A novel fabrication process for CMUTs in air

Hord, Samuel Kay
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A novel fabrication method for producing capacitive micromachined ultrasonic transducers (CMUTs) is presented. The process uses conductive silicon on insulator (SOI) substrates to produce an unstressed transducer diaphragm. By etching release holes through the device layer and selectively removing the underlying buffered oxide (BOX) layer, an ultrasonic transducer can be made using only two photolithography steps. The process is described in detail, including models predicting the modal behavior and the collapse voltage of the device. The acoustical behavior of a perforated plate over a sealed cavity is modeled using mechano-acoustical circuit analysis. The device is found to produce sound despite the perforations so long as the holes are sufficiently small and the frequency of operation is sufficiently high. A pitch-catch measurement verifies the transduction of the device. To the author’s knowledge, this is the simplest method for CMUT fabrication to date.