Process monitoring with residual gas analyzers (RGAs): limiting factors

Abstract

Many vacuum processes are limited by low-level impurities introduced with process gases or generated within the process chamber. Specific impurities are often monitored using residual gas analyzers (RGAs) or partial pressure analyzers (PPAs), most commonly, mass spectrometers of the quadrupole type. Unfortunately, the performance of these instruments can be affected significantly by a number of instrument and vacuum environment variables, so that the sensitivities of even “calibrated” instruments can differ significantly from expected values. These variables include ion source parameters, total pressure and prior exposure to active gases. The magnitude of the deviations in sensitivity varies for different instruments, but can reach orders of magnitude in extreme cases. This paper reviews the important factors affecting instrument performance, illustrates the magnitude of the effects for different instruments, recommends instrument test procedures, and suggests operating parameters and procedures that can minimize these effects.