“Lollipop-Shaped” High-Sensitivity MEMS Vector Hydrophone Based On Parylene Encapsulation

The MEMS vector hydrophone, proposed by North University of China, combines with the MEMS technology and bionics principle and it has good performances in underwater acoustic detection, such as vector detection, small scale and low price. Based on the previous research work, the encapsulation and receiving sensitivity are crucial problems: reduce the influence of encapsulation on the frequency response and significantly increase the receiving sensitivity.In order to eliminate polyurethane hat resonance frequency intervention and reduce fluid influence, Whisker-inspired MEMS vector hydrophone(WIVH) encapsulated with Parylene is proposed to broaden frequency bandwidth and improve frequency response performance. Parylene that is conformally deposited on the device surface replaces polyurethane encapsulating hat and silicon oil existing in current encapsulation technology. The main advantage of WIVH as demonstrated by characterization and modeling is the enhanced bandwidth response, which is the critical factor in hydrophone design. The influence of polyurethane hat on frequency response is analyzed according to acoustic pressure gradient principle, where WIVH is proved to be a standard pressure gradient hydrophone in accord with an increment of 6 dB per octave in linear region. Moreover, the interactions of Parylene membrane with fluid and the influences on vibrating performance are investigated. Resonance measurement and frequency response analysis demonstrate the frequency bandwidth of WIVH could be promoted twice in contrast to that of lateral line-inspired MEMS vector hydrophone(LLIVH).This paper presents methods of promoting the sensitivity of MEMS vector hydrophone by increasing the sensing area of cilium and perfect insulative Parylene membrane. Firstly, a low-density sphere is integrated with the cilium to compose a “lollipop shape”, which can considerably increase the sensing area. A mathematic model on the sensitivity of the “lollipop-shaped” MEMS vector hydrophone(LVH) is presented, and the influences of different structural parameters on the sensitivity are analyzed via simulation. Secondly, the MEMS vector hydrophone is encapsulated through the conformal deposition of insulative Parylene membrane, which enables underwater acoustic monitoring without any typed sound-transparent encapsulation. Finally, the characterization results demonstrate that the sensitivity reaches up to-174dB(500Hz 0dB @1 V/μPa), which is increased by more than 10 dB, comparing with the previous cilium-shaped MEMS vector hydrophone(CVH). Besides, the frequency response takes on a sensitivity increment of 6dB per octave. The working frequency band is 20~500Hz and the concave point depth of 8-shaped directivity is beyond 30 dB, indicating that the hydrophone is promising in underwater acoustic application.