicability of configuration in any way. Rather, the foregoing detailed description provides those skilled in the art with a convenient roadmap for implementing the preferred exemplary embodiment of the invention. Narious changes may be made in the function and arrangement described above without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

1. A method for forming a pixel cell in a liquid crystal display having a high contrast, the method comprising the steps of: providing a mirrored surface in the pixel cell; forming a first dielectric layer on the mirrored surface, the first dielectric layer having a first thickness and a first index of refraction; and forming a second dielectric layer on the first dielectric layer, the second dielectric layer having a second thickness and a second index of refraction, wherein the first thickness and the second thickness are selected to maximize contrast between a dark state reflection and a bright state reflection in the liquid crystal display while providing a predefined level of bright state reflection.

2. The method of claim 1 wherein the first thickness and the second thickness are selected to maximize contrast in an operational color band.

3. The method of claim 2 wherein the operational color band comprises approximately 500 nm to 575 nm for a green pixel cell.

4. The method of claim 2 wherein the operational color band comprises approximately 430 nm to 498 nm for a blue pixel cell.

5. The method of claim 2 wherein the operational color band comprises approximately 600 nm to 670 nm for a red pixel cell.

6. The method of claim 1 wherein further comprising: forming a third dielectric layer on the second dielectric layer, the third dielectric layer having a third thickness and a third index of refraction; forming a fourth dielectric layer on the third dielectric layer, the fourth dielectric layer having a fourth thickness and a fourth index of refraction, wherein the third thickness and the fourth thickness are also selected to maximize contrast between the dark state reflection and bright state reflection in the liquid crystal display while providing a predefined level of bright state reflection.

7. The method of claim 6 wherein the first dielectric layer comprises a silicon dioxide layer, and wherein the second dielectric layer comprises a silicon nitride layer, and wherein the third dielectric layer comprises a silicon dioxide layer, and wherein the fourth dielectric layer comprises a silicon nitride layer.

8. The method of claim 7 wherein the first dielectric layer has a thickness of approximately 750 angstroms, wherein the second dielectric layer has a thickness of approximately 640 angstroms, wherein the third dielectric layer has a thickness of approximately 840 angstroms, and wherein the fourth dielectric layer has a thickness of approximately 1500 angstroms.

9. The method of claim 1 wherein the wherein the first thickness and the second thickness are selected to maximize contrast between the dark state reflection and bright state reflection in the liquid crystal display by: computing the dark state reflection for a plurality of thickness combinations; computing the bright state reflection for the plurality of thickness combinations; and determining which of the plurality of thickness combinations results in maximized ratio between the dark state reflection and the bright state reflection for an operational color band of the pixel cell.

10. The method of claim 9 wherein the step of computing the dark state reflection comprises determining a director configuration for the dark state.

11. The method of claim 1 wherein the mirrored surface comprises a pixel mirror.

12. A liquid crystal display pixel cell, the liquid crystal display pixel cell comprising: a mirrored surface; a first dielectric formed on the mirrored surface, the first dielectric layer having a first thickness and a first index of refraction; a second dielectric layer formed on the first dielectric layer, the second dielectric layer having a second thickness and a second index of refraction, wherein the wherein the first thickness and the second thickness are selected to maximize contrast between the dark state reflection and bright state reflection in the liquid crystal display pixel cell while providing a predefined level of bright state reflection.

13. The liquid crystal display pixel cell of claim 12 wherein the first thickness and the second thickness are selected to maximize contrast in an operational color band

14. The liquid crystal display pixel cell of claim 12 wherein further comprising: a third dielectric layer on the second dielectric layer, the third dielectric layer having a third thickness and a third index of refraction; a fourth dielectric layer on the third dielectric layer, the fourth dielectric layer having a fourth thickness and a fourth index of refraction, wherein the third thickness and the fourth thickness are also selected to maximize contrast between the dark state reflection and bright state reflection in the liquid crystal display while providing a predefined level of bright state reflection.

15. The liquid crystal display pixel cell of claim 14 wherein the first dielectric layer comprises a silicon dioxide layer, and wherein the second dielectric layer comprises a silicon nitride layer, and wherein the third dielectric layer comprises a silicon dioxide layer, and wherein the fourth dielectric layer comprises a silicon nitride layer.

16. The liquid crystal display pixel cell of claim 15 wherein the first dielectric layer has a thickness of approximately 750 angstroms, wherein the second dielectric layer has a thickness of approximately 640 angstroms, wherein the third dielectric layer has a thickness of approximately 840 angstroms, and wherein the fourth dielectric layer has a thickness of approximately 1500 angstroms.

17. The liquid crystal display pixel cell of claim 12 wherein the wherein the first thickness and the second thickness are selected to maximize contrast between the dark state reflection and bright state reflection in the liquid crystal display by: computing the dark state reflection for a plurality of thickness combinations; computing the bright state reflection for the plurality of thickness combinations; and determining which of the plurality of thickness combinations results in maximized ratio between the dark state reflection and the bright state reflection for an operational color band of the pixel cell.

18. The liquid crystal display pixel cell of claim 17 wherein the step of computing the dark state reflection comprises determining a director configuration for the dark state.

19. The liquid crystal display pixel cell of claim 12 wherein the mirrored surface comprises a pixel mirror.

20. A method for forming a pixel cell in a liquid crystal display having a high contrast, the method comprising the steps of: providing a mirrored surface in the pixel cell; forming a first dielectric layer on the mirrored surface, the first dielectric layer having a first thickness and a first index of refraction; forming a second dielectric layer on the first dielectric layer, the second dielectric layer having a second thickness and a second index of refraction; forming a third dielectric layer on the second dielectric layer, the third dielectric layer having a third thickness and a third index of refraction; forming a fourth dielectric layer on the third dielectric layer, the fourth dielectric layer having a fourth thickness and a fourth index of refraction, selecting the first thickness, the second thickness, the third thickness and the fourth thickness to maximize contrast between a dark state reflection and a bright state reflection for an operational color band in the pixel cell, the selecting comprising: computing the dark state reflection for a plurality of thickness combinations; computing the bright state reflection for the plurality of thickness combinations; and determining which of the plurality of thickness combinations results in maximized ratio between the dark state reflection and the bright state reflection for the operational color band while providing a predefined level of bright state reflection.

21. The method of claim 20 wherein the step of computing the dark state reflection comprises determining a director configuration for the dark state.

22. The method of claim 20 wherein the first dielectric layer comprises a silicon dioxide layer, and wherein the second dielectric layer comprises a silicon nitride layer, and wherein the third dielectric layer comprises a silicon dioxide layer, and wherein the fourth dielectric layer comprises a silicon nitride layer.

3. A liquid crystal display pixel cell, the liquid crystal display pixel cell comprising: a mirrored surface; a first dielectric formed on the mirrored surface, the first dielectric layer having a first thickness and a first index of refraction; a second dielectric layer on the first dielectric layer, the second dielectric layer having a second thickness and a second index of refraction; a third dielectric layer on the second dielectric layer, the third dielectric layer having a third thickness and a third index of refraction; a fourth dielectric layer on the third dielectric layer, the fourth dielectric layer having a fourth thickness and a fourth index of refraction, wherein the first thickness, the second thickness, the third thickness and the fourth thickness are selected to maximize contrast between a dark state reflection and a bright state reflection for an operational color band in the pixel cell while providing a predefined level of bright state reflection.