Fundamentals of Vacuum Technology (1248463), страница 45
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Among all the conceivable solutions it is now necessary to+identify set Ig+* which after inverse calculation to the partial ion flows Im*will exhibit the smallest squared deviation from the partial ion currents i+mactually measured. Thus:∑(im − im*) = min++2This minimization problem is mathematically identical to the solution ofanother equation system4.7.3 Process-oriented software –Transpector-Ware for WindowsTranspector-Ware is based on an entirely new philosophy. During thecourse of the process (and using settings Ð the ÒrecipeÓ Ð determinedbeforehand) data will be recorded continuously Ð like the individual framesin a video. These data can be stored or otherwise evaluated.
It is possiblein particular to analyze interesting process sections exactly, both during theprocess and retroactively, once the process has run to completion, withouthaving to interrupt the measurement operations which are running in thebackground. Where ongoing monitoring of identical processes isundertaken the program can generate statistics (calculating mean valuesand standard deviations) from which a bandwidth for Òfavorable processoperationÓ can be derived. Error reports are issued where limit values areexceeded.108HomeMass spectrometry4.7.4 Development software –TranspectorViewThis software used to for develop custom software versions for specialsituations.
It is based on the LabView development package and includesthe drivers required to operate the Transpector.seconds (thus by an overall factor of 100), respectively. By comparison, inthe sequence a-d-e-f, at constant integration time, the total pressure wasraised in three steps, from 7.2 á 10-6 mbar to 7.2 á 10-5 mbar (or by a factorof just 10 overall).4.94.8Partial pressure regulationSome processes, such as reactive sputter processes, require the mostconstant possible incidence rates for the reacting gas molecules on thesubstrate being coated.The Òincidence rateÓ is the same as the Òimpingement rateÓ discussed inChapter 1; it is directly proportional to the partial pressure. The simplestattempt to keep the partial pressure for a gas component constant isthroughput by regulating with a flow controller; it does have thedisadvantage that the regulator cannot determine whether, when and wherethe gas consumption or the composition of the gas in the vacuum chamberchanges.
The far superior and more effective option is partial pressurecontrol using a mass spectrometer via gas inlet valves. Here the significantpeaks of the gases being considered are assigned to channels in the massspectrometer. Suitable regulators compare the analog output signals forthese channels with set-point values and derive from the differencebetween the target and actual values for each channel the appropriateactuation signal for the gas inlet valve for the channel. A configuration ofthis kind has been realized to control six channels in the QUADREX PPC.Gas inlet valves matching the unit can also be delivered.The gas used to measure the impingement rate (partial pressure) mustnaturally be drawn from a representative point in the vacuum chamber.When evaluating the time constant for a regulation circuit of this type it isimportant to take into account all the time aspects and not just the electricalsignal propagation and the processing in the mass spectrometer, but alsothe vacuum-technology time constants and flow velocities, as illustrated inFigure 4.17.
Pressure converters or unfavorably installed gas inlet linesjoining the control valve and the vacuum vessel will make particularly largecontributions to the overall time constant. It is generally better to establish afavorable S/N ratio with a large signal (i.e. through an inlet diaphragm witha large opening) rather than with long integration periods at the individualchannels. Contrasted in Figure 4.18 are the effects of boosting pressureand lengthening the integration time on signal detectability.
In depictions a,b and c only the integration period was raised, from 0.1 to 1.0 and 10Maintenance(Cathode service life, sensor balancing, cleaning the ion source and rodsystem)The service life of the cathode will depend greatly on the nature of theloading. Experience has shown that the product of operating periodmultiplied by the operating pressure can serve as a measure for theloading. Higher operating pressures (in a range of 1 á 10-4 to 1 á 10-3 mbar)have a particularly deleterious effect on service life, as do certain chemicalinfluences such as refrigerants, for example. Changing out the cathode isquite easy, thanks to the simple design of the sensor. It is advisable,however, to take this opportunity to change out or at least clean the entireion source.Sensor balancing at the mass axis (often erroneously referred to ascalibration) is done today in a very easy fashion with the software (e.g.SQX, Transpector-Ware) and can be observed directly in the screen.Naturally, not only the arrangement along the mass axis will be determinedhere, but also the shape of the lines, i.e.
resolution and sensitivity (seeSection 4.5).It will be necessary to clean the sensor only in exceptional cases where itis heavily soiled. It is usually entirely sufficient to clean the ion source,which can be easily dismantled and cleaned. The rod system can becleaned in an ultrasonic bath once it has been removed from the configuration.
If dismantling the system is unavoidable due to particularlystubborn grime, then the adjustment of the rods which will be requiredafterwards will have to be carried out at the factory.cbat4t5Mass spectrometerdPressure stageRegulation valvet6ee^ef_t1Vacuum vesselt2eft3efSensorTMP50CFFig. 4.17 Partial shares for overall time constantsFig. 4.18 Improving the signal-to-noise ratio by increasing the pressure or extending theintegration time109HomeLeak detection5. Leaks and theirdetectionApart from the vacuum systems themselves and the individual componentsused in their construction (vacuum chambers, piping, valves, detachable[flange] connections, measurement instruments, etc.), there are largenumbers of other systems and products found in industry and researchwhich must meet stringent requirements in regard to leaks or creating a socalled ÒhermeticÓ seal.
Among these are many assemblies and processesin the automotive and refrigeration industries in particular but also in manyother branches of industry. Working pressure in this case is often aboveambient pressure. Here Òhermetically sealedÓ is defined only as a relativeÒabsence of leaksÓ. Generalized statements often made, such as Ònodetectable leaksÓ or Òleak rate zeroÓ, do not represent an adequate basisfor acceptance testing. Every experienced engineer knows that properlyformulated acceptance specifications will indicate a certain leak rate (seeSection 5.2) under defined conditions.
Which leak rate is acceptable is alsodetermined by the application itself.5.2No vacuum device or system can ever be absolutely vacuum-tight and itdoes not actually need to be. The simple essential is that the leak rate below enough that the required operating pressure, gas balance and ultimatepressure in the vacuum container are not influenced. It follows that therequirements in regard to the gas-tightness of an apparatus are the morestringent the lower the required pressure level is. In order to be able toregister leaks quantitatively, the concept of the Òleak rateÓ with the symbolQL was introduced; it is measured with mbar á l/s or cm3/s (STP) as the unitof measure.
A leak rate of QL = 1 mbar á l/s is present when in an enclosed,evacuated vessel with a volume of 1 l the pressure rises by 1 mbar persecond or, where there is positive pressure in the container, pressure dropsby 1 mbar. The leak rate QL defined as a measure of leakiness is normallyspecified in the unit of measure mbar á l/s. With the assistance of the statusequation (1.7) one can calculate QL when giving the temperature T and thetype of gas M, registering this quantitatively as mass flow, e.g. in the g/sunit of measure. The appropriate relationship is then:QL =5.1Types of leaksLeak rate, leak size, mass flow∆(p ⋅ V ) R ⋅ T ∆m⋅=∆tM ∆t(5.1)Differentiation is made among the following leaks, depending on the natureof the material or joining fault:where R = 83.14 mbar á l/mol á K, T = temperature in K; M = molar mass ing/mole; ∆m for the mass in g; ∆t is the time period in seconds. Equation5.1 is then used• Leaks in detachable connections:Flanges, ground mating surfaces, coversa) to determine the mass flow ∆m / ∆t at a known pV gas flow of ∆p á V/∆t(see in this context the example at 5.4.1) or• Leaks in permanent connections:Solder and welding seams, glued jointsb) to determine the pV leak gas flow where the mass flow is known (seethe following example).• Leaks due to porosity: particularly following mechanical deformation(bending!) or thermal processing of polycrystalline materials and castcomponentsExample for case b) above:• Thermal leaks (reversible): opening up at extreme temperature loading(heat/ cold), above all at solder joints• Apparent (virtual) leaks: quantities of gas will be liberated from hollowsand cavities inside cast parts, blind holes and joints (also due to theevaporation of liquids)• Indirect leaks: leaking supply lines in vacuum systems or furnaces(water, compressed air, brine)• ÒSerial leaksÓ: this is the leak at the end of several Òspaces connected inseriesÓ, e.g.
a leak in the oil-filled section of the oil pan in a rotary vanepump• ÒOne-way leaksÓ: these will allow gas to pass in one direction but aretight in the other direction (very seldom)An area which is not gas-tight but which is not leaky in the sense that adefect is present would be the• Permeation (naturally permeability) of gas through materials such asrubber hoses, elastomer seals, etc. (unless these parts have becomebrittle and thus ÒleakyÓ).A refrigeration system using Freon (R 12) exhibits refrigerant loss of 1 g ofFreon per year (at 25 ¡C). How large is the leak gas flow QL? According toequation 5.1 for M(R12) = 121 g/mole:QL =∆( p ⋅V )8314. mbar ⋅ `⋅ 298K ⋅ 1g=∆tmol ⋅ K ⋅ 121g ⋅ mol –1⋅ 1year=. ⋅ 2.98 ⋅102⋅ 1 mbar ⋅ `8314⋅121⋅1.
⋅107s315=8314. ⋅ 2.98⋅102 –7 mbar ⋅ `⋅ 10 ⋅s1.21⋅102⋅ 315.= 65 ⋅10–7⋅mbar ⋅ `sThus the Freon loss comes to QL = 6.5 á 10Ð6 mbar á l/s. According to theÒrule of thumbÓ for high vacuum systems given below, the refrigerationsystem mentioned in this example may be deemed to be very tight.Additional conversions for QL are shown in Tables VIIa and VIIb inChapter 9.The following rule of thumb for quantitative characterization of high vacuumequipment may be applied:Total leak rate < 10-6 mbar á l/s:Equipment is very tight110HomeLeak detectionTotal leak rate 10-5 mbar á l/s:Equipment is sufficiently tightTotal leak rate > 10-4 mbar á l/s:Equipment is leakyA leak can in fact be ÒovercomeÓ by a pump of sufficient capacity because itis true that (for example at ultimate pressure pend and disregarding the gasliberated from the interior surfaces):pend=QLS(5.2)eff(QL Leak rate, Seff the effective pumping speed at the pressure vessel)Where Seff is sufficiently great it is possible Ð regardless of the value for theleak rate QL Ð always to achieve a pre-determined ultimate pressure ofpend.
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