Metal Vacuum Coating Technology
The process in which metal, alloy or compound is evaporated or sputtered in a vacuum and is allowed to condense and deposit on the object to be coated is defined as vacuum coating. Currently, two coating techniques i.e. vacuum evaporation and sputtering coating are often used in industrial applications. Vacuum coating technology was put into practical use after 1930 when oil diffusion pump and mechanical pump were invented. Anti-reflection coating based on vacuum deposition technology was developed in 1935 and was used on lens of glasses in 1945. As science and technology are constantly advancing, vacuum coating technology and equipment are evolving rapidly, and are widely used in industrial applications, including even coating in buildings, metalwork, locksets, watch making and large parts etc., in order to enhance transmittance or reflectance of optical lens and to make aesthetic, durable and anti-corrosion metal parts, locksets and plastic products etc.
Application of Quartz Monitor Crystals in High-heat-load Metal Coating
High heat load often exists in evaporating the materials with very high melting point e.g. molybdenum (Mo), tantalum (Ta) and tungsten (W) or in large evaporation sources of diode sputtering type, where the substrate may be heated to higher than 300°C. The crystals used in these applications must have very high heat conductivity so as to transfer heat from the evaporation source to the water-cooled sensor body, thus maintaining the working temperature of crystals. In order for long life and low noise, the crystals used in high-heat-load applications must have good adhesion to the materials used in optical coating. Silver crystals are very suitable for these applications. In case of these applications with high heat load such as sputtering, it is necessary to use water-cooled crystals. In case of ALD applications, it is recommended to use high-temperature crystals.
Low-stress coatings e.g. aluminum, gold and silver Low-stress coatings are often used in electrodes or in cold mirrors to reflect heat and light from light sources. They are used in protective devices and other lighting applications e.g. lighting system for store’s display, automotive lights, flash lamps, reflectors and indicators. Gold crystals are commonly used in these applications.
High-stress coatings High-stress materials include those used in semiconductor processes: chromium (Cr), germanium (Ge), molybdenum (Mo), nickel-chromium alloy, nickel (Ni), tantalum (Ta), titanium (Ti), silicon oxycarbide (SiOC), zirconium (Zr), and dielectric materials used in precision optical coating. In these applications, it is preferable to use alloy crystals. Silver crystals are also usable.
Process for Vacuum Coating of Metal
Vacuum coating of metal requires good match between the substrate and the basecoat. The basecoat is typically of thickness 10-20 μm and is designed primarily to prevent release of moisture, organic solvents and plasticizers etc. from the substrate which may affect the adhesion of metal. It requires high hardness of coating and requires that the basecoat can repair the defects in material, can provide a smooth and even surface to facilitate vacuum coating, and can firmly adhere to the substrate and metal layer. Two-component polyurethane and epoxy coating which sets at normal temperature, amino coating which is heated at low temperature and thermoplastic acrylate coating are often used.
Metal coating is prone to become dull by oxidation in air. In addition, there may be defects such as tiny voids. Therefore it is necessary to apply a layer of protective topcoat of 5-10 μm thickness. The requirements for topcoat are: transparency, excellent water resistance, abrasion resistance, weatherability, no work on metal coating, firm adhesion to metal coating. Commonly, acrylate varnish, polyester varnish and polyurethane varnish etc. can be used. In case thermoplastic acrylate coating is used as the basecoat, the solvent in topcoat may seep through the defects in metal coating and erode the basecoat. To prevent this erosion, the fast-drying topcoat with weak solvent e.g. acrylic modified alkyd varnish, polyvinyl butyral varnish and urethane oil can be used.
The process flow for vacuum coating of metal is: treatment of substrate surface (cleaning, activation) → application of basecoat → drying → vacuum coating of metal → application of topcoat → drying