Sammendrag
Capacitor Micromachined Ultrasonic Transducers (CMUTs) are being developed and fabricated to be integrated in a 1mm 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 5.7um 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.
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