Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Utility Patent
1999-02-11
2001-01-02
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S486000, C438S487000, C438S488000
Utility Patent
active
06169013
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates generally to thin film transistor (TFT) processes and fabrication, and more particularly, to a polycrystalline film, and method of forming the polycrystalline film, from a microcrystalline film.
The demand for smaller electronic consumer products with higher resolution displays, spurs continued research and development in the area of liquid crystal displays (LCDs). The size of LCDs can be decreased, and the performance enhanced, by incorporating the large scale integration (LSI) and very large scale integration (VLSI) driver circuits, presently on the periphery of LCDs, into the LCD itself. The elimination of externally located driving circuits and transistors will reduce product size, process complexity, a number of process steps, and ultimately the price of the product in which the LCD is mounted.
The primary component of the LCD, and the component that must be enhanced for further LCD improvements to occur, is the thin film transistor (TFT). TFTs are typically mounted on a transparent substrate such as quartz or glass. TFT performance is improved by increasing the electron mobility in the devices. Increased electron mobility results in brighter LCD screens, lower power consumption, and faster transistor response times. Many of these performance enhancement features are due to the improved switching characteristics associated with TFTs. In addition, further LCD enhancements require uniform TFT performance. That is, display and driver transistors across the entire display must operate at substantially the same level of performance.
The carrier mobility of transistors formed from amorphous silicon is poor, insufficient for LCD circuit driver circuits. The carrier mobility of transistors is improved by using crystallized silicon. Uniformity in transistor performance requires that the crystalline film, from which the TFTs are formed, include areas of substantially uniform crystalline structure. Ideally, the crystalline film from which the TFT semiconductors are made would be crystallized in one uniform crystallographic pattern. But, single crystal silicon films, for use on LCDs are difficult to fabricate when adhered to relatively fragile transparent substrates.
A workable compromise between single crystal films, and amorphous silicon are polycrystalline silicon films, also referred to herein as polycrystalline films. Typically, polycrystalline films include multiple areas of crystallization that are adjacent, but of different crystallographic orientations. That is, the film is composed of many different crystallized areas with somewhat random shapes and random crystallographic orientations. Improved performance across a polycrystalline film is enhanced by making larger areas of uniform crystallization. The average size of the areas of uniform-grain crystallization in a polycrystalline film is referred to in the art as the grain size or average grain size of the film.
Large areas of uniform crystallization inside the polycrystalline film (i.e., large grain size) improves the uniformity of the performance of the film. In addition, the performance of transistors manufactured from the polycrystalline film can be enhanced by decreasing the number of grain boundaries, or areas of intersecting between different crystal grains. The boundary areas between crystal grains form electron traps which reduce electron mobility in TFITs. As a result, the device stability is decreased as the threshold voltages and leakage currents of such devices are increased.
One problem in making polycrystalline film with large grains, and therefore improved TFTs, is that conventional silicon deposition methods used on glass yield only amorphous material. Another problem is the relatively low temperatures that the glass and quartz substrates are able to withstand before degrading. Typically, the transparent substrate is covered with a film of amorphous matter such as silicon or a silicon-germanium compound. The amorphous matter is heated, or annealed, so that the amorphous material takes on a crystalline form. Typically, the annealing process is limited by the requirement that the amorphous material not be heated above a temperature of approximately 600° C. Above that temperature the transparent substrates are often damaged.
Various annealing methods exist for turning amorphous silicon into polycrystalline silicon. Solid phase crystallization (SPC) is a popular method of crystallizing silicon in a furnace. In this process, amorphous silicon is exposed to heat approaching 600° C. for a period of at least several hours. The heat is typically generated from a resistive heater heat source, although SPC can also be induced by other annealing methods well known to those skilled in the art, some of the annealing methods being described below. A rapid thermal anneal (RTA) heats the film to a higher temperature than non-RTA furnace annealing, but for very short durations of time. The amorphous film and transparent substrate are mounted on a relatively low temperature heated surface, or susceptor. The silicon is rapidly heated to a temperature in the range between 700° C. and 800° C.; the substrate heats more slowly which avoids degrading the transparent substrates upon which the film is mounted. One method of performing this anneal is using the IR rays of a heat lamp, such as a halogen heat lamp. Annealing a silicon film with a heat lamp is alternatively referred to herein as lamp anneal or lamp annealing.
An excimer laser process (ELC), such as, for example, excimer laser anneal (ELA) or laser anneal, has also been used with some success in annealing amorphous silicon. Excimer laser energy or laser energy directed at the silicon film allows areas of the amorphous film to be exposed to very high temperatures for very short periods of time. Theoretically, this offers the possibility of annealing the amorphous silicon at its optimum temperature without degrading the transparent substrate upon which it is mounted. However, use of this method has been limited by the lack of control over some of the process steps. Typically, the aperture size of the laser is relatively small. The aperture size, power of the laser, and the thickness of the film may require multiple laser passes, or shots, to finally anneal the silicon. Since it is difficult to precisely control the laser, the multiple shots introduce non-uniformity's into the annealing process.
For convenience herein the terms excimer laser process, excimer laser anneal (ELA), and laser anneal will be used interchangeably to mean heating by means of laser energy. It should be understood that when reference is made to ELA or excimer laser anneal or processes the laser used need not be limited to pulsed or intermittent operation and that the present invention described herein can employ continuous laser energy in every step where ELA or other laser processes are specified.
The physical processes which take place when amorphous silicon is annealed to form crystallized silicon is not entirely understood, and research on the subject continues. Variations such as temperature, film thickness, the degree to which the amorphous matter melts, impurities in the film, and a range of other factors influence the annealing of amorphous silicon. Generally, the largest grains of crystallization occur in a polycrystalline film at a specific temperature near the melting point. Temperatures below this preferred temperature do not melt the amorphous silicon enough to form large grain areas. Temperatures above the preferred temperature rapidly lead to bulk nucleation. The bulk nucleation of amorphous matter results in relatively small grain sizes and poor quality polycrystalline silicon.
The method of depositing amorphous silicon on the transparent substrate is also crucial in the fabrication of polycrystalline films having large crystal grains. Chemical vapor deposition (CVD) processes are available for depositing silicon on a transparent substrate. In a suitable CVD chamber the substrate is mounted on a heated susceptor. The
Elms Richard
Lebentritt Michael S.
Rabdau Matthew D.
Ripma David C.
Sharp Laboratories of America Inc.
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