The overall goal of this procedure is to fabricate high aspect ratio, nano gap electrodes using nanos skiving. This is accomplished by first depositing a thin gold film on a silicon wafer and transferring it to an epoxy substrate. The second step is to form a self-assembled monolayer of Alcan thiols on the metal film.
Next, a second film of gold is deposited. That is offset with respect to the first. The final step is to section thin slices of the structure with an ultra microtome.
The main advantage of this technique over existing methods like photo or mili photographies, is that thousands of nano gap electrodes can be fabricated with submeter resolution on demand and positioned on any arbitrary substrate without the use of a clean room or any conventional photolitographic processes. The shape and aspect ratio of the nano gaps allows them to be directly addressed in a probation without any intermediate fabrication steps, such as defining contact paths. To start the fabrication of a structure, begin with a technical grade three inch silicon wafer.
Treat it in an air plasma cleaner for 30 seconds. Next, expose it to perfluorinated sine vapor for one hour after the exposure, use a Teflon mask appropriate to the desired length of wire to evaporate a layer of gold onto the pretreated wafer. The thickness of the evaporated layer a hundred nanometers in this experiment defines the width of the wire with the evaporation complete, cover the entire wafer with approximately 8.5 milliliters of epoxy pre polymer.
Place the wafer and epoxy in an oven at 60 degrees Celsius to cure for three hours after curing, and with the sample at room temperature, insert the edge of a razor blade at the interface between the silicon wafer and epoxy. Gently peel the epoxy layer from the silicon wafer so that the gold remains attached to the epoxy. Be careful not to break the silicon wafer to achieve gaps below five nanometers.
Choose an appropriate Alcan diol for the desired gap width. Prepare a one millimolar solution of the Alcan YL in ethanol. Immerse the gold on epoxy in this self assembling monolayer solution.
Place the container in a closed chamber that is purged with nitrogen. Leave it to sit overnight. After at least eight hours, retrieve the container from the closed chamber and remove the golden epoxy substrate from the self assembling monolayer solution.
Rinse it with ethanol and dry it with nitrogen. Then dry it at 60 degrees Celsius for two minutes. Longer drying times may damage the Sam layer.
After drying, place the Teflon mask back onto the epoxy substrate with a lateral offset of about 80%of the shorter macroscopic dimension of the gold features evaporate a second layer of gold through the mask in this experiment. The second layer thickness is the same as the first layer, 100 nanometers. Once the evaporation is complete, remove the Teflon mask.
Taking care not to scratch the features. Reed the entire substrate in 8.5 milliliters of epoxy pre polymer. Place it in a 60 degree Celsius oven to cure for three hours.
Once the sample is cured and cooled, use a jeweler saw to cut out the features in four millimeter by 10 millimeter pieces. Place each in a separate well in a polyethylene coffin microtome mold. Next, fill the mold with epoxy pre polymer.
Finally, place it in a 60 degree Celsius oven to cure overnight. To section a sample, remove a block from the polyethylene mold mount the sample in the sample holder. Attach the sample holder to the trimming attachment and mount it in the ultra microtome.
Next, clean a razor blade with ethanol. Inspect the blade under a stereoscope to ensure there are no metal fragments present. Use the razor to trim the block to the width of the diamond knife.
In a trapezoid shape, equip the ultra microtome with a glass knife. The knife should be aligned parallel to the bottom edge of the block face. Start pre-cutting to define a smooth surface on the top face of the block.
Once done to fabricate a metallic structure, replace the glass knife with a diamond knife. Again, align the diamond knife so that it is parallel to the bottom face of the block. In this video, the block is sectioned to 100 nanometers at one millimeter per second.
The thickness of these sections can be verified using color reference charts. If electrical measurements are to be made, place a silicon dioxide substrate under the water in the knife reservoir. Raise the substrate slowly to collect the epoxy sections containing the structures from the surface of the water.
Dry the sections at 60 degrees Celsius for three hours to improve their adhesion to the substrate. Electrical measurements begin with mounting the cleaned and dried epoxy sections under a light microscope, the embedded metallic structures will be visible as either a black line or directly visible. In the case of the thicker gold structures, contacts are made by applying drops of silver paste on the two ends of wires.
In each section shown here are scanning electron micrographs of the gaps of three different nano gap structures, each prepared with a different ol. These images were taken after ashing. The organics with oxygen plasma from top to bottom are the gaps produced using one 12 do DECANE OL one 14 tetra decane OL and one 16 HEXA decane OL respectively.
The gaps are qualitatively larger as the length of the molecules increases. All of the gaps are below the four nanometer resolution limit of the instrument. Quantitative support for increasing gap size is seen in the log current density versus voltage measurement for each of the structures in this plot, the black squares represent data for the one 12 do DECANE HY devices.
The red triangles are for the one 14 tetra decade HY devices, and the blue circles are for the one 16 HEXA decade AL devices. The inset shows the log of the current density at 500 millivolts versus the length of the AL molecule use to create the gap. The good linear fit supports the expectation of an exponential decrease in current with the increase of molecular length and the slope is commensurate with other measurements of tunneling through al Canes.
After watching this video, you should have a good understanding of how to fabricate nano gap electrodes with different gap sizes using nano skying.
