Restgase

Restgase is a term used in vacuum technology to describe the small fraction of gas molecules that remains inside a sealed volume after evacuation. In practice, it refers to the residual gas present at very low pressures, whose composition depends on chamber materials, prior processes, and the presence of leaks or permeation. In German-speaking contexts the word Restgase is common, while English literature often uses residual gas.

The composition of restgase typically includes water vapor, hydrogen, nitrogen, carbon monoxide, carbon dioxide, hydrocarbons, and sometimes oxygen or noble gases. The relative amounts are governed by outgassing from surfaces, desorption of adsorbed species, diffusion through seals, and occasional leaks. Temperature changes and chamber history can significantly alter the residual gas mix over time.

Measuring and monitoring restgase is essential in high and ultra-high vacuum systems. Residual Gas Analysis (RGA) using a mass spectrometer is commonly employed to identify dominant gas species, while ionization gauges provide overall pressure readings. The presence of certain gases can influence surface chemistry, contaminate coatings, or increase background signals in sensitive experiments.

Reduction and control of restgase involve bake-out to desorb adsorbed species, selection of low-outgassing materials, proper sealing and cleaning, and maintaining adequate pumping capacity. Additional techniques include gettering and using staged pumping (roughing, turbomolecular, and ion pumps) to achieve lower base pressures. Understanding restgase is crucial for applications in semiconductor fabrication, surface science, electron microscopy, and other disciplines relying on stable vacuum conditions.