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 thermophoresis⁸ over the last fifteen years. Recently, SAW has been shown⁹ to produce nanoscale fluid pumping and draining with sufficient acoustic pressure to overcome the dominance of surface and viscous forces that otherwise prevent effective fluid transport in nanochannels. The key benefit of acoustic streaming is its ability to drive useful flow in nanostructures without concern over the details of the chemistry of the fluid or particle suspension, making devices that utilize this technique immediately useful in biological analysis, sensing, and other physicochemical applications.

Fabrication of SAW-integrated nanofluidic devices requires fabrication of the electrodes—the interdigital transducer (IDT)—on a piezoelectric substrate, lithium niobate¹⁰, to facilitate generating the SAW. Reactive ion etching (RIE) is used to form a nanoscale depression in a separate LN piece, and LN-LN bonding of the two pieces produces a useful nanochannel. The fabrication process for SAW devices has been presented in many publications, whether using normal or lift-off ultraviolet photolithography alongside metal sputter or evaporation deposition¹¹. For the LN RIE process to etch a channel in a specific shape, the effects on the etch rate and the channel's final surface roughness from choosing different LN orientations, mask materials, gas flow, and plasma power have been investigated^{12,13,14,15,16}. Plasma surface activation has been used to significantly increase surface energy and hence improve the strength of bonding in oxides such as LN^17,18,19,20. It is likewise possible to heterogeneously bond LN with other oxides, such as SiO₂ (glass) via a two-step plasma activated bonding method²¹. Room-temperature LN-LN bonding, in particular, has been investigated using different cleaning and surface activation treatments²².

Here, we describe in detail the process to fabricate 40 MHz SAW-integrated 100-nm height nanochannels, often called nanoslit channels (Figure 1A). Effective fluid capillary filling and fluid draining by SAW actuation demonstrates the validity of both nanoslit fabrication and SAW performance in such a nanoscale channel. Our approach offers a nano-acoustofluidic system enabling investigation of a variety of physical problems and biological applications.

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 lithography can be used to fabricate channels with nanoscale lateral dimensions; Ga⁺-based ion beam lithography causes swelling and uneven substrate profiles²³. The orientation of the two LN wafers should match, otherwise, thermal stress may cause the wafers or the bond between them to fail.
2. Sputter deposition to protect regions from dry etching: Place the wafer into the sputter deposition system. Draw down the chamber vacuum to 5 x 10^-6 mTorr, allow Ar to flow at 2.5 mTorr, and sputter Cr at 200 W to produce a 400-nm thick sacrificial mask where reactive ion etching will be prevented when used in step 3 below.
3. Lift-off: Transfer the wafer into a beaker with sufficient acetone to completely immerse the wafer. Sonicate at medium intensity for 10 min. Rinse with DI water and dry the wafer with dry N₂ flow.
4. Dicing: Use a dicing saw to dice the entire wafer into individual chips with (typically) one nanoslit pattern per chip.
   NOTE: The protocol can be paused here.

2. Nano-height channel fabrication

1. Reactive ion etching (RIE): Use RIE to etch nanoscale depressions into the uncovered regions of the LN substrate. Regions left covered by sacrificial Cr will be protected from etching. Set the RIE power to 200W, heat the chamber to 50 °C, draw down the chamber vacuum to 20 mTorr, set the SF₆ flow rate to 10 sccm, and etch for 20 min to produce a 120-nm deep nanoslit in LN.
2. Hole drilling for channel inlets and outlets: With double-sided tape, attach an etched LN chip to a small steel plate and the plate to the bottom of a petri dish. The Petri dish should be large enough to permit complete immersion of the LN chip and steel plate. Fill the Petri dish with water to fully immerse the chip. Attach a 0.5-mm diameter diamond drill bit to a drill press, and drill at a high speed of at least 10,000 rpm to machine the desired inlets and outlets. Drilling through a 0.5 mm thick substrate should take about 10 to 15 s²⁴ (Figure 1B).
   NOTE: Immersion while drilling prevents excessive local heating and particulate jamming at the drill site. Other types of drill bits are unlikely to work, and hand drilling is not possible at any speed to our knowledge. Drill bit rotation speeds of 10,000 rpm or greater are recommended to avoid shattering the LN.
3. Cr wet etching: Use a diamond tip engraving pen to clearly mark the flat, unetched face of the drilled LN to keep track of which side the nanoheight channel is located in the remaining steps. Sonicate the chips in Cr etchant.
   NOTE: The protocol can be paused here. It is exceedingly difficult to determine which side of the LN chip has the etched nanoscale depression after the Cr is removed. The sonication time depends upon the etching rate and the Cr mask thickness.

3. Room-temperature Plasma Activated Bonding

1. Solvent cleaning LN chips: Collect chip pairs—one SAW device (fabricated by normal photolithography, sputter deposition, and lift-off procedures) and one etched nanoscale depression chip—together to prepare them for bonding. Immerse the chip pairs in a beaker of acetone placed in a sonication bath and sonicate for 2 min. Transfer the chips to methanol and sonicate for 1 min. Transfer the chips to DI water.
2. Piranha cleaning: Prepare piranha acid in a glass beaker in a well-ventilated hood, dedicated to the use of acid, by adding H₂O₂ (30% in water) to H₂SO₄ (96%) at a ratio of 1:3. Place all chips in a Teflon holder. Place the holder in the beaker and immerse all chips into the piranha solution for 10 min, then rinse the chips and holder sequentially in two separate DI water baths. Dry the chips with dry N₂ and immediately transfer them into oxygen (O₂) plasma activation equipment, keeping them covered during handling to avoid contamination.
   CAUTION: Piranha solutions are highly corrosive, are strongly oxidizing, and are dangerous. Follow the specific rules handling them at your institution, but at least take extreme care and wear the proper safety equipment. Upon completion of the work, the piranha solution must be cooled for at least one hour before pouring into a dedicated waste container.
   NOTE: It is necessary to rinse the LN chips twice in two DI water baths. Rinsing them once leaves residue behind that will likely ruin the bonding. Gold electrodes are used for IDTs because of their good resistance to piranha solution.
3. Plasma surface activation: Activate the chip surfaces using plasma with 120 W of power while exposed to O₂ flow at 120 sccm for 150 s. Immediately transfer the samples to a fresh DI water bath for at least 2 min.
   NOTE: Plasma surface treatment quickly followed by DI water immersion will form hydroxyl groups upon the LN surface, increasing its free surface energy to later promote bonding.
4. Room-temperature bonding: Dry the samples with dry N₂ flow and carefully lay the nanoslit chip onto the SAW device chip in the desired position. Realign to produce the desired orientation. Then use tweezers or similar to push down upon the sample from its center to initiate the bond. Gently push down in areas that failed to bond after the initial push.
   NOTE: The bonding may be easily seen through the transparent LN. Bonded regions are entirely transparent. LN that is not double-side polished will be more difficult to assess.
5. Heating after bonding: Place bonded samples in a sprung clamp to safely exert loads on it despite thermal expansion, and place the clamped samples into an oven at room temperature (25 °C). Set the oven heating temperature to 300 °C, ramp rate to 2 °C/min maximum, dwell time to 2 h, and then to automatically shut off to allow it and the clamped samples within to naturally cool to room temperature.
   NOTE: The protocol can be paused here. The bonding between hydroxyl groups produces water at the bond, and heating removes the water to drastically increase the bond strength. Modest clamping forces are sufficient. Attempting to bond two chips of different orientations or materials may cause cracks due to mismatched thermal expansion and consequent stress.

4. Experimental setup and testing

1. Observation: Observe the nanoslit under an inverted microscope. Include and rotate a linear polarizing filter in the optical path to suitably block birefringence-based image doubling in the LN. Use ultrapure DI water via the inlet to observe fluid motion in the completed nanoslit.
   NOTE: Ultrapure liquid is strongly recommended to prevent clogging, especially after evaporation.
2. SAW actuation: Attach absorbers at the ends of the SAW device to prevent reflected acoustic waves. Use a signal generator to apply a sinusoidal electric field to the IDT at its resonance frequency of around 40 MHz. Use an amplifier to amplify the signal. Use an oscilloscope to measure the actual voltage, current, and power applied to the device. Record the fluid motion during SAW actuation within the nanoslit using a camera attached to the microscope.

Results

We perform fluid capillary filing and SAW-induced fluid draining in nano-height LN slits after successful fabrication and bonding of SAW integrated nanofluidic devices. Surface acoustic waves are generated by IDTs actuated by an amplified sinusoidal signal at the IDTs' resonance frequency of 40 MHz, and the SAW propagates into the nanoslit via a piezoelectric LN substrate. The behavior of the fluid in the nanoslit interacting with SAW may be observed using an inverted microscope.

We demonstrat...

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

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
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"