Gas Flow Patterns in Horizontal Epitaxial Reactor Cells Observed by Interference Holography

Abstract

Interference holography is used to visualize gas flow patterns and temperature profiles in epitaxial systems. It is demonstrated that in water-cooled horizontal reactor cells the carrier gases H2 and He give dynamically stable laminar flow profiles throughout the reactor. There is no indication of a stagnant or boundary layer for flow velocities up to 80 cm/sec in this type of cell. In air-cooled cells, H2 and He also give stable laminar flow profiles, but beyond velocities of 40 cm/sec a cold gas finger appears in these flows due to undeveloped flow and temperature profiles. In contrast to the stable flow characteristics of H2 and He, the flows of N2 and Ar always are unstable due to convective motions. Besides this intrinsic instability, these flows are accompanied by severe entrance effects (especially undeveloped flow profiles), which dominate the flows for flow rates higher than 4 cm/sec. This is observed for both air-and water-cooled cells. Another phenomenom which is observed for N2 and Ar is that beyond 4 cm/sec the convective gas breaks up into a thin (8 ram) laminar layer close to the susceptor across which the entire temperature gradient is present and a highly turbulent/convective part above this laminar layer. Analysis shows that for Ar and N~ about eight times longer entrance lengths in the reactor cell are needed to achieve fully developed velocity and temperature profiles as compared with H.2 and He. This explains the dominant influence of this effect on the flows of Ar and N~. When the influence of the entrance effect on the profiles is taken into account, all the observed flow patterns and temperature gradients are in agreement with the theoretical flows which can be predicted on basis of the respective Reynolds and Rayleigh (or Grashof) numbers.Although chemical vapor deposition of silicon is widely used, the flow characteristics and the temperature profiles inside the epitaxial reactor are still a point of discussion. The system which is best studied in this connection is the horizontal epitaxial reactor cell. For this type of cell Eversteijn et al. (1, 2) studied the flow pattern of a stream of hydrogen in a horizontal water-cooled reactor by looking at the streamlines of a smoke of TiO2, obtained from TiCl~ and H20. From these experiments Eversteijn developed the stagnant layer model in which an immobile thin layer of gas of a few millimeters thickness was assumed to be present just above the heated susceptor. Ban (3, 4), in addition to flow visualization experiments with TiO2 smoke, measured temperature profiles with a moving thermocouple, while concentration measurements were performed by mass analysis. He concluded that the flows and temperatures in an air-cooled reactor were complicated by entrance effects. Sedgwick (5) determined temperature and concentration profiles by means of Raman scattering. He also used an air-cooled reactor and came to the conclusion that temperature profiles did depend on the position along the susceptor. In addition a steep temperature gradient was measured ab...

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