Characterization of acoustic vibrations on micro- and nanostructures with picometer sensitivity

摘要

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_3 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.