VAPOUR DEPOSITION

Abstract: The method according to the invention is intended for piece by piece metal coating of in particular thermally sensitive objects. An example of such an object is the so called optical discs (for instance compact discs). The objects are coated piece by piece by being located particularly close to an evaporation source for evaporated metal, for instance aluminium, the evaporation source being constituted by a melt (12) of for instance aluminium. The melt is enclosed in a crucible (11) that is heated by inductive means (13). The distance between the object and the melt is kept to a few centimeters; for instance three centimeters. The exposure time, that is the time when the object is held over the melt, is held very short, a few seconds; for instance three seconds. A time long enough to allow, in a practical way, for a piece by piece handling of the objects. Also in this time sufficient metal will be evaporated, transferred and condensed onto the disc at the same time as the heat radiation from the melt is uncapable of raising the temperature in any damageable way. The metal vapour source is about as large as the object that is to be coated and during the depositing time the melt as well as the object that is to be coated are enclosed in an evacuated space, that for instance can be provided with gates or valves for entering and exiting the objects that are to be coated.

Description: VAPOUR DEPOSITION This invention is related to vapour deposition of m tal, particularly on discrete separate objects. Vapour d position of discrete objects is today exercised industrial in such a way that in a .large closed space or room the o jects that are to be coated with metal are positioned in fixture along the walls of the room and centrally in th a number of vapour sources are placed, constituted by great number of small Wolfram filaments. These are ea loaded with a short piece of aluminium wire and are th heated to a very h.ight temperature by a large electr current through the Wolfram filaments. At this the alumini is first vapourized and then deposited on the objects th are to be coated. Prior to the vapourizing process the ro has been evacuated. By using several vapour sources and oveing and/or rotating the objects in different ways it possible to obtain a comparatively even coating of t objects . This is a comparatively time consumeing process sin some time is necessary for the vapour source to de iver sufficient amount of metal to obtain the desired coati thickness on the subjected objects. All metal vapour wi not be deposited on the intended objects, but instead on t walls of the room and on the fixture. With time the deposi on walls and fixture will become so great that metal flak easily result when for instance air once again is releas into the large evaporation room. Such metal flakes, even they have a size that is smaller than in the air free suspended small dust particles, can be an absolute and i mediate reason for rejection of very sensitive objects, ev if the disturbance is almost impossible to detect, even wi the aid of a magnifying glass. An exampel of such sensiti objects are compact discs for data storage or correspondi discs for the storage of picture images "video discs". Also the need to evacuate and refill air into the ev poura ion room result in additional time losses. Also th positioning of the objects that are to be coated is ver time- and/or resource demanding. It is furthermore necessar to calculate and test very carefully the time for which the objects must be subjected to the emitted vapours in order to obtain the correct and even deposite that is desired. It is furthermore often difficult to secure an even and conform coating for succeessive objects. A possible malfunction in the industrial batch coating of entire loads of objects will result in a great number of objects that have to be discarded. To sum up it can be stated that even if the evaporation technique as such result in a good result, is it today too timeconsumeing and circu stancial to enable use for rapidly massproduced articles, as for instance compact discs, where instead cathod desintegration or sputtering is used. This technique however limits the choice of material in the discs or objects that are to be coated to thermally comparatively insensitive materials that are also regretfully therefor a- terials that are difficult to shape, as for instance poly¬ carbonate (PC). This in turn limits the disc size due to the shaping problems at larger discs as for instance video discs. It is quite simply difficult satisfactorily to cor¬ rectly fill out the entire matrix at the edges of the disc. As a result of these difficulties and the resulting large discard percentage the video discs have not yet reached any larger market. In view of the above problems the invention has as its object to provide a method for metal coating that is quick and efficient, and that requires a minimum of space and machine equipment for use and that can be used also on ther¬ mally sensitive and very easily shaped materials and that allows high rate coating of separate discrete objects piece by piece and in cooperation with for instance presure moul- ding machines. The above object has in accordance with the invention surpr singly simple solution, that however is complete controversal to the conventional teachings of evaporatio in particular in view of its appliability for sensiti materials and shapes. In accordance with the invention t metal vapour source and the object to be coated are name placed very close (a few centimeters) to each other. T evapouration or metal emission rate from a vapour source dependent of the temperature and a high temperature is n cessary in order to obtain any noticeable evaporation. is therefore unlikely in view of the known technique th plastics and then in particular heat sensitive plastic with for instance micro structures, could be coated in th way since they have a comparatively low melting point a since the heat load increase with the temperature and t reduction in distance. In particular in the coating of called compact discs and the like where very fine mic structures must be retained, it would according to the kno technique be impossible to locate the vapour source close to the object that it is to be coated without th being damaged. The improbability of the inventive thought or teachi is however only an illusion since it in practise been esta lished that despite a high temperature of the evaporati source and a very short distance between this and the obje for instance compact discs can be coated with a high quali metal coating without inluencing the micro structure of t coated surface. Preferably and contrary to the prior art the evapor tion source is made large, that is to say with a surfa that more or less corresponds to that of the compact dis The large surface will provide a long life for the evapor tion source and a very even high quality coating of f instance compact discs. The evaporation source will of course radiate heat th in turn heats the object that is to be metal coated. It has however been found that with the inventive method the metal coating of the object that is to be coated can be obtained very rapidly (only a few seconds) and while the coating is done the temperature increase remains at a moderate value, in practice measured to about 30 degrees CCelcius) . The reason for this is probably not only that the exposure time can be kept short, but when researching for probable expla¬ nations as to why the inventive method is possible at all we believe it to be the fact that when an evaporation source as for instance molten aluminium is well molten a mirror-like melt surface is obtained. The metal surface is like a mirror which in turn means that it will become a bad heat radiator. The mirror-like surface of the melt has only a few percent of the heat radiation that would result otherwise. Due to this the metal evaporation will be more efficient and faster than the heat radiation. When additionally successively the surface to be coated becomes coated even this will become reflecting and reflect the heat radiation. In the case of aluminium also the low specific emission factor of the heat radiation contribute. By using a large surface for the evaporation melt in practice the temperature of this melt can be permitted to be lower than what otherwise would be the case in order to ob- tain sufficient evapouration rate. This also diminish the heat load of the object that is to be coated. The above method will provide a very efficient coating. Since object and source are very close very little metal va¬ pours will pass the object. In order to adapt the method to a pressure moulding line each coating including transports may take a few seconds. Preferably the movements of the ob¬ ject are fast and for the major part of the coating cycle the object is kept still in the coating position. In this way the movements will cause no impairing of the eveness of the coating and furthermore very little metal vapours will have time to pass the location of the object to be coate These features in combination with what has been said abo does actually allow a lower temperature than expected. T fact is that even if the coating, as carried out today, accordance with the invention, seems extremely fast, it h actually been slowed down from its possible rate to ada to the other machines in the fabrication line. By means of the invented method it is possible to met coat compact discs and the like fabricated of polymethy etacrylat PMMA (Plexiglas^) or other plastics with a l melting point since the heat load is so small that the su face structure do not risk being softened or distroyed. U til now it has been necessary to use polycarbonate PC th is less sensitive. This material is however in itself n suited to rapid forming of large objects (i.a. video disc and the invention therefor allows a very great gain from practical point of view since it allows the use of mo easily shapeable PMMA plastics. These are furthermore le sensitive to scratches, which is also of interest for o tical discs as compact discs, video discs, CD roms etc. Not unessential is also that PMMA is obviously less e pensive than PC and has better optical properties than P The latter is particularily important when the stored i formation is stored in an analogous way as for instan pictures on video discs, but less important for sound dis with a digital storeing of the information, why it is po sible to use sputtering and PC for these discs. Un undirect effect of the method according to the i vention with a short distance between the evaporation sour and the object to be coated is that the target accuracy the evaporated metal will be great and the waste small, th is to say a very good use will be made of the metal. In accordance with the inventive thought it is th possible to allow even comparatively thin plastic objec with a corresponding low heat storage capacity for a sho time to be exposed to metal vapour very close to the meta melt emitting the vapour. The distance can actually be al lowed to be only a few centimeters with a vapour source tha has a surface that is as large as the surface of the objec that is to be coated or even greater. In a preferable further development of the inventiv method a steady and good coating can be obtained from objec to object by measuring the thickness of the coating. Thi can for instance be made by using a photo transistor tha via a mirror measures the heat or light emission passin from the metal melt through a plastic object to be coate and out on its rear or unexposed side. With the use of mirror the photo transistor is protected from the metal va¬ pour and the mirror will continue to work as a mirror eve if it becomes metal coated. Although its reflection proper¬ ties will change with time this is no problem since a rela¬ tive measuring is sufficient to check and control the pro¬ cess. Alternatively the heat/light radiation reflected against the object that is to be coated can be measured but this will in all probability result in a lesser measuring accuracy and is difficult to arrange. Due to the very short exposure time it is possible to execute the invented method as a part step in a greater fab¬ rication process, where in order to secure vacuum in the metal coating chamber this can be provided with vacuum gates or locks on entrance and exit sides. The execution of the method is faciliated also by the fact that the vacuum cham¬ ber can be small and thus rapidly can be evacuated. Further advantages and features of the invention are apparent from the following description of a preferred em¬ bodiment shown on the enclosed drawings. In the drawings fig 1 schematically shows a device for metal coating of compact discs and fig 2 more in detail the evaporation source for metal in accordance with the invention. The metal coating device shown in fig 1 includes a first gate chamber 1, an evaporation room 2 and a seco gate chamber 5. To each of the chambers 1, 2 and 5 is co nected a vacuum pump 3 via a valve 4. The gate chambers and 5 are in their outer ends provided with tightly closea le 1 ids 6 and between the gate chambers 1 and 5 and the m tal coating chamber 2 respectively valves or locks or ga doors 7 are arranged. The coating chamber 2 is at its bottom provided with evaporation unit for aluminium in accordance with fig This unit has been designated 8 and is connected to a dr ving and controling unit 9. At the use of the device thi works in such a manner that at first a compact disc is in serted into the left gate chamber 1. The lid 6 is closed an the air evacuated by means of the vacuum pump 3. Then th valve or gate port 7 is opened and the compact disc i brought into the coating chamber 2 above the evaporatio source for aluminium. The compact disc then remains in th coating chamber the short time necessary to obtain a prop coating. Thereafter the compact disc is returned to the gat chamber 1, the valve 7 is closed and the compact disc i removed via the id 6 of the chamber 1. The chamber 5 i loaded in the same way as the chamber 1 with compact disc and in the coating chamber 2 alternative discs from the gat chamber 1 and the gate chamber 2 respectively are coated the valves or gate ports 7 being closed against the chambe that is loaded meanwhile. Since discs are fed alternatively from right and lef a high production rate is obtained with a calm and secur handling of the discs. Of course, a device of the above described type c also be adapted for objects that are to be coated that al the time pass in one and the same direction in steps. Sinc the example refers to compact discs, for instance from PMM or PC the different chambers have a flat look. If othe objects, as for instance headlight reflectors or the lik are to be coated the shape must of course be different. The pumps 3 that are connected to the gate chambers 1 and 5 can preferably be rotating two stage vane pumps while the pump that is connected to the coating chamber preferably is turbo molecular pump. The evaporation source of aluminium shown in fig 2 in¬ cludes a crucible 11 with* olten aluminium 12. The crucible rests on an induction heating device, a coil 13, which also is water-cooled. The coil 13 is fed with current and cooling from the driving unit 14. Around the crucible this is sur¬ rounded by heat insulation 15 and outside this a cooling shield 16, which also extends below the major part of the crucible and into the induction heat means. Finally crucible and induction heat means are surrounded by a vacuum enclo- sure 17. Above the melt of aluminium a compact disc 18 has been depicted in dashed lines when being coated with alu¬ minium. When coating takes place the light/heat radiation that can be detected by the detector 19, consituted by a photo transistor is diminished. This reduction can be detec- ted by a control unit 20 that in turn controls the drive unit for the induction heat means so that a stable coating rate is obtained for the heating means. Furthermore the control unit 20 controls the feeding of the compact discs 18. This is done in such a way that when the detector 19 has noted a sufficient lowering of the radiation in relation to the radiation level obtained when no disc is on place the disc is sufficiently coated and removed. In this way the feeding rate can be adapted to the coating rate and a conti¬ nuous and sufficient but not exaggerated coating is obtained for the discs. Alternatively time can be kept more or less constant (which faciliates the connection to the other pro¬ duction steps) and instead the coating rate is influenced by controling the temperature of the melt based on the thickness of the coating as measured by the photo tran- sistor. Also above the crucible and the aluminium melt a shie 21 is arranged. This partly acts as a cooling shield part as a collector for aluminium vapours that have not adher to the intended objects. In this way it is simple to ke the coating chamber clean from metal deposits that otherwi would contaminate by changeing the shield 21. Since the coating is done from below on the compa discs the risk of enclosing dust and the like is diminish since the dust prefer falling down to falling up. With the described device it is estimated that 120.0 compact discs can be coated within a time of about 1 hours. The time for coating one compact discs properly c be approximated to about 3 seconds. It goes without saying that the vapour source accordi to the invention has a considerable improved length of lif as compared to the Wolfram filaments according to the kno technique and that the method according to the invention a its evaporation source is far better adapted for industri use than the prior art is. The vapour source according the invention as it has been described above does howev not require continuous use but can be switched on and o comparatively quickly. This can be done by lowering th temperature of the melt. Through a moderate lowereing th evaporation rate is namely reduced and the aluminium wil remain in the melt. When evaporation once again is to ta place the temperature is once again raised to working tem perature. The establishment of these temperature variation is far quicker than if one would have to shut down the d vice totally. Since the melt is enclosed in vacuum it c be kept heated and fluid for a comparatively long ti (three weeks!) at a low evaporation rate. The starting time from normal room temperature can estimated to about twenty minutes, while a change from wo temperature to a lower stand by temperature is in the siz of three minutes and a return to working temperature c then be obtained in about ten minutes. At these decrease and increases respectively of the temperature between th working temperature and stand-by temperature aluminiu evaporated during this time can be collected by a particula shield intended for this. Even if, as has been said above, the evaporation an the coating takes place in an evacuated surrounding it is n catastrophy if some air should happen to enter into the eva poration source for instance in connection with service o due to lacking tightness. At the prior art now used evapora¬ tion sources with Wolfram filaments etc. one can count on practically complete destruction of the evaporation sources if air should enter, in other words the increase in produc¬ tion security is large. To sum up, the invention enables fast continuous piece by piece metal coating of discrete objects and surfaces with a good and durable adhering and even coating on very sensi¬ tive objects of for instance plastics with a low melt and shaping temperature, with a device that in comparison to the known technique is small and simple and has a good produc¬ tion security and an essentially reduced risk for rejected objects. Of course the inventive thought can also be used for the metal coating of less sensitive materials and/or ob- jects.

Claims: CLAIMS 1. A method for metal coating of in particular discret sensitive objects, as for instance compact discs of materi als with a low melting point, as for instance PMMA (polymet hylmetacrylat) by means of evaporation at a low pressure o in vacuum by metal, characterized in that the object that i to be coated by metal a short time is held closely to th evaporation source of the metal vapour. 2. Method according to claim 1, characterized in tha the evaporation source for metal is constituted by a * meta melt of for instance aluminium or an aluminium alloy or som other substance with a low heat radiation factor. 3. Method according to any of the claims 1 or 2 charac terized in that the evaporation source is great, that is t say has a surface more or less corresponding to that of th object that is to be metal coated. 4. Method according to any of the claims 1-3, characte rized in that the evaporation source corresponds to or cons titute a mirror image of the area that is to be coated. 5. Method according to any of the claims 1-4, characte rized in that the object is held on a distance from the eva poration source, which distance is in the same size as, o less than the dimensions of the area that is to be coated corresponding for instance to a few centimeters. 6. Method according to any of the claims 1-5, characte rized in that the object is held so close to the metal eva poration source that the heat load from the metal evapora tion source would distroy the object in due time, but tha the temperature is sufficiently low or the exposure time i kept sufficiently short to prevent this and yet sufficientl long in practice to allow a piece by piece handling an deposition and onto each piece to obtain the desired meta coating . 7. Method according to any of the claims 1-5, characte rized in that the thickness of the coating is measured by measuring of the heat/light radiation preferably via a mir ror from the melt through the object that is to be meta coated. 8. Evaporation source for metal vapours for use at va cuum coating characterized in that it is constituted by metal melt, that is heated by means of induction. 9. Evaporation source according to claim 8, characteri zed in that it is surrounded by shields, partly for tempera ture restriction, partly for receiving metal vapours tha are not deposited on intended objects. 10. Evaporation source according to claim 8 and 9, cha racterized in that it is arranged in an evaporation chamber, that via valves or gate ports are connected to loading cham¬ bers that can be closed from the surrounding, the evapora¬ tion chamber as well as the connecting chambers being eva- cuateable.

Inventors: NORDLANDER JOHAN, NORDLANDER, JOHAN
Applicants: APPLIED VACUUM TECH, APPLIED VACUUM TECHNOLOGIES 1 AB
Application year: 1992
Main classification: C23C14
IPC codes: C23C14, G11B7