The overall goal of this procedure is to allow for the examination and characterization of the contact substrate and contact nanowire interfaces for single gallium nitride nanowire devices. This is accomplished by first preparing a nanowire suspension and dispersing it onto a substrate. The second step is to deposit the nickel gold contacts on the nanowires and then a kneel the sample.
The final step is to remove the anal to nickel gold contacts from the substrate by adhering them to carbon tape. Ultimately, a scanning electron microscope is used to examine the contact interfaces for void formation. This method can help answer questions about how to process semiconductor nanowires.
In particular, it gives us a way to assess how well the metal contacts are sticking to the nano wires into the surrounding substrate. Visual demonstration of this method is important because the nanowire dispersal and film transfer techniques can be difficult to learn as the small details of handling and placement can have a large effect on the yield. I was looking at the sample with an SEM when I noticed an area of the metal contact had start lifting up away from the substrate.
When I looked at it closer, I noticed the underside of the contact had this very unusual morphology. So in order to get a better look at that, the underside morphology, I developed this technique I To begin cleave a small piece of the as grown gallium nitride nanowires on a 1 1 1 silicon substrate and place it into a small vial with one milliliter of isopropanol. Sonicate the vial for 30 seconds in order to remove the nano wires from their anchoring substrate.
Then clean a silicon wafer by submersing it face down in acetone using a tripod holder. After five minutes, remove the wafer from the acetone and rinse off both sides of the wafer using a squirt bottle of isopropanol. Next, submerge the wafer into a beaker with 100 milliliters of isopropanol.
After five minutes, remove the wafer and rinse it with deionized water before placing it into yet another beaker with deionized water. After five minutes in the water, remove the wafer and dry it with a stream of nitrogen. Again, sonicate the vial of nano wires for 30 seconds.
Then pipette 30 microliters of the solution onto each of the three regions on the oxidized surface of a clean quarter wafer, make sure that the wafer is level so that the nanowire suspension does not migrate away to the edges. After the final drop of the nano wire suspension has evaporated. Place the sample in successive baths of acetone and isopropanol to remove unwanted impurities.
Then rinse the quarter wafer in deionized water and dry with nitrogen. Do not use squirt bottles or sonicate the solvent during the cleaning process. To avoid excess removal of nanowires from the surface, add a layer of LOR five A liftoff.
Resist onto the silicon wafer with the dispersed nanowires by adding one to two milliliters of the resist on top of the nanowires spin coat the sample at a rate of 300 RPM for 10 seconds, followed by 2000 RPM for 45 seconds. After spinning, remove LOR five A remnants from the edges of the sample by swabbing it with micro chem nano EBR pg. Then transfer the sample onto a hot plate and bake it 150 to 170 degrees Celsius.
After five minutes, remove the sample from the hot plate and allow it to cool for 30 seconds. Once cooled, place the sample back into the spin coder and add one to two milliliters of mega deposit SPR 6 61 0.0. Photo resist to cover the surface spin, coat the sample for 35 seconds at 3000 RPM.
Then transfer the sample onto a hot plate and bake it for one minute at 95 degrees Celsius. Next, load the mask and then the sample into the mask aligner. Expose the sample with a dose of 240 millijoules per square centimeter.
Then develop the photo resist by swirling the sample in a beaker of micro posit MF 26, a developer for about 60 seconds or until fully developed. Once developed, rinse the sample in deionized water and then dry it with a stream of nitrogen. Load the samples into a UV ozone generator and expose them to ozone for 10 minutes while flowing an ultra high purity oxygen at a flow rate of 80 standard cubic centimeters per minute through the device.
After OZ zone treatment, place the samples in a 10%hydrochloric acid mixture for one minute at room temperature. Then remove the samples and rinse them with deionized water and dry them with a stream of nitrogen. Mount the dry samples to the electron beam evaporator by securing it to the platin using screws and clips.
Then attach the platin into the e-beam evaporator and load the nickel and gold crucibles into the device. Pump down the chamber until the pressure is below one micro tour. Depending on pump strength, this may take a few hours to overnight.
Then set the high voltage to 10 kilovolts, ensure the shutter is closed and begin to rotate samples at five RPM. Next, select the nickel crucible and set the deposition rate to 0.1 nanometers per second and deposit 500 nanometers of nickel onto the samples. When finished, allow the source to cool down for 15 minutes and then switch to the gold crucible.
Set the gold deposition rate at 0.1 nanometers per second and deposit 100 nanometers onto the samples. Once deposition is finished, wait 10 minutes for the gold crucible to cool down, and then vent the chamber and unload the sample Platin. Remove the samples from the PLA and place them into a bath of photo reist.
Stripper at room temperature for a few hours to lift off the metal deposited on the photo, resist if desired, elevate the bath temperature to around 50 to 60 degrees Celsius to accelerate. Lift off after a few hours. Check to see if the metal separated from the photo resist.
Use a pipette or a squirt bottle of the photo. Resist stripper to forcefully remove any remaining undesired metal from the surface. Then rinse the entire sample with isopropanol, followed by deionized water, and then dry the samples with nitrogen following removal with at least 24 hours before annealing the samples.
Then purge the rapid thermal annealer with three parts nitrogen to one part oxygen at a flow rate of 1.4 liters per minute at 650 degrees Celsius for five minutes without any samples in it. Next, reduce the temperature to 550 degrees Celsius. Load the samples and anal them for 10 minutes.
Begin by carefully placing a piece of carbon tape on an SEM pin stub mount so that the tape is as smooth as possible. Before removing the backing of the tape, use a finger to press hard on the tape so that it is firmly adhering to the mount. Take the sample and gently lay the area of interest directly onto the carbon tape along the edge of the mount, such that the area of the sample that was used for handling with tweezers is hanging off the edge.
Once sample has been placed onto the tape, do not remove it for repositioning. In order to avoid destroying the film, press firmly down onto the back of the sample using forceps. Take care not to press too hard around the mount edges because this can cause the sample to break.
Then take a clean razor blade or scalpel and gently nudge it in between the substrate and the tape along the edges of the sample until the substrate can easily be peeled off by grabbing it with tweezers. Using too much force can break the sample to keep the tape from coming off with the substrate. Place a pair of tweezers onto the area of the tape that is adjacent to the substrate.
When prying it off, the ailed nickel gold film should remain adhered to the tape image, the newly removed nickel gold film using SEM as as possible so as to observe the newly exposed interface of the film before it becomes contaminated. Shown here are SEM images of the Al Nickel gold film before and after being removed from the silicon dioxide substrate using carbon tape. The sample on the right received no cleaning prior to the metal deposition and the residual contamination produced non-uniform void distribution, and large macro voids that resemble blisters.
One important application of this technique is analyzing the contact nano wire interface morphology as CM images of the underside of a kneeled nickel gold films that had been deposited onto nano wires dispersed onto silicon dioxide. Silicon substrates are shown here. High magnification images like those seen on the right hand side allows for the examination of the contact nano nanowire interface.
The SCM image on the bottom right is missing the nanowire, allowing for the interface between the contact and the missing wire to be more easily studied. The presence of this residual contamination can cause a significant increase in the number of voids observed at the contact substrate interface. By quantifying the degree of void formation at the contact substrate interface, the effectiveness of different cleaning methods can be evaluated.
These experiments focused on the effectiveness of the various cleaning methods prior to deposition of the nickel gold onto the silicon dioxide for the removal of residual contamination. This points out the obvious advantage of using UVO Zone and HCL cleaning prior to the deposition of the metals. After watching this video, you should have a good idea of how to disperse nano wires onto a new substrate deposit, into kneel metal contacts to the nanowires, and remove the metal contacts to examine the nanowire contact interface Using this procedure.
Additional techniques like energy dispersive x-ray spectroscopy can be done in order to answer additional questions like layer composition.
