Abstract:
An LCD reflection display array implementing two or more layers of reflecting front surface mirrors with an upper layer mirror(s) having absorbing back surface(s). The mirror surfaces associated with each pixel are electrically connected to the pixel output electrode. The lower mirrors are appropriately positioned in the three dimensions to achieve nearly 100% aperture fill.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of reflective array display devices, and, more particularly, to a novel reflective array structure that provides a novel multi-mirror structure for maximizing aperture ratio while minimizing optical power absorption. 
     2. Discussion of the Prior Art 
     In transmissive or reflection display arrays, it is desirable to have the aperture ratio of the cell as high as possible to minimize the amount of illumination required and optical power absorption by the array. A better display results from higher brightness and efficiency. 
     FIG. 1 illustrates the physical layout of an absorbing gap cell  10 , e.g., 10 μm pitch, for a LCD reflection display light valve with FIG. 2 illustrating its equivalent circuit. As shown in FIG. 2, in an active matrix array LCD display, each pixel “cell” comprises a (thin-film) transistor  15 , and a capacitance  20 , and other components (not shown) and, may be fabricated using well-known CMOS fabrication techniques. 
     With more particularity, as shown in FIGS. 1 and 2, the absorbing gap cell  10  includes the following important functional layers: a conductive “P 1 ” layer (doped polysilicon) providing a control signal to the gate of the transistor  20  row for determining the optical properties of the cell and forming one electrode of capacitor “C sub ” and having another electrode return via the substrate; a first metal layer “M 1 ” for carrying data signals to the source terminal of the active transistor  15 ; a top-surface aluminum mirror layer “M 2 ” located beneath the liquid crystal material (not shown) and forming one electrode of the liquid crystal display capacitor element C LC  and having a top plate electrode formed of a transparent conductor such as ITO. Additionally, as part of the fabrication of the absorbing gap cell design, there is an anti-reflecting “AR” layer which forms a capacitance C AR  with the M 2  and M 1  metal layers. 
     FIG.  3 ( a ) illustrates a cross-sectional view of the absorbing gap cell  10  of FIG. 1 taken along line X 1 -X 1 ′. FIG.  3 ( b ) illustrates a cross-sectional view of the absorbing gap cell  10  of FIG. 1 taken along line Y 1 -Y 1 ′. As shown in FIGS.  3 ( a ) and  3 ( b ), the cell includes: regions of implanted Silicon, for example, N +  regions, indicated as region “RX” and forming the gate and drain/source regions for the thin-film transistor  20 ; 
     the P 1  poly-Si conductive layer forming a gate for the transistor and one electrode of capacitance C sub  with the other electrode formed of the implanted Si (RX layer); the first metallization layer M 1  for carrying data control signals to the source terminal of the active transistor layer RX and providing another end of capacitor C AR ; the second metallization layer which is an light energy absorber layer “AR”, e.g., formed of a tri-layer composite of titanium-nitride, aluminum, and titanium; and, the third level metallization layer M 2  which is a top-surface aluminum mirror layer located beneath the liquid crystal material (not shown) and providing the liquid crystal cell with reflective optical properties. As shown in FIG.  1  and FIG.  3 ( a ), a contact “CA” is provided for connecting the M 1  layer to the P 1  contact. 
     As further shown in FIG.  3 ( b ), the titanium-nitride, aluminum, titanium anti-reflective or absorbing layer AR is provided between the M 1  and M 2  layers throughout the cell. The anti-reflective or light absorption is provided by the top titanium nitride layer, with the aluminum core layer providing the conductivity and the titanium underlayer providing the good contact and a barrier between the aluminum and the underlying SiO 2 . This AR layer is held at the top plate electrode potential (connection not shown) and, typically is fabricated at a depth below the M 2  mirror surface that is equal to an integer number of λ/(2*n) for polarized illumination oriented in the normally black mode where λ is the wavelength of the illuminating light. The aluminum mirror M 2  is shown contacting the M 1  metal layer underneath the AR absorbing layer by the provision of via “V 1 ”, which may be a tungsten plug, for example, connecting M 1  and M 2  layers. As shown in FIG.  1  and  3 ( a ), a region AR of the AR layer is removed about the via V 1  so as to electrically isolate the AR layer. 
     In an active matrix array comprising absorbing gap pixel cells (of c-Si technology) shown in FIG. 1, the M 2  reflecting mirror surface area covers a fraction of the pixel surface area with exposed gaps “G” remaining within the cell. Disposed underlying the gaps “G” is the AR layer between the mirrors M 1  and M 2  that absorbs illumination energy. Thus, if the illumination directed at the cell is of high enough intensity, then optical power absorbed and heat removal from the array may be a design problem because the light valve array is typically packaged for compactness and accommodating heat sink sizes may expand the packaging, and/or require additional fan cooling which adds system weight and noise. This additionally applies to absorbing gap cells fabricated of p-Si technology which utilize a glass substrate (not shown). These problems are compounded in reducing cell pitch or incorporating binary area weighted mirrors. Thus, the prior art absorbing gap cell design exhibits a decreased aperture ratio, i.e., decreased light reflection efficiency. 
     It is the case then that an increase in aperture ratio is very desirable as this would reduce the illumination requirements and reduce array power absorption, thereby saving cost. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a reflection display array implementing two or more layers of reflecting front surface mirrors with upper layer mirror(s) having absorbing back surface(s). The mirror surfaces associated with each pixel are electrically connected to the pixel output electrode. The lower mirrors are appropriately positioned in the three dimensions to achieve nearly 100% aperture fill. Thus, the previous absorbing gap has been replaced with mirror(s). 
     Thus, according to the principles of the invention, there is provided a pixel structure for a reflective LCD display comprising a first layer of reflecting material for reflecting light directed at the cell structure in accordance with a control signal; a second layer of reflecting material disposed above the first layer of reflecting material for reflecting light directed at the cell structure in accordance with the control signal; and, a means for providing control signal to the first and second layers in said the structure for controlling amount of reflection thereof; whereby provision of the first and second layers of reflecting material results in reflective LCD display having substantially increased aperture ratio. 
     Advantageously, the fabrication of this multi-mirror structure for reflective array displays need not require any additional masks than that used by reflecting cell absorbing gap fabrication technique since the number of metal layers are the same. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will become more readily apparent from a consideration of the following detailed description set forth with reference to the accompanying drawings, which specify and show preferred embodiments of the invention, wherein like elements are designated by identical references throughout the drawings; and in which: 
     FIG. 1 illustrates the physical layout of an existing absorbing gap cell for a LCD reflection display. 
     FIG. 2 illustrates the equivalent circuit of the absorbing gap cell design of FIG.  1 . 
     FIG.  3 ( a ) illustrates a cross-sectional view of the absorbing gap cell of FIG. 1 taken along line X 1 -X 1 ′. FIG.  3 ( b ) illustrates a cross-sectional view of the absorbing gap cell of FIG. 1 taken along line Y 1 -Y 1 ′. 
     FIG. 4 illustrates the physical layout of the reflecting gap pixel cell  100  of the invention. 
     FIG. 5 illustrates the equivalent circuit of the reflecting gap cell design of FIG.  4 . 
     FIG.  6 ( a ) illustrates a cross-sectional view of the reflecting gap cell  100  of FIG. 4 taken along line X 2 -X 2 ′. FIG.  6 ( b ) illustrates a cross-sectional view of the reflecting gap cell  100  of FIG. 4 taken along line Y 2 -Y 2 ′. 
     FIG. 7 illustrates a 2×2 active matrix array  200  comprising four reflecting LCD cells of FIG.  4 . 
     FIG. 8 illustrates a cross-sectional view of the reflecting gap cell active matrix array  200  of FIG. 7 taken along line Z 1 -Z 1 ′. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4 shows the superimposed layers of the reflecting gap pixel cell  100  of the invention for a projection LCD display using, e.g., crystalline silicon (c-Si) CMOS technology. As shown, the P 1 , RX, M 1  and M 2  layers and the CA contact in the reflective cell design  100  are the same as in the absorptive cell design  10  of FIG.  1 . For example, the top level mirror (M 2 ) is patterned the same as the absorptive cell design (FIG.  1 ). However, according to the invention, the AR level in between the M 2  and M 1  layers is fabricated with reflective material, e.g., aluminum, with an underlying titanium barrier level. Thus, exposed within gaps between reflective mirrors M 2  in an active matrix array is an AR level of reflective material. 
     FIG.  6 ( a ) illustrates a cross-sectional view of the reflecting gap cell  100  of FIG. 4 taken along line X 2 -X 2 ′ and FIG.  6 ( b ) illustrates a cross-sectional view of the reflecting gap cell  100  of FIG. 4 taken along line Y 2 -Y 2 ′. As shown in FIGS.  6 ( a ) and  6 ( b ), underlying the cell M 2  metallization (mirror) layer is a layer  101  of absorbing material, e.g., about 50 nm of TiN (titanium-nitride). Since the illuminating light directed at the cell and entering the gap G isn&#39;t perfectly parallel, possibly due to secondary reflections from non-planar surfaces and sidewalls of, for example, the M 2  layer, the underside or back surface of upper mirror(s) needs to be absorptive to keep light away from the silicon substrate. The absorbing TiN layer  101  thus functions to absorb light that is transmitted through the gaps between mirror M 2  in the active matrix array and reflected from the AR or any other underlying layer in each pixel, and then reflect back upward towards the underside of M 2 . The non-parallel reflection may be caused, for example, by the surface roughness of the AR layer or any other layer in the light ray path. It should be understood that the SiO 2  layer directly beneath the AR layer may be planarized by the common chemical-mechanical polishing (Chemech) procedure. 
     In the preferred embodiment, the depth of the reflective mirror material at the AR level is preferably an integer number of λ/(2*n) deep where λ is wavelength and n is the index of refraction of the dielectric, e.g., Si 3 N 4 , separating the mirrors. The reflecting gap cell design is easily achievable with CMOS device fabrication techniques such as used to manufacture the absorbing gap cell (of FIG. 1) and does not require any additional masks. Specifically, as shown in FIG.  6 ( a ), the titanium-nitride underlying the M 2  layer is etched in the M 2  gap region. The M 2  layer provides the etch mask for the removal of titanium nitride in the M 2  gap and thus does not require any additional photolithographic masks or steps. 
     More particularly, in the cross-sectional view of the absorbing gap cell of FIG.  3 ( a ) the AR absorptive layer consists of aluminum cladded between titanium and titanium nitride. In the reflecting cell gap design of the invention, the top surface reflecting and back surface absorbing mirrors M 2  may be fabricated by changing the sequence of the titanium nitride deposition. As a result, the same processes used for gap absorbing design (FIGS.  3 ( a )- 3 ( b )) with the AR composed of a trilayer composite of titanium nitride, aluminum and titanium, may be used for the multi-mirror gap reflecting design (FIGS.  6 ( a )- 6 ( b )) of the invention. Consequently, the reflecting gap design does not require a new process to be developed. 
     In the fabrication of the reflecting LCD pixel cell  100  of FIG. 4, the AR layer must have an exposed gap since the same pattern is stepped in x and y and must have electrical isolation from adjacent pixels. To achieve the electrical isolation, an L-shaped cut  101  is designed in the AR level of reflective material level to form a region {overscore (AR)} that electrically isolates each pixel of the array. Electrical connection for controlling light reflection at the AR level is provided by an extra via “V 2 ” that connects the AR reflecting level with the M 2  mirror layer of that pixel. FIG. 5 illustrates the equivalent circuit of the reflecting gap cell design of FIG.  4 . Due to the presence of the reflecting AR layer, and its connection to the M 2  layer by via V 2 , no additional capacitance is present between the M 2  layer and the AR layer. Additionally, the tungsten plug V 1  connecting M 1  and M 2  layers removes capacitance between M 1  and AR, in effect, eliminating the prior art capacitance C AR  in FIG.  5 . 
     FIG. 7 illustrates a 2×2 reflecting gap cell arrangement  200  having reflecting pixel cells of FIG. 4 each electrically isolated from each other by the four L-shaped AR layer cuts  201   a - 201   d  (indicated as layer {overscore (AR)}). In FIG. 7, one reflecting gap pixel  100 ′ is shown. With respect to pixel  100 ′, the portion  301  of the AR cut  201   a  as shown encircled, is exposed in the gap 
     G between mirrors M 2  of adjacent column pixels. Likewise, the portion  302  of the AR cut  201   d  as shown encircled, is exposed in the gap G between mirrors M 2  of adjacent row pixels. All other areas within the pixel cell  100 ′ have exposed AR reflecting layer. Thus, locations  301 ,  302  are the only areas within a reflecting gap LCD pixel cell that may permit light to pass through into the cell. It should be understood however that, any decrease in aperture ratio due to the presence of the {overscore (AR)} cut portions (AR layer cuts)  301 ,  302  in the design is virtually inconsequential. 
     FIG. 8 illustrates a cross-sectional view of the 2×2 absorbing gap cell  100  of FIG. 4 taken along line Z 1 -Z 1 ′ of FIG.  7 . As shown in FIG. 8, there is a small area  301  where the AR layer is not continuous, i.e., the AR cut, so that illuminated light L 1  may find its way, in the M 2  gap G past the AR level. Only the M 1  layer is left to protect the underlying c-Si circuitry. As shown in FIG. 8, the M 1  has a flat topology directly underneath the AR cut,  301 , but there may be a portion of the M 1  layer topology that may redirect oblique light rays, e.g., light ray L 2 , further into the c-Si circuitry. It should be understood however, in actual designs, the light source and the resulting light rays are very perpendicular to the light valve, e.g., collimated and less than 1° angle from the perpendicular to the light valve. Thus, light ray L 2  represents the only light loss (light not reflected back), which in practical light valve designs may approach zero. 
     The amount of M 2  mirror overlap depends upon spacing between the mirror surfaces and amount of absorption per reflection. If spacing between mirrors M 2  of adjacent pixels is λ/n, where λ is the lightwave and n is the index of refraction of the dielectric, e.g., Si 3 N 4 , separating the mirrors, e.g., about 0.55 μm/1.9=0.29 μm, 30% reflection off the back surface of the mirror and 0.15 N.A. illumination optics being assumed, then a 0.5 μm mirror overlap will reduce the amplitude of the maximum angle ray on the order of more than 10 4=l .    
     It should be understood that the principles of the invention as described herein could be readily applied to binary weighted mirror cell designs where two or more different sized mirrors per cell area is provided. 
     While the invention has been particularly shown and described with respect to illustrative and preformed embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention which should be limited only by the scope of the appended claims.