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