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產品與技術 > 電漿化學氣相沈積

　

Plasma-enhanced chemical vapour deposition (PECVD) is a technique wherein a plasma cloud is set up near the surface of a substrate such that pre-cursor gases are decomposed into chemically active molecules which then bombard and coat the substrate's surface. The nature of the resulting coating depends in part on the combination of pre-cursor gases. The technique is usually employed to deposit coatings onto hard substrates. For the XBHCOAT consortium, the challenge was to extend the application of PECVD to softer plastic materials such as the polycarbonate and CR39 (diethyleneglycol-bis-allylcarbonate) used in the production of ophthalmic spectacle and sunglass lenses. The aim was to improve scratch resistance, to reduce reflectivity and increase hydrophobicity. During the course of the project, its original leader, FACT Optique, was acquired by the Finnish company SCS Coatings - which had a particular interest in producing display covers for the GSM market. Consequently, polymethyl methacrylate (PMMA), a material widely used for this application, was added to the list of target polymers for coating.

Need for progress
　

There were several good reasons to look at the PECVD process. One was to find a replacement for current lacquering methods, which involve the use of environmentally objectional solvents. Another was to reduce manufacturing costs by increasing production speeds and improving reproducibility. Finally, it was expected that superior protection could double the lifetime of lenses, while multiple coatings would enhance their multifunctionality.

In order to add a hard silicon-based scratch-resistant layer to a soft plastic, a technique was developed whereby the composition of the coating was varied progressively as the deposition proceeded. This allowed a smooth transition from a polymer-like structure that bonded effectively to the substrate, to a tough, oxide-like outer protective surface, with a total layer thickness of around 10 mm.
The partners also demonstrated that scratch coatings on polycarbonate and CR39 were compatible with antireflective treatments applied by widely-used ion-assisted deposition and sputtering techniques. The coating of PMMA proved less successful, as no reliable adhesive could be found. Hydrophobic coating was also abandoned, due to a lack of commercial interest.

A further problem was in adapting the process to ophthalmic lenses, which vary in their curvature according to the required vision correction properties. PECVD is essentially best suited to volume production, involving the placement of a metallic cathode behind the lens. This cathode must fit the lens closely in order to ensure that the vaporised coating deposits evenly on the plastic surface. Consequently, the handling of mixed batches of lenses requiring individually optimised cathodes would not be a cost-effective option.

Strength through partnership

　

Progress towards an industrial technology for coating sunglasses has been considerable, and the outlook for the coating of display covers is equally promising. Italian research partner played a major role in the project, having built a prototype deposition machine, now installed at ASM, essentially the main project deliverable. They were also instrumental in the strategic reorganisation of the project as it changed directions from an ophthalmic to a GSM application. Other industrial SME participants , which will install a hybrid plating/PECVD plant for coating sunglasses, to perfect a process for one-step antiscratch/antireflective treatment. If this can be accomplished, it could well become economically feasible to coat progressive ophthalmic lenses - which have an intrinsically high added value, thus justifying the cost of treatment in smaller batches.

　

Successful applications

　

The prototype reactor is now working well, delivering homogeneous and reproducible coatings on substrates with surface areas of up to more than 1 m2. Its design is robust, and will readily accommodate extensions for automated loading and unloading.

ASM is now able to produce GSM display covers at a rate of 20-30,000 per week, and expects to be able to move towards a targeted 1 million per week within around one year. This would open the door to an industry with a potential annual turnover of 100 million. However, penetration of the market will depend on the company's success in persuading telephone manufacturers to introduce the new technology.

At present, PMMA is the preferred material for display covers. Untreated, it is a little more transparent than polycarbonate. But coating the latter polymer also adds a degree of antireflectivity, which largely eliminates the difference - while wear resistance is a remarkable 2 000 times greater than that of uncoated PMMA.

Meanwhile, ASM has also identified another interesting niche application in the coating of 1.2 metre-long polycarbonate tubes that form the covers of inspection lights used when servicing aero engines. These have been accepted by the British Royal Air Force, and are now in small-scale production.
According to ASM's Dr. Bosschelle, XBHCOAT was very worthwhile for all of the partners, with excellent co-operation and a positive outcome in terms of an innovative and readily exploitable technology.