research activities in 2007 - CSEM

Micro-Vibration Analysis Setup for MEMS and MOEMS Characterization
J.-M. Mayor, I. Kjelberg, P. Masa, J. Babarowski
A new test station for the measurement of the resonance frequencies of micro-structures is presented. The vibrations are detected either along the
optical direction or perpendicular to it. The absolute resonance frequency is measured with accuracy better than 5 ppm (changes to 1 ppm). In
2008, the station will be equipped with a climatic and a vacuum chamber to measure the samples in a controlled environment.
Figure 1: The test station with a 100 mm wafer
Measurement of the resonance frequency of test samples with
a high accuracy is required in order to ascertain mechanical
properties of the materials used, for instance Young’s
modulus as a function of temperature.
The need to have a reliable and cost-effective system suitable
for operation in the clean room production area as quality
control during the manufacturing process was recognized.
Based on CSEM’s development, the system was built by the
company OCB (CH-Marin) with commercially available
standard mechanical blocks so that any change of the optical
configuration could easily be implemented (see Figure 1).
Particular attention has been given to the mechanical rigidity
of the system. Contrary to usual microscopes, where the fine
adjustment is done by moving the whole optical system, here
the fine adjustment is performed by a small vertical shift of the
objective only. As a matter of fact, the stability of the
observing system has been checked and found to be better
than 1 nanometer in a one hour observing time. For this
purpose, the method developed by P. Masa within the frame
of the ENCODER project [1] was used.
Microscope objectives with a magnifying power of 5 x, 10 x
and 20 x and a working distance of 39 mm also allows the test
of encapsulated microsystems. The working distance can be
further enhanced with the use of commercially available relay
lenses which permit operation of the measuring equipment
when the test sample is in a vacuum or climatic chamber (to
be developed in 2008).
The investigated test structures are expected to have quality
factors well above 1’000 therefore an excitation with amplitude
in the nanometer range, obtained with only 1 V on a PZT
stack actuator, will bring test structures into resonance with
fully detectable movement signals. The frequency response
can be obtained with a SRL lock-in, a HP spectrum analyzer
72
and a rubidium stabilized clock, directly connected to the
station.
The probe laser wavelength is above 650 nm so that the color
rendering of the microscope is not affected by the dichroic
beam splitter in the observation path of the camera: any
defect of the test sample is clearly recognizable on the
display.
The detection of the movement can be measured either along
the optical direction (perpendicular to the wafer plane) or
perpendicular to it (in the wafer plane).
• For the detection of the movement in a horizontal plane
the investigated part must have a sharp edge on which the
laser beam is focused. Any movement of the edge will
change the amount of light reflected or transmitted.
• For the detection along the optical axis a surface of
reasonable optical quality is needed. Then an optical fiber
is put at the image point of the laser spot. Any deviation in
height will affect the size of the spot and thus the quantity
of light transmitted into the optical fiber.
With both configurations absolute resonance frequencies of
test samples and of microsystem structures were determined
with accuracy better than 5 ppm. Resonance frequency
change (for instance with temperature) can be detected with a
sensitivity down to 1 ppm by measuring the phase difference
between the excitation and the position signal.
Figure 2: Mechanical response of a microstructure after a shock
As the optical signal is a direct position signal the station can
monitor aperiodic movements as well. An example is depicted
in Figure 2, where the microsystem was subjected to a shock
by a mechanical impact on the sample holder. Two impacts
corresponding to the moving part of the sample touching the
mechanical stop, then coming back to the equilibrium position
with oscillations at its resonance frequency can clearly be
seen. The damping of the movement can be also determined.
The equipment was financed by the Hans Wilsdorf foundation.
CSEM thanks them for their support.
[1] P. Masa, et al., “Encoder – Nanometric Optical Absolute Position
Encoder”, in this report, page 14