Development of mechanically-robust piezoelectric micromachined ultrasonic transducer based on island-shaped monocrystalline PZT thin film partially covered with polyimide

Abstract

This paper describes a new design of piezoelectric micromachined ultrasonic transducer (pMUT) with a monocrystalline Pb(Zr,Ti)O3-based thin film (Mono-PZT) for enhancing the mechanical robustness. In this study, we investigated two design concepts to suppress crack generation in Mono-PZT. First, the area of Mono-PZT is limited by leaving an island Mono-PZT pattern only on the membrane of the pMUT. Second, the edge of Mono-PZT is covered with a polyimide (PI) thin film as a protection layer to prevent the peeling. We prepared three different designs, a new design with both concepts, another design with the first concept, and the conventional one with neither of them for comparison. The robust mechanical analyses of these devices were performed by driving resonantly and increasing the displacement of the membranes. As these results, the proposed structure with both concepts still stands well at the highest displacement, 1600 nm, while the undesired cracks occur on other structures with approximately 800 nm of displacement. The robustness could improve 50% in comparison to the other designs thanks to this unique design. Finite element method simulation results showed that this PI layer contributed to decrease the stress concentrated at the edge both in the static condition and in the dyanamic vibarion of the membrane. This layer also probably played a role in prevention of the peeling of the edge of the island-shaped Mono-PZT. In conclusion, the new design is useful for the Mono-PZT pMUT in terms of excellent mechanical robustness.

Piezoelectric Micromachined Ultrasound Transducer Technology: Recent Advances and Applications

“…The resonant frequency of the ringshaped PMUT was found to be insensitive to the mean radius. Thao et al [35] designed an island-shaped PMUT with a monocrystalline PZT-based thin film for improving the mechanical robustness. The robust mechanical analysis of the PMUT was carried out by driving resonantly and increasing the displacement of the membranes.…”

Fabrication and characterization of annular-shaped piezoelectric micromachined ultrasonic transducer mounted with Pb(Zr,Ti)O3-based monocrystalline thin film

“…The bottom electrode fully covered the Si layer, while the optimal diameter of the top electrode was investigated through simulation to maximize its actuation ability. A polymer insulation layer is patterned to cover the edge of the PZT to prevent PZT from peeling from the edge [13].…”

“…Thus far, most of the previously reported pMUTs have a design in which a transducer material and a driving electrode are formed at the center of a vibrating diaphragm [13][14][15]. However, it is questionable whether this design is optimal for improving both the resonance frequency and displacement sensitivity.…”

“…In this study, first, we designed the 'annular-shaped' pMUT using the finite element method (FEM). An 'island-shaped' pMUT [13] with the same diameter of the diaphragm was also designed as a conventional design for comparative evaluation. Second, these designed devices were fabricated together.…”

Sputter deposition and characterization of “epi-poly” Pb(Zr, Ti)O3 thin film on (100) Si substrate for MEMS applications

“…We prototyped a mono-PZT-based pMUT, and successfully demonstrated that it could provide high performance with a high signal-to-noise ratio. [30][31][32] However, such a monocrystalline thin film exhibits weak mechanical robustness. Theoretically, monocrystalline materials without defects are intrinsically robust.…”