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

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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.

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Zhang, N., Friend, J. Fabrication of Nanoheight Channels Incorporating Surface Acoustic Wave Actuation via Lithium Niobate for Acoustic Nanofluidics. J. Vis. Exp. (156), e60648, doi:10.3791/60648 (2020).

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 fields5,6,7, 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 investigated12,13,14,15,16. Plasma surface activation has been used to significantly increase surface energy and hence improve the strength of bonding in oxides such as LN17,18,19,20. 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.

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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.

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Representative 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 demonstrate fluid capillary filling in 100-nm tall channels of different widths. Figure 2 shows capillary filling of ultrapure DI water into two 100-nm tall channels, one 400 μm wide and the other 40 μm wide. The drop of ultrapure water is delivered into the nanoslit through the inlet. Capillary forces drive fluid filling of the entire nanoslit, and the filling occurs more quickly with the narrower channel due to its larger capillary force. Capillary force-driven fluid filling using other fluids of different viscosities and surface tensions could be used, as could nanoslits of other heights to produce different results.

We also demonstrate SAW-induced fluid draining in a nanochannel by overcoming capillary pressure. Water in a 100-nm height slit has been drained to show a water-air interface with the maximum length at the middle (Figure 3), indicating maximum acoustic energy at middle of the SAW device. With strong acoustic pressure generated in the nanoslit, it also indicates good bonding strength using our plasma-surface-activated room-temperature LN bonding method. A threshold applied power of around 1 W is required to force the acoustic pressure to be larger than the capillary pressure and drive a visible draining phenomenon (Figure 4). The maximum length of the air cavity which represents fluid surface energy shows a linear relationship with the applied acoustic power. It offers an effective tool for fluid actuation and potentially macro-biomolecule manipulation at the nanoscale. The effect of draining various fluids using SAW with different channel heights and widths could be further investigated.

Figure 1
Figure 1: Images of fabricated devices. (A) Left: Gold electroded IDTs with a 0.7 mm aperture on LN substrate for 40 MHz SAW generation and propagation. Middle, Right: Bonded LN nanoslit device integrated with SAW for fluid actuation. A one-penny coin is shown as a scale reference at bottom. (B) Various reactive-ion-etched nano-height channel LN chips are shown with chromium sacrificial mask structures and after drilling 500-μm diameter holes for fluid inlets and outlets. Scale bar: 5 mm. Please click here to view a larger version of this figure.

Figure 2
Figure 2: Fluid capillary filling in 100 nm-height channels. (A1-A4) Ultrapure water is drawn into a 400-μm wide nanoslit via capillary force over time, shown at the start (0 s) and 1, 2, and 4 s later, respectively. Small water drops can be seen at the top of the superstrate. (B1-B4) Ultrapure water is drawn into a 40 μm wide nanoslit via capillary force over time, shown at the start (0 s) and 0.1, 0.3, and 1 s later, respectively, indicating more rapid filling due to greater capillary force on a smaller amount of fluid. The small depressions at the top of the superstrate are evidence of hitting the surface with tweezers. Scale bar: 400 μm. Please click here to view a larger version of this figure.

Figure 3
Figure 3: SAW-induced fluid draining in 1 mm-width 100 nm-height nanoslit. (A-C) A water-filled nanoslit is drained by 40 MHz SAW at an applied power of 1.31 W, 2.04 W, and 2.82 W, respectively. The SAW is propagating from top to bottom in the images. The interfacial line between the bonded and nanoslit regions is visible: note the color change. Scale bar: 200 μm. Please click here to view a larger version of this figure.

Figure 4
Figure 4: SAW-induced air cavity length with respect to SAW applied power. The dewetting cavity length is approximately linearly dependent upon the applied power. The applied power should offer an acoustic pressure greater than the capillary pressure in the nanoslit, causing fluid drainage. The threshold applied power at which drainage appears is around 1 W in this case. Please click here to view a larger version of this figure.

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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 the nanoslit fabrication and fluid transport performance. It has been shown that the plasma surface activation time will not help increase the surface energy after a certain amount of time21. Thus, the plasma activation time and power need to be defined to maximize the surface energy but not at the expense of increased surface roughness.

Cleaning chips before bonding
Since there is only a nanoscale height channel after bonding, any micro-size particle will be an enormous obstacle and cause bonding failure. Piranha cleaning is used to remove all organic debris on the chip surfaces. After cleaning, it is strongly recommended to use a clean container to cover the chips and prevent contamination.

Orientation of the LN chip pairs prior to bonding
Due to the anisotropy of LN, bonding the upper and lower LN chip currently requires identical material orientation. Failing to do so will cause residual stress and possibly cracking during fabrication. It will also cause different SAW characteristics between the top and bottom surfaces of the nanoslit because of anisotropy. Therefore, bonding two LN chips with identical material orientation is highly recommended.

Alignment of the upper and lower chips
We visually perform the manual alignment and bonding. Introducing fiduciary markers and proper microscope-aided aligned bonding would surely improve device quality and yield.

Oven heating temperature after initiating room-temperature bonding
Heating at higher temperatures will help strengthen the bond. Heating to 300 °C for our LN bonding process produces at least 1 MPa bonding strength since it remains intact against comparable capillary and acoustic pressures in the nanoslit with SAW.

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Disclosures

The authors have nothing to disclose.

Acknowledgments

The authors are grateful to the University of California and the NANO3 facility at UC San Diego for provision of funds and facilities in support of this work. This work was performed in part at the San Diego Nanotechnology Infrastructure (SDNI) of UCSD, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS–1542148). The work presented here was generously supported by a research grant from the W.M. Keck Foundation. The authors are also grateful for the support of this work by the Office of Naval Research (via Grant 12368098).

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"

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