Fabrication of CMUTs with substrate-embedded springs

A capacitive micromachined ultrasonic transducer (CMUT) with substrate-embedded springs, called post-CMUT (PCMUT), decouples the spring constant and the mass of the system by realizing the former using relatively long and narrow posts. The PCMUT improves on the fill factor and average volume displacement of the conventional CMUT as shown in our previous work using 3-D finite element analysis (FEA). This work reports on second-generation PCMUT devices designed according to our 3-D FEA simulation results and manufactured using an improved fabrication process flow. This improved fabrication process is composed of three critical steps: wafer bonding of two silicon-on-insulator (SOI) wafers, deep reactive-ion etching (DRIE) of posts (i.e. springs), and precision wafer polishing. The improved fabrication process results in a flexible platform for 82-element 1-D arrays, a single element device for high-intensity focused ultrasound (HIFU), and test PCMUT element structures. The new fabrication process also provides better post high uniformity as well as a clear path toward making 2-D arrays.