Thesis Title:Multiphysics and Cavity Mechanics Study in Micro/Nano Resonating Devices
Name of the Student:Mr. Vishnu Kumar
Degree Registered: Ph.D. Engineering
Advisor:Prof. Saurabh Chandorkar , CeNSE
Date: 9th May 2024 (Thursday), Time: 3:30 pm
Abstract
Micro/Nano resonating devices offer enormous potential and have distinctive features for building sensors and actuators. One of the key parameters that control the response of these devices is Quality Factor (Q). Q is defined as the ratio of maximum energy stored in a resonator and energy dissipated per cycle. The primary motivation of this thesis is to explore the energy loss mechanisms in the micro/nano-electromechanical systems through the prism of various fundamental physical phenomena.
First, we explore one-dimensional electrostatically actuated phononic crystals (PnCs), employing fixed-fixed beams interconnected via coupling beams to induce mechanical vibrations. We developed an analytical model as a simulation-based model to design a phononic crystal with requisite band-gap at the frequency of our choice. We fabricated this device and, using a custom-designed trans-impedance amplifier, measured the transmission response of the PnC. The response showed the formation of band-gap regions resulting from the interactions within the phononic crystal structures.
Subsequently, we placed a resonator in a PnC cavity such that the resonant frequency of the device lay within the bandgap of the PnCs to reduce energy loss via anchor loss, particularly anchor loss. Employing a double-ended tuning fork (DETF) resonator as the cavity resonator, we observed distinct behaviors of its degenerate modes (in-phase and out-phase) compared to a DETF resonator fixed with an anchor. Notably, the in-plane mode, situated within the bandgap, exhibited an enhancement in quality factor by approximately 200% compared to the anchored resonator at a temperature where the thermal expansion coefficient approaches zero (i.e., 110K) which zeroes out the thermoelastic dissipation. On the other hand, the out-phase mode displayed minimal improvement as its frequency resided outside the band gap region.
While anchor loss is one of the major loss mechanisms, there are several other significant energy loss mechanisms in MEMS/NEMS devices with piezoelectric films. We elucidate the impact of these mechanisms on the system, particularly focusing on the structures where piezoelectric material is deposited on a silicon-on-insulator substrate, termed thin-film-piezoelectric-on-substrate (TPoS). Traditionally, estimating energy loss parameters in bulk piezoelectric devices involves either direct measurement by studying hysteretic behavior between applied fields and resultant responses or by using a resonant structure purely composed of pure piezoelectric material. TPoS devices are composite devices consisting of silicon and piezoelectric material, and estimating energy loss from different components requires complex fabrication. We came up with a measurement technique for simultaneously measuring the optical and electrical responses of a TPoS-based cantilever device using a Laser Doppler Vibrometer and Lock-in-Amplifier. Leveraging a physics-based model, we demonstrate the extraction of piezoelectric parameters and associated energy losses by conducting measurements at only two frequency points: one at sufficiently low frequency compared to resonance frequency and the other at resonance frequency. The validation of our model is further bolstered by independent measurements.
Next, we explore parametric excitation as another way to control the effective Quality Factor of a device. We designed and fabricated a unique resonator device that enables selective excitation of the coupling spring between two resonators and has the property that the nonlinear dynamics of the system are also governed by the coupling spring. We demonstrate the consequences of these properties through experiments and explore subharmonic parametric excitation. We also show the oscillatory nature of apparent Quality Factor enhancement when harmonic and subharmonic excitation are carried out together with varying phases between the two.
Finally, we show by employing superharmonic parametric excitation in micro/nano resonating devices with piezoelectric means of actuation, we can carry out a direct measurement of spontaneous polarization of piezoelectric materials. We demonstrate this technique for PZT film based TPoS devices and validate it by comparing results from PUND measurement of the same film. Such an approach proves particularly advantageous for piezoelectric materials like AlN, GaN, and HfO2, characterized by non-switching domains during polarity transitions thus making it impossible to directly measure their spontaneous polarization.
Thus, through our investigations, we have explored the engineering of quality factors through the study of different types of energy loss mechanisms and their magnitude reduction by various means, as well as parametric excitation that adds energy to the resonator at its resonance. Our work on nonlinear parametric excitation has led to the uncovering of an exciting new measurement technique for the measurement of spontaneous polarization in arbitrary piezoelectric materials.