Fundamentals of Vacuum Technology (1248463), страница 57
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In the case of XTM/2, XTC/2 and XTC/C units,the AutoZero and AutoTune functions are not available, and measurementwith several sensors simultaneously as well as simultaneous control of twovacuum coating sources are not possible.However, the IC/5 offers all comfort functions available today: measurementwith up to eight sensors with AutoZero and AutoTune as well as capabilityof simultaneous control of two evaporator sources. Moreover, it offers 24material programs, with which 250 coatings in 50 processes can beprogrammed.
To simplify operation and avoid errors, the unit also has adiskette drive. All types of crystal holders can be connected here. Thethickness resolution is around 1 •, the rate resolution for rates between 0and 99.9 •/s around 0.1 •/s and for rates between 100 and 999 •/s around1 •/s. A particularly attractive option offered by the IC/5 is a microbalance131HomeThin film controllers/control unitsboard with a highly stable reference quartz.
This oscillator is 50 times morestable than the standard oscillator; long-term stability and accuracy arethen 2 ppm over the entire temperature range. This option is speciallydesigned for coatings of material with low density and at low coating rates.This is important for space contamination and sorption studies, forexample.132HomeApplications of vacuum technology7. Application ofvacuum technologyfor coatingtechniques7.1addition to metal and alloy coatings, layers may be produced from variouschemical compounds or layers of different materials applied in sandwichform. A significant advantage of vacuum coating over other methods is thatmany special coating properties desired, such as structure, hardness,electrical conductivity or refractive index, are obtained merely by selecting aspecific coating method and the process paramaters for a certain coatingmaterial.Vacuum coating techniqueVacuum technology has been increasingly used in industrial productionprocesses during the last two decades.
Some of these processes and theirtypical working pressure ranges are shown in Fig. 7.1.Since a discussion of all processes is beyond the scope of this brochure,this section will be restricted to a discussion of several examples ofapplications in the important field of coating technology.Deposition of thin films is used to change the surface properties of the basematerial, the substrate. For example, optical properties such astransmission or reflection of lenses and other glass products, can beadjusted by applying suitable coating layer systems.
Metal coatings onplastic web produce conductive coatings for film capacitors. Polymer layerson metals enhance the corrosion resistance of the substrate.Through the use of vacuum it is possible to create coatings with a highdegree of uniform thickness ranging from several nanometers to more than100 mm while still achieving very good reproducibility of the coatingproperties. Flat substrates, web and strip, as well as complex moldedplastic parts can be coated with virtually no restrictions as to the substratematerial. For example, metals, alloys, glass, ceramics, plastics and papercan be coated.
The variety of coating materials is also very large. InUltrahigh vacuum7.2Coating sourcesIn all vacuum coating methods layers are formed by deposition of materialfrom the gas phase. The coating material may be formed by physicalprocesses such as evaporation and sputtering, or by chemical reaction.Therefore, a distinction is made between physical and chemical vapordeposition:• physical vapor deposition = PVD• chemical vapor deposition = CVD.7.2.1 Thermal evaporators(boats, wires etc.)In the evaporation process the material to be deposited is heated to atemperature high enough to reach a sufficiently high vapor pressure and thedesired evaporation or condensation rate is set.
The simplest sources usedin evaporation consist of wire filaments, boats of sheet metal or electricallyconductive ceramics that are heated by passing an electrical currentthrough them (Fig. 7.2). However, there are restrictions regarding the typeHigh vacuumMedium vacuumRough vacuumAnnealing of metalsDegassing of meltsElectron beam meltingElectron beam weldingEvaporationSputtering of metalsCasting of resins and lacquersDrying of plasticsDrying of insulating papersFreeze-drying of bulk goodsFreeze-drying of pharmaceutical products–1010–710–310010310Pressure [mbar]Fig.
7.1Pressure ranges for various industrial processes133HomeApplications of vacuum technology6Hairpin-sheped evaporatormade of twisted tungsten wire574Evaporator made of electrically conductiveceramics832Spiral evaporator made of twistedtungsten wireBoat-shapedevaporator191 Substrates2 SputteredatomsFig. 7.33 Anode4 Electrons5 Target6 Cathode7 Magneticfield lines8 Argon ions9 SubstrateSchematic diagram of a high-performance cathode sputter arrangement7.2.3 Cathode sputteringTrough-shapedevaporatorTrough-shaped evaporator with ceramiccoatingFig.
7.2Evaporator with ceramicinsertBasket-shaped evaporatorVarious thermal evaporatorsof material to be heated. In some cases it is not possible to achieve thenecessary evaporator temperatures without significantly evaporating thesource holder and thus contaminating the coating. Furthermore chemicalreactions between the holder and the material to be evaporated can occurresulting in either a reduction of the lifetime of the evaporator orcontamination of the coating.7.2.2 Electron beam evaporators (electronguns)To evaporate coating material using an electron beam gun, the material,which is kept in a water-cooled crucible, is bombarded by a focusedelectron beam and thereby heated. Since the crucible remains cold, inprinciple, contamination of the coating by crucible material is avoided and ahigh degree of coating purity is achieved.
With the focused electron beam,very high temperatures of the material to be evaporated can be obtainedand thus very high evaporation rates. Consequently, high-melting pointcompounds such as oxides can be evaporated in addition to metals andalloys. By changing the power of the electron beam the evaporation rate iseasily and rapidly controlled.In the cathode sputtering process, the target, a solid, is bombarded withhigh energy ions in a gas discharge (Fig.
7.3). The impinging ions transfertheir momentum to the atoms in the target material, knocking them off.These displaced atoms Ð the sputtered particles Ð condense on thesubstrate facing the target. Compared to evaporated particles, sputteredparticles have considerably higher kinetic energy. Therefore, the conditionsfor condensation and layer growth are very different in the two processes.Sputtered layers usually have higher adhesive strength and a densercoating structure than evaporated ones. Sputter cathodes are available inmany different geometric shapes and sizes as well as electrical circuitsconfigurations. What all sputter cathodes have in common is a large particlesource area compared to evaporators and the capability to coat largesubstrates with a high degree of uniformity.
In this type of process metals,alloys of any composition as well as oxides can be used as coatingmaterials.7.2.4 Chemical vapor depositionIn contrast to PVD methods, where the substance to be deposited is eithersolid or liquid, in chemical vapor deposition the substance is already in thevapor phase when admitted to the vacuum system. To deposit it, thesubstance must be thermally excited, i.e. by means of appropriate hightemperatures or with a plasma. Generally, in this type of process, a largenumber of chemical reactions take place, some of which are takenadvantage of to control the desired composition and properties of thecoating.
For example, using silicon-hydrogen monomers, soft Si-H polymercoatings, hard silicon coatings or Ð by addition of oxygen Ð quartz coatingscan be created by controlling process parameters.134HomeCoating methods7.3Vacuum coating technology/coating systems7.3.1 Coating of partsFor molded-plastic parts, vacuum coating techniques are increasinglyreplacing conventional coating methods, such as electroplating.
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