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Aberration-Corrected Electron Beam Lithography at the One Nanometer Length Scale Vitor Riseti Manfrinato, Aaron Stein, Lihua Zhang, ChangYong Nam, Kevin G. Yager, Eric A. Stach, and Charles T Black Nano Lett., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017
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Aberration-Corrected Electron Beam Lithography at the One Nanometer Length Scale Vitor R. Manfrinato1, Aaron Stein1, Lihua Zhang1, Chang-Yong Nam1, Kevin G. Yager1, Eric A. Stach1,*, and Charles T. Black1,*
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Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
*Correspondence to:
[email protected],
[email protected] Abstract: Patterning materials efficiently at the smallest length scales is a longstanding challenge in nanotechnology. Electron-beam lithography (EBL) is the primary method for patterning arbitrary features, but EBL has not reliably provided sub-4 nanometer patterns. The few competing techniques that have achieved this resolution are orders of magnitude slower than EBL. In this work, we employed an aberration-corrected scanning transmission electron microscope for lithography to achieve unprecedented resolution. Here we show aberrationcorrected EBL at the one nanometer length scale using poly(methyl methacrylate) (PMMA), and have produced both the smallest isolated feature in any conventional resist (1.7 ± 0.5 nm) and the highest density patterns in PMMA (10.7 nm pitch for negative-tone and 17.5 nm pitch for positive-tone PMMA). We also demonstrate pattern transfer from the resist to semiconductor and metallic materials at the sub-5 nanometer scale. These results indicate that polymer-based nanofabrication can achieve features sizes comparable to the Kuhn length of PMMA and ten times smaller than its radius of gyration. Use of aberration-corrected EBL will increase the resolution, speed, and complexity in nanomaterial fabrication.
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Key Words: nanofabrication, electron-beam lithography, aberration correction, electron microscopy, nanomaterials, pattern transfer, poly(methyl methacrylate).
Although patterning materials at ever-higher resolution has long been an engine that fuels new technologies, designing materials at the one nanometer length scale still remains in the realm of chemical synthesis.1-3 Although electron-beam lithography (EBL) has been the principal patterning method to fabricate arbitrary-shaped sub-50 nanometer structures, it has not reliably provided sub-4 nanometer patterns, except under specialized conditions such as with assist structures,4 and using long-exposure of self-developing resists.5 Nanopatterning techniques that can surpass the 4 nanometer resolution limit, such as electron-beam induced deposition,6 or scanning probe lithography7,
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require orders-of-magnitude longer exposure time than EBL.
Finally, transferring patterns from the resist to a material at the sub-4 nanometers length scale remains an unsolved roadblock.9 Extending EBL resolution and processes to the one nanometer scale would allow for the arbitrary design and efficient fabrication of a wide array of nanoscale systems. Significant efforts have been made toward pushing EBL into the sub-10 nm scale,10-15 but the few reports of sub-4 nm patterning have all involved compromises that render their practical implementation for material design difficult, or impossible. Using specific, self-developing resists such as NaCl, feature sizes of 2 nm and pitch (pattern periodicity) of 4 nm have been achieved.5 However, NaCl is not practically implemented as a template for pattern transfer into other materials. One route for improving resolution and uniformity is using resists with molecular size comparable to the final feature size.16, 17 Hydrogen silsesquioxane (HSQ) resist is one example that makes possible patterning of 4 nm features.15 Simultaneously, there have been
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many improvements in the exposure system, such as increasing the acceleration voltage and reducing the beam size. We previously demonstrated that the ultra-small electron beam from an aberration-corrected scanning transmission electron microscope (STEM)18 can be used to expose resists at sub-5 nm length scales.4 In that demonstration, we reported isolated features in HSQ of 4 nm and arrays with pitch of 10 nm. Further, we patterned sub-3 nm wide lines supported by larger structures that provide mechanical stability. However, such larger-scale ‘assist-features’ limit the pattern uniformity, positioning control, and yield, as well as the type of structures that can be designed using this technique. Here, we exploit aberration-corrected STEM, augmented with a pattern generator,19 in order to provide a fully-functional aberration-corrected EBL system. We used the tool to pattern the most conventional EBL resist, poly(methyl methacrylate) (PMMA), using both positive- and negative-tone exposures — mechanisms that create an image in PMMA by chain-scission and crosslinking, respectively. We used aberration-corrected EBL to form arbitrary patterns in a PMMA resist in both positive- (Figure 1a) and negative-tone exposures (Figure 1b) – see Supplementary Information. The average of the smallest isolated feature printed in positive-tone PMMA was 2.5 ± 0.7 nm (Figure 1c and 1d), which is smaller than the previously reported record of 3.6 nm.11 (In imaging the exposed patterns for positive-tone PMMA in Figure 1, we increased contrast and fidelity by coating the PMMA with metal. We discuss further in the Supplementary Information the possible impact on our reported resolution, and show in Figure S1 that the metal deposition causes minimal artifacts). The smallest pitch pattern printed in PMMA was 17.5 nm (Figure 1f), significantly smaller than the previous report of 25 nm.10 The average of the smallest isolated printed feature in negative-tone PMMA (Figure 1g) was 1.7 ± 0.5 nm, a factor of 2–3 times smaller than the smallest previously reported features of 4–5 nm in negative-tone PMMA,12 and
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our own previous report of 2–3 nm in HSQ.4 The smallest pitch pattern printed in negative-tone PMMA was 10.7 nm (Figure 1j), significantly smaller than the previous report of 16 nm.12 We have included additional patterning results and statistics from the samples shown in Figure 1g-h in the Figure S2. In addition, we discuss practical aspects of aberration-corrected EBL – such as beam current, write field, focus, and samples – in the Supplementary Information. We also considered if the exposure mechanism of our EBL system (200 keV) would be significantly different from conventional EBL systems at lower energies (
