Name: fabrication of nanoheight channels incorporating surface acoustic wave actuation via lithium niobate for acoustic nanofluidics
Uploaded: 2020-02-05T13:30:00
Description: Watch this Scientific Journal Video about Fabrication of Nanoheight Channels Incorporating Surface Acoustic Wave Actuation via Lithium Niobate for Acoustic Nanofluidics at JoVE.com

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&#8211;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).

1. Nano-height channel mask preparation
<ol>
	<li>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...

<ol>
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M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+parametric+study+of+ICP-RIE+etching+on+a+lithium+niobate+substrate.">A parametric study of ICP-RIE etching on a lithium niobate substrate.</a> 10th IEEE International Conference on Nano/Micro Engineered and Molecular Systems. , 485-486 (2015).</li><li>Queste, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Deep+reactive+ion+etching+of+quartz,+lithium+niobate+and+lead+titanate.">Deep reactive ion etching of quartz, lithium niobate and lead titanate.</a> JNTE (Journ&#233;es Nationales sur les Technologies) Proceedings. , (2008).</li><li>Xu, J., Wang, C., Tian, Y., Wu, B., Wang, S., Zhang, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glass-on-LiNbO3+heterostructure+formed+via+a+two-step+plasma+activated+low-temperature+direct+bonding+method.">Glass-on-LiNbO3 heterostructure formed via a two-step plasma activated low-temperature direct bonding method.</a> Applied Surface Science. 459, 621-629 (2018).</li><li>Tulli, D., Janner, D., Pruneri, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Room+temperature+direct+bonding+of+LiNbO3+crystal+layers+and+its+application+to+high-voltage+optical+sensing.">Room temperature direct bonding of LiNbO3 crystal layers and its application to high-voltage optical sensing.</a> Journal of Micromechanics and Microengineering. 21 (8), 085025 (2011).</li><li>Sridhar, M., Maurya, D. 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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...

Controllable nanoscale fluid transport in nanochannels&#8212;nanofluidics1&#8212;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&#8212;the interdigital transducer (IDT)&#8212;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&#39;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.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Absorber</td>
			<td>Dragon Skin, Smooth-On, Inc., Macungie, PA, USA</td>
			<td>Dragon Skin 10 MEDIUM</td>
			<td></td>
		</tr>
		<tr>
			<td>Amplifier</td>
			<td>Mini-Circuits, Brooklyn, NY, USA</td>
			<td>ZHL&#8211;1&#8211;2W&#8211;S+</td>
			<td></td>
		</tr>
		<tr>
			<td>Camera</td>
			<td>Nikon, Minato, Tokyo, Japan</td>
			<td>D5300</td>
			<td></td>
		</tr>
		<tr>
			<td>Developer</td>
			<td>Futurrex, NJ, USA</td>
			<td>RD6</td>
			<td></td>
		</tr>
		<tr>
			<td>Diamond tip engraving pen</td>
			<td>Malco, Memphis, TN, USA</td>
			<td>Malco A50 USA Made Carbide Tipped Scribe</td>
			<td></td>
		</tr>
		<tr>
			<td>Dicing saw</td>
			<td>Disco, Tokyo, Japan</td>
			<td>Disco Automatic Dicing Saw 3220</td>
			<td></td>
		</tr>
		<tr>
			<td>Heating oven</td>
			<td>Carbolite, Hope Valley, UK</td>
			<td>HTCR 6/28</td>
			<td>High Temperature Clean Room Oven - HTCR</td>
		</tr>
		<tr>
			<td>Hole driller</td>
			<td>Dremel, Mount Prospect, Illinois</td>
			<td>Model #4000</td>
			<td>4000 High Performance Variable Speed Rotary</td>
		</tr>
		<tr>
			<td>Inverted microscope</td>
			<td>Amscope, Irvine, CA, USA</td>
			<td>IN480TC-FL-MF603</td>
			<td></td>
		</tr>
		<tr>
			<td>Lithium niobate substrate</td>
			<td>PMOptics, Burlington, MA, USA</td>
			<td>PWLN-431232</td>
			<td>4&#34; double-side polished 0.5 mm thick 128&#176; Y-rotated cut lithium niobate</td>
		</tr>
		<tr>
			<td>Mask aligner</td>
			<td>Heidelberg Instruments, Heidelberg, Germany</td>
			<td>MLA150</td>
			<td></td>
		</tr>
		<tr>
			<td>Nano3 cleanroom facility</td>
			<td>UCSD, La Jolla, CA, USA</td>
			<td></td>
			<td>Fabrication process is performed in it.</td>
		</tr>
		<tr>
			<td>Negative photoresist</td>
			<td>Futurrex, NJ, USA</td>
			<td>NR9-1500PY</td>
			<td></td>
		</tr>
		<tr>
			<td>Oscilloscope</td>
			<td>Keysight Technologies, Santa Rosa, CA, USA</td>
			<td>InfiniiVision 2000 X-Series</td>
			<td></td>
		</tr>
		<tr>
			<td>Plasma surface activation</td>
			<td>PVA TePla, Corona, CA, USA</td>
			<td>PS100</td>
			<td>Tepla Asher</td>
		</tr>
		<tr>
			<td>Polarizer sheet</td>
			<td>Edmund Optics, Barrington, NJ, USA</td>
			<td>#86-182</td>
			<td></td>
		</tr>
		<tr>
			<td>RIE etcher</td>
			<td>Oxford Instruments, Abingdon, UK</td>
			<td>Plasmalab 100</td>
			<td></td>
		</tr>
		<tr>
			<td>Signal generator</td>
			<td>NF Corporation, Yokohama, Japan</td>
			<td>WF1967 multifunction generator</td>
			<td></td>
		</tr>
		<tr>
			<td>Sputter deposition</td>
			<td>Denton Vacuum, NJ, USA</td>
			<td>Denton 18</td>
			<td>Denton Discovery 18 Sputter System</td>
		</tr>
		<tr>
			<td>Teflon wafer dipper</td>
			<td>ShapeMaster, Ogden, IL, USA</td>
			<td>SM4WD1</td>
			<td>Wafer Dipper 4&#34;</td>
		</tr>
	</tbody>
</table>

fabrication of nanoheight channels incorporating surface acoustic wave actuation via lithium niobate for acoustic nanofluidics

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.

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&#39; 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 demons...

Watch this Scientific Journal Video about Fabrication of Nanoheight Channels Incorporating Surface Acoustic Wave Actuation via Lithium Niobate for Acoustic Nanofluidics at JoVE.com

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

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.

Controlled nanoscale manipulation of fluids is known to be exceptionally difficult due to the dominance of surface and viscous forces. Megahertz-order ...

Our protocol provides a detailed fabrication method of nano-height channels incorporating surface acoustic wave actuation via lithium niobate for acoustic nanofluidics. This technique can be used to perform room temperature plasma surface activated multilayer bonding of single crystal lithium niobate, a process equally useful for bonding lithium niobate or silicon dioxide and other oxides. Any debris and particulates should be removed during the cleaning and plasma surface activated processes to prevent bonding failure in the nano-height channel formation.Visual demonstration of this method can capture the entire fabrication process in detail resulting in a clear presentation of the protocol for other researchers. To prepare a nano-height channel mask, place a wafer inscribed with a pattern designed to be a normal photolithography in liftoff procedures into a sputter deposition system and draw down the chamber vacuum to five times 10 to the negative six millitorr. Allow argon to flow at 2.5 millitorr and sputter chromium at 200 watts to produce a 400 nanometer thick sacrificial mask within 18 minutes.At the end of the deposition, completely submerge the wafer in a beaker of acetone and sonicate the wafer at medium intensity for 10 minutes. At the end of the sonication, rinse the wafer with deionized water and dry the wafer with dry nitrogen flow. Then use a dicing saw to divide the wafer into individual chips with one nano-slit pattern per chip.To fabricate the nano-height channel, place the wafer into a reactive ion etching chamber. Set the chamber parameters as indicated to produce a 120 nanometer deep nano-slit in the lithium niobate. To drill the channel inlets and outlets, use double-sided tape to attach a small steel plate to the bottom of a Petri dish and the etched chip to the plate.Fill the dish with water to fully immerse the chip and attach a 0.5 millimeter diameter diamond drill bit to a drill press. Then drill at a speed of at least 10, 000 rotations per minute to machine the desired inlets and outlets. For chromium wet etching, use a diamond tip engraving pen to clearly mark the flat unetched surface of the drilled lithium niobate to keep track of which side the nano-height channel is located and sonicate the chips in chromium etchant.For solvent cleaning of the chips, place chip pairs consisting of one surface acoustic wave device and one etched nanoscale depression chip and immerse the pairs in a beaker of acetone placed with a sonication bath. After two minutes of sonication in acetone, sonicate the chips in methanol for one minute. At the end of the methanol sonication, rinse the chips in deionized water.Next, add hydrogen peroxide to sulfuric acid at a one-to-three ratio in a well-ventilated hood and place all of the chips into a Teflon holder. Carefully place the holder into the beaker of piranha acid for 10 minutes before rinsing the chips and holder in two sequential deionized water baths. After the second rinse, dry the chips with dry nitrogen flow and immediately place the samples into oxygen plasma activation equipment keeping them covered during handling to avoid contamination.Using 120 watts of power while exposing the chips to oxygen flow at 120 standard cubic centimeters for 150 seconds, activate the chip surfaces with the plasma. At the end of the activation, immediately submerge the samples in a fresh deionized water bath for at least two minutes. After drying the chips with dry nitrogen flow, carefully lay the nano-slit chip onto the surface acoustic wave device chip in the desired position with the chips aligned in the appropriate orientation.Then use tweezers or similar to press down upon the sample from its center to initiate the bond, applying gentle pressure to areas that failed to bond after the initial depression. Next, place the bonded samples in a sprung clamp to safely exert loads despite thermal expansion and place the clamped samples into a room temperature oven. Then set the oven temperature to 300 degrees Celsius with a ramp rate of two degrees Celsius per minute maximum with a dwell time of two hours before automatic shutoff.To observe fluid motion in the completed nano-slit, place the nano-slit chip under an inverted microscope and rotate the chip through a linear polarizing filter in the optical path to suitably block birefringence-based image doubling in the lithium niobate. Then add ultrapure deionized water to the inlet and image the fluid progression. For surface acoustic wave actuation, attach absorbers to the ends of the surface acoustic wave device to prevent reflected acoustic waves and set the resonance frequency on a signal generator to around 40 megahertz.Use an amplifier to amplify the signal and use an oscilloscope to measure the actual voltage, current and power applied to the device. Then apply a sinusoidal electrical field to the interdigital transducer and record the fluid motion during the actuation within the nano-slit. In these images, capillary filling of ultrapure deionized water into one 100 nanometer tall 400 micrometer wide channel and one 100 nanometer tall 40 micrometer wide channel is shown.Capillary forces draw fluid filling of the entire nano-slit with a drop of ultrapure water delivered through the inlet and the filling occurred more quickly within the narrower channel due to its larger capillary force. In this experiment, water in a 100 nanometer height slit was drained to show a water-air interface with the maximum length at the middle indicating a maximum acoustic energy at the middle of the surface acoustic wave device. A threshold applied power of around one watt is required to force the acoustic pressure to be larger than the capillary pressure to drive a visible draining phenomenon.Most fabrication processes should be conducted in a clean room to prevent microscale particulate contamination and fluid used for filling should be ultrapure to prevent clogging of the nano-slit. Our approach offers a nano-acoustic fluidics system for the investigation of a variety of physical problems and biological applications at the nanoscale.

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