Fabrication of CMUTs Using Sacrificial Release Process with Ashing Assisted Polysilicon Release

Capacitive Micromachined Ultrasonic Transducer (CMUT) is a type of ultrasonic transducer that can be applied in many fields like chemical and mechanical sensing, physical imaging, and bio-imaging. Sacrificial layer release is one of the fabrication methods and cavity releasing is the key part of this method. The stress of the sacrificial material is one of the causes making membranes break in this procedure. The previous studies had a phospho-silicide-glass (PSG) layer deposited between polysilicon sacrificial layer and silicon nitride membrane with LPCVD partially solved the problem. However, its processing temperature is relatively high, which makes it not compatible with CMOS integrated circuits (ICs). This work fabricated CMUTs in the sacrificial layer release process, with polysilicon as the main sacrificial layer. Utilizing chemical reaction of polysilicon in ashing process, a thin oxide was formed between the sacrificial layer and membrane while removing the remaining photoresist with ashing. The result showed that it could also help produce large membranes with a lower processing temperature, which can be more compatible with CMOS ICs. After fabrication, the impedance of an element with 100 µm diameter cells was measured to be several hundred ohms in air with 30 V bias voltage and the resonance frequency was 4.7 MHz.