Total Pressure Measurement Using Residual Gas Analyzers
M. Maskell, Old Dominion University; G. Myneni, P. Adderley, Jefferson Lab;
G. Brucker, Stanford Research Systems; C. Day, Forschungszentrum, Karlsruhe
BACKGROUND
The aim of this work is to improve measurement techniques using Residual Gas
Analyzers (RGAs) through better understanding of the effect of gas species on pressure
measurement. The behavior of ionization gauges (IGs), such as the extractor gauge, is
quite linear into the 10^-11 Torr range; however the accuracy of these instruments suffers
for gases other than the calibration standard, Nitrogen. Different ionization cross sections
may cause greater or lesser sensitivity, and fragmentation may cause ion currents to
become multiplied in the detector, giving total pressures that are too high. Some
instruments, such as Ulvac’s Axtran, include correction factors for various gases in their
documentation. This information improves the accuracy of these devices for single gas
species; however they are often calculated rather than found experimentally with the
device itself, and can only be applied one at a time, thus limiting their usefulness for
gases containing more than one species. Residual Gas Analyzers (RGAs) contain a
quadrupole mass filter, which makes their calibration much more complicated than ion
gauges with relatively static configurations, but also allows them to determine the relative
concentrations of various species of gas and apply the appropriate corrections before
summing to find the total pressure. This method of using an RGA as a total pressure
gauge is cumbersome due to the need for calibration specific to its settings, but in
situations where a high degree of accuracy is needed in a mixed gas species environment,
the benefit may merit the effort. RGAs typically include a total pressure measurement
display in their software; however, responsible manufacturers admonish the user that this
total pressure reading is inherently inaccurate for any gas other than that which was used
to obtain the total pressure calibration factor.
The sensitivity of an RGA is dependent on its ion source configuration and its
operating parameters, including ionization energy (electron energy), focus voltage,
resolution setting, and ion energy. When used with an electron multiplier (as is advised
at UHV pressures), the sensitivity also varies with the mass of the gas being detected and
the age of the detector. Sensitivity is gas dependent, and for a gas g it is defined as (Use
Sg instead of S)
, where I and I0 are the RGA reading at a given partial pressure
and the background reading for that mass peak, respectively, and P and P0 are the
reference gauge readings for the pressure and background pressure. The sensitivity
values presented in this paper are specific to the devices and parameters that were used.
Thus the preferred method for using an RGA as a total pressure gauge is to operate in
conjunction with a reference, such as the SRG and extractor gauge used in this
experiment, so that the sensitivity for each gas can be determined for the particular device
and parameters being used. Once the sensitivity has been found, the extractor can be
removed and the RGA used as a stand alone total pressure gauge so long as the operating
parameters remain the same as when the sensitivity was determined. The gain of the
electron multiplier must remain fixed for sensitivity calibrations to remain valid. It is
therefore recommended that the electron multiplier voltage be routinely tuned to maintain
a constant gain. This procedure is easily accomplished through RGA operating software.
It should be noted that the removal of the extractor changes the system, and thus can
create a source of error for future measurement. A SRG does not alter the vacuum
system, so it is the preferred reference gauge for determining sensitivity. The SRG
measures pressure directly and passively via molecular drag, it does not emit heat or
ionizing electrons into the system.
The following section describes in more detail the experimental setup and
procedure. The basic premise is that the RGA sensitivity is normalized to 1 at ~4E-7
Torr by correcting both the RGA and the other ion gauges to match the SRG, and then
observing the change in sensitivity of the RGA relative to the other ion gauges as
pressure is decreased.
EXPERIMENTAL PROCEDURE
The experiment is performed in a cylindrical stainless steel vacuum chamber with
gauge ports situated symmetrically around the circumference. The gas inlet is designed
such that the gas is introduced at the top of the chamber and equidistant from all gauges.
The chamber is pumped by a Pfeiffer Balzers TPU-180H turbo pump, and backed by a
Varian scroll pump. The gas inlet is connected by a leak valve to a system of inlets
pumped by its own turbo pump, which can be valved off, and backed by a diaphragm
pump. Each of the inlets in this system has a leak valve leading to the gas samples to be
studied. Stable pressures are achieved by establishing a flow through the vacuum
chamber. The gauges installed on the vacuum chamber include three SRS 100 AMU
RGAs, an MKS Spinning Rotor Gauge (SRG), an Ionivac IM 520 extractor gauge, and an
Ulvac Axtran gauge. The RGAs are labeled RGA A, RGA B, and RGA C. RGA A is a
unit that has been used extensively in systems at Jlab, with no factory reconditioning.
RGA B is a used RGA that has undergone factory reconditioning. RGA C is a new RGA.
The RGA resolutions are set to the standard 1 AMU peak width at 10% peak height. All
RGA settings are kept at factory defaults or optimized for the particular device, unless
otherwise noted. The characteristics of the RGAs are shown in Table 1. The same
number of ionization gauges are active in each test (one RGA, the extractor, and the
AxTRAN), so that addition or removal of ion gauges does not cause a temperature
difference between tests.
The SRG does not rely on ionization for its operation, and its mass dependence is
completely calculable, making it easily corrected for by entering the molecular mass into
the SRG controller; therefore, calibrating the extractor with the SRG eliminates these
factors from the extractor reading, making the extractor an acceptable transfer standard to
lower pressures. The RGAs are calibrated using the SRG as the reference gauge under
~10^-6 Torr of nitrogen, and except where otherwise stated, they remain calibrated for
nitrogen. At the beginning of each trial, the extractor and Axtran are corrected to match
the reading of the SRG at approximately 4*10^-7 Torr. Since these gauges are known to
be quite linear into the UHV range, the correction factor applied to accomplish this is
used throughout the trial. The RGA being tested is given a correction from the SRG at
this pressure as well. The sensitivities calculated from the data corrected by the SRG are
normalized sensitivities, all starting at 1 at the highest pressure measured; this allows the
percent change in sensitivity to be easily read from the graphs. The actual sensitivity of
the RGA (when calibrated as above to have a sensitivity of 1 for nitrogen) at the highest
pressure is equal to the inverse of the correction factor determined from the SRG.
Nitrogen, Carbon Dioxide, Methane, Hydrogen and Helium are individually
introduced into the vacuum chamber in order to obtain normalized sensitivity plots as a
function of pressure.
RESULTS
LINEARITY OF SENSITIVITY
The results of the relative sensitivity measurements are shown in Figure 1. While
all of the RGAs show some variations in relative sensitivity for each gas species, these
variations are small for the most part, indicating linear behavior.
FRAGMENTATION EFFECTS
Fragmentation of mass spectra by dissociation or multiple ionization of molecules
complicates pressure measurement for both total pressure gauges and partial pressure
gauges such as the RGA. Gauges such as the extractor detect all ions produced; thus a
single CH4 molecule, for example, may dissociate into two ions, resulting in an ion
current disproportionate to the number of molecules present in the sample. RGAs have
an advantage in that they sort these fragmentary ions into different mass channels,
allowing the user to deal with them. A general quantitative description of the relation
between fragmentary peak heights and pressure is difficult, and this is one reason why
RGAs are considered by some to be solely qualitative devices as explained in the SRS
RGA documentation. If a constant fragmentary pattern can be assumed, however, the
previously discussed calibration procedure can be performed for any desired peak in a
particular gas species cracking pattern. The choice of the peak to use requires intuition
on the part of the user, and some prior knowledge of the likely gas species present. By
correcting the chosen peak based on this procedure and ignoring other peaks from that
species, the RGA can provide a more precise measurement of total pressure than gauges
without a mass filter. This method also provides the user with a means of dealing with
overlapping peaks from different gases. For example, when both CO2 and CH4 are
present, the primary peak of CH4 at mass 16 will have a contribution from the secondary
peak of CO2; in this case, the problem can be circumvented by using the sensitivity
correction for the mass 15 peak for CH4 (which can be assumed to be about 85% of the
sensitivity for mass 16, or can be directly measured), then adding the corrected mass 15
and 44 peaks to get the total pressure.
The above discussion is meaningful only if the fragmentation pattern remains
constant with pressure. Fortunately, for the pressure range measured here at least, this is
the case. Figure 2 shows the abundance of the secondary mass 15 peak relative to the
primary mass 16 peak for CH4 as a function of pressure. Fragmentation percentage may
be altered by adjustment of electron emission current, but should remain constant at least
down to 10^-10 Torr. The linearity of ionization total pressure gauges is further evidence
of constant fragmentary patterns, as a nonlinear fragmentation would result in a nonlinear
relationship between ion current and pressure.
SPACE CHARGE EFFECTS AND RGA EMISSION CURRENT
Space charge effects affect the RGA in two places: the ionizer and the quadrupole.
In the ionizer, the electronic space charge can decrease the sensitivity of the detector by
altering the trajectory of ions that otherwise would have entered the detector. This effect
has been shown to effectively decrease the ionization volume2. In the quadrupole, the
ionic space charge causes the ion beam to become defocused. This effect depends on the
number of ions in the quadrupole, and the time an individual ion spends in the
quadrupole. Because higher velocity ions exit the quadrupole sooner, lower mass ions
experience less defocusing, resulting in higher sensitivities for lower mass ions. Both the
electronic and ionic space charge effects can be reduced by lowering the emission
current. This decreases the number of electrons in the ionizer, and the number of ions in
the quadrupole. Although lowering the emission current also lowers the sensitivity of the
detector, it improves the linearity of the sensitivity, especially at pressures above 10^-7
Torr. The loss of sensitivity due to lowering the emission current can be corrected by
recalibrating the gauge from the standard once the emission current has been changed.
Figure 3 shows the effect of changing emission current on the sensitivity for Hydrogen;
this figure shows that reducing the emission current by a factor of 10 does not necessarily
change the sensitivity by the same factor, thus recalibration is necessary. This is because
while lowering the emission current decreases the ion production rate, it also increases
the sensitivity by lowering space charge effects, and the exact net effect can only be
experimentally determined. Operating with a lower emission current can allow an RGA
to be used at higher pressures without loss of linearity.
MASS FILTER THROUGHPUT
RGAs can be viewed as containing a “window” that is only open to ions of a
certain charge to mass ratio at a given time, and this “window” slides over a range of
masses at a certain scanning rate. Because the ions collected are produced to possess a
certain uniform energy, lower mass ions will travel through the filter faster than higher
mass ions; therefore, more mass 2 ions will make it through the “window” before it
closes to them than will mass 28 ions. In order to maintain a constant resolution as mass
increases, RGAs a relationship between the d.c. voltage U and the a.c. max voltage V
such that U=KV-Uoffset, where K is a dimensionless constant and Uoffset is an offset
voltage. This method of maintaining constant resolution (important for enabling the
device to resolve very close mass peaks) has the drawback of increasing mass
dependence of sensitivity3. The degree of this mass dependence is determined by K and
Uoffset, which are set by the manufacturing technician when initially tuning the
instrument, and so this particular portion of the mass dependence must be determined by
measurement, since K and Uoffset are not typically available to the end user for
alteration. Table 1 shows the Faraday Cup sensitivities for the three RGAs to Hydrogen
and Helium, relative to Nitrogen. A total pressure ion gauge, whose sensitivity is
determined primarily by ionization cross section, would have a typical sensitivity around
0.5 for Hydrogen relative to Nitrogen. Table 2 shows that RGAs are capable of having
Hydrogen sensitivities exceeding 1 relative to Nitrogen, requiring that mass peak to be
actually multiplied rather than divided (as would be the case for a total pressure gauge
with no mass spectrometer).
CONCLUSIONS AND SUMMARY
There are factors other than pressure and emission current that can affect RGA
sensitivity, and overall sensitivity changes with time; however, as shown here, it can be
demonstrated that RGAs can be calibrated to provide accurate and consistent
measurements over a wide pressure range for most experimental time scales. The
throughput of the quadrupole is related to its resolution setting and the ion energies used.
While these were both kept constant for this experiment, their effect should be noted so
that users will check and maintain consistent resolution as sensor age causes the value to
drift, and so that users will redetermine calibration values if a resolution change becomes
necessary. Additional work would be useful to establish the minimum pressure RGAs
are capable of measuring, and the linearity of the gauges as the pressure approaches this
minimum. While many RGAs, especially high-end units, are very sensitive (some
claiming miminum measureable pressures down to as low as 10^-13 Torr), the total
pressure gauges that were used here to determine linearity have difficulty measuring
pressures that low. In addition, while the chamber used in this experiment provides very
good vacuum (as low as around 10^-11 Torr), specially treated chambers to reach even
lower pressures and ion gauges capable of measuring those pressures are needed if a true
lower bound of RGA total pressure measurement capability and sensitivity is to be
determined.
We have seen here that RGAs behave quite linearly into the UHV pressure range
for the gases studied, and that the linearity can be improved by adjusting the emission
current of the gauge. In typical UHV applications, generally only a few gas species are
present; in the ideal case only Hydrogen is present. RGAs can therefore serve as a good
total pressure gauge for UHV applications once the sensitivities specific to the gauge and
gases present have been found. Residual Gas Analyzers are complex gauges, both in
their construction and their use. Accurate measurements require the user to consider
many different physical factors involved in the operation of the gauge.
FIGURES
Figure 1: Sensitivity data for all RGAs and gases studied – Generally there is little more
than 10% deviation in sensitivity over the pressure range studied. The factory
reconditioned RGA (RGA B) seems to provide the greatest degree of linearity.
1a: Nitrogen:
RGA N2 SENSITIVITY VS EXT READING 1 MA EMIS CURRENT
1.4
1.2
SENSITIVITY
1
0.8
RGA A
RGA B
0.6
RGA C
0.4
0.2
1.00E-10
1.00E-09
1.00E-08
1.00E-07
0
1.00E-06
EXT PRESSURE
1b: Methane:
RGA CH4 MASS 16 SENSITIVITY VS EXTRACTOR READING
1.2
SENSITIVITY
1
0.8
RGA A
0.6
RGA B
0.4
RGA C
0.2
1.00E-10
1.00E-09
1.00E-08
EXT PRESSURE (TORR)
1c: Carbon Dioxide:
1.00E-07
0
1.00E-06
RGA CO2 SENSITIVITY VS EXTRACTOR READING 1 MA EMIS
CURRENT
1.2
SENSITIVITY
1
0.8
0.6
0.4
RGA A
RGA B
RGA C
0.2
1.00E-10
1.00E-09
1.00E-08
1.00E-07
0
1.00E-06
EXT PRESSURE
1d: Helium:
RGA He SENSITIVITY VS AXTRAN READING 1 MA EMIS CURRENT
1.2
SENSITIVITY
1
0.8
RGA A
0.6
RGA B
RGA C
0.4
0.2
1.00E-10
1.00E-09
1.00E-08
AXT PRESSURE (TORR)
1e: Hydrogen:
1.00E-07
0
1.00E-06
RGA H2 NORMALIZED SENSITIVITY VS PRESSURE, 1 MA
EMISSION CURRENT
1.20
SENSITIVITY
1.00
0.80
RGA A
0.60
RGA B
0.40
RGA C
0.20
1.00E-10
1.00E-09
1.00E-08
1.00E-07
0.00
1.00E-06
PRESSURE (TORR)
Figure 2: Fragmentation percent of Methane – It can be seen here that the fragmentation
percentage for Methane remains nearly constant as pressure is
decreased.
Fragmentation Percent vs. Pressure
1
0.9
Fragmentation
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.00E-010
1.00E-009
1.00E-008
1.00E-007
1.00E-006
Pressure
Figure 3: RGA sensitivity to background H2 as a function of emission current –
sensitivity can be seen to decrease almost linearly with emission current
RGA NORMALIZED H2 SENSITIVITY VS EMISSION CURRENT
SENSITIVITY
1.20E+00
1.00E+00
8.00E-01
RGA A
6.00E-01
RGA B
4.00E-01
RGA C
2.00E-01
0.00E+00
0
0.2
0.4
0.6
0.8
1
1.2
CURRENT (MA)
Table 1: Properties of the SRS 100 AMU RGAs
Filament Type
ThO2/Ir
Electron Energy
25-105 eV
Ion Energy
8-12 eV
Focus Voltage
0-150 V – Standard 90 V setting used
Emission Current
0-3.5 mA -- 1 mA used except where noted
Table 2: RGA Faraday Cup H2 sensitivities relative to N2
RGA A
RGA B
RGA C
1 mA
2.69
2.04
2.25
0.1 mA
0.459
0.658
0.443
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