Transport processes in materials processing plasmas: Particulate behavior and neutral gas transport.

摘要

This research work focuses on two transport problems in low pressure plasma reactors which are used in thin film manufacturing processes. Computer simulation techniques are used to study particulate behavior in processing discharges and neutral species transport in high plasma density discharges.; Particulate behavior is predicted by combining models of charging and transport with numerical plasma simulation. A charged particulate is influenced by discharge electric fields and by momentum transfer collisions with drifting plasma species. A particulate is also subject to other forces including neutral gas drag, thermophoresis, and gravity. For radio frequency capacitively coupled discharges, several forces which act on a particulate may be of comparable magnitude. This results in particulate trapping at plasma-sheath boundaries in many cases. For high plasma density discharges, high ion fluxes make the ion drag force dominate particulate behavior. This means that it is more difficult for particulates to be suspended in the plasma, compared to the situation in parallel plate systems. However, particulate contamination of a wafer can still occur in high density plasmas because particulates may be ejected from chamber walls and reach the wafer after residing very briefly in the gas phase.; The direct simulation Monte Carlo method is applied to follow transition regime neutral gas transport in high plasma density processing discharges. Three effects are evaluated: neutral depletion by ionization; neutral heating by collisions with energetic plasma species; and gas flow and pumping. These effects are important for discharges that operate at relatively high fractional ionization. Results show the magnitude of these effects in an electron cyclotron resonance reactor and in an inductively coupled reactor operated under a range of typical conditions.; The neutral gas transport simulation is extended to investigate neutral beam processing. A high density inductively coupled plasma is used as a source of energetic directed neutral molecules which take part in the etching of a wafer in a downstream chamber. The transport simulation shows how the chamber geometry and operating conditions should be set to balance conflicting demands for process rate, anisotropy, and uniformity.