The present invention relates to color filter arrays, and more specifically to color filters using interference filters. BACKGROUND OF THE INVENTION

Liquid crystal mosaic display technology is being developed as a possible successor to color cathode ray tubes (CRT's) in many applications. This technology offers important advantages such as higher reliability along with reduced power, size and weight. While different types of color liquid crystal display devices exist, such devices generally include two spaced panels which define a sealed cavity filled with a liquid crystal display material. A transparent common electrode is formed inside the defined cavity on one of the glass panels. Individual electrodes, also on the inside of the defined cavity, are formed on the other glass panel. Each of the individual electrodes has a surface area corresponding to the area of one, or part of one, picture element. Each picture element is too small to be easily seen by the unaided human eye. If the device is to have color capabilities, it must also include color filters with red, green and blue color areas. Each color area is aligned with one of the electrodes. Each set of red, green, and blue color areas is grouped into a triad, repetitive stripe, or other consistent pattern within the picture element.

In a typical LCD, each of the individual electrodes can be addressed by means of a thin film transistor. Depending upon the image to be displayed, one or more of the electrodes is energized during the display operation to allow full light, no light, or partial light to be transmitted through the color filter area associated with the electrode. The image perceived by the user is a blending of colors formed by the transmission of light through adjacent color filter areas.

In many cases the LCD may be backlighted by locating a light source on the opposite side of the display, away from the viewer. Alternatively, the display may include a reflective layer at its rear surface and rely on the light source located on the same side of the display as the viewer.

Color filters for use on such devices have been fabricated using a number of different approaches. One approach has been to spin or deposit a light sensitized

adhesive film onto the glass panel. The film is then patterned in three sequential steps. During each step, dye of a specific color is applied to the predetermined regions of the film. According to another approach, organic pigments are deposited by vacuum evaporation. These pigments are then photolithographically patterned by conventional lift-off techniques. According to still another approach, a dyed and patterned stretched film material is used to create an internal color polarizing filter.

Another type of color filter is of the interference type. Optimum performance is obtained using multilayer thin film dielectric color filters, which are more efficient than the alternative dye filters, do not bleach or degrade with time, and are relatively hard. The conventional method for producing patterned filters of this type is to use a lift-off technique to pattern two or more thin film dielectric stacks of well-defined transmission characteristics. The stacks are typically made of dielectric materials such as ZnS/MgF2 or the harder, more durable, and optically stable materials Siθ2/Tiθ2- In order to provide suitable transmission characteristics for individual colors, the dielectric materials are assembled according to a precise design which specifies each layer thickness and refractive index. In one method in particular, filter stacks are deposited sequentially and uniformly over a glass substrate. A selective etching or lift-off procedure is then performed on the filter stacks to separate the stack filters and thus define pixel color location. While this sequence is simpler and less expensive to produce than brute force separate mapping of discrete filter designs in pixel zones, it does not appear possible to achieve the true color separation filters necessary in LCD displays for full range color gamut.

SUMMARY OF THE INVENTION The invention herein is a mosaic color filter array for use in such devices as a liquid crystal display. An array of interference filters is disclosed which achieves color separation through the use of single cavity Fabry-Perot type filters. Specifically, common broadband dielectric mirrors are used for all the color filters with tuned spacer layers positioned between the mirrors in order to control the transmission of a particular wavelength of light. The method of fabricating the filter array comprises the steps of first providing a transparent glass substrate. On the substrate a first layer sequence, Ml, of predetermined reflectance is deposited. Ml is comprised of alternating layers of high

(e.g., Tiθ2) and low (e.g., Siθ2) index of refraction dielectric materials. A transparent spacing layer of known index of refraction is then deposited over Ml and its thickness is tuned at individual pixel positions for a mosaic layout of red, green and blue filters. A second reflective layer, sequence M2, which may be similar but not necessarily identical to Ml , is then deposited over all the pixel positions, thus creating an array of single cavity Fabry-Perot type filters. The bandwidth and peak transmittance properties of these Fabry-Perot filters are determined by the reflectance properties of Ml and M2, while their tuning (i.e., center wavelength) is determined by the optical thickness of the spacer layer. The advantage of the above described process is that common first and second reflective layers can be used at all pixel positions. This fabrication sequence is simpler and less expensive to produce than separate mapping of discrete filters designed into three pixel zones.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross section of an embodiment of a liquid crystal display.

Figure 2 is a possible pattern for mapping pixels positions. Figure 3 is a cross section of a portion of the color filter array. Figure 4 is a graph showing the spectral characteristics of the red filter. Figure 5 is a graph showing the spectral characteristics of the green filter. Figure 6 is a graph showing the spectral characteristics of the blue filter.

Figure 7 is a graph showing the spectral characteristics of a second embodiment of the color filter array.

Figure 8 is a graph showing the spectral characteristics of a third embodiment of the color filter array. Figures 9a-9d show the sequence of depositing reflective layers, photolithographic masking, and transparent space tuning layers.

Figures 10a and 10b show the creation of the color filter array using standard lift-off technique.

PREFERRED EMBODIMENT OF THE INVENTION The invention described herein is a mosaic color filter array. Although this array can be used with TN. cameras and a variety of projection devices, it is described herein in relation to a liquid crystal display (LCD).

Figure 1 is a cross section of an embodiment of a liquid crystal display in which the present color filter array could be incorporated. The LCD includes a backlight 12 which transmits white light to components which are positioned between rearward glass panel 11 and forward glass panel 22. These components include the color filter array 14, thin film transistor 16, liquid crystal 18, and absorptive filters 20. In the present embodiment, the color filter array 14 acts to allow passage of light of a particular wavelength while retroreflecting light of all other wavelengths into a chamber which encloses the backlight 10. The liquid crystal in combination with the thin film transistors act as an on/off switch for the passage of light so that when the switch is on, the liquid crystal is transparent and allows all wavelengths of light to pass. When it is off it is opaque allowing no wavelengths to pass. The absorptive filters 20 absorb all wavelengths of light except those of a particular pass band. The combination of the reflective color filter array with the absorptive color filters acts to improve color saturation of the display conserve light energy normally lost by absorption in conventional construction, and maintain good color at off normal viewing angles. This combination of color filters is described in more depth in international patent application number PCT/US94/02668, entitled "Patterned Dichroic Filters for Color Liquid Crystal Display Chromaticity Enhancements", which is hereby incorporated by reference. As seen in Figure 1, different portions of the liquid crystal display allow different wavelengths of light to pass. In the example shown, the colors of red, green and blue are allowed to pass through particular portions of the liquid crystal display. A plan view of color pixels in a typical color filter array are shown in Figure 2. This typical mosaic layout allows for the viewing of full color in liquid crystal displays. As is seen the array is laid out in separate Blue 30, Red 32, and Green 36 pixel positions. The invention herein is focused on color filter array 14. The design of such optical interference filters is common in the optics industry. The filters can be fabricated from a variety of refractory inorganic evaporated or sputtered thin films. The filter typically consists of three layers, the second layer interposed between a first and third layer. The first and third layers are highly reflective, slightly transmitting film such as silver. The second or spacer layer is a dielectric layer such as zinc sulfide. The thickness of the spacer layer determines the wavelength of the output light and hence the color. This type of filter is more commonly known as a Fabry-Perot optical

interference filter with the spacer layer thickness of the filter adjusted to provide transmission of a particular color. To insure that the transmitted light has a narrow pass band and that there is minimum transmission outside of the passband frequency, the reflecting layer should have a reflectivity close to unity. A higher performance alternative to single layer metal mirrors (which absorb some of the light) is to use low absorption all-dielectric layers arranged in properly designed thickness sequences to form the Fabry-Perot cavity mirrors. The simplest all-dielectric mirror designs are just quarter wavelength tuned stacks (i.e., the optical thickness of each layer is a quarter wavelength of the wavelength for peak reflectance) of alternating high and low refractive indices. The result of this is that alternating layers of materials such as titanium dioxide (Tiθ2) and silicon dioxide (Siθ2) can be used to manufacture the entire filter.

Shown in Figure 3 is a cross sectional view of the present color filter array. Shown in particular is the repeating nature of the red, green and blue filters for each pixel position within the array. The filter is comprised of a transparent glass substrate

40 with a first mirror stack 42 disposed thereon. As was described above, first mirror stack 42 is comprised of alternating layers of Tiθ2 and Siθ2- This first mirror stack has a uniform thickness for the entire filter array. On top of the first mirror stack 42 are tuned spacer layers 43 and 44, both made of Siθ2- The tuned spacer thicknesses are dependent on the type of color filter desired. On top of the first mirror stack 42 and tuned spacer layers 43 is second mirror stack 46. In this embodiment of the invention the second mirror stack 46 is of the same composition and thickness as the first mirror stack 42.

As was described above, the basic design for each of the three color filters within the array is known as a single cavity Fabry-Perot filter. A Fabry-Perot filter is simply a tuned cavity structure of two mirrors separated by a spacer layer. In this filter, resonance peaks occur when the spacer thickness is an integer multiple of one half the wavelength of resonant frequency. So if the same mirror stack is used for both mirrors, the basic filter designs are then of the form:

-, _., -r red, green, or blue . -

M\ \ M

where p is an integer, M denotes the thickness and structure of the mirror stacks, and λ is the wavelength of either the red, or blue light. In this embodiment M = 1.1(HLH).8(LH) ² , where L denotes a quarter wave optical thickness (QWOT) of Siθ2 and H, a QWOT of ΗO2 at a wavelength of 550 nm (i.e., the middle of the visible spectrum). Note that the QWOT of a material of index refraction n, corresponds to a physical thickness of λ/4n. So at λ =550 nm, H is equal to 57 nm and L is equal to approximately 92 nm.

The red filter is obtained from a layer structure M/1.2L/M or 1.1(HLH).8(LH) ² / 1.1(HLH)/1.1(HLH).8(LH) ² . A green filter is obtained from the layer structure M/2.5L/M, or 1.1(HLH).8(LH) ² /2.5L1.1/(HLH).8(LH) ² . In the green case and in the red filters, the harmonic spacer thickness is 1.2L and 2.5L, respectively. In this embodiment, the blue filter does not have the tuned spacer layer and is obtained from a structure that looks like 1.1(HLH).8(LH) ² 1.1(HLH).8(LH) ² . This design has the primary advantage of achieving color separation without optical absorption loss and thus greatly improved efficiency.

Figures 4-6 show the spectral characteristics for the red, green and blue filter designs, respectively. In each figure a graph is shown of the transmissibility of a particular color filter versus the wavelength of light. In each case the passband is narrow enough to achieve true color separation, but wide enough to help mitigate the shift to shorter wavelengths with viewing angle, which all interference coatings are subject to.

Should filters with different optical characteristics be required for a particular application, the approach described herein offers the advantage of flexible adaptability. For example, if filters of narrower/wider spectral bandwidths are needed, the second embodiment of:

M=1.1H1.1L1.1H.7L.7H.7L

Red = M ²

Green = M/1.5L/M

Blue = M/L/M whose performance is shown in Figure 7, and the third embodiment

M=1.1H1.1L.8H.8L

Red = M/1.8L/M

Green = M/1.2L/M Blue = M/1.5L M whose performance is shown in Figure 8, illustrate how the filter optical performance can be tailored as required One method of constructing the color filter array is depicted in Figures 9 and 10 and is as follows:

The procedure described below is a conventional lift-off procedure which is used in the creation of integrated circuit devices. The lift-off technique is extremely useful for fine geometries and when perpendicular sidewalls are required. Figure 9 shows the glass substrate 40 which the color filter is constructed upon.

In the first step shown in Figure 9b the first mirror stack 42 is blanket coated over the whole substrate 40. The mirror stacks are comprised of alternating layers of Siθ2 and ΗO2 using the thicknesses described above. In accordance with using conventional lift off techniques for constructing the first embodiment, blue pixel locations are masked off photolithographically leaving areas open in the red and green pixel locations. The blue photolithographic masking 60 is deposited right on the mirror stack 42. As seen in Figure 9b a tuned spacing layer 44 made of Siθ2 is deposited over the blue masking 60 and the first mirror stack 42 at the red tuning thickness. In manufacturing the third embodiment, a spacing layer is deposited over the mirror stack at the green tuning thickness before masking is laid. Returning again to the first embodiment, as shown in

Figure 9c, pixel areas are masked off for the red filters with masking 62 leaving only the green pixel areas exposed. In the next section in Figure 9d, a second spacing layer 44 is disposed over the red masking 62 and the first spacing layer 43. The combined thicknesses of layers 44 and 43 are sufficient to create the Fabry-Perot cavity for green light once the filter array is fully constructed.

Figures 10a and 10b show the final steps in the lift-off process. In Figure 10a, once the array is immersed in warm acetone or a solvent, the red photorestive coating r the bl; .:. and red filters swell and portions of the tuned spacer layer are removed. In e final step shown in Figure 10b, a second filter stack 46 is then formed over the entire substrate 44 using a vacuum deposition process. As can be seen, three distinct filter thicknesses have been created in a repetitive fashion. The first area has only spacer layer 43, the second area has the combination of spacer layer 43 and 44, and the last area

has neither spacer layer 44 or 43. Although a conventional lift-off technique is used herein to describe the construction of the color filter array, it would be obvious to one skilled in the art that other techniques such as photo masking along with etching could also be used to construct the filter. Also, it would be obvious to use other materials such as ZnS and MgF2 to construct the filter array. Both etching and lift-off techniques are well known in the art.

The foregoing is a description of a novel and nonobvious color filter array. The applicant does not intend to limit the invention through the foregoing description, but instead define the invention through the claims dependent hereto.