Nanofabrication (nanofab) is the very important procedure in the fabrication technique of quantum materials, in which raw quantum materials (heterostructures or others) are integrated into measurable quantum devices. It involves several techniques that tailored to different experimental characterizations (such as the nanofab techniques of electronic & magneto transport characterization are much complicated and mature among others). This article mainly presents some universal and important nanofab techniques among most 2D quantum materials experiments.
Lithography
The first phase of nanofabrication is often a certain type of nano-lithography. (Nano-) lithography is the technique of creating fine patterns onto a substrate. There are two main lithography techniques: photolithography and electron beam lithography (EBL). Here we will focus on EBL.
EBL, often known as e-beam lithography, is a method that uses a focused electron beam to create patterns on a substrate. An electron-sensitive layer, also known as a resist, is applied to the substrate’s surface. It doesn’t deposit the material on the surface; rather, it alters the solubility of the resist that is already there. The solubility of the resist varies when it is subjected to an electron beam, allowing it to be selectively removed. (copied from Photolithography vs. Electron Beam Lithography – TheOmniBuzz)
The electron-sensitive layer can be divided into positive resist and negative resist. The solubility of the positive resist increases when exposed by an electron beam, so the exposed part can be selectively removed by dissolving into a certain solution, called developing solution. On the contrast, the solubility of the negative resist when exposed by an electron beam decreases thus it will be remained but other unexposed parts will be removed during the developing process. A common positive resist we choose is the polymethyl methacrylate (PMMA). For a typical sample that needs to do EBL, we first spin coat a thick PMMA layer onto the Si wafer. The process involves two coatings, after the first coating with spin parameter 50r for 10s + 250r for 50s, we bake the sample at 120℃ for 3 min on a hotplate, and do the second coating with the same implementation (spin and bake). Through this way, a thick and uniform PMMA layer can be coated on the Si wafer.
The second step is doing EBL. EBL is implemented by a scanning electron microscope (SEM), a microscope that uses an electron gun to see the sample. A high speed electron beam is ejected from the electron gun, shaped and focused by an electron column, bombarded on the sample, and scattered back. By decoding the scattered electrons that received back to the microscope, one can see the surface sample with nm level resolution.
Here, the electron beam can also be used to do the lithography on the resist. After we put the sample with PMMA resist on the loader inside the SEM, we first use the Au particles to adjust the astigamatism and focusing of the microscope. These small Au nano-particles are scattered on the sample loader, and they serve as a reference. By adjusting the system to get a clear and not distorted image of the Au particles, we reach a good astigamatism and focusing level. The next step is just find the target region, write the designed pattern and do the e-beam exposure. The exposure resolution (minimum feature) of EBL can be down to 6nm, at least two orders finer compared to photolithography. Since EBL uses electrons not photons to do the lithography, it does not suffer the diffraction limit of optical lithography like photolithography. It does suffer from the uncertainty of the electron scattering process and various aberrations in the electron optics.
The third step is developing. After the exposure is done, we soak the sample in the developing solution. Developing is the process to dissolve the solutable part of the resist i.e. the exposed part of PMMA here. The developing process should be controlled around 30s and no more than 40s to prevent over-developing, which will make some unexposed parts dissolved. After developing is the fixing process, where the developed sample is again soaked in the fixing solution. Fixing is the process to dissolve the remaining developing solution on the sample and is often processed around 50s. After developing-fixing, the EBL process is completed.
Etching
The second phase of nanofabrication is etching. After we use the electron gun to draw the nanostructure outlines on the sample, it is still just outlines formed by PMMA and can be washed away. Etching is the process to actually shape the actual heterostructure according to the EBL drawing. For a usual hBN-graphite-based device of the structure graphite back gate-hBN dielectric-graphene active layer-hBN dielectric-graphite top gate-top hBN,
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