mass_spectrometer Pfeiffer обзор (1248468), страница 9
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40a). Here the pressure is reducedonly after ionization of the process gas, i.e.the pressure in the process chamberequals the pressure in the ion source,which essentially eliminates the contribution of the background spectrum of the analyzer unit to the measured results. Thisdesign provides limits of detection downto the ppb range, even for reactive gasesQuadStarTMa) process pressure1 · 10–2 mbar > p > 1 · 10–9 mbarp (ion source)< 1 · 10–2 mbarRS-232-C orfiber-optic cablep < 1 · 10–5 mbarRS-232-C orLAN interfaceprocess chamberPrismaTMSPM ion sourceC-SEM2 x relay output,2 x analog input,4 x analog output2 x relay input24 VDCTMU 07190–240 V, 50/60 HzMVP 015b) process pressure10 mbar > p > 1 · 10–3 mbarp (ion source)< 1 · 10–2 mbarp < 1 · 10–5 mbarRS-232-C orLAN interfaceprocess chamberPrismaTMSPM ion sourceC-SEMTMU 071–3,SplitFlowTM Turbo2 x relay output,2 x analog input,4 x analog output2 x relay input24 VDC90–240 V, 50/60 HzMVP 015c) process pressure10 mbar > p > 1 · 10–9 mbarFig.
40:Differentially pumpedmass spectrometerwith a SPM ion sourcep (ion source)< 1 · 10–2 mbarp < 1 · 10–5 mbarRS-232-C orLAN interfaceprocess chamberPrismaTMSPM ion sourceC-SEM2 x relay output,2 x analog input,4 x analog output2 x relay inputa) SPM 200b) SPM 200with additionalpressure stage witha fixed screenc) SPM 200with additionalpressure stage witha removable screen36TMU 071–3,SplitFlowTM Turbo24 VDC90–240 V, 50/60 HzMVP 015pprocess = pion source > panalyzerSuch an interstage port in combination withthe purge gas mode is also recommendedfor the analysis of corrosive gas mixtures.The following diagram shows furtherexamples of pump combinations for theanalyzer unit.The pump combinations shown in Fig. 40 aare used for the analysis of gas mixtureswith corrosive or explosive components.The injection of purge gas into the turbodrag-pump dilutes the sample gas so thatthe corrosive loading of the backing pump(here an oil-free diaphragm pump) is reduced and an inert gas mixture is producedat the outlet of the system.
The injection ofpurge gas also suppresses memory andenrichment effects of the sample gas inthe backing pump.a)b)c)d)Fundamentalswith a dynamic measuring range of > 8decades. The working range of such anSPM ion source is from < 10-9 up toapprox. 1–2 · 10-2 mbar. Starting from theconditioning phase of the sputtering chamber, ignition of the sources, the coatingprocess itself and back to the initial state,the gas composition of the main components as well as any possible trace components can be monitored continuously without the need to switch any valves. Additional pressure stages are only necessary ifthe process pressure exceeds 10-2 mbar(Fig.
40 b and 40 c) .To generate an intermediate vacuum forthe additional pressure stage, the 2nd intakeport of a SplitFlowTM turbo-drag-pump(TMU 071-3) can be used to advantage.Fig. 41:Examples of speciallyoptimized pumpsystems for the analyzer unit.371 Fundamentals of mass spectrometryThe in-line connection of an additionalturbopump (Fig. 41 b and d) is a means toincrease the compression ratio of gaseswith a low molecular weight (H2, He). Thisleads to a reduction in the partial pressure of these gases in the residual gas spectrum and thus to considerably betterdetection limits for these gases. The influence of the additional turbopump on thebackground of the residual gas is shownin Fig.
42. The experiment was carried outwith the set up shown in Fig. 41 b. Theeffective pumping speed on the analysisvacuum chamber is not altered by the inline connection of an additionalturbopump, so that a reduction of thetotal pressure in the vacuum chamber isonly then attained if it is dominated bythe partial pressure of hydrogen or helium.As can also be seen from Fig. 42, theattainable partial pressure of hydrogen inthis case is determined by the backingvacuum pressure or the compression ratioof the TMU 071 (KH2 > 105). The final partialpressure of components with a highermolecular weight is determined by thepumping speed of the TMU 071, since thecompression ratio of > 1011 for these components is already sufficient.E-06Fig. 41 c and 41 d show the pump combinations for the analysis vacuum chamberof the QMA 410 that have a greater effective pumping speed.
Such combinations,particularly turbo-drag-pumps withmagnetic bearings, are mainly used in theanalysis of very pure gases and trace analysis. Their advantages include high compression and freedom from lubricants androutine maintenance.In most cases, the total pressure in theanalysis vacuum chamber is measuredindependently of the mass spectrometer.This serves as an independent monitoringof the maximum permissible inlet pressure, as the triggering sensor for the automatic closure of inlet valves as well as thepressure control of the baking and optimization cycles. In the selection of the gaugetype and the positioning of the gauges tomeasure the total pressure, it must beensured that there is no influence betweenthe total-pressure gauge and the quadrupole analyzer via electrostatic or magneticfields. In the case of very sensitive measurements of the gas composition with themass spectrometer, the total-pressure gauge is frequently switched off or ispositioned further away for this reason.Ion Current [A]528.00E-075start-up time of TPD 011E-08high speed time of TPD 0115Fig.
42:The effects of anadditional turbopump on the partialpressures of hydrogen (m/e = 2) andnitrogen (m/e = 28).382.00TPD 011nominal speed reachedE-09061218243036424854606672788490t [s]The utilization of mass spectrometers forprocess gas analysis is not confined tovacuum technologies. In numerous applications the gas mixture to be analyzed isat higher pressures. In most of these casesthe parameter of interest is not the absolute partial pressure of the individual gascomponents, but instead their concentration. These concentrations are independentof the total pressure or inlet pressure ofthe gas mixture, so they can be compareduniversally with other measurements. Thistask is generally summarized under theterm of ”quantitative gas analysis”. Inquantitative gas analysis the mass spectrometer systems are operated either asstand alone analyzer unit, or in combination with other measuring methods such asgas chromatography, infra red spectroscopy, thermogravimetry and liquid phasechromatography.
The particular advantages of the mass spectrometer systems liein their universal connectivity, fast acquisition times, small sample gas consumptionas well as their adaptability to various andvariable inlet pressure ranges. Mass spectrometer systems are consequently especially suitable for online monitoring of thegas composition in rapidly fluctuating processes with large dynamic ranges. For thegas inlet systems described below, onlysmall amounts of sample gas are neededand the requirements for conditioning thesample gas are relatively simple, so that inmost applications they are either alreadyfulfilled or can be achieved with littleeffort.The connection to the gas sampling pointmust be direct and have limited volume andinternal surface area in order to avoid affecting the gas composition before the measurement is taken and to achieve a fast response time for the mass spectrometer.Reduction of the inlet pressure of thesample gas to the operating pressure of thequadrupole analyzer is usually achieved inseveral pressure stages to avoid or minimize any mass discrimination effects that canotherwise change the gas composition.
Thegas is drawn from the high pressure sidethrough a suitably sized capillary in laminarflow into a small volume from where it iswithdrawn, again in laminar flow, by athrottled pumping system. The gas throughput of the capillary lies in the range of1–10 sccm (1.7 · 10-2 –1.7 · 10-1 mbar · l/s).A small fraction (0.01% – 1%) of the gas flows via a defined conductance value into thevacuum chamber in molecular flow. Heretoo, separation of the components does nottake place in spite of the mass-dependentmolecular flow, because the gas is pumpedout of the vacuum chamber in molecularflow too. This two stage pressure reductionensures that the composition of the gasmixture remains unchanged. Chemical reactions and condensation effects are largelysuppressed by the following measures:Fundamentals1.3.2 Mass spectrometers – set upsfor inlet pressures > 10 mbar1) The capillaries and orifices or valves inthe high pressure stage are heated.2) Suitable selection of all constructionmaterials in contact with the gas mixture.3) Suitable dimensioning of the conductance values for producing optimumpressure gradient and gas throughput.4) Suitable selection and dimensioning ofthe pumping system.Requirements for the sample gas:Temperature rangeRoom temperature to 200 °CPressure range1–1200 mbar (absolute), depending onthe inlet systemMoisturenon-condensing under the inlet conditionsParticle size< 1 µmConsumption (in continuous measurement) 1–10 sccm at 1000 mbar inlet pressure391 Fundamentals of mass spectrometryThe response time to changes of the gascomposition at the input of the capillaryuntil the ion currents and the concentration changes are displayed by the massspectrometer depends on the type of gas,on the temperature of the capillary andon the gas flow rate through the capillary.For most gases, typical response times arein the range of 0.3 s to 1 s at 100°C andwith a flow rate of 2 sccm.
The greater theflow rate, the shorter will be the responsetime, but this is accompanied by a corresponding increase in sample gas consumption.To avoid contamination of the capillaryand the inlet apertures with hydrocarbonsoil-free backing pumps should be used forpumping of the gas inlet system. In present day practice, combinations of turbopumps and diaphragm pumps, scrollpumps and dry piston pumps are utilizedfor this task. The use of diaphragm pumpsas the only pumping stage is limitedbecause in most cases the compressionratio is poor for light gases (H2 and He)and this leads to a higher final pressure.
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