Fundamental theoretical studies of gas phase and surface kinetics in the chemical vapor deposition of microelectronic materials

R Ravi, Purdue University

Abstract

Gas phase and surface kinetics play an important role in determining the relationships between processing parameters and material quality in Chemical Vapor Deposition (CVD) reactors. In this work, the focus is on two important aspects of kinetics in CVD systems: Unimolecular decomposition in the gas phase and chemisorption. Unimolecular decomposition of a source species is often the first step in the sequence of reactions in the gas phase and has been found to be of primary kinetic significance in CVD systems. Phosphine decomposition is important in the in situ CVD of phosphorous-doped silicon and in the CVD of III-V materials containing phosphorous. Using the master equation formalism of Gilbert and co-workers, we compute the rate constants for phosphine decomposition under both polysilicon and epitaxial silicon deposition conditions; rate constant values are provided for decomposition at 900 K and 1300 K in a pressure range of 0.2 to 760 Torr. It is shown that the hydrogen molecule elimination channel dominates over the hydrogen atom removal channel under all conditions studied. Significantly, our rate constant values are about three orders of magnitude lower than the upper bound estimates reported in the literature. These differences stem primarily from our accounting of the barrier to reverse reaction of the hydrogen molecule elimination reaction. A dimensional analysis of the species balance equation shows that phosphine decomposition could influence reactor performance significantly at pressures greater than about 10 Torr at 1300 K. Chemisorption is the first in the sequence of steps that take place at a substrate surface. The change in energy that accompanies chemisorption, called the chemisorption energy, of a species is crucial to determining the course of surface reactions. Using a surface molecule model for chemisorption, we calculate the chemisorption energies of H and Cl on a Si (111) surface to be 4.1 eV and 5.3 eV respectively. These values are higher than those obtained from theoretical calculations that emphasize the band structure of silicon. The presence of reactive sites that provide a channel for the strong local bonding of the adsorbates seems to favor the surface molecule picture.

Degree

Ph.D.

Advisors

Takoudis, Purdue University.

Subject Area

Chemical engineering

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