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. 2022 May 18;8(5):e09475.
doi: 10.1016/j.heliyon.2022.e09475. eCollection 2022 May.

The effect of electron dose on positive polymethyl methacrylate resist for nanolithography of gold bowtie nanoantennas

Caroline Campbell  1 , Abigail Casey  2 , Gregory Triplett  3
Affiliations

The effect of electron dose on positive polymethyl methacrylate resist for nanolithography of gold bowtie nanoantennas

Caroline Campbell et al. Heliyon. .

Abstract

Plasmonic structures, such as bowtie nanoantennas, may be used in Surface Enhanced Raman Spectroscopy (SERS). Nanoantennas can be employed to amplify the biomolecular and chemical reactions, which is useful for biomedical applications. The electric field created by nanoantennas are optimized when the resonant wavelength of the probed laser light closely matches the resonant wavelength of the plasmonic structure. In this work, we fabricated several bowtie nanoantennas with varying geometric spacing for use with a 532 nm wavelength laser line in Raman Spectroscopy. The fabrication utilized nanolithography by electron beam lithography on a Raith Voyager, development, deposition, and metal lift-off. This study explored a specific bowtie nanoantenna geometry of 270 nm equilateral sides triangle pairs with 3 varying gap sizes, 50 nm, 20 nm, and 10 nm, and the effect of varying electron beam doses on the final structure of the nanoantenna. The results presented here, will show that the working dose factor range is 6.5-10.3 (650-10,300 μC/cm2) for 120 nm thick polymethyl methacrylate (PMMA), and with a 44.78% increase in dose, the footprint area increases between 5.9% and 10.7%.

Keywords: Bowtie nanoantennas; Dose; Electron beam lithography; Exposure; Fabrication; Monte Carlo simulation; Nanopatterning; Polymethyl methacrylate (PMMA); Surface Enhanced Raman Spectroscopy (SERS).

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
A drawing detailing the (a) side length, (b) tip-to-tip gap, and (c) total width for the bowtie nanoantennas.
Figure 2
Figure 2
A cross section of PMMA on the ITO/SiO2 substrate with the (A) active exposed area during exposure, but before development and (B) after exposure, after development.
Figure 3
Figure 3
The Dose factor Map is an illustration showing twenty dose factors, ranging from 6.5 to 10.3, for three separate gap sizes of 50, 20, and 10 nm in the A., B. and C. schematic, respectively. The (a,b) axis belongs to each dose array and is denoted with a number within the 6.5–10.3 range, where there are 7,000 bowtie nanoantennas internally, and is approximately 150 μm by 200 μm. The (i,j) axis belongs to each geometrical spacing, illustrated as either green (A.), blue (B.), or purple (C.), and each geometrical spacing is approximately 1,100 μm by 1,600 μm (1.1 mm, 1.6mm). The (x,y) axis belongs to the entire pattern. The entire pattern is approximately 5.8 mm by 1.6 mm.
Figure 4
Figure 4
A simplified cross section of deposition with (a) ideal exposure and (b) exposure impacted by severe electron backscattering.
Figure 5
Figure 5
Images taken from CASINO showing the interaction of the 6.7 (821), 7.7 (944), 8.7 (1066), and 9.7 (1189) Dose factor (electrons) interacting with PMMA, ITO, and SiO2 with absorbed or transmitted (blue) and backscattered (red) electrons.
Figure 6
Figure 6
Images were taken with the Raith Voyager showing a 2 μm field of view (FOV) 6.7 (670), 7.7 (770), 8.7 (870), and 9.7 (970) Dose factor (μC/cm2) set of gold bowtie nanoantennas with a programmed geometry of 270 nm equilateral side length triangle and a 50 nm tip-to-tip gap on an ITO coated glass coverslip.
Figure 7
Figure 7
Images were taken with the Raith Voyager showing a 2 μm field of view (FOV) 6.7 (670), 7.7 (770), 8.7 (870), and 9.7 (970) Dose factor (μC/cm2) set of gold bowtie nanoantennas with a programmed geometry of 270 nm equilateral side length triangle and a 20 nm tip-to-tip gap on an ITO coated glass coverslip.
Figure 8
Figure 8
Images were taken with the Raith Voyager showing a 2 μm field of view (FOV) of 6.7 (670), 7.7 (770), 8.7 (870), and 9.7 (970) Dose factor (μC/cm2) set of gold bowtie nanoantennas with a programmed geometry of 270 nm equilateral side length triangle and a 10 nm tip-to-tip gap on an ITO coated glass coverslip.
Figure 9
Figure 9
Images were taken with the Raith Voyager showing a 10 μm field of view (FOV) of 6.7 (670), 7.7 (770), 8.7 (870), and 9.7 (970) Dose factor (μC/cm2) set of gold bowtie nanoantennas with a programmed geometry of 270 nm equilateral side length triangle and a 50 nm tip-to-tip gap on an ITO coated glass coverslip.
Figure 10
Figure 10
Images were taken with the Raith Voyager showing a 10 μm field of view (FOV) of 6.7 (670), 7.7 (770), 8.7 (870), and 9.7 (970) Dose factor (μC/cm2) set of gold bowtie nanoantennas with a programmed geometry of 270 nm equilateral side length triangle and a 20 nm tip-to-tip gap on an ITO coated glass coverslip.
Figure 11
Figure 11
Images were taken with the Raith Voyager showing a 10 μm field of view (FOV) of 6.7 (670), 7.7 (770), 8.7 (870), and 9.7 (970) Dose factor (μC/cm2) set of gold bowtie nanoantennas with a programmed geometry of 270 nm equilateral side length triangle and a 10 nm tip-to-tip gap on an ITO coated glass coverslip.
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