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:
1b: Methane:
1c: Carbon Dioxide:
1d: Helium:
1e: Hydrogen:
Figure 2: Fragmentation percent of Methane – It can be seen here that the fragmentation percentage for Methane remains nearly constant as pressure is decreased.
Figure 3: RGA sensitivity to background H2 as a function of emission current – sensitivity can be seen to decrease almost linearly with emission current