Fundamentals of Vacuum Technology (1248463), страница 53
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When using a plastic bag as the envelope, the bag shouldbe pressed against the test specimen before filling it with helium in order toexpel as much air as possible and to make the measurement with thepurest helium charge possible. The entire outside surface of the test objectis in contact with the test gas.
If test gas passes through leaks and into thetest specimen, then the integral leak rate will be indicated, regardless of thenumber of leaks. In addition, it is necessary to observe when repeatingtesting in enclosed areas that the helium content of the room will rise quiterapidly when the envelope is removed. Using plastic bags is thus moreadvisable for Òone-offÓ testing of large plants.
The plastic envelope usedhere is often referred to as a ÒtentÓ.b) Rigid envelopeThe use of a solid vacuum vessel as the rigid envelope, on the other hand,is better for repetitive testing where an integral test is to be made. Whensolid envelopes are used it is also possible to recover the helium once thetest has been completed.5.7.4 “Bombing” test,“Storage under pressure”The ÒbombingÓ test is use to check the tightness of components which arealready hermetically sealed and which exhibit a gas-filled, internal cavity.The components to be examined (e.g. transistors, IC housings, dry-reedrelays, reed contact switches, quartz oscillators, laser diodes and the like)are placed in a pressure vessel which is filled with helium.
Operating withthe test gas at relatively high pressure (5 to 10 bar) and leaving the systemstanding over several hours the test gas (helium) will collect inside the123HomeLeak detectionleaking specimens. This procedure is the actual ÒbombingÓ. To make theleak test, then, the specimens are placed in a vacuum chamber followingÒbombingÓ, in the same way as described for the vacuum envelope test.The overall leak rate is then determined. Specimens with large leaks will,however, lose their test gas concentration even as the vacuum chamber isbeing evacuated, so that they will not be recognized as leaky during theactual leak test using the detector.
It is for this reason that another test toregister very large leaks will have to be made prior to the leak test in thevacuum chamber.5.8Industrial leak testingIndustrial leak testing using helium as the test gas is characterized aboveall by the fact that the leak detection equipment is fully integrated into themanufacturing line. The design and construction of such test units willnaturally take into account the task to be carried out in each case (e.g. leaktesting vehicle rims made of aluminum or leak testing for metal drums).Mass-produced, standardized component modules will be used whereverpossible. The parts to be examined are fed to the leak testing system(envelope test with rigid envelope and positive pressure [5.7.3.1b] orvacuum [5.7.3.2b] inside the specimen) by way of a conveyor system.There they will be examined individually using the integral methods andautomatically moved on.
Specimens found to be leaking will be shunted tothe side.The advantages of the helium test method, seen from the industrial point ofview, may be summarized as follows:• The leak rates which can be detected with this process go far beyond allpractical requirements.• The integral leak test, i.e. the total leak rate for all individual leaks,facilitates the detection of microscopic and sponge-like distributed leakswhich altogether result in leakage losses similar to those for a largerindividual leak.• The testing procedure and sequence can be fully automated.• The cyclical, automatic test system check (self-monitoring) of the deviceensures great testing reliability.• Helium is non-toxic and non-hazardous (no maximum allowableconcentrations need be observed).• Testing can be easily documented, indicating the parameters andresults, on a printer.Use of the helium test method will result in considerable increases inefficiency (cycling times being only a matter of seconds in length) and leadto a considerable increase in testing reliability.
As a result of this and due tothe EN/ISO 9000 requirements, traditional industrial test methods (waterbath, soap bubble test, etc.) will now largely be abandoned.124HomeThin film controllers/control units6Thin film controllersand control unitswith quartzoscillators6.1IntroductionIt took a long time to go from the coating of quartz crystals for frequencyfine tuning, which has long been in practice, to utilization of frequencychange to determine the mass per unit area as a microbalance with thepresent-day degree of precision.
In 1880 two brothers, J. and P. Curie,discovered the piezoelectric effect. Under mechanical loads on certainquartz crystal surfaces, electrical charges occur that are caused by theasymmetrical crystalline structure of SiO2. Conversely, in a piezocrystaldeformations appear in an electrical field and mechanical oscillations occurin an alternating field. A distinction is made between bending oscillations,thickness shear mode and thickness shear oscillations. Depending on theorientation of the cut plane to the crystal lattice, a number of different cutsare distinguished, of which only the so-called AT cut with a cut angle of35¡10" is used in thin film controllers because the frequency has a very lowtemperature dependence in the range between 0 and 50 ¡C with this cut.Accordingly, an attempt must be made not to exceed this temperaturerange during coating (water cooling of crystal holder).Since there is still a problem with Òquartz capacityÓ (i.e.
the maximumpossible coating thickness of the quartz at which it still oscillates reliably)despite refined technology, a number of approaches have been developedto expand this capacity:1. The use of several crystals, one behind the other, in a multiple crystalholder with automatic change and data updating in the event ofimminent failure of a quartz: CrystalSix.rel. frequency change (ppm)2.
The RateWatcher function, in which the quartz is alternately exposed tothe coating beam for a short time until all measurements and regulationhave been carried out and then remains covered by a shutter for alonger period of time.6.2Basic principles of coatingthickness measurement withquartz oscillatorsThe quartz oscillator coating thickness gauge (thin film controller) utilizesthe piezoelectric sensitivity of a quartz oscillator (monitor crystal) to thesupplied mass. This property is utilized to monitor the coating rate and finalthickness during vacuum coating.A very sharp electromechanical resonance occurs at certain discretefrequencies of the voltage applied.
If mass is added to the surface of thequartz crystal oscillating in resonance, this resonance frequency isdiminished. This frequency shift is very reproducible and is now understoodprecisely for various oscillation modes of quartz. Today this phenomenon,which is easy to understand in heuristic terms, is an indispensablemeasuring and process control tool, with which a coating increase of lessthan one atomic layer can be detected.In the late 1950s Sauerbrey and Lostis discovered that the frequency shiftconnected with the coating of the quartz crystal is a function of the changein mass due to the coating material in the following way:Mf ∆F∆F=or Mf = Mq ·withMq FqFq(6.1)Mfmass of the coatingMqmass of the quartz prior to coatingFqfrequency prior to coatingFcfrequency after coating∆F = Ff Ð Fc ... frequency shift due to coatingIf the following are now applied: Mf = (Mc Ð Mq) = Df á ρf á A andMq = Dq á ρq á A, where T = the coating thickness, ρ = density and A standsfor area while the index q stands for the state of the Òuncoated quartzÓ andc for the state after Òfrequency shift due to coatingÓ, the following results areobtained for the coating thickness:Df =K=Temperature (¡C)Fig.
6.1The selection of the ÒrightÓ crystal holder thus plays an important role in allmeasurements with quartz oscillators. Various crystal holder designs arerecommended for the different applications: with or without shutter,bakeable for UHV, double crystal holder or crystal six as well as specialversions for sputter applications. In addition to these important and moreÒmechanicalÓ aspects, the advances in measuring and control technologyand equipment features will be discussed in the following.FqFq· Dq · ρ q ·Dq · Fq · ρq2Fq=∆F∆F=K·ρfFq · ρ fwithNAT · ρq2FqwhereN = Fq á Dq is the frequency constant (for the AT cut NAT = 166100 Hz á cm)and ρq = 2.649 g/cm3 is the density of the quartz. The coating thickness isthus proportional to the frequency shift ∆F and inversely proportional to theNatural frequency as a function of temperature in an AT cut quartz crystal125HomeThin film controllers/control units→↔ENodeFig.
6.2Thickness shear oscillationsFig. 6.3density ρf of the coating material. The equationDf = K ·∆Fρffor the coating thickness was used in the first coating thickness measuringunits with Òfrequency measurementÓ ever used. According to this equation,a crystal with a starting frequency of 6.0 MHz displays a decline infrequency of 2.27 Hz after coating with 1• of aluminum (de = 2.77 g/cm3).In this way the growth of a fixed coating due to evaporation or sputteringcan be monitored through precise measurement of the frequency shift ofthe crystal. It was only when knowledge of the quantitative interrelationshipof this effect was acquired that it became possible to determine preciselythe quantity of material that is deposited on a substrate in a vacuum.Previously this had been practically impossible.6.3The shape of quartz oscillatorcrystalsRegardless of how sophisticated the electronic environment is, the maincomponent for coating measurement remains the monitor quartz crystal.Originally monitor quartzes had a square shape.
Fig. 6.4 shows theresonance spectrum of a quartz resonator with the design used today (Fig.6.3). The lowest resonance frequency is initially given by a thickness shearoscillation, which is called the fundamental wave. The characteristicmotions of the thickness shear oscillation are parallel to the main crystalboundary surfaces. In other words: the surfaces are shift antinodes, seeFig. 6.2.
The resonance frequencies slightly above the basic frequency arecalled ÒanharmonicÓ and are a combination of thickness shear andthickness rotation oscillation forms. The resonance frequency at aroundthree times the value for the fundamental wave is called Òquasi-harmonicÓ.Near the quasi-harmonic there are also a number of anharmonics with aslightly higher frequency.The design of the monitor crystals used nowadays (see Fig. 6.3) displays anumber of significant improvements over the original square crystals. Thefirst improvement was the use of round crystals.
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