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. 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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><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&amp;ndash;1&amp;ndash;2W&amp;ndash;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&amp;rdquo; double-side polished 0.5 mm thick 128&amp;deg;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&quot;</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 ...

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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 of Silica Ultra High Quality Factor Microresonators

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q&gt;100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
Introduction
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (&tau;o) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/&tau;o). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q&gt;100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.

We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the ...

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 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 and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

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 ...

Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared on transparent membranes which are very fragile and difficult to manipulate. We present processes for the fabrication of samples with magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for Lorentz transmission electron microscopy and magnetic transmission x-ray microscopy studies. The samples are prepared on silicon nitride membranes and the fabrication consists of a spin coating, UV and electron-beam lithography, the chemical development of the resist, and the evaporation of the magnetic material followed by a lift-off process forming the final magnetic structures. The samples for the Lorentz transmission electron microscopy consist of magnetic nanodiscs prepared in a single lithography step. The samples for the magnetic x-ray transmission microscopy are used for time-resolved magnetization dynamic experiments, and magnetic nanodiscs are placed on a waveguide which is used for the generation of repeatable magnetic field pulses by passing an electric current through the waveguide. The waveguide is created in an extra lithography step.

A protocol for the fabrication of magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for transmission electron microscopy (TEM) and magnetic transmission x-ray microscopy (MTXM) studies is presented.

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared ...

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 ...

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 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 ...

Fabrication and Characterization of Thickness Mode Piezoelectric Devices for Atomization and Acoustofluidics

We present a technique to fabricate simple thickness mode piezoelectric devices using lithium niobate (LN). Such devices have been shown to atomize liquid more efficiently, in terms of &#64258;ow rate per power input, than those that rely on Rayleigh waves and other modes of vibration in LN or lead zirconate titanate (PZT). The complete device is composed of a transducer, a transducer holder, and a &#64258;uid supply system. The fundamentals of acoustic liquid atomization are not well known, so techniques to characterize the devices and to study the phenomena are also described. Laser Doppler vibrometry (LDV) provides vibration information essential in comparing acoustic transducers and, in this case, indicates whether a device will perform well in thickness vibration. It can also be used to &#64257;nd the resonance frequency of the device, though this information is obtained more quickly via impedance analysis. Continuous &#64258;uid atomization, as an example application, requires careful &#64258;uid flow control, and we present such a method with high-speed imaging and droplet size distribution measurements via laser scattering.

Fabrication of piezoelectric thickness mode transducers via direct current sputtering of plate electrodes on lithium niobate is described. Additionally, reliable operation is achieved with a transducer holder and &#64258;uid supply system and characterization is demonstrated via impedance analysis, laser doppler vibrometry, high-speed imaging, and droplet size distribution using laser scattering.

We present a technique to fabricate simple thickness mode piezoelectric devices using lithium niobate (LN). Such devices have been shown to atomize liquid ...

Fabrication of Surface Acoustic Wave Devices on Lithium Niobate

Summary

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Chapters in this video

Fabrication of Silica Ultra High Quality Factor Microresonators

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

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

Fabrication and Characterization of Superconducting Resonators

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

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

Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

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

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

Fabrication and Characterization of Thickness Mode Piezoelectric Devices for Atomization and Acoustofluidics