2006crz8207
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ABSTRACT
The present work focuses on the preparation and characterization of Silicon
Carbide (SiC) and Aluminum Nitride (AlN) films prepared by RF magnetron
sputtering for micro-electro-mechanical systems (MEMS) application using their
respective ceramic targets. SiC and AlN films were selected respectively as structural
and piezoelectric materials. Sputtered SiC films have been reported for application as
protective coatings but their application as structural material has not been
investigated in significant manner. In the present work, we aim to produce low stress
SiC films with high Youngs modulus (E) and Hardness (H) values for structural
material in MEMS. Such SiC films could also be useful for high frequency
resonating MEMS. The objective of the present work is to prepare SiC films by RF
sputtering, preferably without external substrate heating. This is considered
important, if the MEMS are to be realized on low-cost substrates such as glass. The
residual stress was measured using wafer curvature technique. The sputtering process
was optimized for obtaining a low stress SiC films. The structural and mechanical
properties of these films were investigated. For this purpose, characterizations tools
such as: XRD, Fourier Transform Infrared Spectroscopy (FTIR) and nano-
indentation techniques are used. The chemical inertness of low stress SiC films in
buffered hydrofluoric acid (BHF), KOH / tetra-methyl-ammonium hydroxide
(TMAH), was investigated and it was observed that the films were stable in these
solutions for prolonged exposure. To demonstrate the feasibility of sputter deposited
SiC films for MEMS, suspended microstructures such as cantilever beams,
diaphragms, and bridges were fabricated using Si bulk micromachining process.
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For the AlN films, the objective was defined to prepare c-axis oriented AlN films
without external substrate heating by RF magnetron sputtering. FTIR was used to
study the Al-N bonds in the films. The films were characterized by XRD and TEM
(transmission electron microscopy) to investigate the preferred orientation. AlN films
with preferred (002) orientation were prepared on a variety of substrates by RF
magnetron sputtering using ceramic target. The c-axis orientation in the films is
achieved without external substrate heating when sputtering was carried out in Ar-N2
(1:1) ambient.
For applications as piezoelectric material in MEMS and other acoustic devices,
these films are required to be deposited on metal electrode. Therefore we investigated
the effect of various metal films such as: Al, Cr, Au-Cr on preferred orientation of
AlN films. The quality of c-axis oriented AlN films prepared by this method was
investigated for a chosen set of sputtering parameters using techniques such as UV-
VIS spectroscopy, PL (photoluminescence) spectroscopy, and SIMS (secondary ion
mass spectroscopy). The electrical resistivity of AlN films was found in the range of
1011
-1012
-cm for moderate fields (~50 kV/cm). Such high resistivity of these films
is essential for the application of AlN as a piezoelectric material. A prototype film
bulk acoustic resonator (FBAR) device was fabricated to demonstrate that the films
are suitable for acoustic / piezoelectric MEMS.
The thesis concludes by suggesting the scope of further research in this area.
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