Fundamentals of Vacuum Technology (1248463), страница 40
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In this way the unthrottled cross-section of thediaphragm controller is available again for rapid final drying. At the sametime the final drying procedure can be monitored by means of the pressuresensor PS.2) Pressure regulation by means of diaphragm controller with externalautomatic reference pressure adjustment (see Fig. 3.31)For automatic vacuum processes with regulated process pressure,presetting of the desired set pressure must often function and be monitoredautomatically.
If a diaphragm controller is used, this can be done byDC Diaphragm controllerPS Process pressure sensorRS Reference pressure sensorV1 Gas inlet valveV2 Pump valveV3 Gas inlet variable-leak valveTH ThrottleM Measuring and switching devicePP Process pumpRC Reference chamberPC Process chamberAP Auxiliary pumpCV Internal reference pressure controlvalveFig. 3.31 Diaphragm controller with external automatic reference pressure regulation93HomeVacuum measurementequipping the reference chamber with a measuring and switching deviceand a control valve block at the reference chamber.
The principle of thisarrangement is shown in Fig. 3.31.Mode of operation: Starting with atmospheric pressure, gas inlet valve V1 isclosed at the beginning of the process. Pump valve V2 opens. The processchamber is now evacuated until the set pressure, which is preset at themeasuring and switching device, is reached in the process chamber and inthe reference chamber.
When the pressure falls below the set switchingthreshold, pump valve V2 closes. As a result, the pressure value attained isÒcaughtÓ as the reference pressure in the reference chamber (RC) of thediaphragm controller (DC). Now the process pressure is automaticallymaintained at a constant level according to the set reference pressure bymeans of the diaphragm controller (DC).
If the reference pressure shouldrise in the course of the process due to a leak, this is automaticallydetected by the measuring and switching device and corrected by brieflyopening pump valve V2. This additional control function enhances theoperational reliability and extends the range of application.
Correcting theincreased reference pressure to the originally set value is of special interestfor regulated helium circuits because the pressure rise in the referencechamber (RC) of the diaphragm controller can be compensated for throughthis arrangement as a consequence of the unavoidable helium permeabilityof the controller diaphragm of FPM.To be able to change the reference pressure and thus increase the processpressure to higher pressures, a gas inlet valve must be additionallyinstalled at the process chamber.
This valve is opened by means of adifferential pressure switch (not shown in Fig. 3.31) when the desiredhigher process pressure exceeds the current process pressure by morethan the pressure differential set at the differential pressure switch.94HomeMass spectrometry4. Analysis of gas atlow pressures usingmass spectrometry4.1GeneralAnalyses of gases at low pressures are useful not only when analyzing theresidual gases from a vacuum pump, leak testing at a flange connection orfor supply lines (compressed air, water) in a vacuum. They are alsoessential in the broader fields of vacuum technology applications andprocesses.
For example in the analysis of process gases used in applyingthin layers of coatings to substrates. The equipment used for qualitativeand/or quantitative analyses of gases includes specially developed massspectrometers with extremely small dimensions which, like any othervacuum gauge, can be connected directly to the vacuum system. Their sizedistinguishes these measurement instruments from other massspectrometers such as those used for the chemical analyses of gases.
Thelatter devices are poorly suited, for example, for use as partial pressuremeasurement units since they are too large, require a long connector line tothe vacuum chamber and cannot be baked out with the vacuum chamberitself. The investment for an analytical mass spectrometer would beunjustifiably great since, for example, the requirements as to resolution arefar less stringent for partial pressure measurements. Partial pressure isunderstood to be that pressure exerted by a certain type of gas within a mixof gases. The total of the partial pressures for all the types of gas gives thetotal pressure.
The distinction among the various types of gases isessentially on the basis of their molar masses. The primary purpose ofanalysis is therefore to register qualitatively the proportions of gas within asystem as regards the molar masses and determine quantitatively theamount of the individual types of gases associated with the various atomicnumbers.Partial pressure measurement devices which are in common use comprisethe measurement system proper (the sensor) and the control devicerequired for its operation.
The sensor contains the ion source, theseparation system and the ion trap. The separation of ions differing inmasses and charges is often effected by utilizing phenomena which causethe ions to resonate in electrical and magnetic fields.Initially, the control units were quite clumsy and offered uncountablemanipulation options. It was often the case that only physicists were able tohandle and use them.
With the introduction of PCs the requirements inregard to the control units became ever greater. At first, they were fittedwith interfaces for linkage to the computer. Attempts were made later toequip a PC with an additional measurement circuit board for sensoroperation. TodayÕs sensors are in fact transmitters equipped with anelectrical power supply unit attached direct at the atmosphere side;communication with a PC from that point is via the standard computer ports(RS 232, RS 485). Operating convenience is achieved by the softwarewhich runs on the PC.caba: High-performance sensor with Channeltronb: Compact sensor with Micro-Channelplatec: High-performance sensor with Faraday cupFig.
4.1a TRANSPECTOR sensors4.2A historical reviewFollowing ThomsonÕs first attempt in 1897 to determine the ratio of chargeto mass e/m for the electron, it was quite some time (into the 1950s) beforea large number and variety of analysis systems came into use in vacuumtechnology. These included the Omegatron, the Topatron and ultimately thequadrupole mass spectrometer proposed by Paul and Steinwedel in 1958,available from INFICON in its standard version as the TRANSPECTOR(see Fig.
4.1). The first uses of mass spectrometry in vacuum-assistedprocess technology applications presumably date back to BackusÕ work inthe years 1943 / 44. He carried out studies at the Radiographic Laboratories at the University of California. Seeking to separate uranium isotopes,he used a 180¡ sector field spectrometer after Dempster (1918), which hereferred to as a Òvacuum analyzerÓ. Even today a similar term, namely theÒresidual gas analyzerÓ (RGA), is frequently used in the U.S.A.
and the U.K.instead of Òmass spectrometerÓ. TodayÕs applications in process monitoringare found above all in the production of semiconductor components.Fig. 4.1b TRANSPECTOR XPR sensor95HomeMass spectrometry4.3The quadrupole massspectrometer (TRANSPECTOR)The ion beam extracted from the electron impact ion source is diverted intoa quadrupole separation system containing four rod-shaped electrodes. Thecross sections of the four rods form the circle of curvature for a hyperbolaso that the surrounding electrical field is nearly hyperbolic.
Each of the twoopposing rods exhibits equal potential, this being a DC voltage and asuperimposed high-frequency AC voltage (Fig. 4.2). The voltages appliedinduce transverse oscillations in the ions traversing the center, between therods. The amplitudes of almost all oscillations escalate so that ultimately theions will make contact with the rods; only in the case of ions with a certainratio of mass to charge m/e is the resonance condition which allowspassage through the system satisfied. Once they have escaped from theseparation system the ions move to the ion trap (detector, a Faraday cup)which may also take the form of a secondary electron multiplier pick-up(SEMP).The length of the sensor and the separation system is about 15 cm. Toensure that the ions can travel unhindered from the ion source to the iontrap, the mean free path length inside the sensor must be considerablygreater than 15 cm.
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