Ph.D. Oral Defense Announcement

Ping Zhang pzhang at stanford.edu
Sat Sep 21 18:57:30 PDT 2002


University Ph.D. Oral Examination

Thursday, September 26, 2002, 1:30 pm
CIS-X Auditorium
(Refreshments at 1:15 pm)

"A Micromechanical Study of Free-standing Thin Films
for RF MEMS Switch Application"

Ping Zhang
Department of Materials Science and Engineering


Abstract:

Microelectromechanical systems (MEMS) have a wide range of applications.
In the field of wireless and microwave technology, considerable attention
has been given to the development and integration of MEMS-based RF (radio
frequency) components in an effort to achieve a single-chip RF solution
for future telecom systems.

An RF MEMS switch requires high isolation, low insertion loss, and low
actuation voltage - operational aspects that have been extensively
studied. The mechanical requirements of the switch, such as low
sensitivity to built-in stress and high reliability, greatly depend on
the micromechanical properties of the switch materials, and have not been
thoroughly explored. This research focuses on the study of these
micromechanical issues.

RF MEMS switches are typically in the form of a free-standing thin film
structure. Large stress gradients and across-wafer stress variations
developed during fabrication of the switches severely degrade their
electrical performance. We have built a stress measurement sensor that
can potentially be employed for in-situ monitoring of stress evolution
and wafer-scale stress variation during thin film deposition. The sensors
were micromachined using five masks on two wafer levels, each measuring
5x3x1 mm. They function by means of an electron tunneling mechanism,
where a 2x2 mm silicon nitride deflection membrane elastically deflects
under an applied deflection voltage in an external feedback circuitry.
For the current design, the sensors are capable of measuring tensile
stresses up to the GPa range under deflection voltages of 50-100 V.
Sensor functionality was studied by finite element modeling (FEM) in
conjunction with a theoretical analysis of square membrane deflection.

While the mechanical properties of thin films on substrates have been
extensively studied, studies of free-standing thin films have been
limited due to the practical difficulties in sample handling and testing.
We have successfully fabricated free-standing Al and Al-Ti thin film
specimens and performed microtensile and stress relaxation tests in our
custom-designed micromechanical testing apparatus. A dedicated TEM
(transmission electron microscopy) sample preparation technique allows us
to investigate the microstructures of these thin films both before and
after mechanical testing and to correlate the microstructural findings
with the mechanical behavior. Major studies include grain boundary
strengthening in pure Al and solid solution and precipitate strengthening
in an Al-Ti alloy, plastic deformation in pure Al by inhomogeneous
deformation and localized grain thinning, and stress relaxation of Al and
Al-Ti.














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