Effect of photoabsorptive convection on the rate of cw-CO2-laser induced chemical vapor deposition of amorphous hydrogenated silicon

The deposition rate has been studied as a function of geometric parameters during chemical deposition of silicon from a mixture of monosilane and argon induced by a cw CO₂ laser. The geometric parameters were the beam diameter and the distance between the beam axis and the substrate. Direct thermocouple measurements of the gas temperature were carried out near the laser beam in order to identify the reasons for the observed behavior. The results of these measurements were compared with the results of a numerical simulation. The calculated behavior of the gas temperature as a function of the distance from the beam axis to the substrate agrees well with the experimental data, while the calculated behavior of the gas temperature as a function of the beam diameter does not. It is concluded from an analysis of the results and of data in the literature that the reason for the decrease in the deposition rate with decreasing beam diameter (at a fixed beam power) is an intensification of photoabsorptive convection. Convective cooling of the gas must be taken into account in order to construct an adequate theoretical model for the process.