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The thin film resonator technology has been under development
for over forty years in one form or another. Although the
basic approach is derived from the desire to reach higher
frequencies than those readily achieved by thinning bulk crystals,
there have always been competing technologies or fundamental
material or processing problems that have impeded the development.
Finally, a point was reached in the wireless market wherein
competing technologies appeared unable to meet the demands
of modern wireless applications.
First, thin film transducers using CdS or ZnO, were used
in microwave delay lines as a means of generating the high
frequency wide bandwidth time delays demanded by radar signal
processing applications. Thin film transducers were a viable
alternative, if not the only approach, to obtain high frequency
signals and held that niche application for a considerable
time.
In the background, there were techniques proposed, mostly
in the patent literature, for configurations that would result
in high frequency resonators, not just delay lines. Some of
those ideas greatly exceeded the material science of thin
films at the time and there was little hope of a practical
implementation. Accordingly, successful devices were not demonstrated,
in part because of a lack of materials processing sophistication
but also because there was no real compelling need for the
high frequency devices in the first place.
In the mid 60s surface acoustic devices began to emerge as
a promising technology. This was not because surface acoustic
waves had just been discovered, but rather because a simple
means of transduction was invented that converged with significant
advances in microelectronics having to do with the production
of fine metal lines. Today SAW devices are a major mainstay
of wireless frequency control devices.
Also in competition with thin film resonator technology are
those devices that derive from dielectric electromagnetic
resonators. Advances in ceramic materials science has resulted
in very low cost filters for wireless applications.
This paper will present a tutorial on the thin film bulk
acoustic wave resonator technology. Every effort will be made
to provide an objective analysis of the technology in relation
to applications and competing technologies, and point out
obstacles and promises, as known, for further technology advancement.
Biography
Kenneth Lakin received B.S. degrees in Electrical Engineering,
Engineering Mathematics, and Engineering Physics in 1964,
the M.S. degree in Electrical Engineering in 1965 from the
University of Michigan, and the Ph.D. degree in Applied Physics
from Stanford University in 1969. He was a faculty member
in the Electrical Engineering and Materials Science Departments
at the University of Southern California from 1969 to 1980
where he conducted research on surface acoustic wave devices,
thin film resonators, and piezoelectric film growth and characterization.
From 1980 to 1989 he was affiliated with Iowa State University's
Ames Laboratory and founded the Microelectronics Research
Center. He formed TFR Technologies in 1989 and is acting as
President and CEO while conducting research on thin film resonators,
piezoelectric materials, filters, planar dielectric resonators
and numerical analysis of electromechanical resonators for
microwave frequencies. He has published over 100 technical
papers in areas of acoustic signal processing device and materials
research. Dr. Lakin is a Senior Member of the IEEE. |
