Residual Gas Analysis 101
Definitions:
AMU: Atomic Mass Units, e.g. nitrogen = 28, water vapor = 18.
Atmospheric Gas Analysis: This is worthy of a more detailed
discussion, but essentially is an RGA with differential pumping.
Background: The base partial pressure levels that exist in the
chamber before any gas(es) or outgassing sources (such as
substrates or parts) are added back in. Often users will
perform a "background subtract" using the RGA software to see
only the "new" partial pressures after turning gas flow on
and/or inserting a new batch of parts in the chamber.
Collector: The part of the analyzer "head" which accepts the gas
molecules which have not been filtered out by the quadrupole
rods.
De-gas: A feature on most RGAs which enables the user to
force off built up matter and gas particles from the analyzer
rods and collector(s).
Differential Pumping: Most RGAs cannot operate above 10-5 or
10-4 Torr because the mean free path is too short: too few if
any gas molecules can travel to the collector before colliding
with another molecule. Differential pumping, most commonly
a turbo pump backed by a dry roughing pump, is used to reduce
the pressure down to a workable range.
Electron Multiplier: This solves the opposite problem presented
by atmospheric gas analysis - below roughly 10-8 or 10-9, there
are too few residual molecules available to use only a Faraday
Cup, so an electron multiplier is added to boost the signal.
Faraday Cup: The most common type of collector used in RGAs.
Fingerprint: The summary of the M/E values that comprise any
given chamber condition, whether it be, e.g., "clean and dry,"
"due for maintenance," or "during process run."
M/E: Mass to Energy or Mass to Charge ratio. This is what the
RGA is actually measuring, and what is reported on the
controller display or PC screen.
Partial Pressure: the part of the whole of the residual gas that is
is contributed by any one gas species (water vapor, nitrogen,
oxygen, etc.). This can be expressed in a pressure unit such
as Torr or Pascals, or in PPM (parts per million).
Peak to Valley Resolution: This is a measure of how clearly the "
RGA can distinguish between neighboring peaks. With poor
resolution a large peak will overwhelm its neighbor peak such
that the neighbor peak value cannot be determined.
Quadrupole: Most RGAs use a four-rod scheme for discerning
between the various M/Es as it scans through its range.
Range: Most RGAs are classified by their "range", meaning the
lowest to highest M/E it can detect. The most common is
1-100 but RGAs are also commonly available in 1-65, 1-200,
and 1-300. The number of suppliers of RGAs above 1-300 is
limited. With some RGAs the low end of the range is not
always 1; it might be 2 or higher depending on its "zero blast"
performance.
Residual Gas: the trace amounts of the various gas species that
remain after your chamber has been evacuated.
Zero Blast: The M/E burst that occurs at or around "0" AMU upon
the start of a scan. Often it is big enough to overwhelm the
real contribution of M/E 1 (atomic hydrogen), and 2 (hydrogen).
General Comments on RGAs:
1) Everyone involved with high vacuum should at least have access to an RGA for basic leak detection and troubleshooting.
2) Every critical chamber you use should be fingerprinted under various conditions so you can compare the current condition to "normal" condition.
3) For practical purposes it is best to have an RGA analyzer "head" dedicated to each critical chamber if possible, installed behind an isolation valve so it stays clean between uses. This way it is ready to go as soon as you have a need. The electronics can be moved from head to head if you need to share for cost reasons.
4) Most common M/E values in vacuum systems:
2 = H2 / hydrogen
4 = He / helium
18 = H2O / water vapor
28 = N2 / nitrogen
32 = O2 / oxygen
40 = Ar / argon
44 = CO2 / carbon dioxide
57 = xxx / hydrocarbon pump oil
69 = xxx / PFPE oil (e.g. Krytox, Fomblin)
Please feel free to contact us with any questions on RGAs. IES Technical Sales partners with Hiden Analytical to provide a wide variety of gas analysis solutions from basic RGA to Plasma Energy Analysis.
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