6F-2 Extended Insulation Layer Structure for CMUTs

Electrostatic transducers require an electrically insulating structure between their electrodes. In state-of-the-art transducers, such as capacitive (micromachined) ultrasonic transducers (CUTs, CMUTs), this insulating structure is the main limiting factor in terms of device reliability (electrical breakdown, parasitic charging effects) and device performance (thin gap devices with low parasitic capacitance). We present a configuration, based on an extended insulation layer structure, which addresses all of these issues. A deep-trench-oxidation technique is used, that allows the fabrication of a released thermally-grown silicon dioxide structure, which can be more than 30 mum deep extended into the substrate. Preliminary measurement results from a CUT, featuring a back plate with such a deep extended insulation layer structure, are presented. The approach of using an extended insulation layer structure not only can improve the present CUT and CMUT technology in terms of reliability and performance, it also opens the door to high temperature applications of various types of electrostatic transducers.