Hussain, Sadiq Baker (1971) Cadmium sulphide transducers : thick vacuum-deposited films for ultrasonic shear-mode low-frequencey operation. PhD thesis, University of Warwick.

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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.

Item Type:	Thesis (PhD)
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
Library of Congress Subject Headings (LCSH):	Cadmium sulfide, Transducers, Thick films, Thin films, Electric welding
Official Date:	August 1971
Dates:	Date Event August 1971 Submitted
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Crecraft, D. I.
Sponsors:	Great Britain. Ministry of Aviation Supply PD/187/02/ADM
Extent:	x, 196 leaves : ill., charts
Language:	eng

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