Forming array with metal scan lines to control semiconductor gatelines

Abstract: A method of forming array circuitry at a surface of a substrate, the array including a first conductive layer with M scan lines (20,30,22), a second conductive layer with N data lines (24,32,26), and cell circuitry for a region (34) in which the m th scan line and the n th data line cross. The cell circuitry includes a component (40) with a data lead for receiving signals from or providing signals to the nth data line (32). A first semiconductor layer (42) of the cell circuitry includes a first line with a channel (46) between a connecting point (D1) to the n th data line and a connecting point (D2) to the component's data lead. A second semiconductor layer (44) includes a second line extending from a connecting point (D3) to the m th scan line and crossing the first line (42) at the channel (46). The first and second conductive layers and the cell circuitry are formed with electrical connections at the connecting points so that signals on the m th scan line control conductivity of the first line between the n th data line and the data lead. The semiconductor layers can be polysilicon. The first semiconductor layer (42) can be formed before the second semiconductor layer (44), with the first line electrically connected to the data lead by implanting a dopant. The connections to the m th scan line and the n th data line can be metal/semiconductor interfaces, with the first conductive layer deposited on the second line and with the second conductive layer deposited on the first line through an opening in an insulating layer.

Description: The present invention relates to circuitry formed on substrates. More specifically, the present invention relates to arrays. Lewis, A., and Wu, I-W., "Polysilicon TFTs for Active Matrix Liquid Crystal Displays," IEICE TRANSACTIONS, Vol. J76-C-II, No. 5, May 1993, pp. 211-226, describe fabrication of polysilicon (poly-Si) thin film transistors (TFTs) and poly-Si active matrix liquid crystal display (AMLCD) pixel designs. Wu, I-W., "High-definition displays and technology trends in TFT-LCDs," Journal of the SID, Vol. 2, No. 1, 1994, pp. 1-14, describes various liquid crystal displays (LCDs), focusing on AMLCDs with TFTs. EP 0,484,965 A2 describes an active matrix substrate including on a given substrate a thin film transistor, a scanning line connected to the gate of the thin film transistor, a data bus connected to the source of the thin film transistor, and a picture element electrode connected to the data bus through the thin film transistor, the active matrix substrate comprising a structure of the scanning line having the surface covered with an insulating film. A semiconductor layer covered with a gate insulating film constituting the thin film transistor, and a gate electrode constituting the thin film transistor, which are laid on each other in the stated order. US 5,238,861 describes a method for manufacturing an active matrix display screen with storage capacitors, the method including only two masking levels. During the second masking, capacitive lines are defined which overlap the pixels so as to form storage capacitors. The invention addresses problems that affect arrays of circuitry fabricated on substrates. A two-dimensional (2D) array, for example, can include two sets of conductive lines extending in perpendicular directions, as illustrated by Fig. 3 of the Wu article described above. Each line extending in one direction can provide signals to a column of the array; each line extending in another direction can provide signals to a row of the array. Conventionally, each row-column position in a 2D array includes circuitry, sometimes called a "cell," that responds to or provides signals on the lines for the cell's row and column combination. Through one set of parallel lines, illustratively called "data lines," each cell receives or provides signals that determine or indicate its state. Through the other set of parallel lines, illustratively called "scan lines," each cell along a scan line receives a signal that enables the cell to receive signals from or to provide signals to its data line. The area of each cell that is bounded by data lines and scan lines, referred to herein as the "cell area," can serve as a transducer, providing or receiving signals to or from sources outside the array. In conventional arrays, each scan line provides a periodic scan signal that enables a component in each cell connected to the scan line to receive a signal from or provide a signal to its data line during a brief time interval of each cycle. Therefore, tight synchronization of the scan signals with signals on the data lines is critical to successful array operation. Also, the scan signals must maintain high quality transitions across an array. If a scan line has high resistance, its propagation delay is also high, which causes signal distortion, preventing accurate signal synchronization and causing inaccurate loading or extraction of data. Some conventional techniques address the scan line resistance problem by using metal scan lines. But each cell's circuitry conventionally includes a semiconductor channel, and the conductivity of the channel is conventionally controlled by the scan signals, also referred to as "gate signals" by analogy to the gate of a transistor. Therefore, a metal scan line conventionally extends over the semiconductor channel, causing problems with fabrication processes such as ion implantation and hydrogenation and also causing problems with design such as line width control for scaling. Such fabrication problems are especially serious for poly-Si TFTs: Hydrogenation to passivate defects in a poly-Si channel affects poly-Si differently than metal. Also, self-aligned ion implantation requires a shrunken metal line, leading to line width control problems, especially for high density displays. Some conventional techniques use poly-Si scan lines in poly-Si TFT arrays, providing excellent gate oxide integrity in each TFT because the oxide/semiconductor interface between the gate and insulator layers is stable due to compatibility of materials. This avoids fabrication problems but increases scan line resistance, precluding sufficiently large array sizes or sufficiently high densities for many applications. To reduce poly-Si scan line resistance, it has been proposed to provide an aluminum shunt, since the resistivity of doped aluminum is as low as 0.3 ohms/square or less while that of poly-Si is 30-50 ohms/square. But hydrogenation to passivate channel defects would also degrade the shunted scan line due to atomic mixing at the interface between metal and poly-Si. It is an object of the invention to provide a more elegant solution to the scan line resistance problem, a solution that also avoids fabrication problems. According to the invention, there is provided a method of forming array circuitry as claimed in the accompanying claims. The invention is based on the discovery of a technique that preserves the advantages of a metal scan line and also obtains the advantages of a poly-Si gate line. The technique provides a metal scan line and a semiconductor line connected to the scan line. The semiconductor line crosses a channel and is conductive so that signals on the scan line control conductivity of the channel. The technique can be implemented in a method that forms array circuitry at a surface of the substrate. The method forms one patterned conductive layer that includes M scan lines extending approximately in a first direction and forms another patterned conductive layer that includes N data lines extending approximately in a second direction different than the first. The scan lines include metal. For each value of m from 1 through M and for each value of n from 1 through N, the array circuitry includes a crossing region in which the mth scan line and the nth data line cross. The method also forms cell circuitry for at least one of the crossing regions, connected to the mth scan line and the nth data line. The cell circuitry includes a component that has a data lead for receiving signals from or providing signals to the nth data line. To form the cell circuitry the method forms a first patterned semiconductor layer that includes a first line extending between a first connecting point for connecting to the nth data line and a second connecting point for connecting to the data lead. The first line includes a channel between the first and second connecting points. The method also forms a second patterned semiconductor layer that includes a second line that extends from a third connecting point for connecting to the mth scan line. The second line crosses the first line at the channel and is conductive. In forming the first patterned conductive layer, the second patterned conductive layer, and the cell circuitry, the method also forms electrical connections at the first, second, and third connecting points so that signals on the mth scan line control conductivity of the first line between the nth data line and the data lead. Each of the first and second patterned semiconductor layers can include polysilicon. The first layer can be formed before the second layer so that the channel is between the second line and the substrate and therefore is shielded from implantation of a dopant. The first patterned semiconductor layer can include the data lead, connected to the first line. Therefore, the method can form an electrical connection at the second connecting point by implanting a dopant in the data lead and the connected part of the first line. Further, the method can lithographically produce the first patterned semiconductor layer so that it includes a connected shape with the first line, the data lead, and a capacitor electrode that is along part of the mth scan line. Then, an insulating layer between the capacitor electrode and the mth scan line provides a capacitive element. The second line, channel leads in the first line, and the capacitor electrode can be doped at once, eliminating the need for a separate capacitor mask and a separate capacitor implant. To form an electrical connection at the first connecting point, the method can lithographically produce an opening through the insulating layer to the first connecting point. Then the method can deposit a data line layer that goes through the opening to form a connecting interface between the data line layer and the first line. The method can lithographically produce the data lines from the data line layer. The method can form a metal/semiconductor interface between the mth scan line and the second line at the third connecting point. For example, the method can deposit a scan line layer that includes metal over the second line, forming the metal-semiconductor interface. Then the method can lithographically produce the scan lines from the scan line layer. The technique described above is advantageous because it provides a metal scan line that can have very low resistance, and therefore very low signal delay and low signal distortion. Because the scan line need not be modified to form a gate, it can be a straight, uniform, continuous metal line, with no kink or other modification that would reduce conductivity. The technique is also advantageous because it provides a channel that is controlled by a semiconductor line rather than a metal line. Because the scan line can be spaced apart from the channel region, hydrogenation and other processes that relate to poly-Si TFT fabrication, such as ion implantation to form self-aligned source and drain, do not cause interaction between metal from the scan line and the channel, so that the technique can be used in a poly-Si TFT array. The technique is also advantageous because it permits very efficient use of cell area. Each cell's circuitry can include one or two channels between the edges of the cell's data line, with a semiconductor gate line that connects to the cell's scan line in the region where the scan line crosses the data line. As a result, the channels and the gate line do not occupy cell area. The technique is especially advantageous for arrays in which a cell includes a storage capacitor. As illustrated in Figs. 4 and 5 of the Wu article, each cell of a 2D array can include a capacitor to maintain a voltage within the cell for an appropriate storage time. In an LCD array or a liquid crystal light valve (LCLV) array, for example, the capacitor can hold a voltage that controls orientation of adjacent liquid crystal material. In a sensor array, on the other hand, the capacitor can hold a voltage that indicates a quantity of energy received by a sensor for the cell. Similarly, in an energy emitting array, the capacitor can hold a voltage that indicates a quantity of energy to be emitted by an emitter for the cell. One advantage is that the technique simplifies fabrication of a capacitor under a scan line, with a part of the scan line forming a first electrode and with a second electrode formed in a semiconductor layer between the scan line and the substrate. Conventionally, capacitor fabrication requires an additional masking operation to select the capacitor's electrode area for doping while shielding semiconductor channels from being doped. This requires a tradeoff, however, between the level of capacitor implant doping necessary to prevent voltage dependent capacitance and the damage to the Si/SiO 2 interface from stripping implant doped photoresist. Some conventional techniques extend the second electrode beyond the edges of the scan line to provide regions that receive the additional implant. The technique permits fabrication of the capacitor in the same way as source/drain regions, without an additional masking operation and an additional implantation operation. Therefore, the capacitor is heavily doped, ensuring that capacitance is not voltage dependent. Also, the second electrode need not extend beyond the edges of the scan line, but can have edges aligned with the scan line's edges. Further, the second electrodes of adjacent cells need only be separated by the minimum spacing necessary for isolation without coupling, approximately 1 µm in a wafer-sized array and approximately 2 or 3 µm in a large area array. In other words, the second electrodes can extend as far as possible into the crossing regions, maximizing capacitor area. As a result of its maximized area, the capacitor need not occupy any part of the cell area bounded by the scan lines and data lines. Because the second electrodes are very thin semiconductor layers and are separated by small spacings, a metal scan line formed over them can have a cross section that is nearly uniform across the array, with the same width and thickness at all points along its length. Therefore, conductivity of the scan line is also maximized. As a result of maximizing capacitor area under the scan line, the technique permits greater useful area and greater cell density without sacrificing cell area. Therefore, the technique is especially advantageous in light transmissive, sensing, or emitting applications, such as displays, light valves, and sensors. Furthermore, when implemented with poly-Si TFTs, the technique permits integration of peripheral circuitry around the outer boundary of the array on the same substrate, providing driving or sensing circuitry for the scan lines and data lines. A method in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic layout diagram showing functions performed in fabricating an array in which a cell's circuitry includes a metal scan line and a conductive semiconductor line that provide gate signals controlling conductivity of a channel. Fig. 2 is a flow chart showing general acts that can implement the functions shown in Fig. 1. Fig. 3 is a flow chart showing other general acts that can implement the functions shown in Fig. 1. Fig. 4 is a schematic layout diagram showing semiconductor layers in a cell's circuitry. Fig. 5 is a schematic cross section along line a-a in Fig. 4. Fig. 6 is a schematic cross section along line b-b in Fig. 4. Fig. 7 is a schematic cross section along line c-c in Fig. 4. Fig. 8 is a schematic cross section along line d-d in Fig. 4. Fig. 9 is a flow chart showing acts in producing cell circuitry as in Fig. 4. Fig. 10 is a cross section of a display that includes an array produced as shown in Fig.9. A. Conceptual Framework The following conceptual framework is helpful in understanding the broad scope of the invention, and the terms defined below have the indicated meanings throughout this application, including the claims. "Circuitry" or a "circuit" is any physical arrangement of matter that can respond to a first signal at one location or time by providing a second signal at another location or time, where the second signal includes information from the first signal. Circuitry "transfers" a first signal when it receives the first signal at a first location and, in response, provides the second signal at a second location. The terms "array" and "cell" are related: An "array" is an article of manufacture that includes an arrangement of "cells." For example, a "two-dimensional array" or "2D array" includes an arrangement of cells in two dimensions. A 2D array of circuitry may include rows and columns, with a line for each row and a line for each column. Lines in one direction may be "data lines" through which a cell receives or provides signals that determine or indicate its state. Lines in the other direction may be "scan lines" through which a cell receives a signal enabling it to receive signals from or provide signals to its data line. In an array of circuitry, "cell circuitry" is circuitry connected to a cell's scan line and data line. A "crossing region" is a region in which two lines cross, such as a scan line and a data line. A "channel" is a part of a component through which electric current can flow. A channel is "conductive" when the channel is in a state in which current can flow through it. An operation "forms" an electrical connection by producing an electrical structure through which current can flow between components. Deposition can form an electrical connection by producing a metal/semiconductor interface or a conductive metal line. Doping can form an electrical connection by producing a doped semiconductor line. B. General Features of the Invention Figs. 1-3 show general features of the invention. Fig. 1 shows schematically functions in forming an M × N array in which a cell's circuitry includes a semiconductor line that is connected to a metal scan line and that crosses another semiconductor line at a channel between connections to a data line and a data lead. Fig. 2 shows general acts in implementing the functions shown in Fig. 1. Fig. 3 shows other general acts in implementing the functions shown in Fig. 1. Array 10 in Fig. 1 includes M scan lines, from first scan line 20 through Mth scan line 22, and N data lines, from first data line 24 through Nth data line 26. Cell circuitry connected to mth scan line 30 and nth data line 32 is illustratively shown in more detail. As shown in Fig. 1, array 10 can be produced by performing a number of functions. One function, designated "A," forms mth scan line 30 and the other scan lines, each of which includes metal. Another function, designated "B," forms nth data line 32 and the other data lines, crossing the scan lines in crossing regions such as crossing region 34. Another function is to produce cell circuitry for each crossing region. This function produces component 40, which has a data lead for receiving signals from or providing signals to nth data line 32. In addition, one part of this function, designated "C1," forms semiconductor line 42 connected between nth data line 32 and a data lead of component 40. Another part of this function, designated "C2," forms semiconductor line 44 which is conductive and which crosses semiconductor line 42 at channel 46. Another function is to form electrical connections at appropriate connecting points. One part of this function, designated "D1," forms an electrical connection between nth data line 32 and semiconductor line 42. A second part of this function, designated "D2," forms an electrical connection between semiconductor line 42 and the data lead of component 40. A third part of this function, designated "D3," forms an electrical connection between mth scan line 30 and semiconductor line 44. Because of these connections, signals on m th scan line 30 control conductivity of channel 46. The functions in Fig. 1 could be implemented in many ways using various techniques for producing circuitry on a substrate. Fig. 2 shows a sequence of general acts that can implement the functions in Fig. 1. The act in box 60 deposits and lithographically patterns a first semiconductor layer, producing semiconductor line 42 in Fig. 1. The act in box 60 can also produce the data lead and other parts of component 40, such as a capacitor electrode. The act in box 62 deposits a first insulating layer over the pattern from box 60. Then, the act in box 64 deposits and lithographically patterns a second semiconductor layer, producing semiconductor line 44 in Fig. 1. The act in box 66 then implants a dopant. Because semiconductor line 44 crosses channel 46, channel 46 is shielded from the dopant, but other parts of semiconductor line 42 are doped, forming channel leads for channel 46. The act in box 66 can therefore form an electrical connection between semiconductor line 42 and the data lead of component 40, the function designated D2 in Fig. 1. The act in box 70 deposits and lithographically patterns a scan line layer that includes metal, producing M scan lines 20 through 22. The act in box 70 therefore forms an electrical connection between semiconductor line 44 and mth scan line 30, by forming a metal/semiconductor interface. The act in box 72 deposits a second insulating layer. The act in box 74 then lithographically forms openings in the first and second insulating layers to expose the connecting point at which semiconductor line 42 connects to nth data line 32. The act in box 76 then deposits and lithographically patterns a data line layer, producing N data lines 24 through 26. The act in box 76 therefore forms an electrical connection between semiconductor line 42 and nth data line 32, because the metal in the opening formed in box 74 forms a metal/semiconductor interface. Fig. 3 shows another sequence of general acts that could be used implement the functions in Fig. 1. The sequence in Fig. 3 would produce a structure with a gate under a channel and with a top ITO layer. The act in box 100 deposits and lithographically patterns a scan line layer, producing scan lines. The act in box 102 then deposits and lithographically patterns a layer referred to as the "second semiconductor layer" because it includes a line that serves the same function as semiconductor line 44 in Fig. 1. Deposition of the second semiconductor layer on a scan line forms a metal/semiconductor interface, forming an electrical connection. The act in box 104 deposits a first insulating layer over the pattern from box 102. Then, the act in box 106 deposits a layer referred to as the "first semiconductor layer" because it is subsequently etched to form a pattern that includes a line that serves the same function as semiconductor line 42 in Fig. 1. The act in box 110 deposits a second insulating layer over the first semiconductor layer from box 106. Then, the act in box 112 performs a self-aligned back side exposure followed by an etch to lithographically pattern the second insulating layer so that it includes an island over the channel in the first semiconductor layer. The act in box 114 then implants a dopant, with the channel shielded from the dopant by the island from box 112. The act in box 114 can also anneal to activate the channel and passivate. The act in box 116 then lithographically patterns the first semiconductor layer, producing a line that serves the same function as semiconductor line 42 in Fig. 1. The act in box 116 can also produce the data lead and other parts of component 40, such as a capacitor electrode, as in box 60 in Fig. 2. The act in box 120 then deposits and lithographically patterns a third insulating layer so that it has openings to the first semiconductor layer at the connection points. The act in box 122 then deposits and lithographically patterns a data line layer, producing data lines. The act in box 122 therefore forms an electrical connection between the line in the first semiconductor layer and a data line, because the metal in the opening formed in box 120 forms a metal/semiconductor interface. C. Implementation The general features described above could be implemented in numerous ways in various arrays. An implementation described below employs poly-Si TFTs, and is suitable for an AMLCD array. C.1. Cell Circuitry Figs. 4-8 illustrate cell circuitry. Fig. 4 shows a layout of first and second semiconductor layers. Figs. 5-8 show representative cross sections of cell circuitry in Fig. 4, along lines a-a, b-b, c-c, and d-d, respectively. Fig. 4 shows a part of an M × N array, with m th scan line 200, ( m + 1)th scan line 202, n th data line 204, and ( n + 1)th data line 206 in dashed lines. Fig. 4 also shows part of the cell circuitry for the cell that is connected to mth scan line 200 and nth data line 204. The cell's circuitry includes first semiconductor pattern 210, with a line that extends from first connecting point 212 to second connecting point 214 and with another line extending from second connecting point 214 to capacitor electrode 216. First connecting point 212 is substantially all within the edges of nth data line 204, to which it is electrically connected. The edges of capacitor electrode 216 are aligned with the edges of ( m + 1)th scan line 202, forming a capacitive element. The cell's circuitry also includes second semiconductor pattern 220, a line that crosses first semiconductor pattern 210 at channels 222 and 224. Second semiconductor pattern 220 extends from end 226, at which it is electrically connected to mth scan line 200. The layout in Fig. 4 is designed to have the following dimensions when implemented with poly-Si TFTs on an insulating substrate: Each cell is 30 µm × 30 µm, of which the scan line occupies 6 µm and the data line occupies 5 µm. The lines in first semiconductor pattern 210 are 2 µm wide, and second semiconductor pattern 220 is 2.5 µm wide. In general, the smallest features are 2 µm and the smallest separations are 3 µm, with 1.0 µm overlay and an aperture ratio of 49.7%. The cell circuitry in Fig. 4 is designed so that the capacitive element formed by ( m + 1)th scan line 202 and capacitor electrode 216 has sufficient capacitance that stored voltages are not significantly affected by capacitive coupling with data line voltage fluctuations. Dark matrix is used to improve image quality by blocking stray illumination, as at edges, with minimum sacrifice of aperture. Fig. 5 shows substrate 240, which can be quartz, with surface 242 at which circuitry 244 is formed. Circuitry 244 includes insulating layer 250 on surface 242, on which capacitor electrodes 252 and 254 are formed for the cell circuitry connected to the ( m - 1)th scan line and to the n th and ( n - 1)th data lines respectively. Capacitor electrodes 252 and 254 each include heavily n-doped poly-Si, as indicated by "n+," except that a small area of electrode 252 that is under second semiconductor pattern 220 is undoped, intrinsic poly-Si, as indicated by "i." If the area in which second semiconductor pattern 220 overlaps capacitor electrode 252 is small, as in Fig. 4, this small undoped area has an insignificant effect on capacitance. Circuitry 244 also includes insulating layer 256 between capacitor electrodes 252 and 254 and second semiconductor pattern 220 shown in Fig. 4. Second semiconductor pattern 220 also includes heavily n-doped poly-Si, and is electrically connected to mth scan line 200, which includes aluminum, and which can be implemented as a hybrid TiW/AlCu stack. Because of the small overlap with second semiconductor pattern 220 and because of a slight dip between capacitor electrodes 252 and 254, mth scan line has slight variations in cross section, but in general its cross section remains substantially uniform across the array. Circuitry 244 then includes insulating layer 260 separating m th scan line 200 from n th data line 204, which can also be implemented as a hybrid TiW/AlCu stack. Over n th data line 204 is passivation layer 262 of polyimide. Fig. 6 shows a different cross section of circuitry 244, with many of the same features as Fig. 5, but also showing channel 224 in first semiconductor pattern 210. As shown, second semiconductor pattern 220 provides signals from m th scan line 200 to channel region 270, controlling conductivity of channel 224. Fig. 7 shows a cross section through first connecting point 212. As shown, n th data line 204 forms a metal/semiconductor contact with first semiconductor pattern 210 through an opening in insulating layers 256 and 260. Dark matrix lines 280 and 282 are formed on passivation layer 262 over the edges of nth data line 204, and indium tin oxide (ITO) pixel electrodes 290 and 292 slightly overlap dark matrix lines 280 and 282. Fig. 8 shows a cross section through second connecting point 214. Metal pattern 300 forms a metal/semiconductor contact with first semiconductor pattern 210 through an opening in insulating layers 256 and 260. Metal pattern 300 can be formed from the same metal layer as nth data line 204. Conductive dark matrix pattern 302 can be formed from the same material as dark matrix lines 280 and 282 in Fig. 7. ITO pixel electrode 290 then forms an electrical connection with second connecting point 214 through metal pattern 300 and dark matrix pattern 302. Although metal pattern 300 blocks stray illumination at the edges of insulating layers 256 and 260 and smooths the topology, dark matrix pattern 302 provides process compatibility between ITO pixel electrode 290 and metal pattern 300. C.2. Fabrication Fig. 9 shows acts in producing cell circuitry as described above. The act in box 330 begins by preparing a surface of a quartz substrate. The act in box 330 can include any necessary cleaning. The act in box 332 then deposits a first layer of low temperature oxide (LTO), which can be SiO 2 deposited with plasma chemical vapor deposition. The first LTO layer can be deposited to a thickness of 0.7 µm and then annealed. The act in box 334 deposits a layer of a-Si to a thickness of 0.1 µm, then performs Si self-ion implantation to enhance performance. The act in box 334 also performs crystallization and annealing at 600° C. As a result, the a-Si becomes poly-Si. The act in box 334 performs lithography to produce a pattern of mask material that covers the parts of the poly-Si that form first semiconductor pattern 210. Then, the act in box 334 etches to remove areas not covered by the pattern of mask material, leaving first semiconductor pattern 210. The act in box 336 deposits a second layer of LTO to a thickness of 0.085 µm. The act in box 336 also performs oxidation at 950° C under 150 atmospheres and anneals the second LTO layer. The act in box 340 deposits a layer of poly-Si to a thickness of 0.35 µm. The act in box 340 performs lithography to produce a pattern of mask material that covers the parts of the poly-Si that form second semiconductor pattern 220 or another similar pattern that crosses the first semiconductor pattern at one or two channels. Then, the act in box 340 etches to remove areas not covered by the pattern of mask material, leaving second semiconductor pattern 220. Then, the act in box 340 removes the mask material. The act in box 342 performs lithography to produce a pattern of mask material that does not cover the cell circuitry, but may, for example, cover areas in which peripheral circuitry is being formed on the substrate. The act in box 342 then implants a heavy concentration of an n-type dopant, making second semiconductor pattern 220 conductive and forming conductive leads in first semiconductor pattern 210. Then, the act in box 342 removes the mask material by an appropriate plasma resist etching. The act in box 344 similarly performs lithography to produce a pattern of mask material that does not cover the peripheral circuitry, but covers the cell circuitry. The act in box 344 then implants a heavy concentration of a p-type dopant to form conductive areas in the peripheral circuitry. Then, the act in box 344 removes the mask material. The act in box 344 can also perform crystallization annealing at 600° C. Because they implant heavy concentrations of dopants, the acts in boxes 342 and 344 may employ any of a number of implantation processes, including non-mass selection processes such as ion showering, plasma extraction, ion bucket, and so forth. The act in box 346 deposits a layer of metal to a thickness of 0.1-0.2 µm to produce a scan line layer. The scan line layer can, for example, be a hybrid TiW/AlCu multilayer stack with three or four layers of 0.01 µm of TiW separated by two or three layers of 0.05 of AlCu. The hybrid films can be sputtered or evaporated using two alloy targets, alternating between the targets. These thicknesses avoid hillock formation in the AlCu layers during processing, yet provide a thin barrier metal to avoid intermixing between the AlCu layers or between an AlCu layer and other layers, avoiding hillock formation, film bubbling, peeling, or spiking into a poly-Si layer, for example. The films can all be wet etched at approximately the same rate to achieve a high degree of dimension control. The ratio of the wet TiW etchant to the wet AlCu etchant can be approximately 50:1; in contrast to the standard Al etchant, this mixed etchant is much lower in viscosity and less violent during etching, removing bubbling. The act in box 346 then performs lithography to produce a pattern of mask material that covers the parts of the scan line layer that form the scan lines. Then, the act in box 346 etches to remove areas not covered by the pattern of mask material, leaving the scan lines. Then, the act in box 346 removes the mask material. The act in box 350 deposits a third layer of LTO to a thickness of 0.7 µm. The act in box 350 also performs hydrogenation to passivate the channels in first semiconductor pattern 210 and an appropriate wet oxide etch to remove the damaged layer resulting from the hydrogenation process. This hydrogenation does not cause degradation of channels in first semiconductor pattern 210 because the scan lines formed in box 346 are not over the channels in first semiconductor pattern 210. The act in box 352 performs lithography to produce a pattern of mask material that does not cover first and second connection points 212 and 214 and any other areas in which metal in the data line layer contacts the layer formed in box 334, but covers all other areas. The act in box 352 then etches to form openings in the second and third LTO layers from boxes 336 and 350 in the areas that are not covered. Then, the act in box 352 removes the mask material. The act in box 354 deposits a data metal layer to a thickness of 0.5 µm. The data metal layer can, for example, be a hybrid TiW/AlCu multilayer stack as described above. The act in box 354 then performs lithography to produce a pattern of mask material that covers the parts of the data line layer that form the data lines and that cover the opening to second connection point 214. Then, the act in box 354 etches to remove areas not covered by the pattern of mask material, leaving the data lines. Then, the act in box 354 removes the mask material. The data line layer over second connection point 214 blocks light leakage due to poor liquid crystal control as a result of the steepness of the topology. This light leakage blocking raises contrast ratio dramatically. The act in box 356 deposits a passivation layer of polyimide to a thickness of 1.5 µm. The act in box 356 performs lithography to produce a pattern of mask material that does not cover second connection point 214 and any other areas in which metal in the data line layer should be exposed for contact. The act in box 356 then etches to form openings in the passivation layer in the areas that are not covered. Then, the act in box 356 removes the mask material. The passivation layer also acts to planarize the surface, and spin-on glass could be used instead of polyimide. Planarization is important because of the steepness of the topology. The act in box 360 deposits a dark matrix layer of TiW to a thickness of 0.1 µm. TiW is advantageous as a dark matrix material because it acts as a mutual etch stop with indium-tin-oxide (ITO) and aluminum and has a high optical density even in a thin layer, and it is advantageously applied over the circuitry rather than on a cover sheet. The act in box 360 performs lithography to produce a pattern of mask material that covers the dark matrix layer only in areas in which light shielding is needed, such as along the edges of each data line and around second connection point 214. The act in box 360 then etches to remove the areas that are not covered. Then, the act in box 360 removes the mask material. The act in box 362 deposits ITO to a thickness of 0.055 µm. The act in box 362 performs lithography to produce a pattern of mask material that covers the ITO layer in light transmissive cell areas. The act in box 362 then etches to remove the areas that are not covered. Then, the act in box 362 removes the mask material and anneals the ITO layer at 280°C. C.3. Display Fig. 10 shows features of a display that includes circuitry produced as described above. Display 400 in Fig. 10 includes array substrate 402 and cover substrate 404. Array substrate 402 is quartz, but cover substrate 404 could be quartz or glass. Circuitry 410 formed at surface 412 of array substrate 402 includes array circuitry 414 and peripheral circuitry 416. Peripheral circuitry 416 is outside the boundary of array circuitry 414, and can include drivers connected to the ends of scan lines and data lines. As described above, array circuitry 414 can have n-doped poly-Si TFTs, while peripheral circuitry 416 can have p-doped poly-Si TFTs. ITO layer 420 is formed at a surface of cover substrate 404 and faces surface 412 of array substrate 402, separated by spacers 422 and 424 at the boundary of array circuitry 414. If display 400 is a projection device, it should be small enough that no other spacers are needed within the boundary of array circuitry 414, because spacers can cause image problems in a projection display. The need for spacers can be reduced by using thicker substrates and by assembling at lower temperatures. Display 400 also includes liquid crystal material 430 in the cavity defined by ITO layer 420, array circuitry 414, and spacers 422 and 424. Liquid crystal material 430 can be a 90° twisted nematic liquid crystal. Display 400 could, for example, be a wafer-scale projection display of approximately 38 mm diagonal. Display 400 could alternatively be a large area direct viewing display. C.4. Results The techniques described above have been simulated and compared with prior art techniques. Simulation results have shown significantly reduced cross talk and improved image quality, in part due to much larger storage capacitance and in part due to smaller gate and parasitic capacitances in the TFTs. A dummy 30 × 30 array of cells with no liquid crystal assembly has been manufactured, tested, and found to be fully functional. 1280 x 1024 arrays have been successfully manufactured, both wafer-scale and large area. The wafer-scale arrays employ conventional 2 pm CMOS technology, while the large area arrays employ conventional 3 µmCMOS technology. Results to date indicate that the wafer-scale and large area arrays will be fully functional when assembled and that the circuitry is scalable to 1 µm or smaller CMOS technology. C.5. Variations The implementation described above provides thin film circuitry on an insulating substrate. The invention could be implemented with other types of circuitry on other types of substrates. The implementation described above provides circuitry with specific geometric and electric characteristics, but the invention could be implemented with different geometries and with different circuitry. The implementation described above includes layers of specified thicknesses, produced from specified materials by specified processes, but other thicknesses could be produced, and other materials and processes could be used, such as thinner semiconductor and gate oxide layers to improve TFT performance or to increase storage capacitance. Rather than poly-Si, other semiconductor materials could be used in the semiconductor layers, including but not limited to a-Si, SiGe, CdSe, or a composite layer of poly-Si and SiGe. Similarly, various conductive materials could be used in the scan lines and data lines, including but not limited to ITO, MoTa, Cr, MoCr, Ta, Cu, Ti, TiN, and organic conductive materials. The implementation described above includes layers in a specific sequence, but the sequence of layers could be modified, such as by producing bottom gate TFT structures. The implementation described above forms a semiconductor line that is heavily doped except in channels at which it crosses a gate line, but other doping techniques could be used. For example, doping could be lowered to reduce leakage current. The implementation described above forms an array with a metal scan line controlling a semiconductor gate line. The invention could, however, be implemented to form other arrays. The implementation described above forms a gate line that crosses a semiconductor line at two channels. The invention could, however, form a single channel or form multiple gates of other kinds. D. Application The invention could be applied in many ways, including arrays for displays, sensors, and light valves. E. Miscellaneous The invention has been described in relation to thin-film implementations, but the invention might be implemented with single crystal technology.

Claims: A method of forming array circuitry at a surface (242) of a substrate (240); the method comprising: forming a first patterned conductive layer (200) that includes M scan lines (20,22), where M is greater than 1; each of the scan lines extending approximately in a first direction across the surface of the substrate (240); the M scan lines being positioned in order from first through Mth; each of the scan lines including metal; forming a second pattern conductive layer (204) that includes N data lines (24,26), where N is greater than 1; each of the data lines extending approximately in a second direction across the surface of the substrate (240); the second direction being different from the first direction; the N data lines being positioned in order from first through Nth; the array circuitry including, for each value of m from 1 through M and for each value of n from 1 through N, a crossing region (34) in which the m th scan (30) and the n th data line (32) cross; and forming cell circuitry for each of a set of at least one of the crossing regions; the cell circuitry of each crossing region (32) in the set being connected to the m th scan line (30) and the n th data line (32); the cell circuitry of each crossing region in the set including a component (40) having a data lead for receiving signals from or providing signals to the nth data line; the cell circuitry comprising a field effect transistor; the act of forming cell circuitry comprising: forming a first patterned semiconductor layer (210); the first patterned semiconductor layer including, for each crossing region in the set, a first line extending between a first connecting point (212) for connecting to the nth data line and a second connecting point (214) for connecting to the data lead; the first line including a channel (222,224) between the first and second connecting points; and forming a second patterned semiconductor layer (220); the second patterned semiconductor layer including, for each crossing region in the set, a second line that extends from a third connecting point (226) for connecting to the m th scan line; the second line crossing the first line at the channel (222,224), the second line being conductive; the acts of forming the first patterned semiconductor layer (210), the second patterned conductive layer (204), and the cell circuitry comprising: forming electrical connections at the first, second and third connecting points so that the signals on the m th scan line control conductivity of the first line between the n th data line and the data lead, characterized in that the act of forming the first patterned semiconductor layer (210) includes, for each crossing region in the set, forming a capacitor electrode (216) connected to the first semiconductor line by the data lead; the act of forming the cell circuitry further comprising: depositing a first unpatterned semiconductor layer; performing lithography to produce a first pattern of mask material; the first pattern covering parts of the first unpatterned semiconductor layer that form for each crossing region in the set, a connected shape that includes the first line, the data lead, and a capacitor electrode (216) along a part of the (m+1)th scan line; etching to remove areas of the first unpatterned layer not covered by the first pattern of mask material so that the covered parts of the first unpatterned semiconductor layer remain; and forming an insulating layer (256) between the capacitor electrode (216) and the (m+1)th scan line (202) so that the capacitor electrode and the (m+1)th scan line can act as a capacitive element. The method of claim 1 in which each first and second patterned semiconductor layer (210,220) includes polysilicon. The method of claim 1 in which the first patterned semiconductor layer (210) is formed before the second patterned semiconductor layer (220) so that the channel (222,224) is between the second line and the substrate (240). The method of claim 1 in which the first patterned semiconductor layer (210) includes the data lead connected to the first line; the act of forming electrical connections comprising, after the first patterned semiconductor layer is formed: implanting a dopant; the act of implanting the dopant doping the data lead and a connected part of a first line so that the data lead and the first line are electrically connected. The method of claim 1 in which the first patterned semiconductor layer (210) is formed before the insulating layer (256) and the insulating layer is formed before the first patterned conductive layer (200), so that the insulating layer (256) and the capacitor electrode (216) are between the (m+1)th scan line and the substrate (240). The method of claim 1 in which the first patterned semiconductor layer (210) is formed before the second patterned semiconductor layer (220), the act of forming electrical connections further comprising, after the second patterned semiconductor layer is formed: implanting a dopant; the dopant doping the second line, parts of the first line other than the channel (222,224), and the capacitor electrode (216) so that the second line and the capacitor electrode are conductive and so that the first line has a conductive first channel lead electrically connected to the first connecting point (212) and a conductive second channel lead electrically connected to the capacitor electrode (216). The method of claim 1 in which the second patterned conductive layer (220) is formed after the insulating layer (256) is formed; the act of forming electrical connections comprising: performing lithography to produce a connection pattern of mask material; the connection pattern covering parts of the insulating layer except, for each crossing region in the set, the first connecting point (212) of the first line; etching to remove areas of the insulating layer not covered by the connection pattern of mask material so that the covered parts of the insulating layer remain, the remaining parts defining, for each crossing region in the set, an opening to the first connecting point (212) of the first line; and depositing a data line layer (204) over the insulating layer so that the data line layer goes through openings defined by the remaining parts of the insulating layer and, for each crossing region in the set, forms a connecting interface between the data line layer (204) and the first line at the first connecting point (212), the act of forming the second patterned conductive layer (204) comprising: performing lithography to produce a data line pattern of mask material; the data line pattern covering parts of the data line layer that form the N data lines; and etching to remove areas of the data line layer (204) not covered by the data line pattern of mask material so that the covered parts of the data line layer remain; the covered parts forming the N data lines. The method of claim 1 in which the act of forming electrical connections comprises forming a metal/semiconductor interface between the m th scan line and the second line at the third connecting point. The method of claim 8 in which the first patterned conductive layer (200) is formed after the second semiconductor layer; the act of forming the first patterned conductive layer comprising: depositing a scan line layer (200) that includes metal over the second line to form the metal-semiconductor interface; performing lithography to produce a scan line pattern of mask material; the scan line pattern covering parts of the scan line layer that form the M scan lines; and etching to remove areas of the scan line layer not covered by the scan line pattern of mask material so that the covered parts of the scan line layer remain; the covered parts forming the M scan lines.

Inventors: WU I-WEI, WU, , I-WEI
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1996
Main classification: H01L21
IPC codes: H01L23, H01L21, H01L27, G02F1, H01L29