Three dimensional photoacoustic imaging using a monolithic CMOS MEMS capacitive ultrasonic sensor

In this study, we demonstrate the feasibility of three dimensional (3-D) photoacoustic imaging using a monolithic CMOS MEMS capacitive ultrasonic sensor. The sensing membranes are released by a post-CMOS metal etch and sealed by silicon dioxide. Nine membranes, each with an inner diameter of 60 μm, form a single detection unit with a capacitance value of 292.5 fF. The measured sensitivity is 151 mV_pp/MPa/V. The equivalent noise pressure is 3.3 Pa/√Hz. A photoacoustic method with a 6-μm carbon fiber was employed to characterize the impulse and frequency responses of our CMOS sensor. To evaluate the 3-D imaging capability and the achievable photoacoustic imaging resolution from our CMOS sensor, carbon fiber and hair phantoms were imaged. A 2-D large-aperture array was emulated by 2-D mechanically scanning of a single sensing element with a scanning step of 120 μm. A 2-D synthetic aperture focusing technique was used for photoacoustic image reconstruction. The -6dB spatial resolution provided by our sensor and scanning and reconstruction scheme was 181 μm in axial and 448 μm in lateral, respectively at the depth of ~2.4 cm and with a synthesized 120-mm by 4.8-mm 2-D array. Future work will focus on fabrication of a true 2-D array and improving the bandwidth and the integrated electronics to offer higher SNR and facilitate real-time imaging.