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
The molecules of the gas
being analyzed are turned into ions by an
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
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
for detecting ions at less sensitive ranges, while ion detection at
higher sensitivity would require electron multipliers.
are usually represented as a chart with the
on the x-axis and the
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
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.
FA Techniques; Hermeticity Testing;
Equipment; Basic FA
Package Failures; Die
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