DESIGN AND CHARACTERIZATION OF A VACUUM SYSTEM FOR
INTERFEROMETRIC MEASUREMENTS OF MEMS RESONATORS
Lauri Kangas
Optics and Molecular Materials, Helsinki University of Technology,
P.O.Box 3500 FIN-02015 TKK Finland
email: [email protected]
Recently there has been great interest of developing silicon-based microelectromechanical
resonators that can be utilized as a frequency reference for e.g. wireless communication
applications. The advantage of these MEMS resonators is their small size and integrability
as opposed to traditional quartz oscillators. In ambient air pressure however, significant
performance losses take place due to air damping the oscillation. Therefore the resonators
must operate in vacuum in their final applications.
Optical interferometric probing has been proven useful in research and development of
MEMS resonators as a tool to obtain direct information of the acoustic phenomena associated with the devices, since only indirect characterization of these devices is possible
by studying their electrical respones.
In this thesis a vacuum system enabling interferometric measurements of MEMS devices
in low pressure has been designed and characterized. The sample under study is positioned inside a custom built vacuum chamber equipped with a window enabling optical
probing. In addition, the system provides the ability to achieve a stable pressure between 0.1 mbar and atmospheric pressure for further analysis of the pressure dependent
behaviour of the resonator.
A square plate resonator sample[1] was used for testing the system. The resonator was
excited with frequencies around its square extensional resonance mode of 13.1 MHz. In
vacuum as compared to atmospheric pressure, a nine-fold increase of the quality factor of
the resonator was observed together with a 250 ppm decrease of the resonance frequency.
A treshold for the pressure-limited behaviour of the sample was found to be 10 mbar,
below which no performance gain of the resonator was achieved by lowering the pressure.
f = 12.1312 MHz
-42
-50
-60
-70
-80
-90
100 µm
-100
100 µm
Left: Microscope image and Lamé
mode relative amplitude field of the
sample. Right: A vacuum casing
on top of the sample.
1 cm
-107 dB
(a)
(b)
[1] V. Kaajakari et al. Square-extensional mode single-crystal silicon micromechanical
resonator for low phase noise oscillator applications. Electron Dev. Lett., 2004