Abstract:
A method and system for fabricating microcircuits occupying large areas on substrates capable of withstanding high semiconductor processing temperatures and then transferring the circuits onto large substrates incapable of withstanding the high processing temperatures. The method and system is particularly suitable for fabricating large area displays.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a method and system for fabricating and transferring microcircuits, and more particularly to a method of fabricating and processing microcircuits at high temperature on a flexible carrier capable of withstanding the high processing temperature and then transferring the circuits to a substrate such as a transparent plastic.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known to fabricate high quality microcircuits, such as TFT circuits, on a silicon substrate which can withstand high processing temperatures, and then to transfer them onto a glass substrate. The microcircuits are adhered to the substrate and then the silicon substrate is removed. The silicon substrate can be removed, for example, by etching the silicon oxide which is formed between the microcircuits and the silicon substrate or by etching away the silicon substrate itself or by thermally shocking the microcircuit and substrate whereby the differential expansion between the circuit and the substrate causes separation. U.S. Pat. Nos. 6,232,136, 6,027,958, 5,702,963, and 5,475,514, among others, describe the above-noted process.  
           [0003]    This technology works well for small displays and for transferring a high temperature polysilicon microcircuit from silicon to glass. However, the use of a silicon substrate limits the size of the microcircuit area and is expensive.  
           [0004]    There is a need to provide a method in which large area microcircuits can be fabricated and processed at high temperatures to provide high quality, high frequency microcircuits, and then transferred to a substrate such as a transparent flexible plastic. Particularly, there is a need for large area flat panel displays in which the TFT matrixes which drive the liquid crystal displays (LCDs) or light-emitting diodes (LEDs) are formed at elevated temperatures to provide enhanced characteristics and then the circuits are transferred to a transparent panel such as a plastic panel.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0005]    It is a general object of the invention to provide a method in which thin film semiconductor microcircuits are fabricated and processed at high temperatures on a flexible carrier capable of withstanding the high processing temperature and thereafter transferred to a flexible substrate, such as a plastic substrate.  
           [0006]    It is another object of the invention to provide a method and system for fabricating high quality thin film transistors (TFTs) at each pixel site of a flat panel display and their high speed drive circuitry.  
           [0007]    It is another object of the present invention to provide a method and system which simplifies the fabrication of large area displays and reduces the costs.  
           [0008]    In one embodiment of the invention the thin film microcircuitry is fabricated and processed at high temperatures on a high temperature steel foil or belt initially coated with a nickel layer. Once the circuitry has been fabricated on the high temperature foil it is tested and prepared for transfer onto a flexible substrate. The microcircuitry is suitably secured to the flexible substrate and the circuit is thereafter parted from the high temperature substrate by etching away the nickel layer leaving the microcircuit attached to the flexible substrate. The method is particularly suitable for the fabrication of large area flat panel displays and their drive circuits. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention will be more clearly understood from the following description when read in conjunction with the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a flow diagram of a system for forming or fabricating and transferring microcircuits in accordance with the present invention.  
         [0011]    [0011]FIG. 2 is an enlarged sectional view of the coated steel belt used in one embodiment of the present invention.  
         [0012]    [0012]FIG. 3 is an enlarged sectional view of a thin film transistor formed on the steel belt taken along the line  3 - 3  of FIG. 1.  
         [0013]    [0013]FIG. 4 is an enlarged sectional view of a flexible transparent substrate taken along the line  4 - 4  of FIG. 1.  
         [0014]    [0014]FIG. 5 is an enlarged view of the flexible transparent substrate bound to the microcircuits fabrication on the steel belt taken along the line  5 - 5  of FIG. 1.  
         [0015]    [0015]FIG. 6 is an enlarged view of the flexible transparent substrate and mounted microcircuits separated from the steel belt taken along the line  6 - 6  of FIG. 1.  
         [0016]    [0016]FIG. 7 is an enlarged view of a thin film transistor mounted on the transparent flexible substrate with an LED applied.  
         [0017]    [0017]FIG. 8 is a schematic diagram showing the pixels and microcircuits of an LED flat panel display formed in accordance with the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    [0018]FIG. 1 is a schematic diagram of a system for forming a flexible active matrix back plane ready for the application of light-emitting diodes (LEDs) or organic light-emitting diodes (OLEDs). The flexible active matrix back plane is then further processed to apply the OLED or LED material and the addition of common connection to the LEDs or OLEDs.  
         [0019]    In the process illustrated the substrate is a steel belt or foil  11  which can withstand high processing temperatures. Alternatively a high temperature plastic belt such as polymide belt can be used as the substrate when the semiconductor material, such as cadmium selenide, requires lower processing temperatures. In one embodiment, a steel belt  11  is coated with a nickel coating or layer  12  by a process such as electrodeposition or sputtering, step  13 . The layer or coating is of a material that can thereafter be dissolved or etched to release the fabricated microcircuits. Alternatively, the layer can be an oxide coating formed by thermal oxidation which likewise can be removed by etching. In order to facilitate the releasing of the microcircuits from the steel belt, the steel belt or alternative belts may have tiny perforations which allow the etching fluid to easily access the release layer. The perforations can be in the order of 0.001 inches in diameter, the fabrication of which is well known in the art. An alternative substrate could be a sintered powder metallurgy substrate which is porous to fluids. This type of metal has been made for years for automobile oil and fuel filters. In the one embodiment, the coated steel belt, FIG. 2, serves as the substrate for the fabrication of thin film microcircuitry on the surface of the belt, step  14 . The thin film microcircuitry may for example include TFTs with conductive vertical and horizontal lines for driving the TFTs and exciting the LEDs or OLEDs at each pixel site. TFT drive and control circuits may also be fabricated during this step. The thin film microcircuits may be fabricated by depositing semiconductor material onto the belt and, by techniques well-known in the semiconductor industry, processing the semiconductor material to form TFTs and their interconnections and the drive and control circuits.  
         [0020]    By way of example, the active matrix circuit for the OLED display of FIG. 7 can be fabricated on the steel belt. For an active matrix display, the belt will have a width corresponding to the size of the display so that all elements of the display are fabricated and transferred to the transparent flexible display substrate.  
         [0021]    A brief description of the display of FIG. 8 illustrates the type of thin film circuits which can be fabricated in accordance with the present invention. Referring to FIG. 7, each pixel of the active matrix display includes light-emitting diodes  24  driven by a circuit including transmission gates  21 , storage capacitors  22  and power FETs  23 . The drain of each power FET  23  is connected to the anode of the corresponding light-emitting diode (LED)  24 . The cathode of LED  24  is connected to ground. In operation, signal data is stored line by line in buffers  26   a  and  26   b . Buffer  26   a  feeds signal data to the odd column lines (1, 3, 5, etc.) represented by  27   a . Buffer  26   b  feeds signal data to the even lines (2, 4, 6, etc.), represented by  27   b . Which pixel is to receive the data from the buffers is determined by row selector  28 . As the signal data arrives at the matrix, first buffer  26  is filled with the first line of the display frame. When the complete first line is in buffer  26 , the row selector places a signal on columns  27 . This row signal opens all the transmission gates  21  in the first row  29 , and the data stored in the buffer  26  is downloaded and stored as a voltage in storage capacitor  22  at each pixel on the selected row. The total storage capacitance is the sum of the metal connection lines, the gate capacitance of output FET  23 , and the capacitance of the storage capacitor  22 . The storage duration is determined by the RC time constant calculated by the reverse resistance of transmission gate  21 , plus the storage capacitance  22 , leakage resistance times the total storage capacitance. The storage RC constant should be at least three times the frame duration in time. For example, if the signal data consists of sixty (60) frames per second, the frame duration time is 16.7 ms and the RC constant should be 49.5 ms or greater. Therefore, frame rate plus the total reverse leakage resistance determines the size of the total storage capacitance.  
         [0022]    The voltage level +V and duration placed on the gate of output FET  23  determines the perceived brightness of LED  24 . this means that there are two ways to effect brightness (gray scale). The first is by storing the value of voltage level of the display voltage on storage capacitor  22 . The second way is to break the display frame into eight (8) binary sub-frames that can be combined in 256 ways to give varying time durations of the voltage signal on storage capacitor  22 . This is called 8-bit gray scale.  
         [0023]    The microcircuitry fabricated in the above-described embodiment of the invention is a combination of active matrix arrays for the LEDs or OLEDs and the row and column drivers for operating the display. Since particularly the row and column drivers require high speed circuitry, high temperature, annealing procedures are required to maximize the electron mobility in the deposited semiconductor material of the microcircuitry. In the case of a polysilicon temperatures as high as 900° are advantageous. Other types of semiconductor thin film, such as CdSe, circuitry may need lower anneal temperatures to maximize the electron mobility.  
         [0024]    An active matrix pixel drive employing a thin film transistor having an active semiconductor layer is now described with reference to FIGS.  3 - 6 . The thin film transistor is fabricated by depositing the semiconductive material and the metal, and by masking and etching using equipment and techniques well-known in the semiconductor industry for forming the transistor elements. First the drain electrode  41 , gate electrode  42  and transparent thin oxide electrode  43  are formed. Thereafter, oxide regions  44  are grown and defined. A semiconductor polysilicon channel material  45  is deposited, defined and subjected to high temperature annealing procedure required to maximize electron mobility. Temperatures as high as 900° C. are advantageous. If semiconductor materials such as cadmium selenide are used, lower annealing temperature are required and the belt can be plastic material such as polymide. Chrome/aluminum source and drain contacts  46  and  47  are then formed and an oxide passivation layer  48  is grown. The foregoing is merely illustrative of a procedure for forming a thin film transistor. It will be apparent that other devices can be formed. The thin film transistors and/or other devices are interconnected by conductive metal lines, not shown, to complete the microcircuit. For example, a flat panel display circuit such as that shown in FIG. 7 can be formed.  
         [0025]    Once the microcircuitry is fabricated on the belt, it is tested and prepared for transfer to a transparent flexible substrate or panel.  
         [0026]    In the next step  51 , FIG. 1, the completed microcircuits are adhered to the flexible transparent substrate  52  having a clear epoxy bonding layer  53 , FIG. 5. The flexible transparent substrate with the epoxy bonding layer and the steel belt with the microcircuits, FIG. 3, travel between rollers  54  and  56  where they are pressed together and adhered to form a sandwich, FIG. 5. The sandwich is then directed over rollers  57 ,  58  to an electro-etch bath  59  where the nickel coating is etched away. This allows the steel foil to separate from the transparent flexible substrate or material and microcircuits, FIG. 6. The transparent belt and microcircuits travel over rollers  61 ,  62  and are, for example, reeled onto a roll  63 . The steel substrate travels over rollers  64 ,  65  to a cleaning bath  67  and is reused. The steel belts can have an oxide separating layer rather than a nickel layer. The oxide layer can be etched with a suitable etchant.  
         [0027]    Referring to FIG. 7, a light-emitting diode  71  and metal cathode  72  are then formed on the transparent thin oxide layer  43 . Electrodes  73 ,  74  and  75  are then applied to the drain, gate and cathode, respectively, to excite the light-emitting diode  71  and emit light  76  at each pixel site through the transparent epoxy bonding layer  53  and flexible plastic substrate  52 .  
         [0028]    Although a continuous process using a belt substrate has been described, it should be apparent that the steel or polymide substrate may be a rigid panel the size of the display. The required microcircuitry can be fabricated on the substrate and transferred to a panel of the same or larger size by pressing. Separation can be by etching.  
         [0029]    Thus, there has been provided a system and method for fabricating integrated large-area flat panel displays.