Simulation of Nanoparticle Transport in a Low Pressure Plasma

Sarah Warthesen, advisors: Steven Girshick (ME), Uwe Kortshagen (ME), Jim Kakalios (Physics)

Plasma enhanced chemical vapor deposition is commonly used to grow films used in the microelectronics and optoelectronics industries. Under certain plasma conditions, particles will form from the gas phase and could potentially be being deposited in the film. The deposition of large "killer" particles could result in defective films, thus particle deposition is undesirable. In solar cell applications, however, improved film properties have been associated with the presence of nanosized particles or crystallites, formed from the gas-phase and deposited on the film substrate. The transport of particles may be controlled by varying deposition conditions such as pressure, temperatures, or gas flow rate. By simulating particle behavior under different conditions, the resulting film composition can be predicted and the plasma parameters can be optimized to produce a film with the desired properties.

A Monte Carlo simulation is being implemented to study the processes of diffusion, gas drag, and thermophoresis (the tendency for particles to move towards cold surfaces in the presence of a temperature gradient) on neutral particles in a plasma. The Monte Carlo method simulates individual collisions between a nanoparticle and the surrounding gas molecules, using weighted probabilities and random numbers to determine the trajectory of the nanoparticle after each collision, while the axial position of the particles is recorded in time. A particle reaching the lower electrode represents deposition of that particle in the film. The transport of several different sized particles are compared. Results indicate that the deposition of larger particles (above 3 nm) can be prevented by applying a temperature gradient large enough to overcome the gas drag force. However, smaller particles (1 nm) are dominated by diffusion, and only slightly influenced by thermophoresis. Diffusion does not prefer any direction, thus approximately half of these small particles are deposited on each electrode.

The above figure shows the particles at three different times during the simulation. The lower plate represents the growing film. At 1.4 Torr, with a gas flow of 140 sccm of H2, a 50 K/cm temperature gradient is applied to silicon particles of 1, 3, and 10 nm diameters. As time proceeds, the 3 and 10 nm particles move with the thermophoretic force and are not deposited on the lower plate. A considerable number of 1 nm particles are deposited due to diffusion.