Characterization of the BeAg interfacial region of silver films deposited onto beryllium using a hot hollow cathode discharge

Abstract

Silver films are physically vapor deposited onto beryllium substrates using a hot hollow cathode discharge. To obtain high BeAg adhesion strengths, an atomically “clean” surface is obtained by ion bombardment cleaning. In this investigation, the relationship of the ion cleaning parameters to contaminants in the BeAg interfacial region and their effect on adhesion strength were evaluated. Specimens were ion cleaned at various bombardment parameters and then flash coated with silver. In-depth film profiles were taken by sputter etching in argon and monitoring the Auger electron peak-to-peak heights. The interface was also analyzed by taking a complete spectrum at the edge of the sputter crater. Impurities found at the interface were tantalum, copper and oxygen. The source of tantalum is evaporation from the hot hollow cathode tube. Copper is deposited by sputtering of the specimen holder. If the copper holder had been predeposited with silver, no copper would have been found in the interface. The results for oxygen show that oxygen can be left in the interface if the ion cleaning period is not long enough to remove completely the oxide from the beryllium surface. Oxygen was also found in the interface when the substrate ion current was below a threshold value. Ion cleaning below a threshold current density allowed oxygen to readsorb on the surface. To produce an oxygen-free interface on a machined beryllium surface, the bombardment cleaning time should be at least 10 min with a current density 1.3 mA cm −2 or more. The results for adhesion strengths showed that asmall amount of oxygen (about 2 at.%) left in the BeAg interface will reduce the adhesion strength of the coating. Silver films deposited in an air leak that was greater than a leak which is easily detectable by residual gas analysis contained only about 0.5 at.% O with no reduction in film adherence strengths.