Fundamentals of Vacuum Technology (1248463), страница 41
Текст из файла (страница 41)
For air and nitrogen, the value is aboutp á λ = 6 á 10Ð3 mbar á cm. At p = 1 á 10-4 bar this corresponds to a meanfree path length of λ = 60 cm. This pressure is generally taken to be theminimum vacuum for mass spectrometers. The emergency shut-downfeature for the cathode (responding to excessive pressure) is almost alwaysset for about 5 á 10-4 mbar. The desire to be able to use quadrupolespectrometers at higher pressures too, without special pressure convertors,led to the development of the XPR sensor at INFICON (XPR standing forextended pressure range).
To enable direct measurement in the range ofabout 2 á 10-2 mbar, so important for sputter processes, the rod system wasreduced from 12 cm to a length of 2 cm. To ensure that the ions canexecute the number of transverse oscillations required for sharp massseparation, this number being about 100, the frequency of the current in theXPR sensor had to be raised from about 2 MHz to approximately 6 timesthat value, namely to 13 MHz. In spite of the reduction in the length of therod system, ion yield is still reduced due to dispersion processes at suchhigh pressures. Additional electronic correction is required to achieveperfect depiction of the spectrum.
The dimensions of the XPR sensor are sosmall that it can ÒhideÓ entirely inside the tubulation of the connection flange(DN 40, CF) and thus occupies no space in the vacuum chamber proper.Fig. 4.1a shows the size comparison for the normal high-performancesensors with and without the Channeltron SEMP, the normal sensor withchannel-plate SEMP. Fig. 4.1b shows the XPR sensor.
The high vacuumrequired for the sensor is often generated with a TURBOVAC 50turbomolecular pump and a D 1.6 B rotary vane pump. With its greatcompression capacity, a further advantage of the turbomolecular pumpwhen handling high molar mass gases is that the sensor and its cathodeare ideally protected from contamination from the direction of the forepump.4.3.1 Design of the sensorOne can think of the sensor as having been derived from anextractor measurement system (see Fig. 4.3), whereby theseparation system was inserted between the ion source and theion trap.4.3.1.1The normal (open) ion sourceThe ion source comprises an arrangement of the cathode, anode andseveral baffles. The electron emission, kept constant, causes partialionization of the residual gas, into which the ion source is ÒimmersedÓ ascompletely as possible.
The vacuum in the vicinity of the sensor willnaturally be influenced by baking the walls or the cathode. The ions will beextracted through the baffles along the direction of the separation system.One of the baffles is connected to a separate amplifier and Ð entirelyindependent of ion separation Ð provides continuous total pressuremeasurement (see Fig.
4.4). The cathodes are made of iridium wire andhave a thorium oxide coating to reduce the work associated with electrondischarge. (For some time now the thorium oxide has gradually beenreplaced by yttrium oxide.) These coatings reduce the electron dischargework function so that the desired emission flow will be achieved even atlower cathode temperatures.
Available for special applications are tungstencathodes (insensitive to hydrocarbons but sensitive to oxygen) or rheniumcathodes (insensitive to oxygen and hydrocarbons but evaporate slowlyduring operation due to the high vapor pressure).AnodeCathodeFocussing plate(extractor diaphragm) Ion source exitdiaphragm(total pressure measurement)CathodeAnodeQuadrupole exitdiaphragmReflectorIon trapShieldingExtractor measurement systemIon sourceFig. 4.2Quadrupole separation systemSchematic for quadrupole mass spectrometerIon detectorTranspector measurement headFig.
4.3Quadrupole mass spectrometer Ð Extractor ionization vacuum gauge96HomeMass spectrometryCathodexz plane1-+-+Rod:+U+V, cos ωTransmission:low-pass+Shielding+Anode2yz planeRod:+UTransmission:full++i++Rod:–UTransmission:none+Rod:–U–V · cos ωTransmission:high-pass-i+3Extractor diaphragmV1Total pressure diaphragmVV1i+Vi+4M1Fig. 4.4i+5The quadrupole separation systemHere the ions are separated on the basis of their mass-to-charge ratio. Weknow from physics that the deflection of electrically charged particles (ions)from their trajectory is possible only in accordance with their ratio of massto charge, since the attraction of the particles is proportional to the chargewhile the inertia (which resists change) is proportional to its mass.
Theseparation system comprises four cylindrical metal rods, set up in paralleland isolated one from the other; the two opposing rods are charged withidentical potential. Fig. 4.2 shows schematically the arrangement of therods and their power supply. The electrical field Φ inside the separationsystem is generated by superimposing a DC voltage and a high-frequencyAC voltage:Φ = (U + V × cos ωt) á (x2 Ð y2) / r02r0 = radius of the cylinder which can be inscribed inside the system of rodsExerting an effect on a single charged ion moving near and parallel to thecenter line inside the separation system and perpendicular to its movementare the forcesFx = − 2e ⋅ x ⋅ cos (ω ⋅ t )r02Fy = − 2e2 ⋅ y ⋅ cos (ω ⋅ t )r0Fz = 0The mathematical treatment of these equations of motion uses MathieuÕsdifferential equations. It is demonstrated that there are stable and unstableion paths.
With the stable paths, the distance of the ions from theseparation system center line always remains less than ro (passagecondition). With unstable paths, the distance from the axis will grow until theion ultimately collides with a rod surface. The ion will be discharged(neutralized), thus becoming unavailable to the detector (blockingcondition).Even without solving the differential equation, it is possible to arrive at apurely phenomenological explanation which leads to an understanding ofthe most important characteristics of the quadrupole separation system.If we imagine that we cut open the separation system and observe thedeflection of a singly ionized, positive ion with atomic number M, moving intwo planes, which are perpendicular one to the other and each passingthrough the centers of two opposing rods.
We proceed step-by-step andM1MSuperimposition of the xy and yz planesOpen ion source4.3.1.2MyzxzIIIU .. Selectivity (resolution)VFig. 4.5( UV fixed)IIIMSensitivityPhenomenological explanation of the separation systemfirst observe the xz plane (Fig. 4.5, left) and then the yz plane (Fig.4.5,right):1. Only DC potential U at the rods:xz plane (left): Positive potential of +U at the rod, with a repellant effecton the ion, keeping it centered; it reaches the collector (→ passage).yz plane (right): Negative potential on the rod -U, meaning that at eventhe tiniest deviations from the center axis the ion will be drawn towardthe nearest rod and neutralized there; it does not reach the collector(→ blocking).2.
Superimposition of high-frequency voltage V á cos ω t:xz plane (left): Rod potential +U + V á cos ω t. With rising AC voltageamplitude V the ion will be excited to execute transverse oscillationswith ever greater amplitudes until it makes contact with a rod and isneutralized.
The separation system remains blocked for very largevalues of V.yz plane (right): Rod potential -U -V á cos ω t. Here againsuperimposition induces an additional force so that as of a certain valuefor V the amplitude of the transverse oscillations will be smaller than theclearance between the rods and the ion can pass to the collector at verylarge V.3. Ion emission i+ = i+ (V) for a fixed mass of M:xz plane (left): For voltages of V < V1 the deflection which leads to anescalation of the oscillations is smaller than V1, i.e. still in the ÒpassÓrange. Where V > V1 the deflection will be sufficient to induce escalationand thus blockage.yz plane (right): For voltages of V < V1 the deflection which leads to thedamping of the oscillations is smaller than V1, i.e.
still in the ÒblockÓrange. Where V > V1 the damping will be sufficient to settle oscillations,allowing passage.4. Ion flow i+ = i+ (M) for a fixed ratio of U / V:Here the relationships are exactly opposite to those for i+ = i+ (V) since97HomeMass spectrometrythe influence of V on light masses is greater than on heavy masses.4.3.1.3xz plane: For masses of M < M1 the deflection which results inescalation of the oscillations is greater than at M1, which means that theions will be blocked.
At M > M1 the deflection is no longer sufficient forescalation, so that the ion can pass.Once they have left the separation system the ions will meet the ion trap ordetector which, in the simplest instance, will be in the form of a Faradaycage (Faraday cup). In any case the ions which impinge on the detector willbe neutralized by electrons from the ion trap. Shown, after electricalamplification, as the measurement signal itself is the corresponding Òionemission streamÓ. To achieve greater sensitivity, a secondary electronmultiplier pickup (SEMP) can be employed in place of the Faraday cup.yz plane: For masses of M < M1 the deflection which results in dampingof the oscillations is greater than at M1, which means that the ion willpass.
At M > M1 the damping is not sufficient to calm the system and sothe ion is blocked.5. Combination of the xz and yz planes. In the superimposition of the ioncurrents i+ = i+ (M) for both pairs of rods (U / V being fixed) there arethree important ranges:Range I: No passage for M due to the blocking behavior of the xz pair ofrods.Range II: The pass factor of the rod systems for mass M is determinedby the U/V ratio (other ions will not pass). We see that great permeability(corre- sponding to high sensitivity) is bought at the price of lowselectivity (= resolution, see Section 4.5).
Ideal adjustment of theseparation system thus requires a compromise between these two properties. To achieve constant resolution, the U/V ratio will remain constantover the entire measurement range. The Òatomic numberÓ M (see 4.6.1)of the ions which can pass through the separation system must satisfythis condition:The measurement system (detector)Channeltrons or Channelplates can be used as SEMPs.
SEMPs arevirtually inertia-free amplifiers with gain of about 10+6 at the outset; this willindeed drop off during the initial use phase but will then become virtuallyconstant over a long period of time. Fig. 4.6 shows at the left the basicconfiguration of a Faraday ion trap and, on the right, a section through aChanneltron.
When recording spectra the scanning period per mass line t0and the time constants of the amplifier t should satisfy the condition thatt0 = 10 τ. In modern devices such as the TRANSPECTOR the otherwiseunlimited selection of the scanning period and the amplifier time constantswill be restricted by microprocessor control to logical pairs of values.mV≈M=e14.438 ⋅ f 2 ⋅ ro2V = High-frequency amplitude,rO = Quadrupole inscribed radiusf = High-frequencyAs a result of this linear dependency there results a mass spectrum withlinear mass scale due to simultaneous, proportional modification of Uand V.Range III: M cannot pass, due to the blocking characteristics of the yzpair of rods.Separation system outputPositive ionCollectorElectron suppressorFaraday cupConnectionto front endof the insidesurfaceAmplifierAmplifierResistance of the inner surfaceResistance ≈ 108 ΩNegative.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
