Abstract
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-m 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. © 2011 IEEE.