The invention relates generally to liquid crystal display (LCD) panels for projection display and more particularly to a substrate structure and method of fabrication for forming high-quality thin film transistor (TFT) controlled pixel electrodes for use in LCDs.
Flat panel displays employ liquid crystal material sandwiched between parallel panels of light-transmissive material. The panels are usually made of quartz, glass, plastic, or the like. One panel has an array of pixels formed on its surface. Each pixel on the panel includes a light-transmissive pixel electrode controlled by a switching transistor. In active matrix displays the transistor, generally a thin film transistor (TFT), is operatively connected by thin metal lines on the panel to driver circuitry which selectively energizes the pixels. In active matrix displays each pixel (i.e., a pixel electrode controlled by a TFT) is addressed and controlled individually.
Light directed through the LCD passes through a first polarizing filter applied to one of the two parallel panels. The other panel has a second polarizing filter oriented in a different direction from the first. The liquid crystal material, which fills the volume between the panels, contains molecules which rotate the incident light in a well-defined manner when the adjacent pixel electrode is energized. Typically, the LCD is configured so that a particular pixel, when turned on, rotates the polarized light as it passes through the liquid crystal material, causing it to pass through the second polarizing filter. When the pixel is turned off, the polarized light is not rotated and thus will not pass through the second polarizer. An array of pixels turned on or off in a predetermined order and pattern produces images. By using multiple color filters, and given a sufficient density of pixels, full-color images are produced on the LCD screen. In projection-type LCDs light is directed through the LCD and is projected onto a screen.
Forming LCDs is a manufacturing challenge because a high density of pixels must be formed on a large area of transparent or translucent flat panel material. The pixels (TFTs and pixel electrodes) are fabricated in a layer of silicon applied to a quartz, glass, or another substrate (referred to herein after as the xe2x80x9ctransparent substratexe2x80x9d). The form of silicon that is easiest to apply to transparent substrates is uncrystallized amorphous silicon, which is widely used in LCD panels. Amorphous silicon yields poor TFT performance because of low electron mobility, but it is adequate for active matrix pixel control in most applications. TFTs formed in amorphous silicon lack the frequency response for display driver circuitry, however. The driver circuitry must be fabricated separately, usually as integrated circuits formed in single-crystal silicon. The separately-formed drivers must then be connected to the LCD, increasing manufacturing costs.
Polycrystalline silicon can be formed on a transparent substrate, as an alternative to amorphous silicon, by partially crystallizing deposited amorphous silicon through heating. Polycrystalline silicon (also known as polysilicon) yields higher-quality TFTs, but the high process temperatures required to crystallize amorphous silicon presents major difficulties, particularly for glass and plastic substrates. Heat sufficient to produce crystallized silicon on a transparent substrate can damage the substrate. Therefore, LCD manufacturers still use amorphous silicon on glass panels for the formation of the TFT pixel arrays used in LCDs. The driver circuitry, which requires a higher frequency response and better performance, is fabricated separately and connected to the panel around its periphery.
Projection-type LCDs, wherein light is directed through the LCD for projection onto a screen at a distance from the LCD, have heretofore been fabricated like direct-view LCDs. A circuit panel, with a plurality of TFT-controlled pixel electrodes formed in amorphous or partially crystallized silicon, is formed on glass or a similar transparent substrate. Liquid crystal material fills the void between the circuit panel and a second panel. Individual pixel electrodes on the circuit panel control whether or not light passes through the LCD and onto a projection screen. Suitable color filters are used to produce full-color images.
The poor performance of TFTs formed in amorphous or partially crystallized silicon is an ongoing problem for LCD manufacturers. As compared with TFTs formed in single crystal silicon (such as in IC chips), TFTs formed on transparent substrates have substantially lower electron mobility and higher leakage currents. If the TFTs used in LCD pixel arrays could be fabricated in single-crystal silicon, the result would be improved frequency response and sharper images. Single crystal silicon would also allow for reduced manufacturing costs because the fast logic required in LCD driver circuits could be integrated into the display panel. Heretofore, the only way to provide single-crystal silicon TFTs on glass is to adhere a layer of silicon, formed separately, to a glass substrate. That solution presents adhesion problems due to differing heat expansion coefficients, particularly during fabrication processing.
It would be advantageous to fabricate LCDs for projection display using single-crystal silicon strongly adhering to, and preferably integrated with, a transparent supporting layer.
It would also be advantageous to form LCD pixel arrays for projection display on a substrate which can be processed, without damage, at temperatures higher than the melting point of glass or plastic.
It would also be advantageous to have a new LCD processing system which employs silicon wafer integrated circuit processing methodologies to form high-quality TFTs and driver circuits in single-crystal silicon.
Accordingly, a liquid crystal display (LCD) array substrate is provided for use in projection-type LCDs. The LCD array substrate comprises a portion or segment of a silicon on insulator (SOI) wafer which is processed to include a first layer of substantially all single-crystal silicon on a first side of the substrate, and an insulating layer beneath the first layer. The LCD array substrate further comprises a plurality of pixel structures formed on the top single-crystal silicon layer. Each pixel structure includes a pixel electrode which, when used in a LCD, controls light transmissivity through a subregion of the LCD. Light directed to pass through the insulating layer and the pixel electrodes is, in a completed LCD structure, controlled (i.e., permitted to pass through or not pass through the LCD) by the pixel structures on the LCD array substrate.
In a preferred embodiment of the invention the SOI wafer further includes areas of a second layer of silicon on a second side of the substrate, which is on the other side of (i.e., opposite) the insulating layer from the first layer. The SOI wafer from which the LCD array substrate is made generally includes at least three layers, a top or first layer of single-crystal silicon, an intermediate layer formed of insulating material, and a bottom or second silicon layer also generally formed of single-crystal silicon. Portions of the second layer, beneath the pixel structures, have been removed to form openings in the second layer. The portions which remain cover parts of the insulating layer which extend generally around the periphery of the segment of the SOI wafer. Thus, the areas beneath the pixel structures are free of the silicon of the second layer.
In a preferred embodiment of the invention, the LCD array substrate forms part of a complete LCD array for projection display. The LCD array substrate incorporates a pixel array, which includes a plurality of thin film transistors (TFTs), wherein each TFT controls a pixel electrode. Conductors formed on the LCD array substrate provide operative connections between each TFT and a pixel controller, which is preferably an active matrix control system, the controller also being formed on the LCD array substrate. The complete LCD further includes a parallel second substrate, spaced apart from the first substrate. Liquid crystal material is placed between the LCD array substrate and the second substrate. Each pixel electrode controls the transmission of light through a subregion of the liquid crystal material in the completed LCD.
A method of forming the LCD array substrate in accordance with the present invention comprises the following steps. A SOI substrate is provided having a first semiconductor layer extending to a first side of the substrate, a second semiconductor layer extending to a second side of the substrate, and a buried insulating layer extending through the substrate. The buried insulating layer extends between and generally parallel to the first and second sides of the substrate, between the first and second semiconductor layers. In its preferred form, the silicon on insulator substrate is a wafer of single-crystal silicon into which oxygen ions are implanted in a process known as SIMOX (Separation by IMplanted OXygen). The oxygen ions are implanted at high energy and come to rest beneath the surface in a distribution pattern centered at a depth determined by the implantation energy. Following implantation, the substrate is annealed at a temperature generally in the range of 1100xc2x0 C. to 1400xc2x0 C. The implanted oxygen ions bond with the silicon in the substrate to form a buried layer of silicon dioxide. The annealing also repairs damage to the crystal structure of the top layer of silicon. The result is a wafer having a top layer of substantially single crystal silicon, a buried insulating layer of silicon dioxide, and a bottom bulk layer of silicon which is also substantially all single crystal silicon.
After providing a SOI substrate, the next step is to form a plurality of pixel structures in a selected area of the first semiconductor layer. Each pixel structure includes a pixel electrode, whereby, when used in a LCD, the pixel electrodes each control light transmissivity through a subregion of the LCD. In its preferred form the pixel structures each include a thin film transistor (TFT) on the first side of the substrate together with a pixel electrode which, when used in a LCD, controls the light transmissivity through a subregion of the LCD.
Another step in the method is to remove the second layer of the substrate, in a selected area of the substrate, on the opposite side of the substrate from the plurality of pixel structures, to form one or more openings in the second layer. Thus, in the selected area, the substrate includes a plurality of pixel structures on the first side supported on the insulating layer, and includes no second semiconductor layer beneath the insulating layer, the second semiconductor layer having been removed from the second side of the substrate. When the LCD array formed on the SOI substrate is used in a LCD, light passes through the body of the substrate, i.e., through the insulating layer on the bottom of the substrate, and through the pixels formed on the top of the substrate, as it is passing into and through the liquid crystal material. The light passes directly through the openings formed in the second layer and through the insulating layer. The transmissivity of light through the LCD array substrate is controlled by the pixel structures and their interaction with the liquid crystal material, in the manner well known to those skilled in the art of LCDs.
The invention is particularly directed to the formation of a TFT array for pixel control used in LCD projection displays. The TFT pixel array is formed on a substrate which is ultimately incorporated into a LCD. The preferred substrate on which the TFTs and pixel electrodes of the array are formed has a first semiconductor layer substantially comprising single-crystal silicon, a buried insulating layer, and a second semiconductor layer also substantially comprising single-crystal silicon. The TFTs and pixel electrodes are formed on the first semiconductor layer. Operative connections to the TFTs for control by an active matrix display system are also formed on the substrate. In one embodiment of the invention the active matrix control system is also formed on the first semiconductor layer of the substrate.
The semiconductor substrate, or SOI substrate, used in the method preferably has a first layer of single-crystal silicon with a thickness generally in the range of 100 xc3x85 to 5,000 xc3x85, and a buried insulating layer of silicon dioxide with a thickness generally in the range of 500 xc3x85 to 5,000 xc3x85. In one alternative embodiment of the method, the buried insulating layer has a beginning thickness greater than 2,500 xc3x85 and the method includes an optional step of removing a portion of the thickness of the insulating layer to reduce the final thickness of the insulating layer. The final thickness is preferably generally in the range of about 500 xc3x85 to 2000 xc3x85. Other alternative substrates which could be used in the step of providing a suitable semiconductor substrate, within the scope of the present invention, include providing a substrate with a first semiconductor layer having a thickness generally in the range of 300 angstroms to 3000 angstroms, and a buried insulating layer of silicon dioxide having a thickness generally in the range of 1000 angstroms to 3000 angstroms. Yet another suitable substrate for use with the method of the present invention is a substrate having a buried insulating layer with a thickness greater than 500 angstroms extending between first and second semiconductor layers of substantially all single-crystal silicon.
The step of removing the second layer of the substrate from beneath the selected area of the first layer where the pixel structures are formed is preferably carried out by etching the second layer. A selective etching method, such as wet etch or plasma etch, which removes the second semiconductor layer to the level of the insulating layer, is preferred. Not all of the second layer need be removed. In the preferred embodiment of the invention a perimeter ridge of unetched silicon is left to help reinforce the mechanical integrity of the final LCD array substrate.
The LCD array of the present invention used in projection type LCDs forms part of a liquid crystal display made up of the array substrate, described above, formed in accordance with the above-described method, together with a second substrate. The two parallel substrates are spaced apart with liquid crystal material disposed between the substrates to form a LCD. An important aspect of the present invention is that the method allows a plurality of LCD array substrates to be formed simultaneously on a SOI wafer using integrated circuit (IC) processing techniques.
An alternative embodiment method included within the scope of the present invention is the formation of one or more LCD arrays on a wafer by steps which include providing a wafer of substantially single-crystal silicon having a buried insulating layer. Then, in one or more regions of the wafer, on the top silicon layer, forming a pixel array made up of a plurality of pixel structures. In each region on the wafer a LCD driver and operative connections between the driver and the pixel structures are then formed, providing an operative pixel array. An additional step is the removal, from the other side of the wafer in each of the regions where the pixel arrays and LCD drivers are formed, substantially all of the bottom silicon layer from beneath the pixel structures. As a result, the wafer cross section where the pixel structures are formed consists of the top layer with the pixel structures formed thereon, and the insulating layer. Then a step is carried out of separating, from other areas of the wafer, each of the regions where a pixel array, the LCD driver, and connections are formed. Each region thereby becomes a separate LCD array substrate, alternatively referred to herein as a first substrate of a LCD. The one or more LCD array substrates thus formed, are then each combined with a second substrate which is provided spaced apart from the first substrate. For each of the first and second substrates spaced apart from one another, the next step is to provide liquid crystal material between the substrates to form one or more LCDs. In each LCD the second substrate operates cooperatively with the LCD array on the first substrate to control light transmission through the first and second substrates and through subregions of the liquid crystal material. The result is a plurality of projection-type LCDs, each providing selective control of light transmission through subregions of the LCD to control a projection display. The pixel array substrates for the plurality of LCDs are fabricated simultaneously on a SOI wafer using IC processing methodologies, greatly reducing manufacturing costs for projection-type LCDs.