Getting the most from your RGA.
Residual gas analyzers do an excellent job of measuring residual gas levels without affecting the gas composition of their vacuum environment. However, you need to be wary of a few potential problems, particularly when the sensor is used to monitor minute trace impurities (i.e., ppm and sub-ppm levels) or ultrahigh vacuum (UHV, [less than][10.sup.-9] torr) environments.
The prototypical RGA consists of an open ion source, a quadrupole mass filter, and a Faraday cup ion detector that may or may not be enhanced by an electron multiplier. Recall that as gases enter an RGA, they become ionized in the ion source by energetic electrons from a heated filament wire. The positively charged ions are then accelerated toward the mass filter region, where they are sorted and finally detected according to their mass/charge ratios.
An open ion source (OIS) is completely open to both the vacuum chamber and the interior regions of the RGA. This openness limits the RGA to pressures [less than][10.sup.-4] torr, since the RGA loses sensitivity above this pressure.
In a high-vacuum environment, most of the thermal energy from the OIS's filament wire is dissipated to the ionizer and its surroundings through radiative processes. The elevated temperatures produce increased outgassing from the OIS itself and from the adjacent chamber walls. These gases can degrade the minimum detectable partial pressure (MDPP) of the OIS RGA for many important species, including [H.sub.2], [H.sub.2]O, [N.sub.2], CO, and C[O.sub.2].
Degassing the ionizer by ramping up its temperature and bombarding it with high-energy electrons can help minimize some of the background signals; however, this usually only works as a temporary solution. A better approach is to choose an OIS having an anode (or perhaps the entire ionizer assembly) that's made out of platinum-clad molybdenum. This highly inert material exhibits decreased adsorption for many gases and provides reduced outgassing.
Water outgassing is a frequent source of interference that can also contaminate many high-vacuum processes. Overnight bakeouts at [greater than]200 [degrees] C are the best option to minimize this problem.
Hydrogen outgassing from the OIS electrodes can be a concern for users operating in the UHV regime. Hydrogen is dissolved in most varieties of 300 series stainless steel and can readily outgas from hot OIS electrodes. To dramatically reduce such outgassing, use platinum-clad components..
Even after an RGA has been thoroughly baked out, peaks are frequently observed at 12, 16, 19, and 35 amu, which are formed by electron stimulated desorption (ESD) from surfaces within the OIS. To minimize EDS:
* Degas with high electron energies
* Gold plate the ionizer or use a platinum-clad molybdenum ionizer
* Reduce the extent of the electron beam
* Use an OIS with wire mesh instead of solid perforated metal
* Avoid exposing the ionizer to chlorinated and fluorinated compounds.
The quadrupole mass filter assembly can also outgas. Water is not the only outgassing species you need to worry about. For example, high Ar backgrounds can be expected if the sensor was recently exposed to large amounts of the gas, since Ar tends to get adsorbed on stainless steel surfaces and desorbs only very slowly.
The open ion source is also sensitive to impurities generated at its hot filament. Gas molecules can suffer thermal cracking and chemical reactions at the filament surface, and the products of the reaction can easily find their way into the ionization region. An overnight bakeout at 200 [degrees] C will usually take care of most of these contamination problems.
RGAs are not limited to the analysis of gases at pressures below [10.sup.-4] torr. Higher gas pressures can be sampled with the help of a differentially pumped, pressure reducing gas inlet system (PRGIS) consisting of a restriction and a vacuum pumping package. Common restrictions are pinholes and capillaries, which can provide pressure reductions of more than six decades. The combined RGA, gas inlet system, and pumping station constitute what is usually referred to as a partial pressure reduction (PPR) system. Properly designed PPR systems can monitor processes from beginning to end, providing essential information every step of the way.
The typical PPR system has two inlet paths to the RGA: a high-conductivity path (Hi-C) for monitoring base vacuum below [10.sup.-4] torr and a low-conductivity path (i.e., a pressure reducing Lo-C path) for monitoring gases at operating pressures above [10.sup.-4] torr. Lo-C apertures are available for operating pressures as large as 10 torr.
For pressures higher than 10 torr, the gas flow rate into the sample inlet side of a single-stage PRGIS becomes extremely small, making the response time too slow for any practical measurements. In those cases, a dual-stage bypass pumped gas-sampling system, with a much larger gas flow rate and faster response, is a better choice than a single-stage PRGIS.
PPR systems do an excellent job of sampling gases at pressures below 10 torr. In fact, a large number of PPR systems are dedicated to the detection of trace impurities in gas mixtures. Here too, however, there are a few background interferences to worry about.
Background gases present in the analyzer chamber can obscure the MDPPs of some important gases (particularly, [H.sub.2], [H.sub.2]O, [N.sub.2], CO, and C[O.sub.2]). This is best illustrated by considering an analysis of water in a [10.sup.-2] torr Ar sputtering process. During process monitoring, the mass spectrometer typically runs at about [10.sup.-5] torr, which represents a three-decade pressure reduction across the Lo-C path of the PRGIS.
The pressure drop brings I ppm of water in the process chamber to a partial pressure in the mass spectrometer of about [10.sup.-11] torr (well within the [10.sup.-14] torr detection limit of a typical RGA). However, with the mass spectrometer isolated from the process gases, the residual pressure in the PPR chamber is, at best, on the order of [10.sup.-9] torr, with most of that being water. This water level is one hundred times larger than the [10.sup.-11] torr corresponding to a ppm of water in the process chamber, meaning that the water vapor concentration cannot be reliably detected or measured to better than 100 ppm under these common operating conditions.
The same limitations must be kept in mind for other potential interfering gases. For any species to be detectable at the ppm level (i.e., [10.sup.-8] torr in a 10 mtorr process), the residual mass spectrum for the PPR must show pressure readings of less than [10.sup.-11] torr at the mass values corresponding to the peaks of that species. Such levels are not easily achieved repeatedly in most vacuum systems unless the necessary precautions are taken to minimize all sources of contamination.
The MDPP limit for air is usually limited by the compression ratio of the pumping station. In most PPR systems, [N.sub.2] levels are usually under [10.sup.-9] torr, with oxygen levels approximately five times lower. This corresponds to MDPP levels of better than 20 ppm for [N.sub.2] at 28 amu and 4 ppm for [O.sub.2] at 32 amu in a 10 mtorr process.
The other limitation to ppm detection levels in a typical OIS RGA-based PPR system is caused by interference from the process gases that are being analyzed. To illustrate this point, let's consider a 10 mtorr Ar sputtering process. Besides a possible interference from water contamination, there is a serious interference at the mass/charge value of 18 from the Ar used in the sputtering system. The isotope 36Ar is present at 0.34%. In the electron ionization process, doubly charged argon is formed, leading to peaks at 20 (40[Ar.sup.++]) and 18 (36[Ar.sup.++]). For 70 eV electron impact energy, a typical level of 36[Ar.sup.++] is 350 ppm.
One way to eliminate the 36[Ar.sup.++] interference is to operate the ionizer with the electron impact energy reduced to [less than]40 eV. This ionization energy is below the appearance potential (43.5 eV) of [Ar.sup.++]. For example, the peaks at masses 18, 19, and 20 due to [Ar.sup.++] disappear while operating an RGA with 35 eV electrons, and this is achieved with minimal reduction in the sensitivity of detection of [Ar.sup.+] at 36, 38, and 40 amu.
In applications requiring the measurement of pressures between [10.sup.-4] and [10.sup.-2] torr, the problem of background and process gas interferences to the mass spectra can be significantly reduced by replacing the OIS PPR with a closed ion source (CIS) sampling system.
The CIS consists of a short, gas-tight tube that is open to the vacuum chamber and has two very small openings for the entrance of ionizing electrons and the exit of ions. Alumina rings seal the tube from the rest of the quadrupole mass assembly. Ions are produced by electron impact directly at the process pressure. A pumping system keeps the filament and the rest of the quadrupole assembly at pressures below [10.sup.-5] torr through differential pumping (i.e., two decades of pressure reduction). Most commercially available CIS systems are designed to operate between [10.sup.-2] and [10.sup.-11] torr, and offer ppm-level detection over the entire mass range for process pressures between [10.sup.-4] and [10.sup.-2] torr.
Because the sampling pressure in the CIS is typically two decades higher than that of the rest of the sensor's vacuum system, the signal-to-background ratio is significantly increased relative to OIS PPR systems. This is particularly important when measuring common residual gases such as water.
To illustrate this point, we return to the water measurement example in a [10.sup.-2] torr Ar sputtering process. The Ar gas is ionized directly at [10.sup.-2] torr (i.e., three orders of magnitude higher than in the OIS PPR) but in the same background ([10.sup.-9] torr) of residual water. This residual water signal now corresponds to a 100 ppb MDPP level for water in the CIS system.
For other common interferences, such as organic contaminants or reaction byproducts of the filament, the gas-tight design of the source reduces the visibility of the ionization region to those gases, providing a very clean residual gas spectrum, free of many of the spectral overlaps that are common in OIS PPR setups.
Interferences from contaminants generated by ESD are also reduced in the CIS because a much smaller electron beam penetrates the ionizing volume. In addition, the inside walls of most commercially available CISs are coated with highly inert materials, such as gold or platinum-clad and pure molybdenum, which adsorb less impurities than stainless steel.
The ability of the CIS to sample gases directly in the mtorr range and provide ppm-level detectability across its entire mass range has made CIS systems the instrument of choice in semiconductor processing applications such as PVD, CVD, and etching.
When properly matched to a process, both OIS PPR and CIS systems are very versatile instruments that provide crucial information throughout an entire gas phase process. A PPR system fitted with a dual-path PRGIS can switch effortlessly from a highly sensitive RGA mode of operation to a process monitoring mode.
Different modes of operation can also be easily achieved in a CIS by simply changing some of the sensor's ionization parameters. A CIS gas analyzer, even though not as sensitive as an OIS RGA, can tackle most residual gas analysis and leak checking tests that are required in process chambers. The sensitivity of the CIS is reduced over the OIS because of the very small holes for electron entrance and ion exit. However, in most cases, running the electron multiplier at higher gain levels than the RGA makes up for the reduction in sensitivity. Typical MDPP values for CIS systems, fitted with an optional electron multiplier and operated in the RGA mode, are in the order of [10.sup.-11] torr. This is about two decades higher than the MDPP values that can be achieved with PPRs operated in the RGA mode with the Hi-C sampling path open.
The CIS ionizer can also be reconfigured for on-line process monitoring and control and for verification of process gas purity at the point of use. The electron emission current is raised during residual gas analysis to increase sensitivity and reduced during process monitoring to avoid space charge saturation effects in the ionizing volume at the higher pressures.
The tight design of the CIS makes it possible to operate the ionizer at lower electron ionization energies than are possible with OISs. Most of the commercially available CIS systems offer at least two electron energy settings of 70 and 35 eV. The 70-eV setting is mostly used for leak testing and routine gas analysis. The spectra collected are virtually identical to those obtained with standard RGAs. The 35 eV setting is used during process monitoring to eliminate process gas interference peaks.
A common application of the low-energy mode is to eliminate the doubly ionized 36[Ar.sup.++] peak that interferes with water detection at 18 amu in sputtering processes. CIS systems with user programmable ionizer voltages offer the highest versatility, since they can be configured to selectively ionize species in a gas mixture by carefully adjusting the electron impact energy.
Gerado Brucker is a design engineer at Stanford Research Systems, Sunnyvale, Calif.
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|Title Annotation:||residual gas analyzers|
|Publication:||R & D|
|Date:||Jun 1, 1998|
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