MEMS-based Low-Frequency Piezoelectric Micromachined Ultrasonic Transducers

A PMUT (Piezoelectric Micromachined Ultrasonic Transducer) is a type of ultrasonic transducer that utilizes piezoelectric materials to generate and receive ultrasonic waves. These devices are miniaturized and are commonly fabricated using MEMS (Micro-Electro-Mechanical Systems) technology.

How PMUT Works:

1. Piezoelectric Effect: PMUTs rely on the piezoelectric effect, where certain materials generate an electric charge in response to mechanical stress or deform when an electric field is applied.
2. Vibration Mode: In PMUTs, a piezoelectric thin film is deposited on a flexible membrane. When an alternating voltage is applied, the membrane vibrates at ultrasonic frequencies, generating sound waves in the surrounding medium.
3. Reception: Similarly, incoming ultrasonic waves can cause the membrane to vibrate, and the piezoelectric material converts this vibration into an electrical signal.

Fabrication Process

PMUT are fabricated in the National Nanofabrication Center (ISO Class 5 standard), Indian Institute of Science. Lead zirconate titanate (PZT) is deposited on a platinized SOI. PZT is spin-coated repeatedly using the sol-gel technique to achieve the desired thickness (0.5–1 μm) and then annealed at 650◦C, leading to the formation of a uniform thin film. Next, the top electrode is patterned with lift-off. The backside SiO2 is etched with RIE. Handle Layer bulk silicone is etched with deep reactive-ion etching (DRIE) process, which is highly anisotropic process. Burried oxide acts as an etch stop for DRIE.

Diced PMUT Dies

Fully Packaged PMUTs ~25 KHz