Reducing the grain size of polycrystalline lead films by the use of barriers to grain growth

Abstract

The mechanical strength of lead-alloy films used in Josephson junctions is improved by reducing the grain size. One way to achieve this goal is to reduce the rate of grain growth. The activation energy for barrierless growth during vapor deposition was estimated to be about 540 cal/mole for lead films and is assigned to the ordering of atoms. Because of its small value, instead of reducing the rate of grain growths, it seemed best to stop growth by the application of a barrier layer. This is followed by the nucleation of new grains. Five requirements for a barrier-to-grain growth are given out of which the requirement of a high interfacial free energy between barrier and lead is stressed the most. In the experiments Nb, PdSn4, AuIn2, and PdIn3 were used as barriers between lead layers. The grain size of layered films was determined by transmission electron microscopy and was found to be about the same as if the total film thickness were equal to the thickness of one layer. Yet there are small differences between the different barrier layers; their effectiveness increases as their melting points increase. This is because all the requirements for a barrier-to-grain growth suggested in this paper are the better fulfilled the higher the melting point. Some of the layered films were tested for hillock formation due to thermal cycles between room temperature and 4.2 K; on some of them, the superconducting transition temperature was measured and nuclear backscattering spectra taken.