1. Field of the Invention
The present invention generally relates to the field of liquid crystal devices, and more specifically to a high speed liquid crystal display and fabrication method in which electrodes and microelectronic electrode driver devices are integrally formed in a transparent monocrystalline silicon semiconductor layer.
2. Description of the Related Art
A liquid crystal display includes a sealed space which is filled with a liquid crystal material. Front and back electrodes are disposed on opposite sides of the space and are selectively energized to apply electric fields to the liquid crystal material to cause it to locally change its orientation resulting in a spatially variant perturbation of the light passing through. The electrodes also provide charge storage for the cell.
Different liquid crystal materials affect light passing through by different mechanisms, such as variable birefringence, scattering, etc. The display may provide only discrete black and white levels, or a continuous gray scale.
Liquid crystal displays can have either transmissive or reflective configurations. The front electrodes can be arranged in segments to provide an alphanumeric display for a calculator or clock, or in a rectangular matrix to provide a continuous graphic image for television, computer and other applications.
U.S. Pat. No. 4,239,346, entitled "COMPACT LIQUID CRYSTAL DISPLAY SYSTEM" issued Dec. 16, 1980 to R. Lloyd discloses a reflective Active-Matrix Liquid-Crystal Display (AMLCD) including a transparent front electrode and electrode back plates formed of single crystal silicon which define a sealed space therebetween which is filled with liquid crystal material. A common front electrode is formed on the inner surface of the front plate, whereas a semiconductor layer includes the inner surface of the back plate.
Back electrodes of reflective metal are formed in a rectangular matrix pattern on the top surface of the semiconductor layer in contact with the liquid crystal. MOSFET electrode driver transistors, interconnected by polycrystalline silicon bus lines, are also formed in the semiconductor layer, and are operatively connected to the electrodes. Electrical potentials are selectively applied between the individual back electrodes and the front electrode via the bus lines and driver transistors to locally polarize the liquid crystal material and form an image complete with gray scale.
The preferred material for the front and back plates is glass, due to its negligible reactivity with liquid crystal materials, low cost and transparency, allowing either reflective or transmissive liquid crystal effects to be used. Although epitaxial deposition of monocrystalline (single crystalline or bulk) silicon is possible on various materials such as sapphire, the temperature required for deposition is on the order of 1,000.degree. C. which is far in excess of the melting point of glass. In addition, the atomic structure of glass is highly irregular, and nonconducive to the growth of an epitaxial silicon layer.
For these reasons, the silicon layer on the front plate of Lloyd's display is formed by a standard silicon wafer. Alternative displays have used silicon layers formed by chemical vapor deposition (CVD) of amorphous or polycrystalline silicon. CVD of these materials can be performed at low temperatures and is not adversely affected by the irregular crystalline structure of the glass material of the plate.
However, the carrier mobility of polycrystalline silicon is one-eighth that of monocrystalline silicon, and the mobility of amorphous silicon is one-hundredth that of monocrystalline silicon. The operating speed of a microelectronic device is linearly proportional to the mobility. The low mobility of polycrystalline and amorphous silicon limits the operating speed of the electrode driver transistors and thereby the displays in which they are incorporated. These devices generally operate at less than 60 Hz, which is a common video display speed. In order to accommodate the leakage current and refresh time of transistors fabricated in polysilicon for 60 Hz operation, two transistors are connected in series to obtain sufficiently high impedance and low current.