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Electron Impact Sputtered Neutral Mass Spectrometry SNMS Using the Hiden EQS Energy Resolving Quadrupole Mass Spectrometer

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Electron Impact Sputtered Neutral Mass Spectrometry SNMS Using the Hiden EQS Energy Resolving Quadrupole Mass Spectrometer Mass SElectronpectros cImpactopy of SputteredMetastabl eNeutral Atomi cMass Spe cies Spectrometryin Gas Ana (SNMS)lysis an dusing Proc theess Hidening Pl aEQSsma Energys aResolvingt High Sp Quadrupoleectrometer SMassourc eSpectrometer. Pressures S.Davies; D.L. Seymour; J.A. Rees; C.L. Greenwood; M.E. Buckley Hiden Analytical Ltd, 42G.A.Cooke,0 Europa Bou lS.Daviesevard, Wa randring tD.L.Seymouron, WA5 7UN, UK. [email protected] Abstract Hiden Analytical Ltd, 420 Europa Boulevard, Warrington, WA5 7UN, UK. [email protected] -4 The availability of particle counters which may be operated at ambient pressures of up to 2x10 Torr is highly desirable in many current research importance in the study of reaction processes during hydrogen (H/HD)-Deuterium (D2) plasma fusion, of which the residual fusion byproduct is helium applications. In one field in particular, it allows mass spectroscopy of gas analysis and processing plasmas to be performed using pressures of this ash. Furthermore, collision processes in the ionisation source of the QMS, (including Penning ionisation), which are insignificant at the more usual order in the quadrupole mass spectrometer (QMS). These pressures are much closer to those of many processing plasmas so that the sampling of source pressures of below 1x10-5 Torr, generate product ion species whose study helps the interpretation of the processes occurring in the plasma neuIntroductiontral species, in particular, from the plasma is improved. The sampling of ion and neutral species from magnetron plasmas is a good example. The reactor. Typical data from neutral gases and plasmas in a range of gas mixtures which include helium, krypton or argon are shown. The majority of the parTheticle cHidenounter EQSused iisn t hae highprese ntransmissiont investigation, quadrupolecould be mou nsecondaryted axially s oion as tmasso have spectrometry,a direct line-of-s iSIMS,ght view detector of the sam includingpling orific ea o 45°f the electrostaticQMS. dat asector presen tfored csimultaneousonsist of electro ionn im energypact thre analysis.shold ioniz a tIonsion e fficiency curves obtained by scanning the energy of the electrons in the QMS source. 1 3 Conareseq collecteduently, ene rgonet icthe ne uaxistral sofpe ctheies devicesuch as mwhichetasta bmakesle atom its overyf hel ipopularum, whic hfor ar efi pttingrodu cased anin h after-marketelium plasmas detectorand have ltoon ga lwideifetim evarietys again soft surfaceThe r eanalysissults pres einstrumentation.nted are discussed iThen ter mstandards of proc eEQSsses wcanhic h include collisions between metastable species such as He (2 S and 2 S) at 20.61eV [1] spoalsontane beous usedde-ex ctoita tmonitorion, may ttherave residuall to the d egastecto usingr and h aanve electronsufficient eimpactnergy to ionizerbe coun tsiteded the withinre. The dtheete cdevice.tion of m eThistasta bcanle h ebelium employed may be of for detectionand 19.82 eofV resputteredspectively neutrals,having ra dalthoughiative lifet itheme sdistancetates of ar ound 3000s and also other plasma constituents. from sample to the ionizer leads to a low sensitivity. Sputtered neutral mass spectrometry, SNMS, is a desirable technique that compliments SIMS by providing fast and quantifi able concentration data in the 0.1 to 100 atomic % regime where the matrix effect of SIMS makes direct quantifi cation non-linear and unreliable. Introduction Results -4 -4 With the availability of particle detectors that can be used at pressures up to 4x10 Torr, it has become Flowing a gas mixture combining Helium (He) and Deuterium (D2) at various test gas mixture ratios, at a source pressure of 1x10 Torr, there are significant numbers of ions formed possible to operate mass spectrometers at pressures that are much closer to those used in many plasma with a mass to charge ratio m/e = 8. These are much less abundant even at the more usual maximum source pressures that quadrupole mass spectrometers typically operate at processing systems. This enables the improved sampling of both neutral and ionised species from plasma (≤ 1x10-5 Torr). This suggests the increased pressure in the source leads to a collision enhanced environment resulting in greater probability of Helium atoms and ions combining + + [3] reactors. Additionally, the Hiden Analytical quadrupole mass spectrometer (QMS) can operated in a mode to form molecular Helium (He2) with He.He ions. Figure 4 shows the mass spectral scans of these He.He ions. An additional pathway as suggested by Van Dyck et al for + Incidentwhere the eionner gbeamy of the (blue)electron ssputters emitted within the ionization source is variable. This mode is called TIMS production of He .He ions is (Threshold Ionization Mass Spectrometry). Different elements have defined ionization energies required to therem targetove an materialorbiting e lwithectro nthe. Th emissionis energy is dependent on the electron orbital, i.e. outer shell electrons ofge nmainlyerally ha veneutrals weaker i onwithizatio na e nsmallergies due to the greater distance and lower electrostatic forces from the fractionnucleus. of positive and negative + + The mass peak at m/e=6 is for D3 ions and/or D . He ions. Figure 4a shows the variation in the count rates for m/e=8 and 6 at an electron energy of 30eV as a function of the ionsThis gandives manyrise to telectronshe electron (black).impact “threshold ionization energy” curve shown in figure 1 helium content of the helium/deuterium mixture. Figure 1 Figure 4 The Fstandardigure 4a EQS analyser SIMS vs SNMS The addition of an SNMS Ionizer Both techniques provide mass resolved information for mass spectra, concentration depth profi les and images. SIMS is suited The electron impact ionizer has been relocated to the front of the probe so that it subtends the largest to trace analysis and SNMS to higher concentrations. Combining the techniques provides a very powerful quantitative materials solid angle possible – as the neutrals cannot be directed until they are charged. In SIMS mode the analysis method. ionizer fi lament is off and the electrode behaves as an extractor for ions. In SNMS mode the ionizer cage is populated with electrons and neutrals are ionized – these then follow a similar path to that which SIMS SNMS The ionization process of neutral particles commences at a minimum (threshold) energy of the impacting SIMS ions would have taken. The SIMS signal is deliberately rejected by placing a bias on the target With the prototype QMS arrangement of figure 3, it is seen that a unique combination of system design, geometry and operational mode of Threshold Ionization Mass •e lecionstrons . createdThis min ibymu mthe en esputteringrgy is depe neventdent and unique to any species present •in theions gas mcreatedatrix, re sbyult iseparateng ionizer so as to shift the SIMS energy spectrum to a level where it can be fi ltered by the electrostatic sector. Spectrometry (TIMS) allows the detection and clear identification of metastable neutral atoms. Typically, the metastable states of rare gas atoms have internal energies •in a smallspectra lfraction “identifi eofr” oallr f inemittedgerprint fparticlesor all atomic or molecular species. For neu•tr al sneutralspecies, f orepresentr example, most of the emitted particles between 12 to 20 eV, and the first excited states have lifetimes in the order of milliseconds to thousands of seconds. Since the interaction time during collisions is in •a paeasilyrticular acollectedpplication obyf t helectrice TIMS te fic heldnique has been to accurately quantify the• d etecanrmin aonlytion obef collected by proximity of the ionizer the order of nanoseconds, the lowest metastable state level may be viewed as an effective ground state. These meThetast afib lamentle specie iss a housedre not ide ninti fitheed i nbox con structureventional on the helium/deuterium ratios during plasma fusion, where helium ash is the by-product. Normally, this • ionized fraction depends on surface chemistry • ionized fraction independent of surfacemass sp echemistryctrometer designs and significantly are mass independent of the resulting ion that is produced by increaundersidesed electron ofim ptheact sprobetimulu sand. Of ahasll m enotas talineble of site to quantification is precluded when using a QMS in conventional mass spectral mode due to the overlapping • non-linear when concentration above a few percent • near linear in all concentrations (0.1syste mtos , 100the h eatomiclium co l%)lision system is particularly interesting because of its small mass and because of the prototype character of its electronic system containing convoluted mass spectral signatures of both D and He at 4amu (the actual mass separation is just the sample. The electrons, with energy variable • quantifi able in range ppb to 5 atomic2 % • requires reference material containingonly f otheur e lelementectrons. of interest 0.02amu). When operating the Hiden Analytical QMS in TIMS mode figure 2 shows the electron energy between 4 and 150 eV, are directed into the • requires reference materials of very similar chemistry [1] Figure 5 Figure 5a spectra for Deuterium (D2) and Helium (He) with ionization onsets at 15.4eV and 24.5eV respectively . ionization cage visible at the end of the device. Figure 2 In operation, the optimum sampling distance is between 5 and 10 mm from the end of the probe. Mass Interference Si+CO CO (sputter gun off) 28Si and CO – 1.E+05 appearancee– ener genergyy (0-150eV) 1.E+04 selection -1 The electron impact source also ionizes CO from 1.E+03 the residual gas which can cause a signifi cant mass interference at mass 28. However, use of an electron In this current presentation, metastable neutral atomic species of helium and deuterium formed in the source are also counted by the particle detector. This is readily seen in energy below the appearance energy of CO provides electron energy scans such as those shown in figure 5. The figure shows typical scans at a source pressure of 1x10-4 Torr for pure helium, pure krypton and a When these two gases are present simultaneously, the resulting electron energy spec1.E+02trum is shown in effifig ucientre 2a .
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