Fabrication technology of micro-machined ultrasonic wideband piezoelectric transducer using multi-resonance

Abstract

Wideband transducers have advantages in various applications such as underwater communication, medical probes, and general loudspeakers. In the underwater communication field, wideband transducers can simultaneously transfer more information owing to the wide bandwidth. In medical probes, higher frequency ultrasonic transducers can acquire higher resolution images, whereas lower frequency ultrasonic transducers can acquire deeper images. In general loudspeakers, it requires a bandwidth of at least 10 kHz, because the audible frequency band is 20 Hz–20 kHz. One of the methods to achieve a wide frequency bandwidth in a single device is by modeling the transducer to a multi-resonance system. However, in one structure or one transducer, it is very difficult to implement the multi-resonances of the desired mode at the required frequency bandwidth. It is because the resonance of the primary mode, which is mainly used in acoustic transducers, is determined by the thickness or diameter of the transducers for simple structure using the thickness or thin film mode. Therefore, it is difficult to implement the additional structures that have another resonance on one transducer. Especially for MEMS-based transducers manufactured by micromachining processes, the structural limitations make them even more difficult. Due to this limitation, the previous pMUT arrays achieved wideband by arranging two pMUTs with different resonances and driving them with out-of-phase, not by multi-resonances. Previous pMUT arrays could reportedly generate wideband ultrasounds of approximately 17 kHz with an electro-acoustic efficiency of 70% direct sounds. Although it achieved a high efficiency, it had a dividing the sound output power at a given resonance, as two pMUTs with different resonance were used. This required additional circuits and complex electrode implementation for different driving of two pMUTs. Furthermore, previous pMUTs had various limitations, such as uniformity issues and mechanically vulnerable thin membrane is exposed to air directly, and has the disadvantage of using a structurally and electrically vulnerable air-bridge structure. In this study, a new pMUT array is designed and fabricated by multi-resonance system. The new pMUT is based on silicon dioxide (SiO2) membrane. It designed using one membrane part and additional two acoustic structures (waveguide part, back chamber part) to achieve wideband. All acoustical structures were designed considering the micro-machining process, and new fabrication concepts were introduced to implement the precise multi-resonance systems, which included a SiO2 footing barrier, PZT sputtering method, cap wafer, TSV, etc. The SiO2 footing barrier essentially prevented the footing effect, thus improving the performance of the array transducer. By using the PZT sputtering method, structurally and electrically vulnerable air-bridge structure could be avoided. Using cap wafer and TSV process, it was able to protect structurally vulnerable thin membrane and improve the ASIC compatibility. The fabricated single pMUT unit demonstrated electro-mechanical efficiency, mechano-acoustic efficiency and electro-acoustic efficiency of 95.6%, 93.3% and 89.1 %, which has developed 5.8%, 18.5% and 21.9% compared to previous pMUTs. And it demonstrated a maximum sound pressure level of 64.6dB at 1st peak of 105 kHz and 62.4 dB at 2nd peak of 126 kHz. And a wideband of 101 – 130 kHz (-7.7 dB). However, -3 dB bandwidth of the wideband was 102.5 – 109 kHz and 124 – 126.3 kHz, and there is deep null between two peaks, the reason of which was considered by the fabrication process issues. And, the pMUT array were not implemented by fabrication issues. In this study, these fabrication process issues was analyzed and a method was proposed to solve these process issues. The potential of performance improvement of pMUT was also discussed.