High molecular weight polystyrene as very sensitive electron beam resist Celal Con, Ripon Dey, Mark Ferguson, Jian Zhang ⇑ , Raafat Mansour, Mustafa Yavuz, Bo Cui Waterloo Institute for Nanotechnology (WIN), University of Waterloo, 200 University Ave. West, Waterloo, ON, Canada N2L 3G1 article info Article history: Available online 16 July 2012 Keywords: Nanofabrication Nanolithography Electron beam lithography Resist Polystyrene abstract Previously we demonstrated ultra-dense patterning using 2 kg/mol polystyrene negative electron beam resist that has low sensitivity [16]. To drastically improve its sensitivity, here we studied the exposure behavior of polystyrene with molecular weight of 90 and 900 kg/mol. Very high sensitivity of 1 lC/cm 2 was obtained for 900 kg/mol when exposed at 2 keV. The sensitivity for 90 kg/mol polystyrene is about one order lower. The resist has a contrast around 1.5 that is nearly independent of molecular weight for the current range of molecular weight. It can achieve fairly well-defined patterns of 150–200 nm period line arrays. Polystyrene is a simple and low-cost resist with easy process control and practically unlimited shelf life. It is also considerably more resistant to drying etching than PMMA. Therefore, the current high molecular weight polystyrene could be employed for applications that need moderate resolution but high sensitivity for a reasonable exposure time. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Electron beam lithography (EBL) [1], focused ion beam (FIB) lithography [2] and nanoimprint lithography (NIL) [3] 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 fabri- cating a mold first. Though not as versatile as FIB, which can do both lithography using a resist and milling, EBL is capable of expos- ing 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. Nevertheless, the throughput of EBL is still very low compared to optical and nano- imprint lithography, which calls for highly sensitive resists and tools capable of exposure at tens of nA beam current. 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 pro- cess. 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 fab- rication of hole arrays in a metal film (the structure for extraordi- nary optical transmission [4]) by using liftoff, negative resist would offer substantially shorter exposure time, except when using a more complicated ‘‘resist tone reversal’’ process [5]. Unfortunately, there is no negative resist that gains similar popularity to PMMA and ZEP520. Bilenberg et al. has 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) [6]. 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) that is limited by the diffusion of the photoacid generator during post exposure baking. 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 re- sist for fabricating using EBL bit-patterned recording media that have achieved areal density of 1.4 and 1.6 Tbits/in 2 (corresponding to a dot array of 20 nm period) [7,8] 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 [9–11]. HSQ is an excellent inorganic EBL resist that has demon- strated the highest resolution of 9 nm period line array patterns [12], thanks to its small molecular size and lack of swelling during development [13]. However, in addition to its low sensitivity, HSQ is not suitable for liftoff unless using a double layer resist stack such as HSQ coated on PMMA. The development process is also self-limiting due to cross-linking of resist by the developer, leading to incomplete removal of unexposed resist, though a salty devel- oper can minimize this effect [12,14]. More importantly, HSQ is unstable, so spin coating, baking, exposure and development must be done quickly (yet this is not possible if the exposure time is long) [15]. 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.07.005 ⇑ Corresponding author. Tel.: +1 519 729 3582. E-mail addresses: bcui@uwaterloo.ca, j242zhan@ecemail.uwaterloo.ca (J. Zhang). Microelectronic Engineering 98 (2012) 254–257 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee