Capacitive micromachined ultrasonic transducers with integrated optoelectronic readout

Capacitive detection methods impose several limitations on the performance of capacitive micromachined ultrasound transducers (cMUT) such as low sensitivity below MHz frequencies and low limits on acoustic power output. Optical interferometric detection of cMUT membrane displacement solves these problems by providing a flat frequency response down to DC and electrical isolation between transmitting and receiving electronics. A phase-sensitive optical diffraction grating structure formed by placing a grated electrode under the cMUT membranes enables integration of optoelectronic readout circuitry to cMUT arrays while allowing efficient transmit operation using capacitive actuation. This structure is used to measure membrane displacements down to 10^-4 Å/√Hz level in the DC-2MHz range on cMUTs fabricated on quartz substrates. The possibility of enhanced displacement sensitivity for immersion applications by forming a multi-pass interferometer is discussed and an optical analysis of the detection method is described. The feasibility of hybrid optoelectronics integration for phased array operation is also demonstrated by experiments on cMUTs using vertical cavity surface emitting lasers and 1×8 array of photodetectors with integrated CMOS amplifiers