THE EFFECT OF PHOTOABSORPTIVE CONVECTION ON THE RATE OF CW-CO2-LASER-INDUCED CHEMICAL-VAPOR-DEPOSITION OF AMORPHOUS HYDROGENATED SILICON

The dependence of deposition rate on geometric meters, that is, the be am diameter and the distance between the beam axis and the substrate, has been investigated for the case of silicon deposition from the mixt ure of monosilane with argon under the action of a cw CO2 laser. Gas t emperatures were directly measured by thermocouples in the vicinity of the laser beam to clarify the nature of the observed dependences. The results of these measurements were compared with the results of compu ter simulation. The calculated gas temperature as a function of the di stance between the beam axis and the substrate agreed well with the ex periment, whereas the calculated dependence of the gas temperature on the beam diameter contradicted the measured one. On the basis of the l iterature data and our results a conclusion has been made that the dec rease of deposition rate observed as the beam diameter decreases (with the power of the beam being constant) is caused by more intense photo absorptive convection. It was pointed out that the convective cooling of the gas should be taken into account in order to establish an adequ ate theoretical model of the considered process.