Table of ContentsIntroductionObservationsResultsConclusionsAbout Hiden Analytical
Among the methods commonly used for mass spectrometry studies of processing plasmas, the “threshold ionisation” (TI) method for examining the neutral species generated in plasma has been specifically useful. In the past, the method has been applied using source pressures in the mass spectrometer of about 10-6 Torr. With the current availability of particle detectors, which can be operated at considerably higher pressures, one can study probable extensions of the TI method. The present data for mass spectrometer pressures of up to 4.10-4 Torr using gas mixtures that include rare gases demonstrate clearly long-lived metastable atoms of the inert gases in both the source of the plasmas and the mass spectrometer.
For gases such as oxygen, generation of metastable species in the mass spectrometer source is also observed. The interpretation of the experimental threshold ionization data is also discussed. The measurements enable new avenues of research for both gas analysis and plasma diagnostics for gases having long-lived, metastable states.
With the availability of particle detectors that can be used at pressures up to 4 x 10-4Torr, mass spectrometers can be operated at pressures that are much closer to those used in many plasma processing systems. This enables the improved sampling of both neutral and ionised species from plasma reactors. Further, the Hiden Analytical quadrupole mass spectrometer (QMS) can operate in a mode where the energy of the electrons emitted within the ionization source is variable. This mode is known as Threshold Ionisation Mass Spectrometry or TIMS. Different elements have specified ionization energies needed to eliminate an orbiting electron. This energy is dependent on the electron orbital, i.e. outer shell electrons generally have weaker ionisation energies due to the greater distance and lower electrostatic forces from the nucleus. This gives rise to the electron impact ionization efficiency curves shown in Figure 1.
Figure 1. Electron impact ionization efficiency curves.
The ionization process of neutral particles begins at minimum threshold energy of the impacting electrons. This minimum energy is dependent and exclusive to any species present in the gas matrix, resulting in a spectral “identifier” or fingerprint for all atomic or molecular species. For neutral species, for example, a particular application of the TIMS technique has been to precisely quantify the determination of helium/ deuterium ratios during plasma fusion, where helium ash is the by-product. Normally, this quantification is done while using a QMS in a traditional mass spectral mode due to the overlapping convoluted mass spectral signatures of both D2 and He at 4 amu (the actual mass separation is just 0.02 amu). When operating the Hiden Analytical QMS in TIMS mode figure 2 shows the electron energy spectra for Deuterium (D2) and Helium (He) with ionisation onsets at 15.4 eV and 24.5 eV respectively.
Figure 2. Electron energy spectra for Deuterium (D2) and Helium (He) with ionisation onsets at 15.4 eV and 24.5 eV respectively.
When these two gases are present together, the resulting electron energy spectrum is shown in figure 3. It can be seen that there is a clear deconvolution of the two species in the TIMS spectra such that the presence of D2 can be accurately detected in Helium down to parts per million (ppm) detection levels. Hiden Analytical TIMS equipped mass spectrometers are now used regularly and in current operation at JET the Joint European Torus experimental nuclear fusion facility, Oxford, UK.
Figure 3. Electron energy spectrum of a mixture of helium and deuterium.
The ionisation potential of helium is 24.6 eV. The section AB of the curve is due to the formation of metastable He*m atoms, which have a long lifetime against spontaneous decay. They have considerable energy to generate pulse counts while impacting on the detector. For electron energies above 24.6 eV the section BC of the curve includes both metastable and ionised helium contributions. Similar data were obtained in other experiments for neon, krypton and argon. Data for krypton are included in Figure 4. The form of the curves shown in figure 4 may be understood by reference to figure 5.
Figure 4. Electron energy spectrum for krypton.
Figure 5. Explanation of how the curve in figure 4 came about.
The data of figure 4 were acquired using the system shown schematically in figure 6. RF plasma can be maintained in the reactor between an electrode and the sampling orifice of the mass spectrometer. Electrodes behind the orifice can be used to control the entrance of ions from the reactor into the Hiden mass spectrometer. The particle detector can be used for pressures of 4.10-4 Torr. Gases were allowed into the reactor or directly into the source of the mass spectrometer. He*m was detected in the plasma when the plasma operated in helium with the internal ionisation source of the mass spectrometer off and its sampling system set to prevent all plasma ions from entering it. The metastable signal is proportional to the plasma power and to the gas pressure in the reactor, as shown in figure 7. When the helium plasma was replaced by oxygen plasma, no energetic particles from the plasma were detected, since metastable oxygen species, although long-lived, have insufficient energy to be recorded by the detector.
Figure 6. Schematic of the system.
Figure 6. Gas pressure in the reactor.
With both plasma and mass spectrometer source operating, and the sampling system again set to reject plasma ions, the recorded signals for a mixture of helium and oxygen were as shown in figure 7. For the helium curve, the section BC shows ions generated from ground-state helium sampled from the reactor, while section AB shows ions produced from sampled metastable helium. There will be a small contribution due to metastable helium atoms generated in the source between 20 and 25 eV. The threshold energy (not shown) is expected to be around 5eV as shown in Figure 4.
Figure 7. Signals for helium and oxygen.
For oxygen, there was no evidence of Penning ionisation in the internal source. For pure oxygen, 15 W plasma at 30 mTorr and a mass spectrometer source pressure of 2.10-4 Torr provided the data shown in Figure 8. The region below 16 eV appears to have two components with onset potentials that differ by about 1 eV. This is to be expected if either the sampled oxygen includes metastable 1Δg oxygen, or if the latter were produced in the mass spectrometer source. For the present experiment, the source process dominated.
Figure 8. Oxygen spectra showing no sign of Penning ionization.
By minimizing the pressure differential between a plasma reactor and an attached mass spectrometer such as the Hiden mass spectrometer allows direct detection of metastable species produced in the plasma if these have long life-times and considerable internal energy. Detection of lower energy, but still long-lived, metastable species and other plasma products is also simplified, such measurements may help consider the role of energetic neutral species in the plasma processing of surfaces.
About Hiden Analytical
Hiden Analytical is a leading manufacturer of quadrupole mass spectrometers for both research and for process engineering. Their products Our products address a diverse range of applications including:
- Precision gas analysis
- Plasma diagnostics by direct measurement of plasma ions and ion energies
- SIMS probes for UHV surface science
- Catalysis performance quantification
- Thermo-gravimetric studies
These analytical instruments are designed to work over a pressure range extending from 30 bar processes down to UHV/XHV.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
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