The solid phase crystallization behaviors of amorphous silicon thin films by conventional furnace heating and by microwave heating were investinagated. For conventional furnace crystallization, amorphous silicon thin films were deposited by plasma enhanced chemical vapor deposition method varing deposition temperature from 100 to 400℃, and annealed at 600℃ in nitrogen. With raised the deposition temperature, the nucleation rate and grain growth rate were increased. The nucleation rate showed strong dependence on deposition temperature. However, the nucleation activation energy was independent of the deposition temperature with a value of 3.8 eV. The results suggest that the pre-exponential factor in the Arrhenius equation is dependent on the deposition temperature. The dependence of the nucleation rate on deposition temperature was mainly due to the increased short range disorder during hydrogen evolution. The grain size was increased by introducing a double-layer structure of amorphous silicon film, where the nucleation occurred other than at $Si/SiO_2$ interface.
For crystallization using microwave heating, the amorphous silicon films were also deposited at the temperature of 100-400℃ and annealed at 550℃. Microwave heating lowered the annealing temperature and reduced the annealing time for complete crystallization. For example, the a-Si film deposited at 400℃ was fully crystallized in 3 h at 550℃. The lower temperature crystallization of a-Si films compared to conventional furnace heating is due to the interaction between microwave and silicon atoms. With microwave heating, the hydrogen in the amorphous silicon films diffused out very quickly, but there was no change in short range disorder following hydrogen evolution. The deposition temperature dependence of crystallization and final grain size were smaller than that of conventional furnace heating because there was no change in structural disorder after quick hydrogen evolution. The grain size of the crystallized silicon films was in the range of 0.55 to 0.78㎛, depending on the deposition temperature. These grain size was not so small compared to those of silicon films crystallized by conventional furnace heating, while the crystallization time is much shorter.
After de-hydrogenation by conventional furnace heating at 450℃ for 6 h, the short range disorder of amorphous silicon films was increased following hydrogen evolution. The crystallization behavior of de-hydrogenated silicon films shows stronger dependence on deposition temperature than that of silicon films without de-hydrogenation process. The dependence of the grain size of crystallized silicon films on deposition temperature was also increased. From these results, it is possible to separate the effects of short range disorder and long range disorder on the crystallization behavior of silicon films, by using microwave heating.
The crystallization behavior of LPCVD amorphous silicon was also investigated by microwave heating varying annealing temperature. The kinetic parameters of crystallization ie. the transient time and characteristic time were one order of magnitude smaller than that of crystallization by conventional furnace heating. The activation energy of the transient time and characteristic time were 3.53 and 2.8 eV, respectively. The poly-Si thin film transistors were fabricated with crystallized silicon films. The characters of devices were similar to those of ploy-Si TFTs fabricated with crystallized silicon films by convention furnace heating at 600℃.
The amorphous silicon thin films deposited on glass were crystallized by microwave heating. Since glass does not absorb microwaves, an additional heat source for the glass substrate was needed.
