Advanced CAD System for Electromagnetic MEMS Interactive Analysis

4. Characterization of MEMS Materials Models
Authors: Dr. Guido Cornella, Dr. Hoo-Jeong Lee, Ping Zhang, Prof. John C. Bravman
4.1 Monotonic and Cyclic Testing of Thin Film Materials for RF Switch Application
In MEMS devices, many of the active components exist in the form of free-standing thin films.
Such components are constantly in motion under various actuation or stimulation. Understanding
the mechanical properties and modeling the mechanical behavior of free-standing thin films is
therefore important for the design of MEMS devices, as well as for predicting their mechanical
performance and reliability.
The mechanical behavior of a free-standing thin film is expected to be different from that of bulk
material or a conventional thin film on a substrate. In a free-standing thin film, the grain size is
typically very small and the absence of a substrate leads to both its top and bottom surfaces being
unconstrained. These microstructural characteristics may result in unique mechanical properties.
Although thin films on substrates have been extensively studied, knowledge of free-standing thin
films was not available until the advent of micromachining techniques and is yet to be further
explored. We have developed a sample fabrication process which batch produces free-standing
thin film samples using micromachining techniques within a cleanroom facility.
Basic materials parameters that are required for mechanical device modeling are, among others,
the stress-strain response for monotonic and cyclic loading. We have built a custom-designed
micromechanical testing system that is capable of providing accurate stress-strain data for
monotonic loading as well as for cyclic loading up to a very large numbers of cycles. In
conjunction with a dedicated sample fabrication process, we are able to carry out mechanical
testing on a wide range of free-standing thin films of different materials and different geometry.
We use Transmission Electron Microscopy (TEM) to reveal the microstructural features of the
samples both before and after testing, which facilitates our understanding of the mechanical
properties of the thin film materials under investigation.
One particular MEMS application we are trying to simulate through our testing is the operation
of an RF switch. We are interested in its one-time switching behavior as well as its cyclic
operation in the kHz frequency range, which correspond to monotonic and cyclic loading
conditions, respectively. We are also interested in the stress relaxation behavior, important for
when the switch is held at the “on” position for an extended amount of time. We have studied
pure Al and Al/Ti-alloyed thin film samples and obtained stress-strain responses as well as stress
relaxation data from both monotonic and cyclic loading, and have simulated the stress relaxation
behavior by anelastic models.
4.1.1 Sample Fabrication
Specimens were fabricated using micromachining techniques at the Stanford Nanofabrication
Facility (SNF). The fabrication process makes use of two masks. The front-side mask defines the
micro beams; the backside mask defines the silicon windows to be etched away to release the
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