Embedded sacrificial layers for CMUT fabrication

Capacitive Micromachined Ultrasonic Transducers (CMUTs) are generally fabricated either by conventional sacrificial release process or by wafer bonding technique. In the former, sacrificial layers are patterned with deposited materials on the substrate. This current work reports a development on the aforementioned technique wherein sacrificial islands are embedded inside grooves opened by DRIE in the substrate itself. The depth of the grooves and the thickness of the sacrificial layer are identical in dimension. As the first membrane layer between the top electrode and vacuum gap reduces the device sensitivity, it needs to be kept as thin as possible. Conformality of the deposition technique, however, requires a deposition thickness at least equal to the sacrificial layer. Hence one cannot go below a certain first membrane layer thickness. The present method is expected to solve such problems in CMUT fabrication. The present technique keeps the substrate completely flat even after sacrificial patterning, hence aiding the consecutive process steps such as electrode deposition and patterning. The described method does not increase process complexity other than an additional RIE step. Using this technique CMUTs with 5.6 MHz center frequency have been manufactured and tested.