Investigation of charge diffusion in CMUTs using optical interferometry

Capacitive Micromachined Ultrasonic Transducers (CMUTs) have been developed and fabricated at our department. The main goal is to use an improved version of these structures to perform medical imaging to detect unstable plaque in the coronary arteries. The CMUTs have a radius of 5.7 mum and a center frequency of about 30 MHz in air. When an RF voltage is applied in addition to a DC bias the membrane will vibrate and generate ultrasound waves. A heterodyne interferometer has been built in order to characterize the CMUTs. The setup can measure absolute phase and amplitudes. By using two acousto-optic modulators in the reference arm of the interferometer we can measure acoustic frequencies in the range 10 kHz-1.0 GHz. The results from the interferometer are supplemented with measurements from a network analyzer. The network analyzer measures several CMUTs in parallel whereas the interferometer inspects individual CMUT cells. The vibrating membrane in the CMUT is made of silicon nitride, which ideally is an insulator. However, we observe charge diffusion through this membrane influencing the response of the CMUTs. We propose two possible mechanisms. One mechanism is that positive charges diffuse from the bottom electrode through the silicon substrate and into the silicon nitride membrane. The other mechanism is that negative charges from the top electrode diffuse into the silicon nitride membrane. An experiment investigating the resonance frequency as a function of time indicated that the latter mechanism is dominant. Measurements from both the interferometer and the network analyzer supported this conclusion.