Name of the Student: Mr. Randhir Kumar
Advisor: Prof. Rudra Pratap, CeNSE
Date & Time: 23 October 2020 (Friday), 11:00 AM
Venue: Online on Microsoft Teams: https://tinyurl.com/y262pjwz
Abstract
Multifunctional zinc oxide (ZnO) is a versatile semiconducting material with desirable optical, piezoelectric, pyro-electric, and electrical properties for sensing and actuation. The nanoscale structural diversity of ZnO in the form of nanorods, nanotubes, nanoribbons, nanoflowers, etc., provides a morphological variety that few materials can match. These various forms impart unique capabilities to ZnO for sensing applications. Although there have been numerous reports on the growth and characterization of various kinds of ZnO nanostructures, there is relatively very little work on their successful integration in useful devices in MEMS. This is not unusual as device architectures generally require integration of nanostructures in a material stack that, in turn, puts far more constraints on the desired properties of the nanostructures. Here, we report the growth of a nanostructured thin film of ZnO on a suitable silicon substrate and fabricate a MEMS ultrasonic resonator to demonstrate the suitability of this film for piezo-MEMS applications. We grow ZnO nanorods using a hydrothermal process in a domestic microwave that ensures ultrafast growth while preserving the high crystal quality of the nanorods. First, we identify the optimal substrate for ZnO growth and then optimize the seed layer for the best crystal orientation, as ascertained by HRXRD ω-scan. The realization of a free diaphragm as a MEMS structure incorporating ZnO as the active piezo layer involves another process innovation where we do not pattern the top electrode, as is typical in such piezo MEMS devices. We measure the central deflection of the diaphragm by LDV and use this measurement to calculate the transverse piezoelectric coefficient (d31) of the ZnO film. Subsequently, we extend the entire growth process to a full 3² wafer and optimize the recipe for large area growth. In this process, we encounter a regime in the parameter space where ZnO etching starts to take place during the film growth due to the simultaneous nucleation and growth of ZnO nanoparticles in the solution. We study the growth parameter space and identify the subspace that leads to the successful growth of ZnO thin films formed by the lateral boundary fusion of highly oriented, crystalline ZnO nanorods over the entire surface of the platinized silicon wafer. We discuss the complete device fabrication and characterization steps of ZnO piezoelectric micromachined ultrasonic transducers (PMUTs). The sound pressure level (SPL) output from these PMUTs are also measured. In addition, we experimentally demonstrate a rare nonlinear response of the ZnO diaphragm, where both hardening and softening nonlinearities co-exist. This nonlinear response at fairly low voltage actuation (2-7 V) results in a wide bandwidth for these ultrasonic transducers. Such a response can also be used in MEMS energy harvesting applications.