Performance of gas saturators in the presence of exit stream temperature gradients and implications for chemical vapor deposition saturator design
✍ Scribed by William L. Holstein
- Publisher
- Elsevier Science
- Year
- 1994
- Tongue
- English
- Weight
- 933 KB
- Volume
- 49
- Category
- Article
- ISSN
- 0009-2509
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✦ Synopsis
When a carrier gas is passed through a gas saturator at temperature To containing liquid or solid reagent under conditions leading to its saturation with an equilibrium vapor pressure p"of the reagent, and then into a tube where the exit stream temperature is increased to r,, the downstream,partial pressure of reagent, pm. may be less than p0 due to thermal diffusion of the reagent driven by the temperature gradient. The mass transfer process has been modeled for two cases: (1) a linear temperature gradient over a tube length L, and (2) a tube length L, of constant temperature To followed by a tube length L, with a linear temperature gradient. Solution of case (1) is defined by the dimensionless Peel& number Pe, = vL,/D and a dimensionless "thermal diffusion number" Td = cc In (T-/T,,), where o is average gas velocity in the tube, D is the reagent gas+arrier gas binary diffusion coefficient, and a is the reagent gas+arrier gas thermal diffusion factor. Solution of case ( 2) is characterized by Td, Pe 2, and Pe, = uL,/D. Chemical vapor deposition processes used in the fabrication of electronic and optoelectronic semiconductor devices require the reproducible control of reagent partial pressures to better than k 0.4%. Gas saturators for these processes are commonly operated under conditions where p_ is dependent on both flow rate and the downstream temperature profile, and where pm can be as much as 10% lower than p". The reproducible control of reagent partial pressure is best brought about by designing the saturator system so that PeI > 10, conditions leading to prngpn independent of Row rate or downstream temperature.