Modeling of epitaxial silicon growth in pancake chemical vapor deposition reactors

In-Hwan Oh, Purdue University

Abstract

Epitaxial silicon growth is the formation of a thin single crystalline silicon film on a substrate by chemical vapor deposition (CVD). Such a growth is an important process in the fabrication of bipolar, CMOS, and other integrated circuits. A fundamental understanding of pancake reactors is required for the establishment of process-property relationships in them and hence, a better control of operating conditions for desired thin film quality such as growth rate uniformity across the susceptor. A mathematical model of transport phenomena and epitaxial silicon growth from the SiH$\sb2$Cl$\sb2$-H$\sb2$ system at reduced pressures between 40 and 150 torr in pancake CVD reactors is presented for the first time. Axisymmetric three-dimensional conservation equations of mass, momentum, energy, and species mass in cylindrical coordinates along with appropriate boundary conditions are solved with Finite Element Methods. Predicted streamlines show that the shearing force of the inlet flow yields recirculation zones inside the reactor and a separation point on the susceptor, which is in a good agreement with reported results of visualization studies and autodoping experiments. Temperature and concentration profiles show that steep thermal and concentration boundary layers develop above the susceptor. The effects of total gas flow rates, magnitude of the deposition rate constant, susceptor temperature, thermal diffusion, temperature variation across the susceptor, reactor wall temperature, inlet size, susceptor rotation, gravitational force, SiH$\sb2$Cl$\sb2$ feed mole fraction, and reactor pressure on growth rate profiles are investigated. The agreement between predicted and observed growth rates at various susceptor temperatures and 150 torr is seen to be satisfactory in the Purdue pancake reactor, a Gemini-I. The above modeling has been applied to bulk (unpatterned wafers) and selective (patterned wafers) epitaxial growth from the SiH$\sb2$Cl$\sb2$-H$\sb2$-HCl system. It is predicted that the growth rate decreases nearly linearly with HCl/SiH$\sb2$Cl$\sb2$ feed flow ratio for both bulk and selective epitaxial growth. It is also predicted that there is an optimal HCl/SiH$\sb2$Cl$\sb2$ feed flow ratio that results in equal growth rates on unpatterned and patterned wafers. The agreement between experimental and predicted growth rate profiles at different temperatures and gas flow rates for bulk epitaxial growth at 150 torr is seen to be satisfactory for the ranges studied. At atmospheric pressure and high susceptor temperatures, gas flow patterns and growth rate profiles show turbulent behavior in a larger pancake reactor, the Gemini-II. Preliminary modeling predictions are seen to be in satisfactory agreement with data obtained at Delco Electronics.

Degree

Ph.D.

Advisors

Takoudis, Purdue University.

Subject Area

Chemical engineering

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