Reliability testing of MEMS resonators has grown significantly in importance since these devices moved into high
volume production. In line with this development, we present an automated phonon detection-based test setup, which
utilizes a piezoelectric transducer to translate resonator mechanical motion into voltage, for investigating the long-term
frequency stability of clamped-clamped beam resonators. The automated test system we have developed is able to
continuously actuate up to four devices and characterize them every 30 minutes to monitor resonance frequency f<sub>0</sub> and
Q-factor changes resultant from long-term actuation. The surface temperature of the devices is also carefully monitored
and the temperature data is used to compensate for the f<sub>0</sub> variations caused by temperature fluctuations. The compensated
f<sub>0</sub> measurements obtained over time can be used to determine the frequency drift of the resonators. Q-factor degradation
and variations in resonator in-plane displacement can also be detected by our system. The test system was used to
monitor the behaviour of a 168.502 kHz resonator over a 225-hour operating period. The device was actuated in its linear
mode at 29 ±1.0 °C and ~10<sup>-1</sup> Pa. It showed an f<sub>0</sub> shift of -1.092 Hz/day with Q-factor remaining at ~27,000 throughout.
Resonator displacement was also consistent over the actuation period.
In this work, three useful techniques for dynamic motion characterization of MEMS devices are presented, namely
network analyzer, acoustic phonon detection and stroboscopic SEM techniques. Proof-of-concept experiments using an
MEMS electrostatic resonator reveal reliable and consistent measurement results from the three techniques. The network
analyzer characterization technique is most widely used in practice due to its convenience, high sensitivity and high
speed. The second acoustic phonon technique features non-invasive and package level testing, but it is still an indirect
characterization method, like the network analyzer. In acoustic phonon detection, mechanical waves (phonons) generated
by the actuated MEMS device are used as the coupling mechanism through which information on the dynamic
mechanical state of the device can be obtained. The third stroboscopic SEM technique is capable of directly measuring
the device motion, but its throughput is low and hence not suitable for high volume testing. The stroboscopic SEM
imaging system is based on time-gated sampling of the analogue secondary electron (SE) signal. Unlike conventional
SEM, stroboscopic SEM is able to detect the actual position of the structure at a specific point in time by taking a
time-gated sample of the SEM SE signal at a specific phase of the structure's motion.