Orals Abstract for Mohammed H. Badi

Mohammed H. Badi mbadi at relgyro.stanford.edu
Sat Nov 15 14:50:06 PST 2003

Special University Oral Examination

Using a MEMS Transducer
To Excite Lamb Waves in Silicon

Thursday, November 20, 2003
1:00 PM, CISX-Aud
(Refreshments at 12:30 PM)

Mohammed H. Badi
E.L. Ginzton Laboratory
Department of Electrical Engineering
Advisor: Professor B.T. Khuri-Yakub

Many sensors and acoustic wave filters rely on the conversion of electrical
energy into the mechanical energy.  These devices traditionally perform this
function using a piezoelectric transducer in an interdigital configuration
that launches acoustic waves in the substrate upon which the device is built.
While almost all of these acoustic wave devices operate efficiently in a
gaseous or vacuum ambient, only those that generate primarily shear motion in
their substrate remain efficient in a liquid environment due to excessive
damping.  The exception to this rule occurs for devices that propagate waves
with a velocity lower than that of sound in the liquid.  Asymmetric Lamb
waves, also known as flexural plate waves, fit this criterion and are
therefore often used in both sensor and filter applications.  This
presentation describes the fabrication, modeling and experimental results of
a novel device for the transduction of Lamb waves using the Capacitive
Micromachined Ultrasonic Transducer (CMUT).

The CMUT is similar to other capacitance transducers in that it employs a
vibrating membrane to send and receive ultrasound in air and in water.  Its
invention was reported in 1994, and it has since found applications in a
variety of arenas.  The presence of Lamb waves in devices built for the
purpose of transmitting an acoustic signal into the surrounding medium has a
deleterious effect on the system behavior.  This is because the wave that is
excited creates a cross-coupling of energy between otherwise independent
cells.  If this excitation of Lamb waves is instead exploited and optimized,
however, the foundation for a new device is created.

The Lamb wave device described in this presentation uses high aspect ratio
CMUTs built using two different fabrication techniques.  The first results in
a capacitive transducer built using the standard sacrificial-layer CMUT
manufacturing process, while the second employs the significantly more robust
and less labor-intensive wafer bonding method.  Both arrays and single
element structures have been built on substrates that have a thickness
ranging from 500 microns down to 8 microns.  They have been characterized
using S-parameter, pitch-catch, and laser doppler vibrometer techniques, and
the resulting behavior is consistent with simulations performed using both
analytical and finite element models.  Measurements further demonstrate an
insertion loss of 16.4 dB at 2.6 MHz for an electrically matched device built
using a single 60 um x 1 cm CMUT at both the transmit and receive ports.

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