mass_spectrometer Pfeiffer обзор (1248468), страница 11
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the concentrations, provide information reflecting the changes of the gascomposition.Extension of the inlet pressure range topressures > 1.5 bar (absolute) is possibleonly with further pressure reduction. In thesimplest case, the gas mixture underexcess pressure is brought to atmospheric44PrismaTM withgas-tightion sourceC-SEMspecial wiringpressure via a reducing valve and thenintroduced into the inlet system describedabove. For selecting the pressure reducingvalve, it is necessary to pay attention tothe dosing range, the heating capability,the gas compatibility of the valve materials and the maximum pressure stresslimit.the capillary connection is < 4 cm3 and canbe evacuated with the oil-free diaphragmpump of the GSS when the sample gas ischanged.
The valves are controlled withadditional software under the control ofQuadStarTM, so that complete measurement, periodic calibrations and test measurements can be automated.outputmeasurementgas bypassQuadStarTM123inletSPS2 x RS-232-Cheated valveand aperturepipe 1/8”inletFundamentalsThe gas switching unit GSS 300 serves forcomputer controlled sequential switchingof up to 12 sample gas or calibration gaslines. It can be combined with all massspectrometer configurations which have acapillary inlet. In order to obtain short response and switchover times, gas mustflow continuously through the inlet valves.The volume between the inlet valves andPrismaTM withgas-tightion sourceC-SEM4561 x relay output2 x analog output2 x analog inputTMU 071-3SplitFlowTM TurboexhaustexhaustGSS 300with 6 inlet valuesBesides the previously described capillaryinlet systems there are a couple of differential pumped inlet systems are used forspecial measurement applications.Examples are skimmer set ups and membrane inlet systems.
A set for analyzing indissolved gases in liquids is shown in Fig.49. The liquid to be analyzed is directed infront of a silicon membrane by a smallfeed pump. The in liquid solved gas is diffusing through the membrane into anevacuated space. Part of this gas reachesthrough an aperture the mass spectrometer in order to get analyzed.Because the permeability depends highlyan the temperature the thickness of themembrane and its condition it is recommended to do a comparison with a reference measurement.
In order to achive thesame measurement conditions a 3 wayvalve is used.OmniStarTMA similar set up can be used for in dissolved gases in liquids [20].diaphragm1000 mbaraperturep < 1 – 4 mbargas tight ion sourceLiquidp < 8 · E-05 mbarGasp < 5 · E-06 mbarFig. 48:Diagram of thesample gas feedwith a GSS 300 incombination with anOmniStarTM.The gas flow of thecurrently selectedsample gas (input 6)is depicted blue.The heated region isdepicted red.Fig. 49:OmniStarTM withmembrane inletsystems .QuadStarTMInlet1 (probe)membrane3 way valveinlet 2(reference)aperturePrismaTM withgas tight ionsource C-SEM40–80 °CRS-232-C2 x Relay output2 x analog input2 x analog outputTMU 071-3SplitFlowTM Turboexhaustfeed pump for liquids451 Fundamentals of mass spectrometry1.4 Mass spectrometer for detection of externallygenerated ions and plasma diagnosticsThe previous chapters described massspectrometers for analysis of neutral gasparticles.If generated ions are to be mass detectedor energy selectively detected, a quadrupole mass spectrometer offers itself as adetector.In such cases, the ion source integrated inthe mass spectrometer often is superfluous.
Instead, ion optics are needed tofocus the available ions in the mass spectrometer.Quadrupole mass spectrometers are usedin differing measurement designs, such asSIMS (Secondary Ion Mass Spectroscopy),ICP-MS (Inductively Coupled Plasma MassSpectrometry), proton transfer reactionsand plasma diagnostics, etc., as a massselective detector.The requirements for sensitivity of themass spectrometer, the performance ofthe ion optics and geometrical alignmentconditions are sharply differentiated inthese tasks.A modular system is imperative in order tomeet all requirements and to utilize testedfunctional units.1.4.1 Detection of externally generatedionsModule:Detection of externallygenerated ionsAnalyzerhardware:Mass spectrometercontrol unit:Mass spectrometersoftware:Ion opticsMass filterDetectorMeasuring amplifierSignal amplifierVoltage supplyCommunication withcomputerSignal processingOperation of the massspectrometer datastorage and processingCan be combinedfreely to optimizethe measuring taskCommunication withperipheral hardwaresignalsThe modular design encompasses thethree areas of analyzer, control unit andsoftware.On the part of hardware, the functionalunits can be combined freely with eachother.
A selection on this level is set by thespecific choice of mass spectrometer control unit. Independent of the selected variant of mass spectrometer, the control unit46Design of the interfaceto other systemsAllowing for complexspecific measurementrecipescan be fitted with analog and digital inputsand outputs.This allows quadrupole mass spectrometers to be arranged according to the requirements and to be integrated in complex measurement designs.The analyzers, detectors and signal amplifiers are standardized.DetectorQuadrupole mass filtere-SEM 217for positive ionsSEM 218for positive andnegative ionsPre-amplifier/counterEP 422 and/or CP 400EP 422 andCP 400 onlyfor negative ionsIonsTwo versions:FaradaycollectorEP 422QMA 400Deflection units for30, 60 and 90° on requestQMA 410The three-lens optic is used as an iontransfer optic.
Among other things, thisoptic allows for a large immersion depth.Due to the relatively high energy dispersion of this optic – approximately 1.5 eVhalf-value width – high-energetic ions andthe primary beam are effectively suppressed in SIMS applications.In other applications, the three-lens opticis used as a simple electrostatic energyfilter.Cross beam ion source:This ion source allows electron energies tobe set down to 4 eV.It therefore is suitable for reception ofexcitation spectra. The energy of the electrons used for ionization is varied and thesignal is measured dependent on the electron energy.
This technique is known as“Appearance Potential Spectroscopy”.The cross beam ion source can be combined with both 2- and 3-lens ion optics.Two-lens optic:This optic is used if you only want to display ions in the mass spectrometer. Thisway ions with low kinetic energy aredetected. This optic can be used withSIMS and SNMS (Secondary Neutral MassSpectroscopy) in combination with thecross beam ion source.Deflection units:An electrical sector field is used to mapions in the instrument that are formed vertically to the axis of the mass spectrometer.
In the direction of the mass spectrometer axis, the design is rotatable by 360degrees so that the largest possible detection range is covered.Converging angles of 30, 45, 60 and 90degrees are possible.Of course, such a sector field also functions as an energy filter.Fig. 50:There are four variations of ion optics andcombinations ofoptics and ion sourcesthat can be used tointerface with the specific measurementconfiguration:The utilized functionalunits of the moduleare outlined below.47FundamentalsIon optics andion sources1 Fundamentals of mass spectrometryA highly precise quadrupole mass filtertogether with a discrete dynode designedSEM is the heart of all mass spectrometersfor ion analysis.Transmission through the quadrupolemass filter is relatively high and the measurement discrimination is low.
Both characteristics are affected by the mechanicalprecision of the filter, the quality of thehigh-frequency generator and the use ofPfeiffer Vacuum field axis technology.Therefore, pre-filters are unnecessary.Due to the 90° off axis arrangement of theSEM, photons, electrons and fast neutralscannot reach the detector. This leads to alow noise background and is a necessaryprerequisite for the high dynamic range ofthese mass spectrometers.Fig.
51:SIMS spectraof positive andnegative ions.This example clearlyshows the high dynamic range and thehigh sensitivity thatcan be achieved withsuch a mass spectrometer in SIMSsystems.Partially oxidizedmolybdenum wasused as a sample.This example alsoclearly shows thehigh measurementrange that can beused for analysis.48The additional conversion dynode with theSEM 218 further reduces an energy-dependent mass discrimination.1.4.2 Application examplesSIMS and SNMS are decades-long established procedures for surface analysis andanalysis of thin layers in the semiconductorindustry and in related industry branches.With SIMS, the sample is bombarded withions or neutral particles from an ion sourcein a high or ultra-high vacuum. Secondaryions that occur when the layer is etchedare measured by time and/or stationaryresolution.Pfeiffer Vacuum delivers mass spectrometers for these systems.Fig. 52:The example shows a signal sequencemeasured with an EPD 400 during an ion beamexperiment in an ion milling system.At any time – even with a process disruption –you can determine in which depth of the filmsystem you are working.You can easily re-tool between both typesat any time.FundamentalsEnd Point Detection:Many manufacturing procedures in thinfilm technology require a film system tobe built up before processing and, in somecases, to be selectively re-etched in thegas phase.A technique that has been used for sometime is “ion milling”.
During ion milling,the substrate is bombarded with argonions and thereby etched in the gas phase.In recent times, the requirements for precision of the process have increased dramatically requiring use of a highly sensitivedetector for monitoring and control duringthe process.One option for monitoring such a processis to continuously measure the secondaryions occurring during the ion milling process. For this purpose, the EPD 400 wasdeveloped as a complete unit. The EPD400 consists of a mass spectrometer, housing, pumping station for pressure reduction and control unit. It is delivered in twoversions with a 90-degree acceptanceangle as displayed below (Fig. 52) and asan in line version.In addition to use as an endpoint detectorin ion milling processes, the EPD is suitable for detection of ions in condensablematerials, such as metal ions in industrialsputtering processes.With conventional differentially pumpedmass spectrometers, you achieve a comparably low sensitivity for condensableneutrals since they for the most part condense in the unit before reaching the ionsource.With the EPD 400, the ions arising fromthe process are mapped directly in the unitwith comparably low losses by means ofthe ion optics.Additional examples for detection of externally generated ions are proton transferreactions and devices using chemicalionization or laser-induced ionization.The process can be set to be finished after adisruption or when reaching a pre-selected filmdepth (material).491 Fundamentals of mass spectrometry1.4.3 Mass spectrometric plasmadiagnosticsPlasma processes are a key component inalmost all current procedures for vacuumcoating, cleaning and modification of surfaces.A plasma (in general a gas discharge) consists of positive and negative ions, electrons and neutral gas particles.
Beside thetype (mass), the power distribution andthe density of the species are characteristic for the plasma.To optimize and monitor such processes,simple or complex expensive plasma diagnostics must be executed depending onthe process. Various techniques are in use;no method covers all plasma parameterscompletely.Langmuir probes can be used to determine the plasma density, the electronenergy distribution, the plasma potentialand the floating potential in low-pressuregas discharges.(Use of such probes is very easy, but interpretation of the results requires morescientific knowledge on behalf of the user.)Some species can be detected very sensitively with optical analysis methods. Withappropriate expenditure, the average energy of the particles can be determined inindividual cases. Calibration is necessaryso that particle densities can be measured.The difficulty of the procedure and its viability depends on the experimental design.Stationary resolution measurements bymeans of optical emission spectroscopyare difficult to impossible to generate.A combination of a quadrupole mass filterand an electrostatic energy analyzer, inshort, a Plasma Process Monitor, can beused to measure the following plasmaparameters:The energy distribution of positive andnegative ions and their massThe energy distribution of the neutralparticlesNeutral particles first must be ionized inorder to be detected via a mass spectrometer.
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