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
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