Fabrication of Surface Acoustic Wave Devices on Lithium Niobate

Summary

Two fabrication techniques, lift-off and wet etching, are described in producing interdigital electrode transducers upon a piezoelectric substrate, lithium niobate, widely used to generate surface acoustic waves now finding broad utility in micro to nanoscale fluidics. The as-produced electrodes are shown to efficiently induce megahertz order Rayleigh surface acoustic waves. 

Abstract

Manipulation of fluids and particles by acoustic actuation at small scale is aiding the rapid growth of lab-on-a-chip applications. Megahertz-order surface acoustic wave (SAW) devices generate enormous accelerations on their surface, up to 10⁸ m/s², in turn responsible for many of the observed effects that have come to define acoustofluidics: acoustic streaming and acoustic radiation forces. These effects have been used for particle, cell, and fluid handling at the microscale—and even at the nanoscale. In this paper we explicitly demonstrate two major fabrication methods of SAW devices on lithium niobate: the details of lift-off and wet etching techniques are described step-by-step. Representative results for the electrode pattern deposited on the substrate as well as the performance of SAW generated on the surface are displayed in detail. Fabrication tricks and troubleshooting are covered as well. This procedure offers a practical protocol for high frequency SAW device fabrication and integration for future microfluidics applications.

Introduction

Relying on the well-known inverse piezoelectric effect, where the atomic dipoles create strain corresponding to the application of an electric field, piezoelectric crystals such as lithium niobate LiNbO₃ (LN), lithium tantalite LiTaO₃ (LT), can be used as electromechanical transducers to generate SAW for microscale applications^1,2,3,4,5,6. By enabling the generation of displacements up to 1 nm at 10-1000 MHz, SAW-driven vibration overcomes the typical....

Protocol

1. SAW device fabrication via the lift-off method

1. Perform wafer solvent cleaning in a Class 100 clean room facility by immersing the 4” (101.6 mm) LN wafer into acetone, followed by isopropyl alcohol (IPA), then deionized water (DI water), each in a sonication bath for 5 min. Pick up the wafer and blow the surface dry with nitrogen (N₂) gas flow to remove the remaining DI water from the wafer. 
   CAUTION: Perform the acetone and IPA immersions in a fume hood. Avoid inhalation and skin contact with IPA. Avoid skin and eye contact with acetone. Do not swallow.
   NOTE: Do not allow any fluid to evaporate upon the wafer; if any ....

Results

The IDT to be measured is designed to have a resonant frequency at 100 MHz, as the the finger width and the spacing between them are 10 μm, producing a wavelength of 40 μm. Figure 1 shows the SAW device and IDT fabricated using this method.

Using an oscillating electrical signal matched to the resonant frequency of the IDT, SAW can be generated across the surface of the piezoelectric material. The LDV measures the vibration via the Doppler effect on the surface, and through si.......

Discussion

SAW devices fabricated from either method are capable of generating useful traveling waves on the surface, and these methods underpin more complex processes to produce other designs. The resonant frequency is usually a little lower than the designed value, due to the mass loading effect of the metal deposited on top. However, there still some points worth discussing to avoid problems. 

Lift-off method
The choice of photoresist is important. It is possible to use a p.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Absorber	Dragon Skin, Smooth-On, Inc., Macungie, PA, USA	Dragon Skin 10 MEDIUM
Amplifier	Mini-Circuits, Brooklyn, NY, USA	ZHL–1–2W–S+
Camera	Nikon, Minato, Tokyo, Japan	D5300
Chromium etchant	Transene Company, INC, Danvers, MA, USA	1020
Developer	Futurrex, NJ, USA	RD6
Developer	EMD Performance Materials Corp., Philidaphia, PA, USA	AZ300MIF
Dicing saw	Disco, Tokyo, Japan	Disco Automatic Dicing Saw 3220
Gold etchant	Transene Company, INC, Danvers, MA, USA	Type TFA
Hole driller	Dremel, Mount Prospect, Illinois	Model #4000	4000 High Performance Variable Speed Rotary
Inverted microscope	Amscope, Irvine, CA, USA	IN480TC-FL-MF603
Laser Doppler vibrometer (LDV)	Polytec, Waldbronn, Germany	UHF-120	4” double-side polished 0.5 mm thick 128°Y-rotated cut lithium niobate
Lithium niobate substrate	PMOptics, Burlington, MA, USA	PWLN-431232
Mask aligner	Heidelberg Instruments, Heidelberg, Germany	MLA150	Fabrication process is performed in it.
Nano3 cleanroom facility	UCSD, La Jolla, CA, USA
Negative photoresist	Futurrex, NJ, USA	NR9-1500PY
Oscilloscope	Keysight Technologies, Santa Rosa, CA, USA	InfiniiVision 2000 X-Series
Positive photoresist		AZ1512	Denton Discovery 18 Sputter System
Signal generator	NF Corporation, Yokohama, Japan	WF1967 multifunction generator	Wafer Dipper 4"
Sputter deposition	Denton Vacuum, NJ, USA	Denton 18
Teflon wafer dipper	ShapeMaster, Ogden, IL, USA	SM4WD1