Fundamentals of Vacuum Technology (1248463), страница 59
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If several plates are placed on one common carrier, the coatingprocesses can be carried out in a system using a similar principle to thatused for glass coating. However, most disks must be coated on both sidesand there are substantially greater low particle contamination requirementsas compared to glass coating.
Therefore, in-line systems for data memoriesuse a vertical carrier that runs through the system (Fig. 7.10). The sputtercathodes in the process stations are mounted on both sides of the carrierso that the front and back side of the disk can be coated simultaneously.An entirely different concept is applied for coating of single disks. In thiscase the different process stations are arranged in a circle in a vacuumchamber (Fig. 7.11).
The disks are transferred individually from a magazineto a star-shaped transport arm. The transport arm cycles one station furtherafter each process step and in this way transports the substrates from oneprocess station to the next. During cycling all processes are switched offand the stations are vacuum-linked to each other. As soon as the arm hasreached the process position, the individual stations are seperated fromeach other by closing seals.
Each station is pumped by means of its ownturbomolecular pump and the individual processes are started. As manyprocess stations as there are in the system as many processes can beperformed in parallel. By sealing off the process stations, excellent vacuumseparation of the individual processes can be achieved. However, since theslowest process step determines the cycle interval, two process stationsmay have to be dedicated for particularly timeconsuming processes.The glass coating process requires high gas flows for the sputter processesas well as low hydrocarbon concentration.
The only vacuum pump whichsatisfies these requirements as well as high pumping speed stability overtime are turbo-molecular pumps which are used almost exclusively.While the transfer and process chambers are constantly evacuated, theentrance and exit chambers must be periodically vented and thenevacuated again. Due to the large volumes of these chambers and theshort cycle times, a high pumping speed is required. It is provided bycombinations of rotary vane pumps and Roots pumps.
For particularly shortcycle times gas cooled Roots pumps are also used.Process chamber1Intermediate chamberL1Z←S1SlitsProcess chamber2LZ2→SZS2to backing pumpsEntrance chamberTransfer chamber 2Transfer chamber 1Exit chamberSputter chambersFig. 7.8Plant for coating glass panes Ð 3-chamber in-line system, throughput up to3,600,000 m2 / yearL1Z, LZ2 = conductance between intermediate chamber and process chamber 1 or 2= pumping speed at intermediate chamberSZS1, S2 = pumping speed at process chamber 1 or 2Fig. 7.9Principle of chamber separation through pressure stages137HomeApplications of vacuum technologyFig.
7.10 Plant for coating data storage disks with carrier transport systemFig. 7.11 Plant for individual coating of data storage disks138HomeInstructions for equipment operation8. Instructions forvacuum equipmentoperation8.1Causes of faults where thedesired ultimate pressure is notachieved or is achieved tooslowlyIf the desired ultimate pressure is not reached at all in vacuum equipmentor if it is attained only after an excessively long pumping period, then thefollowing problems may be the reason:If the desired ultimate pressure is not reached, then• the apparatus may be leaky or dirty,• the pump may dirty or damaged,• the vacuum gauge may be defective.If the desired ultimate pressure is reached only after a very long runningtime, then• the apparatus may be dirty,• the pumping line may be restricted,• the pump may be dirty or of too small a capacity,• the pumping speed may be restricted due to other causes.In order to locate the fault, one normally proceeds by separating theevacuated vessel from the pump system (where this is possible) andchecking the vessel alone for leaks and contamination using the pressurerise method, for example.
If it has been found that the vessel is free ofdefects in this regard, then the measurement system will be checked forcleanliness (see Section 8.38) and ultimately Ð if required Ð the pump or thepumping system itself will be examined.8.2Contamination of vacuumvessels and eliminatingcontaminationIn addition to the pressure rise method (Section 6.1) there is a furthermethod for detecting contamination, based on the fact that condensablevapors will generally account for the major share of the gas mix in dirtyvessels: here the pressure reading at a compression vacuum gauge (partialpressure for the non-condensable gases) is compared with that at anelectrical vacuum gauge, e.g.
a thermal conductivity or ionization vacuumgauge (measuring total pressure). These two vacuum gauges must,however, be clean themselves. Where vapors are present the compressionvacuum gauge will indicate much better pressure than the electrical vacuumgauge. This is a sure sign that the vessel walls are contaminated, usuallywith oil. Another commonly used procedure is to compare the pressureindication of one and the same vacuum gauge (not a compression vacuumgauge) with and without a cold trap inserted in the line: Filling the cold trapwith liquid nitrogen will cause the pressure to drop abruptly, by one powerof ten or more, if the container is contaminated since the vapors will freezeout in the trap.Eliminating contamination for glass equipmentIf the contaminants exhibit a high vapor pressure, then they can be pumpedout relatively quickly.
If this is not successful, then the apparatus will haveto be cleaned. Contaminated glass components will first be cleaned withchromic-sulfuric acid mixture or Ð if this is necessary Ð with dilutehydrofluoric acid (1:30). They are then rinsed with distilled water. If thisdoes not bring about the desired results, then an organic solvent can betried. Then the glass components will again have to be rinsed withmethanol and distilled water. (Do not use denatured alcohol!)Eliminating contamination at metallic equipmentMetal components will usually exhibit traces of contamination by oil andgreases. If these cannot be readily removed by pumping down the vessel,then an appropriate organic solvent (denatured alcohol is unsuitable in allcases) will have to be used for cleaning.
Maximum cleanliness can beachieved with vapor baths such as those commonly found in industry. If onedesires to get down to extremely low pressure ranges (< 10-7 mbar), then Ðafter cleaning Ð the metal components will have to be baked out attemperatures of up to 200 ¡C. Seriously contaminated metal componentswill first have to be cleaned by cutting away or sandblasting the top surface.These methods suffer the disadvantage that the surface area for thesurface thus treated will be increased through roughening and activecenters may potentially be formed which would readily adsorb vapormolecules.
Supplementary cleaning in the vapor bath (see above) isadvisable. In some cases electrolytic pickling of the surface may bebeneficial. In the case of high vacuum components it is necessary to payattention to ensuring that the pickling does not turn into etching, whichwould seriously increase the surface area. Polishing surfaces which havebeen sandblasted is not necessary when working in the rough and mediumvacuum ranges since the surface plays only a subordinate role in thesepressure regimes.8.3General operating informationfor vacuum pumpsIf no defects are found in the vacuum vessels and at the measurementtubes or if the apparatus still does not operate satisfactorily after the faultshave been rectified, then one should first check the flange seals at thepump end of the system and possibly the shut-off valve.
Flange seals areknown to be places at which leaks can appear the most easily, resultingfrom slight scratches and mechanical damage which initially appearsinsignificant. If no defect can be discerned here, either, then it is advisableto check to see whether the pumps have been maintained in accordancewith the operating instructions.Given initially in this section are general instructions on pump maintenance,to be followed in order to avoid such defects from the very outset. Inaddition, potential errors and their causes are discussed.139HomeInstructions for equipment operation8.3.1 Oil-sealed rotary vacuum pumps(Rotary vane pumps androtary piston pumps)8.3.1.1Oil consumption, oil contamination, oilchangeThe oil serves to:• lubricate moving parts,• seal moving parts against atmospheric pressure,• seal the valve,• fill the dead space below the valve,• to seal the various operational spaces one from another.In all pumps it is possible to check the oil charge during operation using thebuilt-in oil level sight glass.
During continuous operation in particular it isnecessary to ensure that the oil charge never falls below the minimumlevel. During a pumping process oil-sealed rotary pumps will emit oil vaporsfrom the discharge port, this being due to the high operating temperature.This leads to oil loss to an extent which will depend on the quantity of gasor vapor which is drawn into the pump. Larger oil droplets can be retainedby installing a coarse separator in the discharge line. This will reduce oilloss considerably. The fine oil mist filter installed in some pumps will alsoretain the finest oil droplets (fine separation) so that no oil at all will leavethe outlet of the pump and oil loss is reduced practically to zero since Ð asin coarse separation Ð the oil which is separated out is returned to thepump. With pumps without an integral fine separator this device is offeredas an optional extra.If an oil-filled rotary pump is operated without an oil separation and returndevice, then it will be necessary to expect a certain amount of oilconsumption, the extent of which will depend on the size of the pump andthe nature of the operations.
In the worst case this can amount to about 2cm3 for every cubic meter of air pumped (at STP and including the gasballast also drawn in). Figure 8.1 makes it possible to predict the amount ofoil loss to be expected in practical situations. The example demonstratesthat greater oil losses must be expected when operating the pump with gasballast. This situation, which is generally valid, is always to be taken intoaccount in practice.If the pump oil has become unusabledue to exposure to the vapors orcontaminants which are encountered in the process, then the oil will haveto be replaced.
It is impossible to formulate any hard-and-fast rules as towhen an oil change will be required since the nature of the operations willdetermine how long the oil will remain good. Under clean conditions (e.g.backing pumps for diffusion pumps in electro-nuclear accelerators) rotaryvacuum pumps can run for years without an oil change. Under extremelyÒdirtyÓ conditions (e.g.
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