Fundamentals of Vacuum Technology (1248463), страница 42
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high voltage6R ≈ 4 · 10 ΩFig. 4.6Left: Principle of the Faraday cup; Right: Configuration of the Channeltron98HomeMass spectrometry4.4Gas admission and pressureadaptationareas for the metallic, glass and ceramic components will be needed alongwith specifications on the material and dimensions of the cathode (andultimately regarding the electron impact energy for the ion source as well).4.4.1 Metering valveThe simplest way to adapt a classical mass spectrometer to pressuresexceeding 1 á 10-4 mbar is by way of a metering valve. The inherentdisadvantage is, however, that since the flow properties are notunequivocally defined, a deviation from the original gas composition mightresult.4.4.2 Pressure converterIn order to examine a gas mix at total pressure exceeding 1 á 10-4 mbar it isnecessary to use pressure converters which will not segregate the gases.Figure 4.7 is used to help explain how such a pressure converter works:a.
Process pressure < 1 mbar: Single-stage pressure converter. Gas isallowed to pass out of the vacuum vessel in molecular flow, through adiaphragm with conduc- tance value L2 and into the Òsensor chamberÓ (withits own high vacuum system). Molecular flow causes segregation but thiswill be independent of the pressure level (see Section 1.5). A seconddiaphragm with molecular flow, located between the sensor chamber andthe turbomolecular pump, will compensate for the segregation occurring atL2.b.
Process pressure > 1 mbar: Two-stage pressure converter. Using a small(rotary vane) pump a laminar stream of gas is diverted from the roughvacuum area through a capillary or diaphragm (conductance value L3).Prior to entry into the pump, at a pressure of about 1 mbar, a small part ofthis flow is again allowed to enter the sensor chamber through thediaphragm with conductance value L2, again as molecular flow.A falsification of the gas composition resulting from adsorption andcondensation can be avoided by heating the pressure converter and thecapillary.To evaluate the influence on the gas composition by the measurement unititself, information on the heating temperature, the materials and surfaceNon-segregating gas inlet systemStage Bp ≤ 10 –4 mbarp = 1 ...
10 mbarL2Massspectrometer^_^_SeffSeff À L1 → Seff ~QPumpingLaminar flow2L1L3QHVL1L2 molecular → λ À dL1~ CDM1~ CDMp = 10 ... 1000 mbarCapillaryMolecular flowL2Stage A4.4.3 Closed ion source (CIS)In order to curb Ð or avoid entirely Ð influences which could stem from thesensor chamber or the cathode (e.g. disturbance of the CO-CO2 equilibriumby heating the cathode) a closed ion source (CIS) will be used in manycases.The CIS is divided into two sections: a cathode chamber where theelectrons are emitted, and an impact chamber, where the impact ionizationof the gas particles takes place.
The two chambers are pumpeddifferentially: the pressure in the cathode chamber comes to about10-5 mbar, that in the impact room about 10-3 mbar. The gas from thevacuum chamber is allowed to pass into the impact chamber by way of ametal-sealed, bakeable valve (pressure converter, ultrahigh vacuumtechnology).
There high-yield ionization takes place at about 10-3 mbar. Theelectrons exerting the impact are emitted in the cathode chamber at about10-5 mbar and pass through small openings from there into the impactchamber. The signal-to-noise ratio (residual gas) via ˆ vis the open ionsource will be increased overall by a factor of 10+3 or more.
Figure 4.8shows the fundamental difference between the configurations for open andclosed ion sources for a typical application in sputter technology. With themodified design of the CIS compared with the open ion source in regard toboth the geometry and the electron energy (open ion source 102 eV, CIS75 or 35 eV), different fragment distribution patterns may be found where alower electron energy level is selected. For example, the argon36++ isotopeat mass of 18 cannot be detected at electron energy of less than 43.5 eVand can therefore not falsify the detection of H2O+ at mass 18 in the sputterprocesses using argon as the working gas Ð processes which are of greatimportance in industry.4.4.4 Aggressive gas monitor (AGM)In many cases the process gas to be examined is so aggressive that thecathode would survive for only a short period of time.
The AGM uses theproperty of laminar flow by way of which there is no ÒreverseÓ flow of anykind. Controlled with a separate AGM valve, a part of the working gas fed tothe processes is introduced as Òpurging gasÓ, ahead of the pressureconverter, to the TRANSPECTOR; this sets up a flow toward the vacuumchamber. Thus process gas can reach the TRANSPECTOR only with theAGM valve closed. When the valve is open the TRANSPECTOR sees onlypure working gas. Fig. 4.9 shows the AGM principle.NosegregationQHV ¿ QPumping(Transition laminar/molecular)1MCDSeff compensates for L2 → No segregationSFig. 4.7Principle of the pressure converter (stage B only in the single-stage version andstages A and B in two-stage units)99HomeMass spectrometryProcess: 10 -2 mbarProcess: 10 -2 mbar(Elastomer)10-510ValveInlet diaphragm-5(Metal)10-510-3Example of the sputterprocessTo be detected is 1 ppm N2 ascontamination in argon,the working gas10 -510 -510 -5Impact chamberCathode chamber„Exit diaphragm“10 -51 ppm N 2 at the inlet:1 ppm N 2 at the inlet:10 -6 · 10 -3 mbar = 10 -9 mbar10 -6 ·10 -5 mbar = 10 -11 mbarExit diaphragmBackground:Residual gas (valve closed) 10 -6 mbar totaltotal, including 1% by mass 28 : 10 -8 mbarFig.
4.8PumpBackground:Residual gas (valve closed) 10 -7 mbar totaltotal, including 1% by mass 28 : 10 -9 mbarPumpBackground noise1 ppmSignal at 1‰ of backgroundcannot be detectedBackground noiseSignal twice the background noiseamplitude; can just be clearly detectedOpen ion source (left) and closed ion source (right)Impact chamberWorking gas for the process (Ar)Cathode chamber“Exit diaphragms”AGM protective gas valve10 -5Process:e.
g. 50 mbar10 -3DiaphragmFig. 4.910 -510 -5PumpPrinciple behind the aggressive gas monitor (AGM)100HomeMass spectrometry4.5Descriptive values in massspectrometry (specifications)A partial pressure measurement unit is characterized essentially by thefollowing properties (DIN 28 410):4.5.4 Smallest detectablepartial pressureThe smallest detectable partial pressure is defined as a ratio of noiseamplitude to sensitivity:Pmin =4.5.1 Line width (resolution)The line width is a measure of the degree to which differentiation can bemade between two adjacent lines of the same height.
The resolution isnormally indicated. It is defined as: R = M / ∆M and is constant for thequadrupole spectrometer across the entire mass range, slightly greater than1 or ∆M < 1.Often an expression such as Òunit resolution with 15% valleyÓ is used. Thismeans that the Òbottom of the valleyÓ between two adjacent peaks ofidentical height comes to 15 % of the height of the peak or, put anotherway, at 7.5 % of its peak height the line width DM measured across anindividual peak equals 1 amu (atomic mass unit); see in this context theschematic drawing in Fig. 4.10.The mass range is characterized by the atomic numbers for the lightest andheaviest ions with a single charge which are detected with the unit.4.5.3 SensitivitySensitivity E is the quotient of the measured ion flow and the associatedpartial pressure; it is normally specified for argon or nitrogen:(mbar)Æ á i+ = Noise amplitudeRExample (from Fig.
4.11):Sensitivity E =1⋅ 10– 4AmbarNoise amplitude Æ á i+ = 4 á 10Ð14 ARpmin(FC) =4 ⋅ 10 – 14A= 4 ⋅10 – 10 mbar1⋅10 – 4A / mbarThe definition is:SDPPR = pmin / pΣ (ppm)This definition, which is somewhat ÒclumsyÓ for practical use, is to beexplained using the detection of argon36 in the air as the example: Aircontains 0.93 % argon by volume; the relative isotope frequency of Ar40 toAr36 is 99.6 % to 0.337 %. Thus the share of Ar36 in the air can becalculated as follows:0.93 á 10Ð2 á 0.337 á 10Ð2 = 3.13 á 10Ð5 = 31.3 ppmi+ A pG mbar(4.1)Figure 4.11 shows the screen print-out for the measurement.
The peakheight for Ar36 in the illustration is determined to be 1.5 á 10-13 A and noiseamplitude Æ á i+ to be 4 á 10-14 A. The minimum detectable concentration isRthat concentration at which the height of the peak is equal to the noiseamplitude. This results in the smallest measurable peak height being1.5 á 10-13 A/2.4 á 10-14 A = 1.875. The smallest detectable concentration isthen derived from this by calculation to arrive at:Typical values are:Faraday cup: E = 1⋅10 – 4+2SEMP: E = 1⋅10E4.5.5 Smallest detectable partial pressureratio (concentration)4.5.2 Mass rangeE=∆ ⋅iR+AmbarAmbar31.3 á 10Ð6 / 1.875 = 16.69 á 10Ð6 = 16.69 ppm.100%i+15%7,5%1 amuMM+1Atomic number∆MFig. 4.10 Line width Ð 15 % valleyFig.
4.11 Detection of Argon36101HomeMass spectrometry4.5.6 Linearity range4.6The linearity range is that pressure range for the reference gas (N2, Ar) inwhich sensitivity remains constant within limits which are to be specified(± 10 % for partial pressure measurement devices).4.6.1 Ionization and fundamentalproblems in gas analysisIn the range below 1 á 10-6 mbar the relationship between the ion flow andpartial pressure is strictly linear.
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