Figure 3.

Piezoelectric micromachined ultrasound transducers (PMUT) technology. (a) Schematic of a cross-section of a PMUT and its working principle, (b) fabrication process flow of PMUTs with diaphragm defined by front-side etching method: (i) deposition of piezoelectric and electrode layer on top of oxidized silicon wafer, (i) SiO₂, Ti, Pt, and PZT layer grown on silicon wafer, (ii) pattern top electrode, (iii) insulation pad deposition, (iv) Ti/Pt deposition, (v) etch through layers to realize bias, and (vi) release the front side diaphragm, reprinted with permission from [130], and (c) steps of backside etching method: (i) silicon wafer, (ii) wet oxidation, (iii) oxide etching, (iv) boron diffusion, (v) low temperature oxide growth, (vi) oxide etch, (vii) Si etch, (viii) Ti-Pt deposition, (ix) PZT deposition, (x) TiW-Au deposition, (xi) top electrode etch, and (xii) PZT etch. Reprinted with permission from [132].