Fabrication and Characterization of Thickness Mode Piezoelectric Devices for Atomization and Acoustofluidics

Summary

Fabrication of piezoelectric thickness mode transducers via direct current sputtering of plate electrodes on lithium niobate is described. Additionally, reliable operation is achieved with a transducer holder and fluid supply system and characterization is demonstrated via impedance analysis, laser doppler vibrometry, high-speed imaging, and droplet size distribution using laser scattering.

Abstract

We present a technique to fabricate simple thickness mode piezoelectric devices using lithium niobate (LN). Such devices have been shown to atomize liquid more efficiently, in terms of flow rate per power input, than those that rely on Rayleigh waves and other modes of vibration in LN or lead zirconate titanate (PZT). The complete device is composed of a transducer, a transducer holder, and a fluid supply system. The fundamentals of acoustic liquid atomization are not well known, so techniques to characterize the devices and to study the phenomena are also described. Laser Doppler vibrometry (LDV) provides vibration information essential in comparing acoustic transducers and, in this case, indicates whether a device will perform well in thickness vibration. It can also be used to find the resonance frequency of the device, though this information is obtained more quickly via impedance analysis. Continuous fluid atomization, as an example application, requires careful fluid flow control, and we present such a method with high-speed imaging and droplet size distribution measurements via laser scattering.

Introduction

Ultrasound atomization has been studied for almost a century and although there are many applications, there are limitations in understanding the underlying physics. The first description of the phenomenon was made by Wood and Loomis in 1927¹, and since then there have been developments in the field for applications ranging from delivering aerosolized pharmaceutical fluids² to fuel injection³. Although the phenomenon works well in these applications, the underlying physics is not well understood^4,5,

Protocol

1. Thickness mode transducer fabrication via DC sputtering

1. Wafer preparation
   1. Place a 100 mm 128YX LN wafer in a clean glass dish of at least 125 mm diameter. Sonicate the wafer in at least 200 mL of acetone for 5 min.
   2. Repeat sonication with isopropyl alcohol and again with deionized water for 5 min each.
   3. Remove visible water from the surface using dry nitrogen.
   4. Completely remove water from the surface by placing the wafer on a hotplate at 100 °C for 5 min. Ensure that there is a sheet of aluminum foil on the hotplate as this helps in dissipation of charge buildup on the wafer.
2. Electr....

Results

Thickness mode piezoelectric devices were fabricated from 128YX lithium niobate. Figure 1 shows a complete assembly to hold the transducer in place with a custom transducer holder used with the passive fluid delivery system developed for continuous atomization. The characterization steps for these devices include determination of the resonant frequency and harmonics using an impedance analyzer (Figure 2). The fundamental frequency of the devices was found to be c.......

Discussion

The dimensions and aspect ratio of a transducer affects the vibration modes it produces. Because the lateral dimensions are finite, there are always lateral modes in addition to the desired thickness modes. The above LDV methods can be used to determine dominant modes in the desired frequency range for a given transducer. A square with dimensions below 10 mm typically gives a close approximation to a thickness mode. Three by ten millimeter rectangles also work well. Movie 1 and Movie 2

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Amplifier	Amplifier Research, Souderton, PA, USA	5U1000
Articulating arm	Fisso, Zurich, Switzerland
CF4 Objective	Edmund Optics, Barrington, NJ, USA		Objective used for high speed imaging
Dicing saw	Disco, Tokyo, Japan	Disco Automatic Dicing Saw 3220
Fiber Fragrance Diffuser Wick	Weihai Industry Co., Ltd., Weihai, Shandong, China	https://www.weihaisz.com/Fiber-Fragrance-Diffuser-Wick_p216.html
High Speed Camera	Photron, San Diego, USA	Fastcam Mini
Laser Doppler Vibrometer	Polytec, Waldbronn, Germany	UHF120	Non-contact laser doppler vibrometer
Laser Scattering Droplet size measurement system	Malvern Panalytical, Malvern, UK	STP5315
Lithium niobate substrate	PMOptics,Burlington, MA, USA	PWLN-431232	4” double-side polished 0.5 mm thick 128°Y-rotated cut lithium niobate
Luer-lock syringes	Becton Dickingson, New Jersey, USA
Nano3 cleanroom facility	UCSD, La Jolla, CA, USA		Fabrication process is performed in it.
Network Analyzer	Keysight Technologies, Santa Rosa, CA, USA	5061B
Oscilloscope	Keysight Technologies, Santa Rosa, CA, USA	InfiniiVision 2000 X-Series
PSV Acquistion Software	Polytec, Waldbronn, Germany	Version 9.4	LDV Software
PSV Presentation Software	Polytec, Waldbronn, Germany	Version 9.4	LDV Software
Signal generator	NF Corporation, Yokohama, Japan	WF1967 multifunction generator
Single Post Connector	DigiKey, Thief River Falls, MN	ED1179-ND
Sputter deposition	Denton Vacuum, NJ, USA	Denton 18	Denton Discovery 18 Sputter System
Surface Mount Spring Contacts	DigiKey, Thief River Falls, MN	70AAJ-2-M0GCT-ND
Teflon wafer dipper	ShapeMaster, Ogden, IL, USA	SM4WD1	Wafer Dipper 4"
XYZ Stage	Thor Labs, Newton, New Jersey, USA	MT3	Optical table stages