薄膜 PMUT：40 年研究回顾

A concise review on the thin-film PMUTs, which has been one of the rather important research topics amongst microultrasound experts is being reported. It has been rigorously surveyed, scrutinized, and perceived that the work in thisdirection began nearly 44 years ago, with the primitive development of functional piezoelectric thin-film material, and nowthere are already three major companies commercializing them on a bulk scale. This fascinating fact illustratesthe enormouscontribution made by more than 70 different centers, research institutes, and agencies spread across 4 different continentsto develop the vast know-how of these devices’ design, make, and function. This review covers such important contributionsin a short but comprehensive fashion and in particular, intends to educate the readers about – the global scenario of PMUTs,the principles governing their design, the ways following their make, all non-conventional useful PMUT designs, and lastly,category wise applications. Crucial comparison charts in terms of thin-film piezoelectric material used in PMUTs, and in termsof targeted applications are also depicted and discussed, which is believed to enlighten any MEMS designer planning to beginworking with PMUTs. Moreover, each relevant section is provided with a crisp future forecast, from the author’s pastknowledge and expertise in this very field of research along with the elements of a careful literature survey. In short, thisreview can be considered a one-stop time-efficient guide for whoever is interested in knowing about these small devices.

The field of transducers has undergone several revolutions.One of those reforms was the creation of nanotechnology,which enabled the making of very small transducers, in therange of sub-micron dimensions. This along with thedevelopment of material research gave birth tomicroelectromechanical systems (MEMS). Soon the macroworld of sensors shifted to micro leading to an everincreasing demand in the field. Today, most smartphoneshave MEMS inertial systems inside. MEMS has alsorevolutionized the field of ultrasound, leading to thecreation of tiny ultrasound transducers. These are popularlyknown as Micromachined Ultrasound Transducers (MUTs)and can be broadly classified into – Capacitive MUTs [1](CMUTs) and Piezoelectric MUTs (PMUTs).

The key component of a MUT is a suspended microdiaphragm. A CMUT’s diaphragm consists of either a singlelayer of a conducting material or a nonconducting materialwith an electrode layer on it, suspended generally with a 0.5to 2 µm gap from the grounded substrate which iselectrostatically actuated with an alternating electric field(AC). Alternatively, a PMUT’s diaphragm consists of at leastfour layers — a passive layer, and a piezoelectric layersandwiched between metal electrodes, which can bedynamically actuated with AC.

Fig1

PMUTs function on the lateral dynamic strain developed inthe piezoelectric thin-film and perform best if the thicknessof the film is below a critical limit, above which thetransverse strain cross-interference becomes noticeable.There are works that define this limit where the thicknesshas been experimentally found to be 3 µm in a PZT {100}-textured thin-film [4], above which there has been asignificant decay in the lateral thin-film based (31,f)piezoelectric coefficient. Thus, in this review, only relevantarticles that satisfy this criterion have been included.