PARTICLE-SIZE DISTRIBUTION IN A LOW-PRESSURE SIH4-O2-HE CHEMICAL-VAPOR-DEPOSITION REACTOR - EXPERIMENTAL AND NUMERICAL RESULTS

Particle number concentrations and size distributions were measured ne ar a heated wafer in a low pressure, SiH4:O-2:He chemical vapor deposi tion reactor. The wafer was heated to 450 degrees C and the pressure w as varied from 650-3250 Pa. The measurements were compared with 2D num erical simulations of particle formation, growth, and transport. A key feature of the model is that pressure-dependent cluster sticking coef ficients are needed to explain the pressure-dependence of the aerosol size distribution. The pressure dependence of the calculated and measu red aerosol size distributions are in good agreement. At a height of 2 cm above the wafer and at a reactor pressure, P, of 650 Pa, the simul ated particle concentration is 10(4) [No./cm(3)], increasing to about 10(8) [No./cm(3)] at 1950 Pa, indicating that particle nucleation rate s depend on approximately P-9. Particle formation rates were shown to become negligible below about P = 650 Pa. At P = 2600 Pa the size dist ribution was unimodal at a height of 2 cm above the wafer and became t rimodal at a height of 7 cm above the wafer. This trimodal size distri bution occurs because particle nucleation occurs both near the center of the wafer as well as near the edge of the wafer, where the temperat ure gradients are steep.