Platinum Metals Rev., 1976, 20, (3), 94
High-Rate Sputtering of the Platinum Metals
Economies in Processing Time
The use of sputtering to produce coatings of the platinum groupmetals has a number of attractions. The coatings have good adhesionto the substrate, are dense, and have properties very close to thoseof the bulk metals. The disadvantage of conventional sputtering isthe low rate of deposition which can be attained; a typical rate forplatinum would be around 500 Å/min. For quantity production,higher rates are desirable, and for many metals recourse could be hadto vacuum evaporation or electroplating, but the refractory nature ofplatinum group metals makes evaporation difficult, and except forrhodium, existing aqueous electroplating baths are not verysatisfactory when electrical properties are important.
Attempts to increase sputtering rates simply by increasing powerinput result in serious heating of the substrate by secondaryelectron bombardment, but a recently developed technique, known asplanar magnetron sputtering and now available commercially throughNordiko Ltd of Havant, Hampshire, avoids this heating problem and leads to an increase in sputtering rate amounting to an order ofmagnitude.
The system employs permanent magnets to set up amagnetic field which traps secondary electrons emitted from thetarget, preventing them from bombarding the substrate but using themto produce greatly enhanced ionisation in the sputtering gas. Thearrangement used is shown in the diagram below.
Two magnets, one in the centre of the target with an upward facing northpole and one around the periphery of the target with a south polefacing upwards, set up a field with lines of force running as shown.Electrons emitted when sputtering takes place are constrained tofollow helical paths round these lines of magnetic force. The lengthof this helical path is very large, the chance of an ionisingcollision with a gas molecule is correspondingly large and highplasma current densities result which give rise to high sputteringrates. Typically, a current density of 40 mA/cm2 isobtained compared with about 1 mA/cm2 in conventionalsystems.
By this technique, typical maximum sputtering ratesare raised from 1000 Å/min to 18,700 Å/min forpalladium, from 750 Å/min to 12,600 Å/min for platinum,and from 600 Å/min to 11,700 Å/min for rhodium. Thislarge increase in sputtering rate can lead to considerable economiesin processing times, and may open the way to new applications ofplatinum group metal coatings which previously would have requiredimpractical processing times.
Circular high-rate sputtering electrodes are available up to 20 cm diameter for use in research and development equipment. These water-cooled electrodes contain therequisite magnets and can be used in place of, or in conjunctionwith, normal sputtering electrodes. For large scale productionequipment, rectangular high rate electrodes are available for usewith a linear travelling work table, drum mounted substrates orcontinuous roll coating.
The use of this modification enablesthe advantages of sputtering such as its controllability,reproducibility, uniformity and low pinhole density to be enjoyedwhile attaining high deposition rates together with low substrateheating and low radiation damage.