Transient analysis of carrier gas saturation in liquid source vapor generators

Bubbler-based chemical vapor generators find wide application in the fabric ation of electronic devices. Bubblers generate a carrier-,gas + chemical-va por mixture to supply the device-processing chamber with reactive chemicals . The ''dry'' carrier gas bubbles through the liquid source chemical, becom ing "humidified" with the chemical vapor in the process. This physical-theo ry-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 pull s free of the sparger tube, to when the bubble breaks the liquid surface. D uring this time period liquid evaporates at the bubble edge, and the vapor then diffuses toward the bubble center. The degree of saturation depends up on the: vapor diffusivity (D-nu), carrier-gas distribution holes size (D-nu ), liquid surface tension (sigma), liquid density (p(t)), and liquid viscos ity (nu). Analysis of a bubbler with 0.51 mm N-2 carrier-gas distribution h ole shows that: the bubbles are approximate to 2 mm in diameter, the bubble s reach a terminal velocity of approximate to0.24 m/s, with Reynolds number s in the range of 290-1403. Under these conditions the bubbles reach > 99% saturation after about 45 ms of liquid exposure, while vertically traversin g 4-10 mm of the liquid. Sensitivity ratios of the 99% saturation liquid he ight 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 su rface tension, and liquid density; and 0.1:1 for viscosity. Similar sensiti vity 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, an d liquid density; and 0.0:1 for viscosity. (C) 2001 American Vacuum Society .