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 skin contact with IPA. Avoid skin and eye contact with acetone. Do not swallow. 
	NOTE: Do not allow any fluid to evaporate upon the wafer; if any dust or debris on the surface, start this step over.&#160;</li>
	<li>Place the wafer onto a hotplate at 100 &#176;C to prebake for 3 min. 
	NOTE: Because of the pyroelectric property of LN, it will generate static charges and associated stress within the wafer during heating and cooling. It is recommended to put the wafer onto a piece of aluminum (Al) foil after removing it from the hot plate to release the static charges and avoid breaking it.&#160;</li>
	<li>Place the wafer onto a spin coater. Using a dropper, cover about 75% of the wafer&#8217;s surface with negative photoresist (NR9-1500PY). Program a speed of 500 rpm with an acceleration of 3,000 rpm/s for 5 sec and then a speed of 3,500 rpm with an acceleration of 3,000 rpm/s for 40 sec, to produce a layer of photoresist around 1.3 &#181;m. 
	CAUTION: Perform spin coating in a fume hood. Inhalation of photoresist fumes can cause headaches.&#160; 
	NOTE: The thickness may vary depending on the condition of the photoresist and the spin coater used, even with the same spin settings. The photoresist may be spun beyond the edge and onto the wafer&#8217;s obverse edge; this must be removed by using an acetone-doused swab. Left present, the photoresist will stick the wafer to the hotplate during the soft bake.</li>
	<li>To soft bake, place the wafer onto a hotplate at 100 &#176;C, ramp the temperature up to 150 &#176;C, hold it at 150 &#176;C for 1 min. Then move the wafer from the hotplate, and let the wafer cool in the air to room temperature (RT). 
	NOTE: Due to the pyroelectric effect, if the temperature of the LN wafer is suddenly changed, for example, by directly transferring the LN wafer onto the hotplate or Al foil at 150 &#176;C, the thermal shock within the wafer will likely shatter it. The presence of nonuniform metal on the surface, such as electrodes, significantly enhances this risk. In applications where the transparency of the LN is not important, consider using so-called &#8220;black&#8221; LN or more accurately reduced LN, which is dark brown and translucent but has negligible pyroelectricity.</li>
	<li>Transfer the wafer to the mask aligner (MLA150) for ultraviolet exposure. Expose the photoresist with an energy dose of 400 mJ/cm2 at 375 nm. The dose required may vary depending on the mask design and the age and condition of the photoresist.&#160; 
	NOTE: The wave propagation direction induced by IDTs should be along the X-propagating direction in order to effectively generate SAW. In other words, this implies the &#8220;fingers&#8221; of the IDT should be perpendicular to the X-axis direction. Typical LN wafer manufacturers place the primary (larger) wafer flat (straight edge alongside of wafer) perpendicular to the X-axis, so your IDT fingers should be parallel to this flat. Some manufacturers introduce a second (smaller) wafer flat to help indicate the Y- and Z-axis directions, but this detail is unimportant for SAW generation. Manufacturers often request specifications for the surface finish of the wafer; if you require a transparent wafer, request double-sided optically polished wafers. However, keep in mind that LN is birefringent, so any object illuminated with standard laboratory light and seen through the material will produce not one but two images. Overcoming this problem is discussed later. Single-side polished LN is a better choice for SAW generation if you do not need to see through the wafer, because spurious acoustic waves are diffused by the rough back surface.</li>
	<li>Place the wafer onto a hotplate at 100 &#176;C for 3 min for a post-exposure bake. Then transfer it onto Al foil and allow it to cool to RT. 
	NOTE:&#160;The patterns should be visible after the post-exposure bake. If not, consider stripping the photoresist and restarting the process over from step 1.1 above.</li>
	<li>Develop the wafer by placing it in a beaker filled with pure RD6 developer for 15 sec. Gently shake the beaker during development. Immerse the wafer into DI water for 1 min, and then rinse the wafer under DI water flow. Finally, use dry N2 flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface.&#160; 
	CAUTION: Develop the wafer in a fume hood. Avoid breathing in vapors or contacting the developer with eyes and skin. 
	NOTE: The photolithography is complete after this step. The protocol can be paused here.</li>
	<li>Hard bake the wafer on a hotplate at 100 &#176;C for 3 min. Then transfer it onto Al foil and allow it to cool to RT.&#160; 
	NOTE: This step is to remove any moisture from the wafer and photoresist to prevent later outgassing during sputtering.</li>
	<li>For electrode sputter deposition, place the wafer into a sputter deposition system. Vacuum the chamber to 5 x 10-6 mTorr. Use a 2.5 mTorr argon flow, sputter chromium (Cr) with a power of 200 W for 5 nm as an adhesion layer, followed by sputtering Al with a power of 300 W for 400 nm to form the conductive electrodes. 
	NOTE: Deposition time should be calculated from the expected thickness and the deposition rate. Titanium (Ti) can be used instead of chromium, though the removal process is more difficult, because Ti is tougher. Gold (Au) is also commonly deposited as electrodes. However, for higher frequency SAW devices, Al should replace Au to avoid the mass loading effects of the Au IDT fingers, which reduce the local SAW resonant frequency under the IDT, forming an acoustic cavity from which the SAW can only escape with significant loss.</li>
	<li>For the lift-off process, transfer the wafer into a beaker and immerse in acetone. Sonicate at medium intensity for 5 min. Rinse with DI water and dry the wafer with N2 flow. 
	CAUTION: Use acetone in a fume hood. Avoid inhalation and skin or eye contact with acetone. Do not swallow. 
	NOTE: The protocol can be paused here.</li>
	<li>Use a dicing saw to dice the entire wafer into small pieces of chips as SAW devices for further applications.&#160; 
	NOTE: The process is complete. The protocol can be paused here.&#160; 
	NOTE: Instead of a saw, a diamond-tipped wafer scribe (or even a glass cutter) can be used to dice the LN wafer with some practice, though due to the anisotropy of LN it is important to scribe and break the wafer first along scribe lines perpendicular to the X-axis, followed by those lines along the X-axis.</li>
</ol>
2. SAW device fabrication via the wet etching method
<ol>
	<li>Wafer solvent cleaning: In a Class 100 clean room facility by immersing the 4&#8221; (101.6 mm) LN wafer in acetone, followed by IPA, then DI water, each in a sonication bath for 5 min. Pick up the wafer and dry the surface using N2 to remove the remaining DI water from the wafer.&#160; 
	CAUTION: Use acetone and IPA in a fume hood. Avoid inhalation and skin contact with IPA. Avoid acetone contact with skin and eyes. Do not swallow.</li>
	<li>Place the wafer onto a hotplate at 100 &#176;C for thermal treatment for 3 min. Then transfer it onto Al foil to cool down to RT.</li>
	<li>Place the wafer into a sputter deposition system. Vacuum the chamber to 5 x 10-6 mTorr. Use argon flow at 2.5 mTorr, sputter Cr with a power of 200 W for 5 nm as an adhesion layer, followed by sputtering Au with a power of 300 W for 400 nm to form the conductive electrodes. 
	NOTE: The protocol can be paused here.</li>
	<li>Place the wafer onto a spin coater. Using a dropper, cover about 75% of the wafer&#8217;s surface with positive photoresist (AZ1512). Program a speed of 500 rpm with an acceleration of 3,000 rpm/s for 10 sec and then a speed of 4,000 rpm with an acceleration of 3,000 rpm/s for 30 sec, ultimately producing a layer of photoresist around 1.2 &#181;m.&#160; 
	CAUTION: Perform spin coating in a fume hood. Inhalation of photoresist fumes can cause headaches.</li>
	<li>To soft bake, place the wafer onto a hotplate at 100 &#176;C for 1 min. Then transfer it onto Al foil and allow it to cool to RT.</li>
	<li>Transfer the wafer to the mask aligner (MLA150) for ultraviolet exposure. Expose the photoresist with an energy dose of 150 mJ/cm2 at 375 nm. The dose required may vary depending on the mask design and the age and condition of the photoresist.</li>
	<li>Place the wafer into a beaker filled with pure AZ300MIF developer for 30 sec. Gently shake the beaker during development. Immerse the wafer into DI water for 1 min, then rinse the wafer under DI water flow. Finally, use dry N2 flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface. 
	CAUTION: Avoid contacting AZ300MIF with skin or eyes. Do not swallow.</li>
	<li>Immerse the wafer into a beaker filled with Au etchant for 90 sec, gently shaking the beaker. After rinse the wafer under DI water flow, dry with N2 flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface. 
	CAUTION: Gold etchant can be hazardous to the eyes and skin, and will cause respiratory irritation. This step requires more personal protective equipment (PPE), such as safety glass, black neoprene gloves, apron, etc.</li>
	<li>Immerse the wafer into a beaker filled with Cr etchant for 20 sec, gently shaking the beaker. After rinse the wafer under DI water flow, dry with N2 flow to remove the remaining DI water from the wafer. Never let any fluid evaporate on the wafer surface. 
	CAUTION: Chromium etchant can cause eye, skin, and respiratory irritation. This step also requires more PPE.</li>
	<li>Clean the (sample) wafer, by putting it into acetone, followed by IPA, and DI water in a sonication bath for 5 min each. Pick up the wafer and dry with N2 gas flow over the surface of the wafer to remove the remaining DI water from the wafer. 
	CAUTION: Use acetone in a fume hood. Avoid inhalation and skin contact acetone with skin and eyes. Do not swallow. 
	NOTE: This step is to remove the undesired photoresist on the wafer. The protocol can be paused here.</li>
	<li>Use a dicing saw to dice the entire wafer into discrete SAW devices for further use.&#160; 
	NOTE: The process is complete. The protocol can be paused here.</li>
</ol>
3. Experimental setup and testing
<ol>
	<li>Observe the SAW device under bright-field optical microscopy. 
	NOTE: There are possibly scratches across the metal layers on the LN. Generally they will not cause a notable influence of the device performance, as long as the scratches are not deep enough to result in an open circuit.</li>
	<li>For SAW actuation, attach absorbers at both ends along the propagation direction of the SAW device to prevent reflected acoustic waves from the edges.&#160;</li>
	<li>Use a signal generator to apply a sinusoidal electric field to the IDT at its resonant frequency of around 100 MHz. An amplifier should be connected to amplify the signal.&#160;</li>
	<li>Use an oscilloscope to measure the actual voltage, current&#160;and power applied onto the device. The amplitude and frequency response of the SAW are measured by a laser Doppler vibrometer (LDV); the SAW-actuated droplet motion is recorded using a high-speed camera attached to the microscope.</li>
</ol>

<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>

The authors have nothing to disclose.

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,...

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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 ...

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11.7K Views.  University of California San Diego. 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. 

Video: Fabrication of Surface Acoustic Wave Devices on Lithium Niobate

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 ...

Methods Article

Fabrication of Surface Acoustic Wave Devices on Lithium Niobate