Name: fabrication of surface acoustic wave devices on lithium niobate
Uploaded: 2020-06-18T13:00:00
Description: Watch this Scientific Journal Video about Fabrication of Surface Acoustic Wave Devices on Lithium Niobate 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-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. SAW device fabrication via the lift-off method
<ol>
	<li>Perform wafer solvent cleaning in a Class 100 clean room facility by immersing the 4&#8221; (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.&#160; 
	CAUTION: Perform the acetone and IPA immersions in a fume hood. Avoid inhalation and sk...

<ol>
	<li>Ding, X., 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=Standing+surface+acoustic+wave+(SSAW)+based+multichannel+cell+sorting.">Standing surface acoustic wave (SSAW) based multichannel cell sorting.</a> Lab on a Chip. 12 (21), 4228-4231 (2012).</li><li>Langelier, S. M., Yeo, L. Y., Friend, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=UV+epoxy+bonding+for+enhanced+SAW+transmission+and+microscale+acoustofluidic+integration.">UV epoxy bonding for enhanced SAW transmission and microscale acoustofluidic integration.</a> Lab on a Chip. 12 (16), 2970-2976 (2012).</li><li>Rezk, A. R., Qi, A., Friend, J. R., Li, W. H., Yeo, L. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Uniform+mixing+in+paper-based+microfluidic+systems+using+surface+acoustic+waves.">Uniform mixing in paper-based microfluidic systems using surface acoustic waves.</a> Lab on a Chip. 12 (4), 773-779 (2012).</li><li>Schmid, L., Weitz, D. A., Franke, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sorting+drops+and+cells+with+acoustics:+acoustic+microfluidic+fluorescence-activated+cell+sorter.">Sorting drops and cells with acoustics: acoustic microfluidic fluorescence-activated cell sorter.</a> Lab on a Chip. 14 (19), 3710-3718 (2014).</li><li>Schmid, L., Wixforth, A., Weitz, D. A., Franke, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Novel+surface+acoustic+wave+(SAW)-driven+closed+PDMS+flow+chamber.">Novel surface acoustic wave (SAW)-driven closed PDMS flow chamber.</a> Microfluidics and Nanofluidics. 12 (1-4), 229-235 (2012).</li><li>Shi, J., Mao, X., Ahmed, D., Colletti, A., Huang, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Focusing+microparticles+in+a+microfluidic+channel+with+standing+surface+acoustic+waves+(SSAW).">Focusing microparticles in a microfluidic channel with standing surface acoustic waves (SSAW).</a> Lab on a Chip. 8 (2), 221-223 (2008).</li><li>Friend, J., Yeo, L. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microscale+acoustofluidics:+Microfluidics+driven+via+acoustics+and+ultrasonics.">Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics.</a> Reviews of Modern Physics. 83 (2), 647 (2011).</li><li>Ding, X., 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=Surface+acoustic+wave+microfluidics.">Surface acoustic wave microfluidics.</a> Lab on a Chip. 13 (18), 3626-3649 (2013).</li><li>Destgeer, G., Sung, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+advances+in+microfluidic+actuation+and+micro-object+manipulation+via+surface+acoustic+waves.">Recent advances in microfluidic actuation and micro-object manipulation via surface acoustic waves.</a> Lab on a Chip. 15 (13), 2722-2738 (2015).</li><li>Connacher, W., 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=Micro/nano+acoustofluidics:+materials,+phenomena,+design,+devices,+and+applications.">Micro/nano acoustofluidics: materials, phenomena, design, devices, and applications.</a> Lab on a Chip. 18 (14), 1952-1996 (2018).</li><li>White, R. M., Voltmer, F. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+piezoelectric+coupling+to+surface+elastic+waves.">Direct piezoelectric coupling to surface elastic waves.</a> Applied Physics Letters. 7 (12), 314-316 (1965).</li><li>Smith, H. I., Bachner, F. J., Efremow, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+High-Yield+Photolithographic+Technique+for+Surface+Wave+Devices.">A High-Yield Photolithographic Technique for Surface Wave Devices.</a> Journal of the Electrochemical Society. 118 (5), 821-825 (1971).</li><li>Bahr, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+techniques+for+surface-acoustic-wave+devices.">Fabrication techniques for surface-acoustic-wave devices.</a> Proc. Int. Specialists Seminar on Component Performance and Systems Applications of Surface Acoustic Wave Devices. , (1973).</li><li>Smith, H. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+techniques+for+surface-acoustic-wave+and+thin-film+optical+devices.">Fabrication techniques for surface-acoustic-wave and thin-film optical devices.</a> Proceedings of the IEEE. 62 (10), 1361-1387 (1974).</li><li>Wilke, N., Mulcahy, A., Ye, S. R., Morrissey, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Process+optimization+and+characterization+of+silicon+microneedles+fabricated+by+wet+etch+technology.">Process optimization and characterization of silicon microneedles fabricated by wet etch technology.</a> Microelectronics Journal. 36 (7), 650-656 (2005).</li><li>Madou, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Fundamentals of microfabrication: the science of miniaturization. , (2002).</li><li>K&#246;hler, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Etching in Microsystem Technology. , (1999).</li><li>Brodie, I., Muray, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> The physics of micro/nano-fabrication. , (2013).</li><li>Dentry, M. B., Yeo, L. Y., Friend, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Frequency+effects+on+the+scale+and+behavior+of+acoustic+streaming.">Frequency effects on the scale and behavior of acoustic streaming.</a> Physical Review E. 89 (1), 013203 (2014).</li><li>Morgan, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Surface acoustic wave filters: With applications to electronic communications and signal processing. , (2010).</li><li>Pekarcikova, 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=Investigation+of+high+power+effects+on+Ti/Al+and+Ta-Si-N/Cu/Ta-Si-N+electrodes+for+SAW+devices.">Investigation of high power effects on Ti/Al and Ta-Si-N/Cu/Ta-Si-N electrodes for SAW devices.</a> IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. 52 (5), 911-917 (2005).</li></ol>

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.&#160;
Lift-off method 
The choice of photoresist is important. It is possible to use a p...

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 applications1,2,3,4,5,6. By enabling the generation of displacements up to 1 nm at 10-1000 MHz, SAW-driven vibration overcomes the typical obstacles of traditional ultrasound: small acceleration, large wavelengths, and large device size. Research to manipulate fluids and suspended particles has recently accelerated, with a large number of recent and accessible reviews7,8,9,10.
Fabrication of SAW-integrated microfluidic devices requires fabrication of the electrodes&#8212;the interdigital transducer (IDT)11&#8212;on the piezoelectric substrate to generate the SAW. The comb-shape fingers create compression and tension in the substrate when connected to an alternating electric input. The fabrication of SAW devices has been presented in many publications, whether using lift-off ultraviolet photolithography alongside metal sputter or wet etching processes10. However, the lack of knowledge and skills in fabricating these devices is a key barrier to entry into acoustofluidics by many research groups, even today. For the lift-off technique12,13,14, a sacrificial layer (photoresist) with an inverse pattern is created on a surface, so that when the target material (metal) is deposited on the whole wafer, it can reach the substrate in the desired regions, followed by a &#8220;lift-off&#8221; step to remove the remaining photoresist. By contrast, in the wet etching process15,16,17,18, the metal is first deposited on the wafer and then photoresist is created with a direct pattern on the metal, to protect the desired region from &#8220;etching&#8221; away by a metal etchant.
In a most commonly used design, the straight IDT, the wavelength of the resonant frequency of the SAW device is defined by the periodicity of the finger pairs, where the finger width and the spacing between fingers are both <img align="center" alt="Equation" src="/files/ftp_upload/61013/61013eq2.jpg" />/419. In order to balance the electric current transmission efficiency and the mass loading effect on the substrate, the thickness of the metal deposited on the piezoelectric material is optimized to be about 1% of the SAW wavelength20. Localized heating from Ohmic losses21, potentially inducing premature finger failure, can occur if insufficient metal is deposited. On the other hand, an excessively thick metal film can cause a reduction in the resonant frequency of the IDT due to a mass loading effect and can possibly create unintentional acoustic cavities from the IDTs, isolating the acoustic waves they generate from the surrounding substrate. As a result, the photoresist and UV exposure parameters chosen vary in the lift-off technique, depending upon different designs of SAW devices, especially frequency. Here, we describe in detail the lift-off process to produce a 100 MHz SAW-generating device on a double-sided polished 0.5 mm-thick 128&#176; Y-rotated cut LN wafer, as well as the wet etching process to fabricate the 100 MHz device of identical design. Our approach offers a microfluidic 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>Chromium etchant</td>
			<td>Transene Company, INC, Danvers, MA, USA</td>
			<td>1020</td>
			<td></td>
		</tr>
		<tr>
			<td>Developer</td>
			<td>Futurrex, NJ, USA</td>
			<td>RD6</td>
			<td></td>
		</tr>
		<tr>
			<td>Developer</td>
			<td>EMD Performance Materials Corp., Philidaphia, PA, USA</td>
			<td>AZ300MIF</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>Gold etchant</td>
			<td>Transene Company, INC, Danvers, MA, USA</td>
			<td>Type TFA</td>
			<td></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>Laser Doppler vibrometer (LDV)</td>
			<td>Polytec, Waldbronn, Germany</td>
			<td>UHF-120</td>
			<td>4&#8221; double-side polished 0.5 mm thick 128&#176;Y-rotated cut lithium niobate</td>
		</tr>
		<tr>
			<td>Lithium niobate substrate</td>
			<td>PMOptics, Burlington, MA, USA</td>
			<td>PWLN-431232</td>
			<td></td>
		</tr>
		<tr>
			<td>Mask aligner</td>
			<td>Heidelberg Instruments, Heidelberg, Germany</td>
			<td>MLA150</td>
			<td>Fabrication process is performed in it.</td>
		</tr>
		<tr>
			<td>Nano3 cleanroom facility</td>
			<td>UCSD, La Jolla, CA, USA</td>
			<td></td>
			<td></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>Positive photoresist</td>
			<td></td>
			<td>AZ1512</td>
			<td>Denton Discovery 18 Sputter System</td>
		</tr>
		<tr>
			<td>Signal generator</td>
			<td>NF Corporation, Yokohama, Japan</td>
			<td>WF1967 multifunction generator</td>
			<td>Wafer Dipper 4&#34;</td>
		</tr>
		<tr>
			<td>Sputter deposition</td>
			<td>Denton Vacuum, NJ, USA</td>
			<td>Denton 18</td>
			<td></td>
		</tr>
		<tr>
			<td>Teflon wafer dipper</td>
			<td>ShapeMaster, Ogden, IL, USA</td>
			<td>SM4WD1</td>
			<td></td>
		</tr>
	</tbody>
</table>

fabrication of surface acoustic wave devices on lithium niobate

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&#8212;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.

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 &#956;m, producing a wavelength of 40 &#956;m. Figure 1 shows the SAW device and IDT&#160;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,...

Watch this Scientific Journal Video about Fabrication of Surface Acoustic Wave Devices on Lithium Niobate at JoVE.com

Fabrication of Surface Acoustic Wave Devices on Lithium Niobate

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.&#160;

Manipulation of fluids and particles by acoustic actuation at small scale is aiding the rapid growth of lab-on-a-chip applications. Megahertz-order ...

Our protocol demonstrates the details of fabricating typical surface acoustic wave devices upon piezoelectric substrates especially valuable for people seeking to enter this burgeoning field. Keeping any debris away from the surface during cleaning is crucial in the process of fabrication. To pre-break the wafer, place it on a hotplate at 100 degrees Celsius for three minutes.Then move the wafer to aluminum foil. Place the wafer onto a spin coater. Using a dropper, place negative photoresist onto the wafer covering about 75%of the wafer surface area.To produce a photoresist thickness of approximately 1.3 micrometers, execute the following program on the spin coater:500 rpm with an acceleration of 3, 000 rpm per second for five seconds, followed by 3, 500 rpm with an acceleration of 3, 000 rpm per second for 40 seconds. Bake the wafer by placing it on a hotplate at 100 degrees Celsius. Increase the hotplate temperature to 150 degrees Celsius and maintain that temperature for one minute.Then move the wafer from the hotplate and let the wafer cool in the air to room temperature. Do not place the wafer directly onto the hotplate at 150 degrees Celsius. Let the water cool down in air after heating.To expose the photoresist to ultraviolet energy, transfer the wafer to the mask aligner. With the mask aligner set to deliver light at 375 nanometers, expose the photoresist to an energy dose of 400 millijoules per square centimeter. To bake the wafer, place it on a hotplate at 100 degrees Celsius.After three minutes, transfer the wafer onto aluminum foil where it will cool to room temperature. Place the wafer into a beaker filled with pure RD6 developer. Leave the wafer immersed for 15 seconds while gently shaking the beaker.Remove the wafer from the developer and immerse it in deionized water for one minute. Then rinse the wafer under deionized water flow. Finally, use dry nitrogen flow to remove the remaining water from the wafer.Bake the water again at 100 degrees Celsius. After three minutes, transfer the wafer onto aluminum foil where it will cool to room temperature. Place the wafer into a sputter deposition system and evacuate the chamber to a pressure of five times 10 to the negative six millitorr.Next, flow argon at 2.5 millitorr. Then sputter chromium with a power of 200 watts for five nanometers as an adhesion layer. To form the conductive electrodes, deposit aluminum at 400 nanometers and a power level of 300 watts.Transfer the wafer to a beaker and immerse it in acetone. Sonicate the beaker at medium intensity for five minutes. Rinse the wafer with deionized water and dry the wafer with nitrogen flow.Place the wafer on a hotplate at 100 degrees Celsius for three minutes. Then transfer it onto a piece of aluminum foil and wait for it to cool to room temperature. Place the wafer into a sputter deposition system and evacuate the chamber to a pressure of five times 10 to the negative six millitorr.Flow argon at 2.5 millitorr and then sputter chromium with a power of 200 watts for five nanometers as an adhesion layer. Next, form the conductive electrodes by sputtering gold for 400 nanometers at a power level of 300 watts. Place the wafer on a spin coater.Using a dropper, deposit positive photoresist onto the wafer covering about 75%of the wafer surface area. To produce a photoresist thickness of approximately 1.2 micrometers, execute the following program on the spin coater:500 rpm with an acceleration of 3, 000 rpm per second for 10 seconds, followed by 4, 000 rpm with an acceleration of 3, 000 rpm per second for 30 seconds. Then place the wafer on a hotplate at 100 degrees Celsius.After one minute, transfer the wafer onto aluminum foil where it will cool to room temperature. Transfer the wafer to the mask aligner. With the mask aligner set to deliver light at 375 nanometers, expose the photoresist to an energy dose of 150 millijoules per square centimeter.Place the wafer into a beaker filled with pure AZ300MIF developer. Leave the wafer in the beaker for 300 seconds gently shaking the beaker. Remove the wafer from the developer and immerse it deionized water for one minute.Then rinse the wafer under deionized flow. Finally, use dry nitrogen flow to remove the remaining water from the wafer. Next, immerse the wafer in gold etchant for 90 seconds, gently shaking the beaker.After rinsing the wafer under deionized water flow, use dry nitrogen flow to remove the remaining deionized water from the wafer. Apart from the acetone, photoresist, and developer, the most dangerous reagents are the metal actions which require higher level protection such as neoprene gloves and an apron. Finally, immerse the wafer in chromium etchant for 20 seconds, gently shaking the beaker.Rinse the wafer under deionized water flow. And again, use dry nitrogen flow to remove remaining water. IDTs were fabricated using the methods described.The spacing between the fingers and the fingers themselves are all 10 micrometers in width, resulting in a wavelength of 40 micrometers. A sinusoidal signal was applied to the IDT and a laser Doppler vibrometer was used to measure the amplitude and frequency of the resulting surface acoustic wave. The resonance frequency was found to be 96.5844 megahertz, slightly lower than the design frequency of 100 megahertz.A plot of the vibration on the substrate surface shows a surface acoustic wave propagating from the IDTs. Based on the ratio between the maximum amplitude and the minimum amplitude, the standing wave ratio was calculated to be 2.06. The motion of a sessile droplet actuated by the SAW device was demonstrated.A water droplet of 0.2 microliters was pipetted on lithium niobate about one millimeter away from the IDT. When a SAW propagates and encounters the droplet, it leaks into the liquid at the Rayleigh angle. The jetting angle confirms the presence of a surface acoustic wave.These techniques can be used for the fabrication of megahertz or the surface acoustic wave devices. The process needs to be adjusted if higher frequency acoustic wave actuators are required. This protocol provides two dependable methods for preparing high-frequency surface acoustic wave devices used for micro to nanoscale acoustofluidics research.

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Fabrication, Operation and Flow Visualization in Surface-acoustic-wave-driven Acoustic-counterflow Microfluidics

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we present the fabrication protocol for a multilayered SAW acoustic counterflow device. The device is fabricated starting from a lithium niobate (LN) substrate onto which two interdigital transducers (IDTs) and appropriate markers are patterned. A polydimethylsiloxane (PDMS) channel cast on an SU8 master mold is finally bonded on the patterned substrate. Following the fabrication procedure, we show the techniques that allow the characterization and operation of the acoustic counterflow device in order to pump fluids through the PDMS channel grid. We finally present the procedure to visualize liquid flow in the channels. The protocol is used to show on-chip fluid pumping under different flow regimes such as laminar flow and more complicated dynamics characterized by vortices and particle accumulation domains.

In this video we first describe fabrication and operation procedures of a surface acoustic wave (SAW) acoustic counterflow device. We then demonstrate an experimental setup that allows for both qualitative flow visualization and quantitative analysis of complex flows within the SAW pumping device.

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we ...

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene&#8217;s charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene&#39;s electronic structure in the presence of a charged impurity.&#160;The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene&#8217;s electronic properties.1-8 Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge2 and/or molecular5 states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies.2-5 These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each ...

Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional component that should have high electrical conductivity, high mechanical properties, and high productivity.
In this regard, a carbon fiber/epoxy resin composite can be an ideal material to replace the conventional graphite bipolar plate, which often leads to the catastrophic failure of the entire system because of its inherent brittleness. Though the carbon/epoxy composite has high mechanical properties and is easy to manufacture, the electrical conductivity in the through-thickness direction is poor because of the resin-rich layer that forms on its surface. Therefore, an expanded graphite coating was adopted to solve the electrical conductivity issue. However, the expanded graphite coating not only increases the manufacturing costs but also has poor mechanical properties.
In this study, a method to expose fibers on the composite surface is demonstrated. There are currently many methods that can expose fibers by surface treatment after the fabrication of the composite. This new method, however, does not require surface treatment because the fibers are exposed during the manufacture of the composite. By exposing bare carbon fibers on the surface, the electrical conductivity and mechanical strength of the composite are increased drastically.

A protocol to expose bare fibers on the composite surface by eliminating resin rich area is presented. The fibers are exposed during fabrication of the composites, not by the post surface treatment. The exposed carbon composites exhibit high electrical conductivity in the through-thickness direction and high mechanical property.

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional ...

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of lithium-ion (Li-ion) batteries. However, a number of setbacks prevent their integration into commercial devices. The main limiting factor is due to nanoscale phenomena occurring at the electrode/electrolyte interfaces, ultimately leading to degradation of battery operation. These key problems are highly challenging to observe and characterize as these batteries contain multiple buried interfaces. One approach for direct observation of interfacial phenomena in thin film batteries is through the fabrication of electrochemically active nanobatteries by a focused ion beam (FIB). As such, a reliable technique to fabricate nanobatteries was developed and demonstrated in recent work. Herein, a detailed protocol with a step-by-step process is presented to enable the reproduction of this nanobattery fabrication process. In particular, this technique was applied to a thin film battery consisting of LiCoO2/LiPON/a-Si, and has further been previously demonstrated by in situ cycling within a transmission electron microscope.

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

A protocol for the fabrication of electrochemically active LiPON-based solid-state lithium-ion nanobatteries using a focused ion beam is presented.

Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for applications in two-dimensional nanoelectronics devices. Although many methods to synthesize SnSe nanocrystals have been developed, a simple way to fabricate large-sized single-layer SnSe flakes remains a great challenge. Herein, we show the experimental method to directly grow large-sized single-layer rectangular SnSe flakes on commonly used SiO2/Si insulating substrates using a straightforward two-step fabrication method in an atmospheric pressure quartz tube furnace system. The single-layer rectangular SnSe flakes with an average thickness of ~6.8 &#197; and lateral dimensions of about 30 &#181;m &#215; 50 &#181;m were fabricated by a combination of vapor transport deposition technique and nitrogen etching route. We characterized the morphology, microstructure, and electrical properties of the rectangular SnSe flakes and obtained excellent crystallinity and good electronic properties. This article about the two-step fabrication method can help researchers grow other similar two-dimensional, large-sized, single-layer materials using an atmospheric pressure system.

A protocol is presented demonstrating a two-step fabrication technique to grow large-sized single-layer rectangular shaped SnSe flakes on low-cost SiO2/Si dielectrics wafers in an atmospheric pressure quartz tube furnace system.

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...

Microparticle Manipulation by Standing Surface Acoustic Waves with Dual-frequency Excitations

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip (LOC) system. The simultaneous excitation of the fundamental frequency and its third harmonic, which is termed as dual-frequency excitation, to a pair of interdigital transducers (IDTs) could generate a new type of standing acoustic waves in a microfluidic channel. Varying the power and the phase in the dual-frequency excitation signals results in a reconfigurable field of the acoustic radiation force applied to the microparticles across the microchannel (e.g., the number and location of the pressure nodes and the microparticle concentrations at the corresponding pressure nodes). This article demonstrates that the motion time of the microparticle to only one pressure node can be reduced ~2-fold at the power ratio of the fundamental frequency greater than ~90%. In contrast, there are three pressure nodes in the microchannel if less than this threshold. Furthermore, adjusting the initial phase between the fundamental frequency and the third harmonic results in different motion rates of the three SSAW pressure nodes, as well as in the percentage of microparticles at each pressure node in the microchannel. There is a good agreement between the experimental observation and the numerical predictions. This novel excitation method can easily and non-invasively integrate into the LOC system, with a wide tenability and only a few changes to the experimental set-up.

A protocol for manipulating the microparticles in a microfluidic channel with a dual-frequency excitation is presented.

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip ...