The lack of commercial equipment for characterization of vibrating micro- and nanostructures has motivated the
development of a heterodyne interferometer. The setup is designed to measure phase and absolute amplitude in the entire
frequency range 0-1.2 GHz. Its transverse resolution is < 1 μm while the present sensitivity for vibrations is 3 pm/(Hz)<sup>1/2</sup>.
Capacitive micromachined ultrasonic transducers (CMUTs) are being developed for diagnostic imaging of vulnerable
plaques in the coronary arteries. The CMUTs have 5.7 μm radii, 100 nm membrane thickness and ~30 MHz center
frequency. Arrays of ~7500 CMUTs have been fabricated. Frequency scan measurements along a row of CMUTs reveal
a variation in resonance frequency. This may be due to variations of material properties, dimensions such as thickness
and transverse dimensions, and other manufacturing variance. The frequency scan revealed the fundamental mode and
two closely spaced higher order modes.
Modeling of individual CMUT elements was performed using the commercial program COMSOL. A finite element
model (FEM) based on symmetry assumptions predicted only one higher order mode. After closer analysis it was found
that the symmetry assumptions were insufficient. By using a complete physical model two higher order modes were
predicted in agreement with the measurements.
Simulations are able to predict transducer characteristics in great detail but are dependent on accurate input parameters.
The optical measurements contribute to validate or complement simulations and assumptions they rely on. The
heterodyne interferometer is therefore a valuable tool for quality control in the conception, design and manufacturing of
new acoustic devices.
Capacitor Micromachined Ultrasonic Transducers (CMUTs) are being developed and fabricated to be integrated
in a 1 mm diameter catheter, aiming to detect vulnerable plaques in the coronary arteries. The structure is built
up of an array of 72x104 CMUTs, where two linear arrays of CMUT cells are bonded together. The CMUTs
have resonance frequencies of about 30MHz. The radius of each CMUT is 5.7 &mgr;m and the vibration amplitude
is in the range 20pm-12nm. A heterodyne interferometer has been built for characterizing the CMUTs. It offers
the possibility of both phase and high resolution absolute amplitude vibration measurements. The setup can
measure vibrations from 0 to 1.2GHz. In this work we present interferometric measurements on the CMUTs and
compare them with electrical measurements performed by using a network analyzer. Using the interferometer
we are able to investigate individual CMUT cells, whereas the electrical measurements are based on a sum
of all currents in the CMUTs bonded together. In addition to a RF voltage at the operating frequency, the
CMUT is supplied with a bias voltage to vibrate. The CMUT resonance frequency can be tuned by varying this
DC voltage. In this article we have investigated the predicted linear relationship between applied AC voltage
and vibration amplitude. Other parameters investigated are the effects of temperature increase in addition to
traveling charges on the CMUT membrane. The interferometric setup can be used to characterize various devices
with small surface movements, such as MEMS- and SAW-devices.
A heterodyne interferometer has been built in order to characterize vibrations on Micro-Electro-Mechanical Systems (MEMS). The interferometer offers the possibility of both phase and high resolution absolute amplitude vibrational measurements, which is of great importance. A frequency shift is achieved by introducing acoustooptic (AO) modulation in one of the interferometer arms. By using a lock-in amplifier a narrow bandwidth detection regime is achieved. This factor improves the amplitude resolution. By using two AO-modulators and varying the frequency inputs of both, the setup is designed to measure vibrations in the entire frequency range 0 - 1.2GHz. The absolute amplitude is obtained by performing two measurements at each sample point. The first step is to measure the first harmonic of the object vibration. The second step is to measure the frequency components of the light reflected from the test device corresponding to the frequency without object modulation. This is obtained by mixing the detector signal with an external signal generator, and adjusting the frequency of the latter. By combining these two measurements we are able to determine the absolute amplitude of the vibration. The interferometric setup can be used to characterize various kinds of micro- and nanostructures. The system is here demonstrated on a Surface Acoustic Wave (SAW) device and on Capacitor Micromachined Ultrasonic Transducers (CMUTs). We have measured absolute amplitudes with picometer resolution.
A heterodyne interferometer with picometer sensitivity for non-destructive characterization of micro- and nanostructures has been built. The setup is designed to measure phase and amplitude in the entire frequency range 0-1.2GHz. The object can be scanned in the x- and y-direction with sub-micrometer precision. Absolute amplitude of vibration is determined by combining separate measurements of the carrier and sideband frequency of the detected signal. The detector signal is mixed with a signal from a generator. By adjusting the frequency of the signal generator, we can choose the carrier or sideband frequency.
We have performed measurements on capacitor micro-machined ultrasound transducers (CMUTs) which are being developed for diagnostic imaging of vulnerable plaques in arteries. Arrays of ~7500 CMUTs with a total area of 1.3mm x 0.9mm are planned used in an intravascular catheter. The CMUTs studied have typical radii of 5.7-12.5μm, membrane thickness of 100nm, and center frequencies 10-35MHz. Characterization of both single and arrays of CMUTs is important to optimize the manufacturing process and the design. Quality control during manufacture is also important to identify imperfect elements. Other structures have been characterized such as a piezoelectric element with excitation frequencies from a few kHz to several hundreds of kHz and a LiNbO<sub>3</sub> surface acoustic wave (SAW) transducer with excitation frequencies from 20MHz to 30MHz. We have performed initial measurements of absolute amplitudes with picometer resolution. Theoretical calculations agree well with the measurements. The setup can be used to characterize a large range of micro- and nanostructures.