Pulse-Atomic Force Lithography: A Powerful Nanofabrication Technique to Fabricate Constant and Varying-Depth Nanostructures

Abstract

The widespread use of nanotechnology in different application fields, resulting in the integration of nanostructures in a plethora of devices, has addressed the research toward novel and easy-to-setup nanofabrication techniques to realize nanostructures with high spatial resolution and reproducibility. Owing to countless applications in molecular electronics, data storage, nanoelectromechanical, and systems for the Internet of Things, in recent decades, the scientific community has focused on developing methods suitable for nanopattern polymers. To this purpose, Atomic Force Microscopy-based nanolithographic techniques are effective methods that are relatively less complex and inexpensive than equally resolute and accurate techniques, such as Electron Beam lithography and Focused Ion Beam lithography. In this work, we propose an evolution of nanoindentation, named Pulse-Atomic Force Microscopy, to obtain continuous structures with a controlled depth profile, either constant or variable, on a polymer layer. Due to the modulation of the characteristics of voltage pulses fed to the AFM piezo-scanner and distance between nanoindentations, it was possible to indent sample surface with high spatial control and fabricate highly resolved 2.5D nanogrooves. That is the real strength of the proposed technique, as no other technique can achieve similar results in tailor-made graded nanogrooves without the need for additional manufacturing steps.

Investigation of the Effects of Pulse-Atomic Force Nanolithography Parameters on 2.5D Nanostructures’ Morphology

“…The technique used for our nanolithography experiments, Pulse-AFL, was first presented in our previous papers [48,49]. Therefore, it is worth briefly remarking on its fundamental aspects to understand the experiments reported in the present work and point out the effective capabilities of the technique.…”

“…The AFM topography images of the channels sculpted with increasing steps are presented in Figure 3a-c. At first glance, for step up to 50 nm, the lines appear continuous and contoured by asymmetrical pileups [48,49]; whereas for higher step values they start to be dotted, and, in correspondence to the highest steps (from 125 nm to 200 nm), single indentations can be well-recognized. In this case, every single hole (nanoindentation) is surrounded by a PMMA ring bulge.…”

“…As described above, the P-AFL technique is based on nanoindentation; therefore, to obtain nanogrooves with continuous profile, the distance between successive indentations, defined as step, is a critical P-AFL parameter. As long as the step was kept as low as possible (10 nm), we were able to realize nanochannels with continuous and homogeneous profiles, and low roughness [48]. Nevertheless, with steps so short, the tip must perform a high number of indentations for patterning a groove with a certain length, which entails a significant increase in the nanolithography time.…”

“…Moreover, more sophisticated types of equipment are required and only specific materials can be targeted [47]. In order to overcome the drawbacks of the conventional m-SPL techniques, we have already proposed a new AFMbased nanolithography technique, termed Pulse-Atomic Force Lithography (P-AFL) [48], as an evolution of AFM nanoindentation.…”

“…In our previous paper [48], we demonstrated the effectiveness of P-AFL in the fabrication of nanochannel with a continuously varying depth profile on a thin polymer layer. The obtained results suggest the tremendous potential of P-AFL in the building of 2.5D structures.…”