Expansion transport regime in pulsed-pressure chemical vapor deposition

Control of transport processes is of critical importance in chemical vapor deposition (CVD), yet conventional steady carrier-gas flow presents challenges in scalability, deposition uniformity and process control. This paper describes the precursor delivery dynamics of pulsed-pressure CVD (PP-CVD) and proposes a mass transport regime in which expansion effects dominate over continuum effects. A modified continuum breakdown parameter for unsteady expansion is presented. Using this parameter an expansion mass transport regime is identified in which unsteady expansion effects become significant compared to continuum flow effects. Experiments using the naphthalene sublimation technique demonstrate the relationship between processing parameters and the viscous-expansion regime transition in a simple PP-CVD reactor. Expansion mass transport is described as a means to achieve 3-D deposition uniformity.