Fig. 4: PMUT fabrication is divided into two parts: making the piezoelectric thin-film material and fabricating the PMUT structure with an internal piezoelectric thin-film by nanofabrication tools.
From: Thin-film PMUTs: a review of over 40 years of research
a Various thin film piezoelectric material used to date: (i) zinc oxide (ZnO)35; (ii) lead zirconate titanate (PZT)4; (iii) polyvinylidene fluoride-trifluoro ethylene P(VDF)-TrFE37; (iv) aluminum nitride (AlN)38; (v) 15% scandium-doped AlN (Sc0.15AlN)26; (vi) PZT fibered epitaxial film27; and (vii) sodium potassium niobate (KNN)42. Fabrication methods divided into b PMUTs fabricated by bulk micromachining techniques: (i) wet etching of bulk silicon43; (ii) dry etching of bulk silicon44; and (iii) dry etching of bulk silicon with a thinned-down bulk ceramic31,45. c PMUTs fabricated by surface micromachining techniques: (i) prefabricated wet-etched trenches followed by wafer bonding46; (ii) surface release by wet etching47; (iii) cavity SOI and surface machining48; (iv) surface release by polysilicon wet etching49; (v) surface micromachining by silicon migration50; and (vi) surface release by wet etching51. d Flexible PMUT array with rigid elements: (i) fabrication of PMUT islands with connected springs52. e All flexible PMUTs: (i) flexible PMUTs made by PDMS stamp53 and (ii) flexible PMUT with flexible piezoelectric material on flexible substrate29. All pictures have been adapted with permission
