A model for transport and agglomeration of particles in reactive ion etching plasma reactors

Abstract

Dust particle contamination of wafers in reactive ion etching (RIE) plasma tools is a continuing concern in the microelectronics industry. It is common to find that particles collected on surfaces or downstream of the etch chamber are agglomerates of smaller monodisperse spherical particles. These observations, and the fact that the forces which govern the transport and trapping of particles are partly determined by their size, place importance on understanding particle growth and agglomeration mechanisms. Since individual particles in plasma etching tools are negatively charged, their agglomeration is problematic since the particles must obtain sufficient kinetic energy to overcome their mutual electrostatic repulsion. In this article, we discuss results from a model for particle agglomeration in RIE plasma tools with which we address the transport of particles and interparticle collisions resulting in agglomeration. These results indicate that the rate and extent of particle agglomeration depend on the particle density, plasma power deposition, and, to a lesser degree, gas flow. The dependence of agglomeration on rf power results from the fact that the kinetic energy of a dust particle is largely determined by its acceleration by ion drag forces. Significant agglomeration may occur in particle traps where the particle density is large.