Cadmium sulphide transducers : thick vacuum-deposited films for ultrasonic shear-mode low-frequencey operation

Abstract

The primary aim of this research project was to deposist thick-film (low-frequency) s-mode CdS piezoelectric transducers directly onto copper or aluminium rods which formed part of the welding-electrode of a spot-welding machine. These transducers were to replace the discrete transducers used in a set-up for the "on-machine" evaluation of spot-welds. A secondary aim was to deposit very thin CdS films on glass slides for use as microwave resonators. The dependence of film adhesion on film thickness, and crystallographic orientation changes with thickness imposed an upper limit on the thickness of transducers with adequate properties for applications. The final goal, established through experience on the project, was to determine how thick piezoelectric films could be deposited to make useful transducers. Highly stoichiometric (deviations from stoichiometry of the order of 1 part in 10^13 ) and highly resistive (> 10^10 Ω.m.), and highly oriented films up to 100 μm thick have been successfully deposited on Ae rod substrates. Two deposition techniques were used : CdS/S electron beam bombardment evaporation and Cd/S isothermal cells. Provided that the temperature of the vapour molecules was less than 400 degrees C and that the pumping speed could be increased at will , then, the faster the deposition rate, the sharper the oblique c-axis preferred orientation, and the better the piezoelectric performance of the films. The pumping speed limited the deposition rate to 10 μm.h^- 1. Appreciable thermal stresses in the films gave rise to large forces which induced the thick-films to flake off or disintegrate. The dependence of film adhesion on film thickness is explained in terms of the inequality between the forces which bind the film to the substrate (independent of thickness) and the forces which induce the film to flake off (proportional to thickness). Thick CdS films were made to adhere to the substrate by making the substrate surface rouqher so that the films "keyed-in". No appreciable temperature gradients existed in the CdS films during growth, either across their thicknesses or along their surfaces. No changes in temperature gradients occurred in the films due to changes in film orientation, and vice versa. Up to a certain critical thickness, the c-axes of most CdS film crystallites aligned themselves with the direction of the vapour beam. When the thickness of the film exceeded the critical thickness, the growth of oblique crystallites was stifled and the film's c-axis tilted towards the substrate-normal and eventually became parallel to it. This was confirmed by etching-back a thick CdS film which was deposited at oblique vapour incidence. A model is presented for the "stifling process” which gives the relation between the critical thickness, the grain size and the deposition angle of the film. For a given deposition environment, the stifling process imposed an upper limit on the thickness of an s-mode transducer. The use of copper substrates, and of copper parts inside the deposition chamber, was abandoned because of the corrosive action of sulphur on copper. Cu/CdS junctions were nearly ohmic, and the anomalous behaviour of these junctions is explained in terms of the reaction between Cu and S to form Cu2S. CdS s-mode transducers with untuned two-way insertion loss of 35 dB in a 50 ohm system have been successfully deposited on glass slides for operation at frequencies down to 20 MHz. The stress in CdS films on glass slides was much less than that on Ae rods. It is possible that the higher stress in films on Ae rods weakened their piezoelectric performance