PERFORMANCE OF GAS SATURATORS IN THE PRESENCE OF EXIT STREAM TEMPERATURE-GRADIENTS AND IMPLICATIONS FOR CHEMICAL-VAPOR-DEPOSITION SATURATORDESIGN

When a carrier ps is passed through a ps saturator at temperature T0 c ontaining liquid or solid reagent under conditions leading to its satu ration with an equilibrium vapor pressure p-degrees of the reagent, an d then into a tube where the exit stream temperature is increased to T (infinity), the downstream partial pressure of reagent, p(infinity), m ay be less than p-degrees due to thermal diffusion of the reagent driv en by the temperature gradient. The mass transfer process has been mod eled for two cases: (1) a linear temperature gradient over a tube leng th L2 and (2) a tube length L1 of constant temperature T0 followed by a tube length L2 with a linear temperature gradient. Solution of case (1) is defined by the dimensionless Peclet number Pe2 = vL2/D and a di mensionless ''thermal diffusion number'' Td = alphaln(T(infinity)/T0), where v is average ps velocity in the tube, D is the reagent gas-carr ier gas binary diffusion coefficient, and alpha is the reagent gas-car rier gas thermal diffusion factor. Solution of case (2) is characteriz ed by Td, Pe2, and Pe1 = vL1/D. Chemical vapor deposition processes us ed in the fabrication of electronic and optoelectronic semiconductor d evices require the reproducible control of reagent partial pressures t o better than +/-0.4%. Gas saturators for these processes are commonly operated under conditions where p(infinity) is dependent on both flow rate and the downstream temperature profile, and where p(infinity) ca n be as much as 10% lower than p-degrees. The reproducible control of reagent partial pressure is best brought about by designing the satura tor system so that Pe1 > 10, conditions leading to p(infinity) congrue nt-to p-degrees independent of flow rate or downstream temperature.