Height patterning of nanostructured surfaces with a focused helium ion beam: a precise and gentle non-sputtering method

Abstract

This work presents a new technique for surface patterning with focused ion beams. The technique is based on chemical decomposition in the bulk of a polymer substrate with negligible surface sputtering effects. By using a focused helium ion beam, generated in a helium ion microscope, we show that the surface height of polymethyl methacrylate substrates can be patterned with nanometer depth precision, while preserving the essential features of the nanostructures prefabricated on this surface. The key factors that control this patterning process are discussed.

Out-of-plane surface patterning by subsurface processing of polymer substrates with focused ion beams

“…The primary reason for this choice was the difference in their microstructures, specifically in the availability of structural defects capable of providing the release of gases from radiolysis. As it has been shown before [4], the as-deposited 15 nm Pt 60 Pd 40 thin films contain arrays of nanoscale cracks. In contrast, our studies have not revealed any cracks or other discontinuities in the as-deposited Au thin films.…”

“…The observed increase in the surface descending rates and in the saturation level upon irradiation with Ne + and Ga + ions ( Figure 2) indicate that, in those cases, both surface sputtering and subsurface volume shrinkage contribute to the changes in depth across the sample. According to our previous study [4], the reduction in surface height of the metal-coated PMMA surface is controlled by two major parameters. The first parameter is the irradiation fluence of He + ions, which determines the total amount of radiation energy dissipated by the ions over their entire path in the sample.…”

“…The dependence of the surface depression depth on the irradiation fluence for the 15 nm Pt 60 Pd 40 /PC sample is shown in Figure 6 (red circles) and compared to the fluence dependence for a 15 nm Pt 60 Pd 40 /770 nm PMMA sample (blue squares) obtained in our previous work [4]. In the latter, the 770 nm thick PMMA layer corresponds to a PMMA bulk substrate since the entire path of He + ions is located within this layer (see Figure S2, in the supplementary material of [4]). The curves in Figure 6 have similar shapes.…”

“…The ion beams deposit their energy and, therefore, affect the structure and properties of materials over the entire depth of their penetration path in a target. In our recent work [4], we demonstrated that, in addition to the direct surface patterning by the abovementioned techniques, the radiation damage generated by He + FIB in the bulk of poly(methyl methacrylate) (PMMA) substrates can be used for well-controlled and nanometer-precise patterning of the height of metal thin films and nanostructures prefabricated on the surface of these substrates. This technique is based on subsurface chemical decomposition, structural reconstruction, and, as a result of these processes, volume shrinkage of the PMMA polymer under ion irradiation [5][6][7].…”

“…To study the effects of the ion mass, ultrathin (5 nm thick) Pt 60 Pd 40 films were used in order to minimize energy losses of Ne + and Ga + ions in these films. In all other cases, such as patterned Pt 60 Pd 40 films on PC and PDMS substrates and patterned Au films on PMMA substrates, 15 nm metal thin films were used to facilitate the comparison with previously published results [4], in which 15 nm Pt 60 Pd 40 films were patterned. Also, for the same reason of a more direct comparison, 200 nm thick PMMA substrates were used in this work to study possible effects of changing ion masses.…”