Polystyrene negative resist for high-resolution electron beam lithography
© Ma et al; licensee Springer. 2011
Received: 12 March 2011
Accepted: 12 July 2011
Published: 12 July 2011
We studied the exposure behavior of low molecular weight polystyrene as a negative tone electron beam lithography (EBL) resist, with the goal of finding the ultimate achievable resolution. It demonstrated fairly well-defined patterning of a 20-nm period line array and a 15-nm period dot array, which are the densest patterns ever achieved using organic EBL resists. Such dense patterns can be achieved both at 20 and 5 keV beam energies using different developers. In addition to its ultra-high resolution capability, polystyrene is a simple and low-cost resist with easy process control and practically unlimited shelf life. It is also considerably more resistant to dry etching than PMMA. With a low sensitivity, it would find applications where negative resist is desired and throughput is not a major concern.
Electron beam lithography (EBL) , focused ion beam (FIB) lithography , and nanoimprint lithography (NIL)  are currently the three most widely employed nanolithography techniques. Among them, EBL is undoubtedly the most popular for R&D. Unlike NIL, EBL can generate arbitrary patterns without the need of fabricating a mold first. Though not as versatile as FIB, which can do both lithography using a resist and milling, EBL is capable of exposing thick (> > 100 nm) resist without ion contamination to the resist. In addition, it is faster than FIB exposure since the electron beam can remain well-focused below 10-nm beam size even with nA beam current, as is needed for fast writing. In recent years, one main trend in EBL development is the effort being made toward ultra-high resolution and pattern density, with the record pattern density of 9-nm period line arrays . Desirable properties for EBL resist include high sensitivity, high contrast, and high dry etching selectivity to the substrate materials. Positive resist is typically used for EBL, largely because of the availability of the benchmark resist poly(methyl methacrylate) (PMMA) that offers high resolution with low cost and ease of process. With its higher sensitivity and etching resistance than PMMA, ZEP520 (positive-tone, Zeon Corp.) is arguably the second most popular EBL resist.
However, for some applications, such as the fabrication of hole arrays in a metal film (the structure for extraordinary optical transmission ) by using liftoff, negative resist would offer substantially shorter exposure time, except when using a more complicated "resist tone reversal" process . Unfortunately, there is no negative resist that gains similar popularity as PMMA and ZEP520. Bilenberg et al. have selected four negative EBL resists and compared their performance: calixarene (Tokuyama Corp.), ma-N 2401 (Microresist Technology), SU-8 (Microchem Corp.), and mr-L 6000 (Microresist Technology) . As chemically amplified resists, SU-8 and mr-L 6000 offer superior sensitivity, but with low contrast and resolution (more strictly speaking, half-pitch for dense periodic line array patterns), which is limited by the diffusion of the photoacid generator during postbaking. Ma-N 2401 has sensitivity comparable to that of ZEP520 resist, but with far inferior resolution. Among the four resists, calixarene offers the highest resolution. Calixarene has been studied as a candidate resist for fabricating using EBL bit-patterned recording media that have achieved areal density of 1.4 and 1.6 Tbits/in2 (corresponding to a dot array of 20-nm period) [8, 9] using very thin (sub-20 nm) film. However, it has low sensitivity despite being a chemically amplified resist, and the acid generated in the exposed area may diffuse into the unexposed area, blurring the latent image.
In recent years, hydrogen silsesqioaxene (HSQ) probably attracted more attention than any other negative tone resist [10–12]. HSQ is an excellent inorganic EBL resist that has demonstrated the highest resolution of 9-nm period line array patterns [4, 13], thanks to its small molecular size and lack of swelling during development . (Metal halides have actually demonstrated better resolution, but they are not practical resists due to their extremely low sensitivity and inability to form arbitrary patterns .) However, in addition to its low sensitivity, HSQ is not suitable for liftoff unless when used with a double layer resist stack, such as HSQ coated on PMMA. The development process is also self-limiting due to crosslinking of resist by the developer, leading to incomplete removal of unexposed resist, though a salty developer can minimize this effect [4, 15]. Moreover, HSQ is unstable, and so spin coating, baking, exposure, and development must be done quickly (yet, this is not possible if the exposure time is long) .
In addition, all the above resists are commercially formulated with typically high cost and short shelf life. Therefore, it is preferable to have a negative resist like PMMA, which is a simple polymer with low cost and practically unlimited shelf life, and can be dissolved easily using various solvents to give the preferred film thickness. Polystyrene is such a resist, as it undergoes crosslinking when exposed to deep UV light or an electron beam. Previously, dense periodic patterns with 40-nm period lines have been demonstrated using low molecular weight polystyrene resist . In this article, we investigate the ultimate resolution (half-pitch for dense periodic structure) that can be achieved with polystyrene, and demonstrate the patterning of 20-nm-period lines and 15-nm-period 2D dot arrays, which are the highest densities achieved using organic EBL resists (inorganic resists like HSQ and metal halides have achieved higher resolution). Besides ultrahigh resolution, polystyrene is more (by approximately 3 ×) resistant to dry etching than PMMA. Its major drawback is its low sensitivity compared with PMMA, which would limit its application to small scale nano-patterning.
Polystyrene powder with a molecular weight of 2000 g/mol (Mw/Mn = 1.10) was purchased from Alfa Aesa, and dissolved in chlorobenzene with a concentration of 1.2 w/v%, which gave a film thickness of 30 nm, as measured by atomic force microscope (AFM), after spin-coating at 2000 rpm for 40 s. The silicon wafer was cleaned using acetone and 2-proponol, followed by short exposure to oxygen plasma. After spin coating, the film was baked at 60°C for 1 h on a hotplate. Unlike the high molecular weight polystyrene, the low molecular weight polystyrene film was found to be unstable, forming a non-uniform "broken" film when baked at higher temperatures (e.g., 80°C). In addition, its adhesion to the silicon substrate was not as strong as PMMA. Therefore, in order to obtain reproducible uniform film, we coated a thin layer antireflection coating (ARC, from Brewer Science), which was further thinned to < 15 nm by oxygen reactive ion etching with 20 W power and 20 mTorr pressure. This crosslinked and insoluble thin under-layer would not affect the pattern transfer by liftoff; although due to lateral etch, certain critical dimension loss is expected when transferring the pattern by direct etch. Other adhesion promoters, such as a self-assembled monolayer or thin/thinned PMMA film, might also improve the adhesion of polystyrene to the silicon substrate.
Exposure was performed using a LEO 1530 field emission SEM equipped with a Nabity nanometer pattern generation system at acceleration voltages of 20 and 5 kV. The beam currents were about 20 pA at 20 kV and 10 pA at 5 kV. For high-resolution study, the lines were exposed as single-pass lines with beam step size 3 nm, and dots as zero-dimensional dots. After exposure, the samples were developed using various solvent developers for 90 s at room temperature or 50°C, followed by a 2-propanol rinse. As crosslinked polystyrene is insoluble, in principle, all solvents that can dissolve (un-exposed) polystyrene can be used as developer. In this study, we have developed the samples using xylene (o-, m-, p-mixed), chlorobenzene, and cyclohexane.
3. Results and discussion
We studied the exposure behavior of the negative EBL resist polystyrene. It demonstrated fairly well-defined patterns of 20-nm-period line arrays and 15-nm-period dot arrays, which are the densest patterns ever achieved using organic EBL resists. Such dense patterns can be achieved both at 20 and 5 keV beam energies, using all the three developers that were studied. The contrast for polystyrene is comparable to that of other popular resists like ZEP and PMMA, but its sensitivity is low. In addition to its high-resolution capability, polystyrene is a simple and low-cost resist with easy process control and practically unlimited shelf life. It is also considerably more resistant to dry etching than PMMA. It would find applications where negative resist is prefered and exposure time is not a major concern.
atomic force microscope
electron beam lithography
focused ion beam
- Grigorescu AE, Hagen CW: Resists for sub-20-nm electron beam lithography with a focus on HSQ: state of the art. Nanotechnology 2009, 20: 292001. 10.1088/0957-4484/20/29/292001View Article
- Tseng AA: Recent developments in micromilling using focused ion beam technology. J Micromech Microeng 2004, 14(4):R15-R35. 10.1088/0960-1317/14/4/R01View Article
- Schift H: Nanoimprint lithography: an old story in modern times? A review. J Vac Sci Technol B 2008, 26(2):458–480. 10.1116/1.2890972View Article
- Yang JKW, Cord B, Duan H, Berggren KK, Klingfus J, Nam SW, Kim KB, Rooks MJ: Understanding of hydrogen silsesquioxane electron resist for sub-5-nm-half-pitch lithography. J Vac Sci Technol B 2009, 27(6):2622–2627. 10.1116/1.3253652View Article
- Ebbesen TW, Lezec HJ, Ghaemi HF, Thio T, Wolff PA: Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998, 391: 667–669. 10.1038/35570View Article
- Hajiaboli A, Cui B, Kahrizi M, Truong VV: Optical properties of thick metal nanohole arrays fabricated by electron beam and nanosphere lithography. Phys Status Solidi A: Appl Mater Sci 2009, 206(5):976–979. 10.1002/pssa.200881294View Article
- Bilenberg B, Schøler M, Shi P, Schmidt MS, Bøggild P, Fink M, Schuster C, Reuther F, Gruetzner C, Kristensen A: Comparison of high resolution negative electron beam resists. J Vac Sci Technol B 2006, 24(4):1776–1779. 10.1116/1.2210002View Article
- Hosaka S, Sano H, Itoh K, Sone H: Possibility to form an ultrahigh packed fine pit and dot arrays for future storage using EB writing. Microelectron Eng 2006, 83: 792–795. 10.1016/j.mee.2006.01.005View Article
- Mohamad ZB, Shirai M, Sone H, Hosaka S, Kodera M: Formation of dot arrays with a pitch of 20 nm × 20 nm for patterned media using 30 keV EB drawing on thin calixarene resist. Nanotechnology 2008, 19: 025301. 10.1088/0957-4484/19/02/025301View Article
- Word MJ, Adesida I, Berger PR: Nanometer-period gratings in hydrogen silsesquioxane fabricated by electron beam lithography. J Vac Sci Technol B 2003, 21(6):L12-L15. 10.1116/1.1629711View Article
- Choi S, Yan MJ, Wang L, Adesida I: Ultra-dense hydrogen silsesquioxane (HSQ) structures on thin silicon nitride membranes. Microelectron Eng 2009, 86: 521–523. 10.1016/j.mee.2008.12.055View Article
- Vila-Comamala J, Gorelick S, Guzenko VA, Farm E, Ritala M, David C: Dense high aspect ratio hydrogen silsesquioxane nanostructures by 100 keV electron beam lithography. Nanotechnology 2010, 21: 285305. 10.1088/0957-4484/21/28/285305View Article
- Cord B, Yang J, Duan H, Joy D, Klingfus J, Berggren KK: Limiting factors in sub-10 nm scanning-electron-beam lithography. J Vac Sci Technol B 2009, 27(6):2616–2621. 10.1116/1.3253603View Article
- Sidorkin V, van Run A, van Langen-Suurling A, Grigorescu A, van der Drift E: Towards 2–10 nm electron-beam lithography: a quantitative approach. Microelectron Eng 2008, 85: 805–809. 10.1016/j.mee.2008.01.024View Article
- Yang JKW, Berggren KK: Using high-contrast salty development of hydrogen silsesquioxane for sub-10-nm half-pitch lithography. J Vac Sci Technol B 2007, 25(6):2025–2029. 10.1116/1.2801881View Article
- Clark N, Vanderslice A, Grove R, Krchnavek RR: Time-dependent exposure dose of hydrogen silsesquioxane when used as a negative electron-beam resist. J Vac Sci Technol B 2006, 24(6):3073–3076. 10.1116/1.2366697View Article
- Austin MD, Zhang W, Ge HX, Wasserman D, Lyon SA, Chou SY: 6 nm half-pitch lines and 0.04 μm 2 static random access memory patterns by nanoimprint lithography. Nanotechnology 2005, 16: 1058–1061. 10.1088/0957-4484/16/8/010View Article
- Ocola LE, Stein A: Effect of cold development on improvement in electron-beam nanopatterning resolution and line roughness. J Vac Sci Technol B 2006, 24(6):3061–3065. 10.1116/1.2366698View Article
- Ku HY, Scala LC: Polymeric electron beam resists. J Electrochem Soc 1969, 116: 980–985. 10.1149/1.2412194View Article
- Reinspach J, Lindblom M, von Hofsten O, Bertilson M, Hertz HM, Holmberg A: Cold-developed electron-beam-patterned ZEP 7000 for fabrication of 13 nm nickel zone plates. J Vac Sci Technol B 2009, 27(6):2593–2596. 10.1116/1.3237140View Article
- Häffner M, Haug A, Heeren A, Fleischer M, Peisert H, Chassé T, Kern DP: Influence of temperature on HSQ electron-beam lithography. J Vac Sci Technol B 2007, 25(6):2045–2048. 10.1116/1.2794324View Article
- Yang XM, Xu Y, Lee K, Xiao S, Kuo D, Weller D: Advanced lithography for Bit patterned media. IEEE Trans Magn 2009, 45(2):833–838.View Article
- Yang X, Xiao S, Wu W, Xu Y, Lee K, Kuo D, Weller D: Challenges in 1 Teradot/in 2 dot patterning using electron beam lithography for bit-patterned media. J Vac Sci Technol B 2007, 25(6):2202–2209. 10.1116/1.2798711View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.