Piezoelectric Micromachined Ultrasound Transducers (PMUTs)

Piezoelectricity is a transduction phenomena, wherein any mechanical strain gets converted to electricity and the other way round. Materials that exhibit this phenomena are called piezoelectric materials. Examples of such materials which are available in nature include - quartz, sugar, DNA, bone, silk, tendon etc. These materials can also be synthesized artificially and are available both as thick film (thickness > 10 µm) or thin film (thickness < 1µm). Examples of such materials are - lead zirconate titanate (PZT), Aluminum Nitride (AlN), Zinc Oxide (ZnO) etc. Piezoelectric materials find a plethora of applications in which one of the important applications is in the making of transducers for medical ultrasonography.

We use thin film piezoelectricity to fabricate both thick and thin film ultrasound transducers. The thin film ones which depend on the d31 mode of operation and produce longitudinal waves into the medium of interest can be called piezoelectric micromachined ultrasound transducers. It is like a microscale Indian classical percussion instrument - the tabla, only played at the ultrasound frequencies.

Principle of operation of a PMUT - A PMUT operates on the d31 mode of transduction. When a D.C. field is applied across the piezoelectric material, due to the d31 piezoelectric coupling, there is a tendency of axial strain getting generated in the material. But since it is a layered composite, and the material is fixed to the silicon substrate below, development of strain won't be permitted. This results in the formation of reaction forces (R) emanating from the

C.G. of the piezoelectric material. Now since there is a finite separation between the C.G. of the piezo and the stack's neutral axis/ C.G., it will result in the formation of a couple (M) which will bend the structure. Now if a D.C. field is replaced by an A.C., it results in vibration of the microplate, which in turn generates acoustics.