Preparation of Pb-Bi film by alloy evaporation I: Film composition control

Abstract

Pb-Bi films in h.c.p. phase (ε-PbBi) suitable for lead alloy Josephson junction counterelectrodes are typically prepared by evaporating a Pb-Bi alloy from a single r.f.-induction-heated crucible. The composition of such films is generally expected to be different from that of the source. In order to obtain a single-phase ε-PbBi film which covers only a small range of bismuth content (about 27-32 wt.% Bi), good control of film composition is required. The relationship between film and source composition can be theoretically estimated, using the Hertz-Knudsen-Langmuir (HKL) equation and assuming comparable sticking coefficients among the alloy constituents. Taking into account the contribution of bismuth monomers and dimers to the total partial pressure of bismuth in Pb-Bi vapor, we predicted that there exists an optimum source temperature To, or equivalently an optimum evaporation rate ro, for alloy evaporation to obtain Pb-Bi films of the same composition as the source. It was also predicted that To should be a function of source composition because of the contribution of bismuth dimers present in bismuth vapor. Experiments were carried out to evaluate these predictions. Several alloy sources of known composition were evaporated to depletion, each at a different fixed rate. Films of 1-2 μm thickness were collected onto silicon substrates at a series of predetermined evaporation time intervals. The film composition was determined by controlled-potential coulometry which is accurate to ±0.2 wt.% Bi for film weights of 2 mg or more. Experimental results confirmed the existence of an optimum evaporation rate, corresponding to To, for a given source composition as predicted from the HKL equation. The optimum evaporation rate was also found to increase as expected with decreasing bismuth content. The optimum evaporation temperatures for 27% Bi and 29% Bi sources were measured to be 1080 ± 10 K and 1022 ± 10 K respectively, which are about 100 K higher than the predicted values. The discrepancy is believed to be mainly due to the use of an open crucible in this work causing the vapor pressure over the melt, at a given temperature, to be lower than the equilibrium value and the To value to increase. The difference in To values for two sources of different bismuth contents used in this experiment is in good agreement with the predicted value. © 1982.