Residual Gas Analysis (RGA)

Residual Gas Analysis (RGA) is an analytical technique used for identifying the gases present in vacuum environments. The equipment used in performing RGA is simply referred to as a residual gas analyzer (using the same acronym, RGA).    

Residual gas analyzers operate by creating a beam of ions from samples of the gas being analyzed.  The resulting mixture of ions are then separated into individual species through their charge-to-mass ratios.  To accomplish these, a typical RGA has three major parts, namely, an ionizer, a mass analyzer, and an ion detector. The output of an RGA is a spectrum that shows the relative intensities of the various species present in the gas.  This output is known as a mass scan or mass spectrum.

The molecules of the gas being analyzed are turned into ions by an ionizer through electron impact ionization, i.e., an electron beam is used to strike the gas atoms to ionize them.  This ionizing electron beam is generated by a hot emission filament and extracted by means of an electric field.  This hot filament is easily destroyed by reactive gases like oxygen, which is why RGA's operate at pressures of less than 10^-4 torr.

The ions from the gas are distinguished from each other in terms of their masses by the mass analyzer of the RGA. There exist various techniques for mass separation, but mass analyzers used in RGA's usually employ the RF quadrupole. The RF quadrupole has four cylindrical rods that are provided with combinations of AC and DC voltages of varying frequency.  Only ions that possess the right mass-to-charge ratio can reach the ion collector for a given applied frequency.  A Faraday cup may be used as RGA detector for detecting ions at less sensitive ranges, while ion detection at higher sensitivity would require electron multipliers.  

RGA mass spectra are usually represented as a chart with the mass-to-charge ratio on the x-axis and the relative intensity on the y-axis. The peaks exhibited by a mass spectrum need to be interpreted properly since these can be ambiguous in certain cases, such as when two different molecules exhibit the same mass. Knowledge of how two different molecules with the same mass would dissociate into smaller fragments of different mass-to-charge ratios (known as cracking patterns) allows absolute identification of the gas.

In the semiconductor industry, RGA is used in identifying gases, vapors, or residues for the purpose of fixing leaks in vacuum systems and eliminating contaminants that cause process problems or product failures. For instance, it is widely used to analyze residual gases inside a hermetically sealed package to pinpoint the cause of problems such as die corrosion.

See Also:  Failure Analysis;  All FA Techniques;  Hermeticity Testing; 

Chromatography;  FA Lab Equipment;  Basic FA Flows; 

Package Failures;  Die Failures