  This paper reports an aluminum nitride (AlN) multi-frequency piezoelectric
micromachined ultrasound transducers (MF-PMUT) array for photoacoustic (PA)
imaging, where the broadened bandwidth is beneficial to improve imaging
resolution. Specifically, PMUT based on micro-electromechanical systems (MEMS)
technology is suitable for PA endoscopic imaging of blood vessels and bronchi
due to its miniature size. More importantly, AlN is a non-toxic material, which
makes it harmless for biomedical applications. In this work, a MF-PMUT array
are designed and fabricated for PAI. The device's vibration mode impedance and
bandwidth are analyzed. The MF-PMUT sensor provides a wider bandwidth (65%)
signal detection, which increases the resolution of PAI compared with
traditional PMUT. We conduct an experiment on agar sample to present sensor's
performance in images' axial resolution.
