ROLE OF ETCH PRODUCTS IN POLYSILICON ETCHING IN A HIGH-DENSITY CHLORINE DISCHARGE

For low-pressure, high-density plasma systems, etch products can play a significant role in affecting plasma parameters such as species conc entration and electron temperature. The residence time of etch product s in the chamber can be long, hence depleting the concentration of the reactants, and leading to a decrease in etch rate. We use a spatially averaged global model including both gas phase and surface chemistry to study Cl-2 etching of polysilicon. Etch products leaving the wafer surface are assumed to be SiCl2 and SiCl4. These species can be fragme nted and ionized by collisions with energetic electrons, generating ne utral and charged SiClx products (x = 0-4). Two limiting cases of the etch mechanism are found: an ion flux-limited regime and a neutral rea ctant-limited regime. The high degree of dissociation in high-density plasmas leads to the formation of elemental silicon, which can deposit on the chamber walls and wafer surface. We include surface models for both the wall and the wafer to better understand the role of etch pro ducts as a function of flowrate, pressure, and input power. A phenomen ological model for the surface chemistry is based on available experim ental data. We consider the two limiting conditions of nonreactive and reactive walls. These models are perfectly reflective walls, where al l silicon-containing species are reflected; and reactive walls, which act as reactive sites for. the formation of SiCl2 and SiCl4 etch produ cts. The two limiting conditions give significantly different results. A decrease in the absolute atomic silicon density and a weaker depend ence of etch rate flowrate are observed for the reactive wall.