A CMUT array based on annular cell geometry for air-coupled applications

An annular cell has been reported with improved transmit sensitivity over a conventional circular cell for air-coupled CMUTs. The particular geometry of an annular cell also allows multi of them to be arranged in a concentric layout such that high-intensity ultrasound can be focused in the depth direction. This paper presents a 9.3-mm-diameter CMUT array consisting of four concentric annular cells. The wafer-bonded CMUT has a plate thickness of 18.3 μm and cell width of 930 μm. Under atmospheric pressure, the ratio of average-to-maximum displacement (RAMD) of the annular cells was measured to be 0.53 using a profilometer. At 200-V DC bias and 20-Vpp AC actuation voltage, the frequency response of the maximum displacement was measured using a laser Doppler vibrometer, and a resonance frequency of 182.5 kHz was observed. At the same DC bias, the resonance frequency of the array was also measured to be 182.5 kHz using a vector network analyzer. Based on the frequency response, the acoustic filed of the CMUT array was simulated to have an 11.4-mm natural focal depth with -3-dB beam width of 3.14 mm. This study demonstrates the potential of using concentric annular-cell CMUT array to generate focused ultrasound in air.