Transient analysis of carrier gas saturation in liquid source vapor generators

Abstract

Bubbler-based chemical vapor generators find wide application in the fabrication of electronic devices. Bubblers generate a carrier-gas+chemical-vapor mixture to supply the device-processing chamber with reactive chemicals. The “dry” carrier gas bubbles through the liquid source chemical, becoming “humidified” with the chemical vapor in the process. This physical-theory-based analysis provides an estimate of the degree of humidification, or saturation, that the dry carrier gas achieves during bubbling. The “single-bubble regime” analysis considers the time period from when the bubble pulls free of the sparger tube, to when the bubble breaks the liquid surface. During this time period liquid evaporates at the bubble edge, and the vapor then diffuses toward the bubble center. The degree of saturation depends upon the: vapor diffusivity (Dv), carrier-gas distribution holes size (Dh), liquid surface tension (σ), liquid density (ρl), and liquid viscosity (ν). Analysis of a bubbler with 0.51 mm N2 carrier-gas distribution hole shows that: the bubbles are ≈2 mm in diameter, the bubbles reach a terminal velocity of ≈0.24 m/s, with Reynolds numbers in the range of 290–1403. Under these conditions the bubbles reach >99% saturation after about 45 ms of liquid exposure, while vertically traversing 4–10 mm of the liquid. Sensitivity ratios of the 99% saturation liquid height to a small change in the controlling variables are approximately: 1.5:1 for vapor diffusivity; [0.93–1.1]:1 for sparger tube hole size, liquid surface tension, and liquid density; and 0.1:1 for viscosity. Similar sensitivity ratios for the 99% saturation liquid-residence time are: 1:1 for vapor diffusivity; 0.67:1 for sparger tube hole size, liquid surface tension, and liquid density; and 0.0:1 for viscosity.