This method can help answer key questions in the field of 2D material device fabrication, related to techniques to precisely locate 2D material samples in the preparation for later fabrication steps. The main advantage of this technique is that this is tailored to the development of small-scale devices, for which finding the location of materials is more challenging. Demonstrating the procedure will be Po-Chun Chen and Kristan, research assistants and graduate students from my laboratory.
The fabrication process requires two prepared substrates. The first is back-gated silicon dioxide on silicon with titanium and gold metal pad arrays. The second substrate is sapphire with a deposited layer of molybdenum disulfide.
Take the sapphire substrate with the molybdenum disulfide to a spin coater. Spin coat PMMA to cover the top of the molybdenum disulfide at 3, 500 rpm for 30 seconds. Then, move the sample to a hot plate and bake it at 120 degrees Celsius for three minutes to strengthen the PMMA coating.
Next, prepare 50 milliliters of ammonia solution. Immerse the sample to separate the molybdenum disulfide from the substrate. Once the film has separated, remove it from the ammonia solution.
Transfer the molybdenum disulfide film to the silicon dioxide on the silicon substrate. To enhance adhesion, bake the sample at 120 degrees Celsius for at least 30 minutes. Recover the sample and place it in 30 milliliters of acetone.
After about 30 minutes, the PMMA will be removed as indicated by a change in color. Before proceeding, rinse the sample in isopropyl alcohol and use nitrogen to blow it dry. Now, prepare to perform electron-beam lithography.
Use an optical microscope to measure the displacement between the target locations and the alignment marks on the sample. Based on these measurements, design the metal electrode pattern layout using software. Spin coat Photo Resist on top of the sample and ensure it covers the entire sample.
Move on to soft-bake the sample at 100 degrees Celsius for 90 seconds to enhance the adhesion. At the electron-beam lithography machine, upload the design and position the sample. The alignment marks in the silicon-silicon dioxide substrate should match the corresponding marks in the design.
Expose the sample to the electron beam. When done, take the sample to a hot plate. Heat the sample to 120 degrees Celsius for 90 seconds in a post-exposure bake.
Next, have a container of TMAH ready as a developer and immerse the sample for 80 seconds. Then, wash the sample in 200 milliliters of deionized water for 10 seconds. Examine the sample with an optical microscope to determine if the pattern is well developed.
If it is well developed, hard-bake the sample at 110 degrees Celsius for 90 seconds. The next step is to use an electron gun evaporator to deposit 100 nanometers of gold on the sample. After deposition, work to remove the Photo Resist.
To dissolve the Photo Resist, prepare 100 milliliters of acetone. Immerse the sample in the acetone to perform lift-off. Monitor the process with an optical microscope and stop when only metal lines and pads remain.
During characterization, choose a source and a drain electrode from those in the device, then use an anatomic force microscope to apply a load to the sample. Here Xs indicate where loads have been applied. The atomic force microscope loads results in a compressive strain on the device.
Here are the current voltage characteristics of the molybdenum disulfide device at different applied forces producing compressive strain. At a given voltage, the device's current decreases with an increase in applied force and vice versa, indicating a change in the resistance of the device, a behavior that is expected for a piezo sensor. These data are for the current response of the molybdenum disulfide device for repeated compressive strains at a fixed-bias voltage of one volt.
The output current barely changes with the repeated application of 10 nanonewtons of applied force, suggesting that the sensor is stable. Once mastered and performed properly, this technique can be done in 20 straight hours, including the fabrication of all transistors. After its development, this technique can serve as the platform for future nanodevice developments as it paves the way towards the production of future advanced nanoscale devices.
After watching this video, you should have a good understanding of how to reliably fabricate 2D back-gated transistors using standard fabrication processes, including electron beam lithography and metal electrodeposition. Though this method caters to the development of 2D nanomaterial devices, it can also be applied to 1D materials. Don't forget that working with TMAH, ammonia solution, PMMA, and other photo resistors can be extremely dangerous and personal protection equipment should always be worn while performing this procedure.
