Stanford Research Systems RGA100, Руководство по эксплуатации и программированию

Страница: 178 / 362

7-2 Introduction
SRS Residual Gas Analyzer
Introduction
Accurate qualitative and quantitative partial pressure measurements can only be assured
by proper tuning of the RGA Head.
Correct calibration of the mass scale is essential during qualitative analysis for the correct
assignment of mass numbers to the different peaks.
The mass resolution of the quadrupole mass filter,
'
m
10%
, must be kept at or under 1 amu
to avoid severe overlap between adjacent peaks. Changes in
'
m
10%
during the
measurements (caused by aging, severe contamination and large temperature changes)
will cause variations in the sensitivity of the instrument and the shapes of the
fragmentation patterns of the molecules, seriously affecting all quantitative
measurements.
For careful quantitative analysis, it is important that the sensitivity of the RGA be
determined for every gas which may be a component of the system and under the same
operating parameters used during the actual measurements. Sensitivity factors change as
a factor of time due to aging and periodic recalibration is necessary.
The gain of the electron multiplier is mass dependent and needs to be determined prior to
performing measurements with the device. The gain characteristics of the multiplier
change with time and periodic recalibrations are very important.
The following sections of this chapter describe several tuning procedures designed to
assure that all the calibration conditions described above are satisfied prior to a set of
partial pressure measurements. All tuning procedures can be executed from RGA
Windows using a set of Tuning Commands built into the program. The RGA On-line
Help files provide all the information necessary to set up and execute the tuning
commands. Users writing their own programs can implement the procedures themselves,
using the RGA Command Set and the instructions of this chapter.
All the tuning procedures require the ability to introduce pure gases (or a mixture of
gases of known composition) into the vacuum system, and a way to measure or calculate
pressures. Bayard-Albert gauges are often used to measure the pressure of the calibration
gases. Hot cathode gauge displays are usually calibrated for a single gas (N
2
or Ar) and a
table of conversion factors will be needed to convert the readings for other gases.
When only a few choices of calibration gases are available choose those that are most
likely to be encountered during measurements (i.e. usual major components of the
residual gas environment of the chamber) and try to sample as much of the RGA’’s mass
range as possible. Obviously, the correct gases will be needed for sensitivity factor and
fragmentation factor determinations.
Important : Tuning should only be attempted after the unit has been warmed up (with
the filament on and under typical operating conditions) for at least one complete hour.