Fundamentals of Vacuum Technology (1248463), страница 58
Текст из файла (страница 58)
Forexample, using vacuum coating methods, automobile reflectors obtain amirror-like surface, plastic articles in the furniture, decoration, clock andwatch as well as electronics industry are metal-coated and optical effectsare created on articles in the decoration industry.Fig. 7.4 shows a type of vacuum system in which large batches of moldedplastic parts can be coated simultaneously. The substrates are placed on acage that rotates past the coating source, a sputter cathode in thisexample.
In some applications, by using a glow discharge treatment, thesubstrates are cleaned and the surface is activated prior to the coatingprocess. This enhances the adhesive strength and reproducibility of thecoating properties. A corrosion protection coating can be applied aftersputtering. In this case, a monomer vapor is admitted into the system and ahigh-frequency plasma discharge ignited. The monomer is actived in theplasma and deposits on the substrates as a polymer coating. In this type ofsystem there may be plastic substrates with a surface area of several10 m2 on the cage, causing a correspondingly high desorption gas flow.The vacuum system must be able to attain the required pressures reliablydespite these high gas loads. In the example shown, the system isevacuated with a combination of a backing and Roots pump.
A diffusionpump along with a cold surface forms the high vacuum pump system. Thecold surfaces pump a large portion of the vapor and volatile substancesemitted by the plastic parts while the diffusion pump basically removes thenon-condensable gases as well as the noble gas required for the sputterprocess.A completely different concept for the same process steps is shown in Fig.1 Vacuumchamber2 HighperformancecathodeFig.
7.43 Substrateholder4 Substrates5 Diffusion pump6 Roots pump7 Rotary pistonpump8 Cold trap9 High vacuumvalveDiagramm of a batch system for coating parts7.5. The system consists of four separate stations made up of a drumrotating around the vertical axis with four substrate chambers and processstations mounted in the vacuum chamber. During rotation, a substratechamber moves from the loading and unloading station to the pretreatmentstation, to the metallization station, to the protective coating station andthen back to the initial position. Since each station has its own pumpingsystem, all four processes can run simultaneously with entirely independentadjustable process parameters. The vacuum system comprises ofturbomolecular pumps and backing pump sets consisting of Roots androtary vane pumps.7.3.2 Web coatingMetal-coated plastic webs and papers play an important role in foodpackaging.
They preserve food longer according to storage and transportlogistics requirements and give packaging an attractive appearence.Another important area of application of metal-coated web is the productionof film capacitors for electrical and electronics applications. Metal-coating iscarried out in vacuum web coating systems. Fig. 7.6 shows a typicalscheme. The unit consists of two chambers, the winding chamber with theroll of web to be coated and the winding system, as well as the coatingchamber, where the evaporators are located.
The two chambers are sealedfrom each other, except for two slits through which the web runs. Thismakes it possible to pump high gas loads from the web roll using arelatively small pumping set. The pressure in the winding chamber may bemore than a factor of 100 higher than the pressure simultaneouslyestablished in the coating chamber. The pump set for the winding chamberusually consists of a combination of Roots and rotary vane pumps.With strongly degassing rolls of paper, it may be necessary to install a coldsurface in the winding chamber to act as a water vapor pump. The rolls ofthe plastic web or paper typically have diameters between 400 and 1000mm and a width of 400 to 3000 mm.
A precise, electronically controlled10 Valve forbypass line11 Foreline valve12 Venting valveFig. 7.5Multi-chamber parts-coating unit (rotationally symmetric in-line system DynaMet 4V)135HomeApplications of vacuum technologyHigh-performanceplasmasourceElectronbeamevaporatorO2Monomer1 Unwinder2 Coating source3 Coating rollerFig. 7.64 Drawing roller5 Take-up reelSchematic diagram of a vacuum web coating systemwinding system is required for winding and unwinding as well as webguidance.During the coating process the web, at a speed of more than 10 m/s,passes a group of evaporators consisting of ceramic boats, from whichaluminium is evaporated. To achieve the necessary Al-coating thickness atthese high web speeds, very high evaporation rates are required.
Theevaporators must be run at temperatures in excess of 1400 ¡C. Thethermal radiation of the evaporators, together with the heat of condensationof the growing layer, yields a considerable thermal load for the web. Withthe help of cooled rollers, the foil is cooled during and after coating so thatit is not damaged during coating and has cooled down sufficiently prior towinding.During the entire coating process the coating thickness is continuouslymonitored with an optical measuring system or by means of electricalresistance measurement devices.
The measured values are compared withthe coating thickness setpoints in the system and the evaporator power isthus automatically controlled.7.3.3 Optical coatingsVacuum coatings have a broad range of applications in production ofophthalmic optics, lenses for cameras and other optical instruments as wellas a wide variety of optical filters and special mirrors. To obtain the desiredtransmission or reflection properties, at least three, but sometimes up to 50coatings are applied to the glass or plastic substrates. The coatingproperties, such as thickness and refractive index of the individual coatings,must be controlled very precisely and matched to each other.
Most of thesecoatings are produced using electron beam evaporators in single-chamberunits (Fig. 7.7). The evaporators are installed at the bottom of the chamber,usually with automatically operated crucibles, in which there are severalArFig. 7.7Coating unit for optical coating systemsdifferent materials. The substrates are mounted on a rotating calotte abovethe evaporators. Application of suitable shieldings combined with relativemovement between evaporators and substrates, results in a very highdegree of coating uniformity. With the help of quartz coating thicknessmonitors (see Section 6) and direct measurement of the attained opticalproperties of the coating system during coating, the coating process is fullycontrolled automatically.One of the key requirements of coatings is that they retain their propertiesunder usual ambient conditions over long periods of time. This requires toproduce the densest coatings possible, into which neither oxygen nor watercan penetrate.
Using glass lenses, this is achieved by keeping thesubstrates at temperatures up to 300 ¡C during coating by means ofradiation heaters. However, plastic lenses, as those used in eyeglassoptics, are not allowed to be heated above 80 ¡C. To obtain dense, stablecoatings these substrates are bombarded with Ar ions from an ion sourceduring coating. Through the ion bombardement the right amount of energyis applied to the growing layer so that the coated particles are arranged onthe energetically most favorable lattice sites, without the substratetemperature reaching unacceptably high values.
At the same time oxygencan be added to the argon. The resulting oxygen ions are very reactive andensure that the oxygen is included in the growing layer as desired.The vacuum system of such a coating unit usually consists of a backingpump set comprising a rotary vane pump and Roots pump as well as a highvacuum pumping system. Depending on the requirements, diffusion pumps,cryo pumps or turbomolecular pumps are used here, in most cases inconnection with large refrigerator-cooled cold surfaces. The pumps must beinstalled and protected by shieldings in a way that no coating material canenter the pumps and the heaters in the system do not thermally overloadthem.
Since shielding always reduces the effective pumping speed, thesystem manufacturer must find a suitable compromise between shieldingeffect and reduction of pumping speed.136HomeCoating methods7.3.4 Glass coatingCoated glass plays a major role in a number of applications: window panesin moderate and cold climate zones are provided with heat-reflectingcoating systems to lower heating costs; in countries with high intensity solarradiation, solar protection coatings are used that reduce air conditioningcosts; coated car windows reduce the heating-up of the interior and mirrorsare used both in the furniture and the automobile industry.
Most of thesecoatings are produced in large in-line vacuum systems. Fig. 7.8 shows atypical system. The individual glass panes are transported into a entrancechamber at atmospheric pressure. After the entrance valve is closed, thechamber is evacuated with a forepump set. As soon as the pressure is lowenough, the valve to the evacuated transfer chamber can be opened. Theglass pane is moved into the transfer chamber and from there at constantspeed to the process chambers, where coating is carried out by means ofsputter cathodes.
On the exit side there is, in analogy to the entrance side,a transfer chamber in which the pane is parked until it can be transferredout through the exit chamber.Most of the coatings consist of a stack of alternative layers of metal andoxide. Since the metal layers may not be contaminated with oxygen, theindividual process stations have to be vacuum-isolated from each other andfrom the transfer stations.
Utilization of valves for separating processchambers is unsatisfactory because it increases plant dimensions. To avoidfrequent and undesirable starting and stopping of the glass panes, theprocess chambers are vacuum-separated through so-called Òslit locksÓ, i.e.constantly open slits combined with an intermediate chamber with its ownvacuum pump (Fig.
7.9). The gaps in the slits are kept as small astechnically possible to minimize clearance and therefore conductance asthe glass panes are transported through them. The pumping speed at theintermediate chamber is kept as high as possible in order to achieve aconsiderably lower pressure in the intermediate chamber than in the process chambers. This lower pressure greatly reduces the gas flow from aprocess chamber via the intermediate chamber to the adjacent processchamber. For very stringent separation requirements it may be necessary toplace several intermediate chambers between two process chambers.All major functions of a plant, such as glass transport, control of the sputterprocesses and pump control, are carried out fully automatically. This is theonly way to ensure high productivity along with high product quality.7.3.5 Systems for producing data storagedisksCoatings for magnetic- or magneto-optic data storage media usually consistof several functional coatings that are applied to mechanically finisheddisks.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
