Thermal stability of Pb-alloy Josephson junction electrode materials. IX. Multilayered Pb-In-Au electrodes

Abstract

Effects of substrate temperature, deposition sequence, and number of layers on the microstructure of Pb-12 wt. % In-4 wt. % Au films were studied using x-ray diffraction and transmission and scanning electron microscopy before and after repeated thermal cycling between 298 and 4.2 K. Significant grain size reduction and smooth film surfaces were obtained when the film deposition was carried out at substrate temperatures below 200 K. The deposition sequence strongly affected the rate of grain growth that occurred during warming from the deposition temperature to ambient temperature. It was found that the sequential Au and In depositions were essential in order to suppress grain growth. Strain introduced into films upon cooling from 298 to 4.2 K due to a thermal expansion coefficients mismatch between the films and Si substrates was also measured. The strain supported elastically by 0.35-μm-thick multilayered films was increased by reducing the thickness of each layer. By studying dependence of thickness of each layer on strain at 4.2 K, the critical grain size, smaller than which strain is not relaxed by dislocation glide, was determined to be ∼90 nm. This size agrees well with the previously measured mean spacing between misfit dislocations. Based on the present study, 10-layered Pb-In-Au films prepared at 200 K were developed for use as Josephson junction electrodes. The films had smooth surfaces and showed no undercut at the edges of lift-off patterned films. After 400 thermal cycles between 298 and 4.2 K, no microstructure changes such as hillock formation, dislocation slip steps, and grain rotation, were observed. These films can be used both as base electrode films for Pb-alloy Josephson junctions and as counterelectrodes for Nb/Pb-alloy junctions.