Multiple pixel driven mirror electrodes for improved aperture ratio of reflective displays

[0002] 1. Field of the Invention

[0003] 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.

[0004] 2. Discussion of the Prior Art

[0005] 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.

[0006] 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.

[0007] With more particularity, as shown in FIGS. 1 and 2, the absorbing gap cell 10 includes the following important functional layers: a conductive “P1” 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 “M1” for carrying data signals to the source terminal of the active transistor 15; a top-surface aluminum mirror layer “M2” located beneath the liquid crystal material (not shown) and forming one electrode of the liquid crystal display capacitor element C_LCand 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_ARwith the M2 and M1 metal layers.

[0008] FIG. 3(a) illustrates a cross-sectional view of the absorbing gap cell 10 of FIG. 1 taken along line X1-X1′. FIG. 3(b) illustrates a cross-sectional view of the absorbing gap cell 10 of FIG. 1 taken along line Y1-Y1′. 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;

[0009] the P1 poly-Si conductive layer forming a gate for the transistor and one electrode of capacitance C_subwith the other electrode formed of the implanted Si (RX layer); the first metallization layer M1 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 M2 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 M1 layer to the P1 contact.

[0010] As further shown in FIG. 3(b), the titanium-nitride, aluminum, titanium anti-reflective or absorbing layer AR is provided between the M1 and M2 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 SiO₂. This AR layer is held at the top plate electrode potential (connection not shown) and, typically is fabricated at a depth below the M2 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 M2 is shown contacting the M1 metal layer underneath the AR absorbing layer by the provision of via “V1”, which may be a tungsten plug, for example, connecting M1 and M2 layers. As shown in FIG. 1 and 3(a), a region AR of the AR layer is removed about the via V1 so as to electrically isolate the AR layer.

[0011] In an active matrix array comprising absorbing gap pixel cells (of c-Si technology) shown in FIG. 1, the M2 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 M1 and M2 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.

[0012] 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.

[0013] 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).

[0014] 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.

[0015] 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.

[0016] 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:

[0017] FIG. 1 illustrates the physical layout of an existing absorbing gap cell for a LCD reflection display.

[0018] FIG. 2 illustrates the equivalent circuit of the absorbing gap cell design of FIG. 1.

[0019] FIG. 3(a) illustrates a cross-sectional view of the absorbing gap cell of FIG. 1 taken along line X1-X1′. FIG. 3(b) illustrates a cross-sectional view of the absorbing gap cell of FIG. 1 taken along line Y1-Y1′.

[0020] FIG. 4 illustrates the physical layout of the reflecting gap pixel cell 100 of the invention.

[0021] FIG. 5 illustrates the equivalent circuit of the reflecting gap cell design of FIG. 4.

[0022] FIG. 6(a) illustrates a cross-sectional view of the reflecting gap cell 100 of FIG. 4 taken along line X2-X2′. FIG. 6(b) illustrates a cross-sectional view of the reflecting gap cell 100 of FIG. 4 taken along line Y2-Y2′.

[0023] FIG. 7 illustrates a 2×2 active matrix array 200 comprising four reflecting LCD cells of FIG. 4.

[0024] FIG. 8 illustrates a cross-sectional view of the reflecting gap cell active matrix array 200 of FIG. 7 taken along line Z1-Z1′.

[0025] 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 P1, RX, M1 and M2 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 (M2) is patterned the same as the absorptive cell design (FIG. 1). However, according to the invention, the AR level in between the M2 and M1 layers is fabricated with reflective material, e.g., aluminum, with an underlying titanium barrier level. Thus, exposed within gaps between reflective mirrors M2 in an active matrix array is an AR level of reflective material.

[0026] FIG. 6(a) illustrates a cross-sectional view of the reflecting gap cell 100 of FIG. 4 taken along line X2-X2′ and FIG. 6(b) illustrates a cross-sectional view of the reflecting gap cell 100 of FIG. 4 taken along line Y2-Y2′. As shown in FIGS. 6(a) and 6(b), underlying the cell M2 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't perfectly parallel, possibly due to secondary reflections from non-planar surfaces and sidewalls of, for example, the M2 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 M2 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 M2. 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 SiO₂layer directly beneath the AR layer may be planarized by the common chemical-mechanical polishing (Chemech) procedure.

[0027] 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., Si₃N₄, 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 M2 layer is etched in the M2 gap region. The M2 layer provides the etch mask for the removal of titanium nitride in the M2 gap and thus does not require any additional photolithographic masks or steps.

[0028] 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 M2 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.

[0029] 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 “V2” that connects the AR reflecting level with the M2 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 M2 layer by via V2, no additional capacitance is present between the M2 layer and the AR layer. Additionally, the tungsten plug V1 connecting M1 and M2 layers removes capacitance between M1 and AR, in effect, eliminating the prior art capacitance C_ARin FIG. 5.

[0030] 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 201a-201d(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 201aas shown encircled, is exposed in the gap

[0031] G between mirrors M2 of adjacent column pixels. Likewise, the portion 302 of the AR cut 201das shown encircled, is exposed in the gap G between mirrors M2 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.

[0032] FIG. 8 illustrates a cross-sectional view of the 2×2 absorbing gap cell 100 of FIG. 4 taken along line Z1-Z1′ 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 L1 may find its way, in the M2 gap G past the AR level. Only the M1 layer is left to protect the underlying c-Si circuitry. As shown in FIG. 8, the M1 has a flat topology directly underneath the AR cut, 301, but there may be a portion of the M1 layer topology that may redirect oblique light rays, e.g., light ray L2, 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 L2 represents the only light loss (light not reflected back), which in practical light valve designs may approach zero.

[0033] The amount of M2 mirror overlap depends upon spacing between the mirror surfaces and amount of absorption per reflection. If spacing between mirrors M2 of adjacent pixels is λ/n, where λ is the lightwave and n is the index of refraction of the dielectric, e.g., Si₃N₄, 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 .}

[0034] 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.

[0035] 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.