Experimental investigation of a supersonic jet in near vacuum*

Most semiconductor processes are carried out in vacuum conditions due to purity requirements, deposition efficiency, and plasma generation. Although the overall conditions are near vacuum, air jets are utilized for various processes within vacuum process chambers. These jets expand rapidly and propagate at a very fast speed when they enter the vacuum environment, and the detailed flow physics of this process are not well understood. Due to the difficulty of measuring such flows in a vacuum environment, previous work has relied mostly on computational simulations, and there is a strong need for experimental validation. This study quantitatively visualized the flow structure of a round supersonic free jet at various vacuum conditions, utilizing particle image velocimetry (PIV) with ultra-fine tracer particles tens of nanometers in size. In addition, the shock structure was imaged via shadowgraphy. These results will provide a baseline for further studies of jets in near vacuum environments.