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