Abstract
In the recent years, micro crystal silicon thin film is more popular on thin film solar cell. Because higher plasma density and lower ion energy, the frequency of PECVD is turning from RF region to VHF region. The goal of our study is to build a VHF-PECVD model and to find out how the plasma behavior change with control variables by simulation. For the VHF plasma in 80 MHz, electron density, electron temperature, and number density of H and SiH3 go to steady state before 60 ms. H and SiH2 are produced by electron collision reaction with SiH4, and SiH3 is produced by the reaction of H and SiH4. Because the different production reaction, the distribution of the density of H, SiH2 and SiH3 is different: the density of H and SiH2 is two-peak distribution and the density of SiH3 is bell-shaped distribution. The plasma is uniform in the radial direction in the range of a radius of 3.0 cm. In one period, the plasma potential changes with the potential of power electrode. The electric field would be created by the voltage between the plasma and electrode, and accelerate electrons and ions. When the power increase, electron density, electron temperature, number densities and fluxes of H, SiH2, and SiH3 are increased. The ratio of H and SiH3 flux is increased, too. By the result, we may say that the deposition rare and crystallization rate would increase with power. It's good for thin film deposition. When the gap between two electrodes is enlarged, the plasma density, number densities and fluxes of H, SiH2, and SiH3 and ratio of H and SiH3 flux are decreased. The result is not good for the deposition rare and crystallization rate. When the pressure is increased, the plasma density and number density of H and SiH2 are decreased. The flux of H, SiH3 and SiH2 and the ratio of H and SiH3 flux are also decreased. It means that the deposition rare and crystallization rate decrease with pressure increased. Only the number density of SiH3 doesn't change with pressure. The plasma density and electron temperature do not change when the SiH¬4 flow rate increase. But the number density of SiH3 and flux of H, SiH3 and SiH2 increase with the SiH4 flow rate. Because the flux of SiH3 increase faster than H, the ratio of H and SiH3 flux is decreased with increased SiH4 flow rate. It makes good deposition rate, but bad crystallization rate. Compare with the case in 27.12 MHz, the result shows that the voltage between plasma and electrode is much lower in 80 MHz. It makes the electron temperature lower, but higher electron density. The SiH3 density in 80 MHz grows faster with power than in 27.12 MHz. It means that the deposition rate is more sensitive with change of power. The ratio of H and SiH3 flux in 80 MHz is higher than in 27.12 MHz. We may say that crystallization rate in 80 MHz is better than in 27.12 MHz. Compare the case with pressure changing in 27.12 MHz and 80 MHz, we can find out that most of the behaviors of plasma have same tendency, beside SiH3 density. The density of SiH3 is increased with pressure in 27.12 MHz, but constant in 80 MHz. The reason might be that the producing rate of SiH3 is increased with pressure in 27.12 MHz but decrease in 80 MHz. From the simulation result, we can say that the deposition rate and crystallization rate are better with higher power, smaller gap, lower pressure and higher frequency. Increasing the flow rate of SiH4 would increase the deposition rate, but decrease the crystallization rate.