Coating Methods for Use with the Platinum Metals a REVIEW of the AVAILABLE TECHNIQUES
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Coating Methods for Use with the Platinum Metals A REVIEW OF THE AVAILABLE TECHNIQUES By Christopher Hood Group Research Centre, Johnson Matthey & Co Limited Th.e economic use of platinum group metal coatings requires that the most appropriate coating method should be used jor any particular applica- tion. This article reviews the techniques available for various substrate materials and outlines some of the more important properties obtained with different types of deposit. There is now a wide variety of methods which can be carried out by two techniques : that can be employed to form coatings of the vacuum evaporation and sputtering. These platinum group metals. The choice of methods are used for the large-scale produc- materials and methods to be used can best tion of micro-electronic components where be made after the manufacturer and the user platinum provides the required properties of of the product have considered the required good electrical conductivity and complete properties and related factors such as the freedom from atmospheric corrosion (I, 2). thickness and physical properties of the coat- Vacuum evaporation is carried out at a ing, the properties of suitable substrates, and pressure of IO-~Torr or below. The metal the cost of alternative processes. This review from which the coating is to be formed is describes processes which can be used to heated to a sufficiently high temperature to deposit a coating of a platinum group metal cause it to volatilise at the extremely low on to a substrate. The properties of the pressure of the chamber, the substrate being coating and the type of substrate to which arranged to condense this vaporised metal. the various methods are applicable, and the Excellent films can be formed by this method thickness normally to be obtained, are also although there are a number of difficulties, discussed. For the purpose of this article the the main one encountered with the platinum main methods of coating are considered to be group metals being the high temperatures as follows: required for evaporation. The most effective heating method is by an electron beam using Vacuum deposition Thermal decomposition and chemical a water-cooled copper crucible to contain the vapour plating platinum group metal and as only the central Electroless plating part of the exposed surface of this metal is Aqueous electroplating Fused salt electroplating heated, contamination is avoided, The high Metallic bonding temperatures involved may, however, cause Metallising additional problems because, as both radiant heat and the evaporated atoms are propagated Vacuum Deposition rectilinearly from the source, it is not possible The most versatile method of depositing to shield the substrate from the radiant heat very thin coatings is vacuum deposition during deposition. The resulting temperature Platinum Metal Rev., 1976, 20, (2), 48-52 48 rise could not only damage a sensitive sub- the substrate is improved because gas ad- strate such as plastic, but may also change sorbed on the substrate surface is displaced the structure and properties of the deposited and the sputtered atoms embed themselves in film. In addition, the pressure in the vacuum the surface layers. Although ion bombard- chamber must be sufficiently low for the ment does raise the temperature of the target mean free path of the evaporated atoms to the radiant heat emission is small compared exceed the source to substrate distance, with that from a thermal source, and in most otherwise the metal will tend to nucleate commercial plants is completely suppressed before reaching the substrate and produce a by cooling the target. One disadvantage of soot-like deposit instead of a continuous film sputtering in comparison with thermal of metal. evaporation is that the deposition rate is much Many substrates have substantial amounts lower, unless powerful radio frequency plasma of adsorbed gas on their surfaces, and this can is employed. take a long time to remove by pumping, water With both types of vacuum deposition, a being particularly troublesome in this respect. vast range of materials can be coated, almost Poor adhesion is often a problem with the only condition being that the substrate evaporated films of the platinum group metals can be safely subjected to vacuum conditions. especially on oxide surfaces such as ceramic Unless the substrate being coated is very and glass. This results from their low small in relation to the source, it is necessary chemical reactivity so can generally be over- to move it around during deposition in order come by coating the substrate with a more to obtain an even coating. The size of the reactive metal such as chromium before the substrate being coated is obviously limited platinum metal is deposited. Adhesion to by the capacity of the vacuum chamber, and plastic substrates, however, is usually very the shape by the rectilinear form of the good. emission from the source; it is not possible Many of the problems associated with to coat surfaces which cannot be placed thermal evaporation can be avoided by sput- approximately perpendicular to the line of tering techniques. Instead of vaporising the emission. coating material by thermal energy, the sur- The thickness of deposit which can usefully face of the target-the source of the platinum be formed is limited by the amount of strain group metal-is bombarded with positive which can be accommodated in the coating. ions from a low pressure plasma of an inert This strain may be caused by structural gas, usually argon. The ions are accelerated imperfections at the atomic level or by towards this target by a potential of several incorporated impurities, and leads to crazing, kilovolts and as they collide with it their cracking and peeling of the coating. Although kinetic energy is transferred to the platinum the strain can be reduced to some extent by group atoms so giving them sufficient energy heating the substrate during deposition, so to leave the surface and travel to the substrate. annealing the deposit, the process is best These sputtered platinum metal atoms have suited to the production of thin coatings in much greater kinetic energy than thermally the range from a few atomic layers up to I volatilised atoms and can undergo collisions micrometre. en route to the substrate without nucleation occurring. This means that higher pressures Thermal Decomposition and in the 10-3 to IO-~Torr range can be used, Chemical Vapour Plating thus easing the pumping requirements. As Thin coatings of platinum and of gold- the sputtered atoms arrive at the substrate palladium alloys can be applied to heat with much higher energies than evaporated resistant substrates by the thermal decom- atoms the adhesion between this coating and position of organometallic compounds of Platinum MetalRev., 1976, 20, (2), 49 these metals to which resins and solvents have chemical reduction methods, often called been added to give the required properties electroless plating. The substrate is usually for application by such methods as brushing, first treated chemically or mechanically to spraying or screen printing. After drying, the roughen the surface, thus providing a substrate is fired in air in the temperature mechanical key which improves the adhesion range 500 to 900°C to decompose the organo- of the coating, before being treated with metallic compound and to oxidise all carbon- palladium chloride followed by stannous aceous residue, leaving the required metal chloride. The resulting layer of colloidal film. Obviously, this process can only be palladium then acts as a catalyst for the form- carried out with a metal which does not ation of the coating proper, which is formed oxidise at the required temperature. by immersion in a bath containing a salt of An alternative method of utilising thermal the coating metal together with a reducing decomposition is known as chemical vapour agent such as sodium hypophosphite. This plating (3). The substrate is heated in an method can be used to produce coatings of atmosphere containing the vapour of a metal base metals such as copper or nickel which compound which decomposes to the metal can subsequently be electroplated with one at the temperature of the substrate. The of the platinum group metals. process is usually carried out under reduced Because of the position of the platinum pressure in order to obtain the necessary group metals at the bottom of the electro- volatility without decomposing the metal- chemical series, coatings can often be obtained bearing compound prematurely. Further- on the base metals simply by immersing the more, the process can be, and normally is, substrate in an aqueous solution of a platinum conducted in an inert atmosphere, so coatings group metal salt. The quality of coating of materials sensitive to oxidation at elevated obtained is not usually good, but the method temperature, such as palladium, can be applied is a simple way of producing composite metal by this method. The coating is deposited powders containing small proportions of over the entire surface of the substrate, appli- platinum group metals. cation to selected areas can therefore be achieved only by masking or by subsequent Aqueous Electroplating etching of unwanted areas. This ability to Aqueous electroplating is a well-established coat the entire surface of the substrate extends and widely practised method of applying to the interior of fine pores and crevices so many metals to metallic substrates, and good the method is well suited to the impregnation results can be obtained with most of the of porous materials, For this purpose, the platinum metals (5, 6). The nature and process can be modified with advantage. The thickness of the deposits obtainable with the material to be treated is first impregnated various members of the group vary consider- with a solution of the volatile organometallic ably.