GB2306512A - Electron beam vacuum coating plant having a crucible of high melting material - Google Patents

Description

2306512 Vacuum coating plant with a crucible disposed in the vacuum

chamber for holding material to be evaporated The invention relates to a vacuum coating plant with a crucible disposed in a vacuum chamber for holding material and an electron beam source which is directed at the crucible for evaporating the material contained in the crucible, for example a metal or a metal oxide, such as Siox, MgO, A1203 or S'021 or a mixture of both, and with a substrate which is held at a distance from the material to be evaporated, for example a film which is guided over rolls.

A known device for the vacuum evaporation of sublimable substances in particular (DPS 26 28 765) consists of a vessel, which is provided with an opening, for the material to be evaporated and an electron beam source with an accelerating anode for producing an accelerated and focussed electron beam, which is directed at the vessel, wherein a horizontal impact plate for the electron beam is disposed in the beam path between the electron beam source and the vessel, the underside of which plate, being remote from the impact side, faces the vessel cavity and the plate covers the vessel while leaving free an outlet opening for the vapour jet which lies outside of the point of impact for the electron beam.

This known device solves the problem of continuously evaporating material in powder form from the outset over a lengthy period by means of an allpurpose electron beam evaporator without spattering or spraying.

Also known is a vacuum coating plant (DOS 42 03 632) with a vessel containing a material to be evaporated, with an 1 evaporator device for evaporating the material contained in the vessel, the material to be coated being disposed at a distance from the material to be evaporated, and with a microwave transmitter which transmits microwaves into the space between the material to be evaporated and the material to be coated.

This known plant enables the properties of a metal oxide coating on a plastics film to be improved.

Transparent plastics films are used to a greater degree for food packaging. Firstly to be considered in this case are films consisting of polymer plastics which, although flexible, have the disadvantage of being permeable to aromatic substances, water and oxygen. If the diffusion of such substances is to be excluded, aluminium foil or plastics films with vapour-deposited aluminium are therefore generally used. However these have the disadvantage of being relatively difficult to dispose of and being opaque to microwaves and light. Yet because microwave ovens are now used in almost all households in the industrialised countries, it is in many cases vital for the packaging material to be transmissive to microwaves.

In order to combine the advantages of plastics films which are transmissive to microwaves with those of aluminium foil, which forms an absolute barrier to aromatic substances, water and oxygen, it is already known to coat polymer films with metal oxides. Silicon oxide is a particularly important coating material in this case. As regards laminated structure and barrier behaviour with respect to oxygen, water vapour and aroma, plastics films which are coated with silicon oxide have properties which are similar to those of plastics films which are coated with aluminium foil or with aluminium.

However the coating of plastics films with metal oxide s such as SiOX requires a process technique which is very different from conventional coating techniques, as - in contrast to 2 metals - metal oxides have to be evaporated from the solid phase.

Siox coatings are produced by evaporating SiO by means of an evaporation furnace or by means of electron beam evaporation (cf. T. Krug, K. RUbsam: Die neue 11glAserne Lenensmittelverpackung11 (The new "glassy food packaging") in llneue Verpackung11 ("new packaging"), published by HUthig, 1991). As SiO sublimes - i.e. evaporates directly from the solid state without passing through a liquid phase - a special crucible is required. In order to achieve film tape speeds of several m/s, which represent a prerequisite for economical production at acceptable costs, crucibles for evaporator temperatures of approximately 1350C are necessary. The Siox-1 vapour oxidizes by means of a controlled, reactive atmosphere, so as to achieve a degree of oxidation of x 1.5 to 1.8. SiOX coatings which are thinner than 2000 have the advantage of being easy to bend. Siox is also chemically inert and resistant to corrosion by water. SiO can be evaporated by means of an electron beam, as it has a relatively high vapour pressure. However, in order also to achieve the required higher deposition rates when using other metal oxides, e.g. MgO + Si02, temperatures of 18000C or above are necessary.

When evaporating metal oxides (e.g. SiOX) directly with the electron beam, spattering due to the high temperatures occurs at the oxide surface at the point of impact of the electron beam. These material particles strike the film and adhere to it or even penetrate it. This causes holes in the film which impair the barrier properties. Particles which adhere to the film have a detrimental effect during subsequent printing.

The object of the invention is to provide a vacuum coating plant which enables films to be coated with metal oxides or with mixtures of metal oxides with metals at a high rate and with high quality in a spatter-free manner. The plant should also have a long service life and a high process stability.

3 Finally, it should be possible to achieve particularly good cross control with respect to conventional evaporator furnaces.

This object is solved according to the invention in that the crucible is formed from a high-melting metal such as, for example, molybdenum, tantalum or tungsten, or from graphite or a graphite compound or a ceramic material.

Further particulars and features are described and characterised in detail in the claims.

The invention permits a wide variety of embodiments, one of which is represented in detail in a purely diagrammatic form in the appended drawing, which is a sectional view of a vacuum coating plant.

The drawing shows a cylindrical container 1, in which an elongate crucible 2 with openings is disposed. A coating roll 3 is provided above this crucible 2 and defines a coating chamber 4 at the top. A film 5, which is wound off a supply roller 6 and onto a wind-up roller 7, runs continuously over this coating roll 3.

The crucible 2 is heated by an electron gun 8, the electron beam 9 of which is deflected towards the crucible 2 by a magnetic coil 10. The material 15 which is to be evaporated (metal oxide such as SiOX or a metal or a metal compound) is contained in the crucible 2, which is provided with openings, apertures or slits towards the coating roll 3.

Microwave energy is supplied to the coating chamber 4 by means of a horn. aerial 11 in order to produce a plasma in the resulting vapour cloud.

A cartridge-shaped or tubular crucible 2 is uniformly heated over the coating width by the electron beam 9 in the described arrangement. Slits are located in the top side of the crucible 2, through which slits the vaporous metal oxide 4 emerges and condenses on the film. As the entire material is uniformly heated and the vapour can only escape though comparatively small holes, spattering cannot occur, even with very high coating rates.

In this case the vessel consists of a material with a high melting temperature such as, e.g. molybdenum, tungsten, graphite, composite graphite fibre material or a ceramic material.

It is also possible to ignite a plasma (e.g. p-wave excitation) between the evaporator and the film in the illustrated plant. The film is thereby stripped of any stray electrons from the electron beam which may have impinged on it.

An oxygen source 12 enables oxygen to be supplied, if required, via the line 13 connected to the coating chamber 4, so that, for example, SiO evaporates and is deposited on the film as SiOx.

claims Vacuum coating plant with a crucible (2) disposed in a vacuum chamber for holding material and with an electron beam source (8) which is directed at the crucible (2) for evaporating the material contained in the crucible (2), such as metal or metal oxide, for example SiOx, MgO, A1203 or Si02 or a mixture of metal and metal oxide, and with a substrate which is held at a distance from the material to be evaporated, for example a film (5) which is guided over rolls (3, 6, 7), the crucible (2) being formed from a highmelting metal such as, for example, molybdenum, tantalum or tungsten, or from graphite or a graphite compound or a ceramic material.

2. Vacuum coating plant with a crucible (2) disposed in a coating chamber (4) for holding material and with an electron beam source (8) which is directed at the crucible (2) for evaporating the material contained in the crucible (2), such as metal or metal oxide, for example SiOxt, Mgo, A1203 or S'02, or a mixture of metal and metal oxide, with substrate which is held at a distance from the material to be evaporated, for example a film (5) which is guided over rolls (3, 6, 7), and with a horn aerial (11) to supply microwave energy to produce a plasma in the vapour cloud produced between the film (5) and the material to be evaporated, the crucible (2) being in the form of an essentially closed vessel, preferably a cartridge or tube which can be sealed at both ends, comprising in its side which faces the film (5) a plurality of openings, slits or apertures (15), through which the evaporated material can emerge in the direction of the substrate (5), and being formed from a high- melting metal such as, for example, molybdenum, tantalum or tungsten, or from graphite or a graphite compound or a ceramic material.

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