Fabrication of Nanoheight Channels Incorporating Surface Acoustic Wave Actuation via Lithium Niobate for Acoustic Nanofluidics

Summary

We demonstrate fabrication of nanoheight channels with the integration of surface acoustic wave actuation devices upon lithium niobate for acoustic nanofluidics via liftoff photolithography, nano-depth reactive ion etching, and room-temperature plasma surface-activated multilayer bonding of single-crystal lithium niobate, a process similarly useful for bonding lithium niobate to oxides.

Abstract

Controlled nanoscale manipulation of fluids is known to be exceptionally difficult due to the dominance of surface and viscous forces. Megahertz-order surface acoustic wave (SAW) devices generate tremendous acceleration 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 manipulation at the microscale, although more recently SAW has been used to produce similar phenomena at the nanoscale through an entirely different set of mechanisms. Controllable nanoscale fluid manipulation offers a broad range of opportunities in ultrafast fluid pumping and biomacromolecule dynamics useful for physical and biological applications. Here, we demonstrate nanoscale-height channel fabrication via room-temperature lithium niobate (LN) bonding integrated with a SAW device. We describe the entire experimental process including nano-height channel fabrication via dry etching, plasma-activated bonding on lithium niobate, the appropriate optical setup for subsequent imaging, and SAW actuation. We show representative results for fluid capillary filling and fluid draining in a nanoscale channel induced by SAW. This procedure offers a practical protocol for nanoscale channel fabrication and integration with SAW devices useful to build upon for future nanofluidics applications.

Introduction

Controllable nanoscale fluid transport in nanochannels—nanofluidics¹—occurs on the same length scales as most biological macromolecules, and is promising for biological analysis and sensing, medical diagnosis, and material processing. Various designs and simulations have been developed in nanofluidics to manipulate fluids and particle suspensions based on temperature gradients², Coulomb dragging³, surface waves⁴, static electric fields^5,6,7, and thermophoresis<....

Protocol

1. Nano-height channel mask preparation

1. Photolithography: With a pattern describing the desired shape of the nanoheight channels (Figure 1B), use normal photolithography and lift-off procedures to produce nanoheight depressions in an LN wafer. These depressions will become nanoheight channels upon wafer bonding in a later step.
   NOTE: The lateral dimensions of the nanoscale depressions are microscale in this protocol. Electron beam or He/Ne ion beam lithogr.......

Discussion

Room-temperature bonding is key to fabricating SAW-integrated nanoslit devices. Five aspects need to be considered to ensure successful bonding and sufficient bonding strength.

Time and power for plasma surface activation
Increasing the plasma power will help increase the surface energy and accordingly increase the bonding strength. But the downside of increasing the power during plasma surface activation is the increase in surface roughness, which may adversely affect t.......

Materials

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
Developer	Futurrex, NJ, USA	RD6
Diamond tip engraving pen	Malco, Memphis, TN, USA	Malco A50 USA Made Carbide Tipped Scribe
Dicing saw	Disco, Tokyo, Japan	Disco Automatic Dicing Saw 3220
Heating oven	Carbolite, Hope Valley, UK	HTCR 6/28	High Temperature Clean Room Oven - HTCR
Hole driller	Dremel, Mount Prospect, Illinois	Model #4000	4000 High Performance Variable Speed Rotary
Inverted microscope	Amscope, Irvine, CA, USA	IN480TC-FL-MF603
Lithium niobate substrate	PMOptics, Burlington, MA, USA	PWLN-431232	4" double-side polished 0.5 mm thick 128° Y-rotated cut lithium niobate
Mask aligner	Heidelberg Instruments, Heidelberg, Germany	MLA150
Nano3 cleanroom facility	UCSD, La Jolla, CA, USA		Fabrication process is performed in it.
Negative photoresist	Futurrex, NJ, USA	NR9-1500PY
Oscilloscope	Keysight Technologies, Santa Rosa, CA, USA	InfiniiVision 2000 X-Series
Plasma surface activation	PVA TePla, Corona, CA, USA	PS100	Tepla Asher
Polarizer sheet	Edmund Optics, Barrington, NJ, USA	#86-182
RIE etcher	Oxford Instruments, Abingdon, UK	Plasmalab 100
Signal generator	NF Corporation, Yokohama, Japan	WF1967 multifunction generator
Sputter deposition	Denton Vacuum, NJ, USA	Denton 18	Denton Discovery 18 Sputter System
Teflon wafer dipper	ShapeMaster, Ogden, IL, USA	SM4WD1	Wafer Dipper 4"