Chapter 2: Basic Device Physics
2.7 Poly-Silicon Recrystallization
In order to increase TFTs performance, various techniques have been used for crystallize a-Si. Historically, solid phase crystallization was the first technology to produce poly-Si films for display applications, followed by rapid thermal annealing crystallization process, metal-induce lateral crystallization process, and laser annealing crystallization. These four major methods of recrystallization are describe as follow:
2.7.1 Solid-Phase Crystallization Process Technology
The most direct method of obtaining poly-Si films from initially amorphous precursor-Si films is via SPC (Solid-Phase Crystallization) in a furnace environment.
Amorphous silicon is a thermodynamically metastable phase possessing a driving force for transformation to polycrystalline phase given sufficient energy to overcome the initial energy barrier. Solid-phase crystallization can be accomplished within a wide annealing temperature range that requires a similarly wide range of annealing times. The relationship between annealing upon the microstructural details of the precursor-Si film, different annealing times have been observed at the same annealing temperature. A key factor affecting crystallization is the nucleation rate in the precursor-Si film. The nucleation rate is strongly influenced by the selected deposition method and conditions. The structural order/disorder in the precursor film
affects the ability of the film to form supercritical nuclei when subjected to thermal annealing.
The structural order is, in turn, affected by deposition parameters such as temperature and deposition rate. As the temperature decreases and the deposition rate decreases, films are formed having a higher degree of structural disorder.
Despite the successes of traditional SPC methods in increasing grain size and reducing crystallization temperature, the crystallization time typically required for complete transformation tends to be rather long. The crystallization-time issue is further compounded by the use of PECVD as the deposition method for the precursor-Si film. The selection of this technique, even though suboptimal, has been driven by the prior application of PECVD technology in a-Si TFT fabrication.
2.7.2 Metal-Induce Lateral Crystallization Process Technology
Recently, the metal-induced lateral crystallization (MILC) process has been studied widely for polycrystalline silicon thin film transistor applications. Compared with the conventional solid-phase crystallization process of amorphous silicon, MILC process offers the advantages of lower annealing temperature and better crystallization film. In addition, poly-Si films crystallized by the MILC process can be used as the basis for developing the low cost integrated circuits on glass substrate. At present, nickel and palladium have been used to induce lateral crystallization of a-Si:H film. Experimental annealing temperatures and MILC rates obtained for Ni and Pd, respectively. However, the annealing temperature
( <=500oC) is still too high for poly-Si TFT devices to be fabricated on conventional glass substrate, and the low MILC rate, gold (Au) has been employed to induce lateral crystallization of a-Si:H film owing its lower eutectic temperature (363oC). The crystallization of Au/a-Si:H film is observed starting from annealing treatment at 175oC, which is a much lower crystallized temperature than for Ni and Pd (500oC). After the discovery of Au-MILC where microtwin-free Si grains are obtained, MILC also has been successfully applied to the low-temperature fabrication of high-mobility N-channel TFTs.
2.7.3 Rapid Thermal Annealing Crystallization Process Technology
To obtain the poly-Si crystalline phase, laser crystallization can be used with very good results, but the process is expensive and difficult to control. On the other hand, for similar results, furnace annealing requires lower temperature and is much simple, can be better checked, and is cheaper. So, to achieve desirable material properties for the poly-Si films, RTA has been used in this work, thermal crystallization of amorphous silicon. For the Si films annealed at 750oC or higher, using RTA, the grain average sizes are reduced whereas the electron/hole mobility are increased. This indicates that the poly-Si film electrical properties depend not only on the grain size, but also on the crystalline quality of the grains.
Moreover, it appears that the large amount of crystalline defects remaining in the so-called
“grains” of the films annealed at 600oC (SPC) are partially annihilated when the films are annealed at higher temperatures. With regards to the TFTs electrical characteristics, the work suggests combining SPC and RTA steps to obtain TFTs with improved electrical performance.
2.7.4 Excimer Laser Annealing Crystallization Process Technology
Pulsed excimer laser annealing is also being investigated as an alternative crystallization technique to replace furnace annealing. For fabricating high performance poly-Si TFTs on a glass substrate, excimer laser crystallization method is very promising for the following reasons. First, it is a low-temperature process introduced no serious thermal shrinkage of the glass substrate caused by the effects of the short pulse and large absorption coefficient of silicon in the UV light regime. Secondly, it can crystallize the film selectively by partially irradiating the film surface, so both poly-Si TFTs and amorphous Silicon TFTs can be formed on the same substrate. The laser process heats the thin silicon film to the melting point on a short time scale (tens of nanoseconds) that allows the film to melt and recrystallized without significantly heating the glass substrate. Since this process achieves higher annealing temperatures than a conventional furnace annealing, significantly
higher-quality poly-Si films can be obtained.