SML resist processing for high-aspect-ratio and high-sensitivity electron beam lithography
© Mohammad et al.; licensee Springer. 2013
Received: 10 January 2013
Accepted: 13 February 2013
Published: 27 March 2013
A detailed process characterization of SML electron beam resist for high-aspect-ratio nanopatterning at high sensitivity is presented. SML contrast curves were generated for methyl isobutyl ketone (MIBK), MIBK/isopropyl alcohol (IPA) (1:3), IPA/water (7:3), n-amyl acetate, xylene, and xylene/methanol (3:1) developers. Using IPA/water developer, the sensitivity of SML was improved considerably and found to be comparable to benchmark polymethylmethacrylate (PMMA) resist without affecting the aspect ratio performance. Employing 30-keV exposures and ultrasonic IPA/water development, an aspect ratio of 9:1 in 50-nm half-pitch dense grating patterns was achieved representing a greater than two times improvement over PMMA. Through demonstration of 25-nm lift-off features, the pattern transfer performance of SML is also addressed.
KeywordsSML resist Electron beam lithography High-aspect-ratio nanolithography Nanolithography Nanofabrication Lift-off Preventing pattern collapse Resists (85.40.Hp) Electron beam lithography (85.40.Hp) Nanolithography (81.16.Nd)
Fabrication of nanoscale structures and devices such as nanoimprint lithography templates, dynamic random-access memory capacitors, zone plates (X-ray lenses), etc. requires a high-aspect-ratio (AR) and high-resolution patterning capability. Utilizing electron beam lithography (EBL) to fabricate such nanostructures further requires that the patterning be performed as rapidly as possible (high throughput) due to the serial writing nature of EBL. The requirement of high throughput often imposes a trade-off between the selection of processing conditions and performance. As an example, using a higher voltage in EBL enables the fabrication of higher AR nanostructures; however, the electron dose increases in proportion to the voltage, thus increasing the time of exposure. Careful selection of other processing parameters such as using a higher performance developer solution can decrease the electron dose requirement (increase the process sensitivity) and, to a certain extent, compensate for such trade-offs.
The well-known positive-tone resists polymethylmethacrylate (PMMA) and ZEP-520 (Zeon Corporation, Tokyo, Japan) can be patterned with sub-20-nm resolution for dense grating patterns. However, the achievable ARs of PMMA on solid substrates are limited to 2:1 to 4:1 at 25 keV [1, 2], to approximately 5:1 at 50 keV [1, 3], and to 12:1 to 20:1 at 100 keV [1, 4, 5]. Similarly, ZEP resist has ARs limited to 4:1 at 20 keV  and to 7:1 at 100 keV , albeit with over three times higher sensitivity than PMMA. Another positive-tone resist, polymethylglutarimide (PMGI), has been patterned with an AR of over 2:1 at 30 keV  and extremely high AR of 38:1 at 100 keV  using an optimized development process. However, the sensitivity of PMGI is four to nine times lower than that of PMMA, requiring up to 18,000 μC/cm2 to expose a single line. Similar trends are observed for negative-tone resists such as hydrogen silsesquioxane (HSQ). Reported ARs for HSQ are 4:1 at 10 keV , 7:1 at 50 keV , and 25:1 at 100 keV [12, 13]. HSQ’s main attraction is its extremely high resolution (<10 nm); however, its sensitivity is usually an order lower than that of PMMA. Other negative-tone resists such as AZ nLOF 2020 (Clariant Corporation, Muttenz, Switzerland)  and high molecular weight polystyrene (PS)  have sensitivities a fraction of that of PMMA; however, their AR performance is limited to 4:1 to 5:1 at 100 keV for AZ nLOF 2020  and to less than 2:1 at 20 keV for PS [15, 16].
Recently, an EBL resist ‘SML’  has been introduced by EM Resist Ltd. (Macclesfield, UK) in thicknesses ranging from 50 to 2,000 nm. SML is a positive-tone, organic resist that has been designed for high-AR patterning. The resist is anticipated to yield ARs of up to 10:1 at 30 keV and exceeding 50:1 at 100 keV . This represents a greater than two times improvement over benchmark PMMA resist; however, its sensitivity and resolution are lower than those of PMMA using supplier-recommended conditions. Similar to other positive-tone resists such as PMMA , PMGI , and ZEP , SML may be developed in methyl isobutyl ketone (MIBK)/isopropyl alcohol (IPA) (1:3) solution and rinsed in IPA .
In this work, a systematic experimental study of SML as a high-performance EBL resist at 30 keV is conducted with the aim of co-optimizing sensitivity, contrast, and AR. A total of six developers (both single- and binary-component) are evaluated by generating the contrast curves and comparing their respective sensitivities and contrast values. After selecting the developer with desired characteristics, high-AR grating patterns at various pitch values are fabricated to obtain a dense, high-AR, and high-sensitivity nanolithography process. The pattern transfer performance of SML is also explored by lift-off experiments. At each stage of this work, the performance of SML resist is compared to that of PMMA.
The SML samples used in this study were provided courtesy of EM Resist Ltd.  as pre-spun and baked chips. The experimental work with SML resist began using supplier-recommended conditions [17, 20] to fabricate grating structures in 300- and >1,500-nm-thick resist samples. Based on the understanding of the resist gained in these experiments, the majority of the work was conducted in three sequential steps: (a) generation of SML contrast curves with six different developers, followed by (b) fabrication and characterization of high-AR gratings using a selected developer, and (c) evaluation of lift-off performance.
To generate the contrast curves, an array of 20 × 75 μm rectangular pads (spaced by 20 μm) with a gradually increasing dose was exposed to 30-keV electrons (Raith 150TWO, Dortmund, Germany) on 300- to 330-nm-thick SML resist samples. The exposed samples were developed for 20 s at ambient temperature in six developers: MIBK, MIBK/IPA (1:3), IPA/water (7:3), n-amyl acetate, xylene, and xylene/methanol (3:1). The developed samples were quickly dried in a nitrogen flow, and no post-development rinsing was performed. The resulting resist surfaces were scanned using a physical profilometer (KLA-Tencor Alpha-Step IQ, Milpitas, CA, USA) having a depth resolution of 10 nm.
To fabricate dense, high-AR gratings, large arrays of 50- to 200-nm-pitch grating patterns were exposed at 30 keV on 300- to 330-nm-thick SML samples. An exposure voltage of 30 keV (the highest voltage on Raith 150TWO EBL system) was selected to maximize the AR while achieving high sensitivity through the development process. The width of the grating arrays were kept sufficient for capturing the contribution of proximity effects. The exposure current was 23 to 24 pA (7.5-μm aperture), and a step size of 2 nm was used. The exposed samples were developed ultrasonically for 20 s in IPA/water (7:3) (developer selected after contrast curve study). Before drying the samples in flowing nitrogen, the developed samples were briefly (approximately 2 s) immersed in a low-surface-tension fluid (pentane or hexane) to reduce the probability of pattern collapse. Prior to scanning electron microscope (SEM) imaging, the samples were coated with a 6-nm chromium layer (Gatan PECS, Pleasanton, CA, USA). Cleaved samples were coated at a 45° tilt with the sample cross section facing the target. The SEM imaging (Hitachi S-4800, Schaumburg, IL, USA) was conducted at 5 keV, 20 μA, and 4-mm working distance. To evaluate the pattern transfer capability of SML resist, metal lift-off was performed. By electron beam evaporation, 50 nm of chromium was deposited on nanoscale SML gratings and the resulting stack lifted-off by immersing for 1 min in an ultrasonic acetone bath.
Results and discussion
Based on the analysis of contrast curves, IPA/water (7:3) was selected as the preferred developer for fabricating dense, high-AR gratings. Similar to PMMA, both IPA and water alone are poor or non-developers for SML resist but are effective in combination. The usage of ultrasonic agitation during development was chosen to help promote the dissolution of SML fragments as inspired by Yasin’s work . Since resist fragments tend to coil in poor solvents and exhibit a smaller radius of gyration, ultrasonic agitation may be expected to promote the rapid removal of these fragments, enabling a narrower grating trench . As described in the ‘Methods’ section, a brief rinse in low-surface-tension fluid was used to reduce the probability of pattern collapse. The surface tension of pentane (approximately 16 dyn/cm) and hexane (approximately 18 dyn/cm) is at least four times less than that of water (approximately 73 dyn/cm).
The SEM imaging with SML is quite challenging. Dense grating structures deform and bend as a result of the scanning accompanied by visible film shrinkage. The gratings shown in Figure 6a,b,c,d had perfectly vertical sidewalls before a 5-s SEM scan. The film shrinkage also reduces the AR measurement. Thick (>1,500 nm) patterned SML films show exaggerated deformation and, in some cases, tearing and de-lamination. An additional document explains the visualization challenge and mitigation strategies in more detail [see Additional file 3]. We would like to re-iterate that the resist deformation is a SEM visualization issue, and not the result of EBL exposure.
Conclusions and recommendations
A detailed characterization of SML electron beam resist has been presented with focus on high-aspect-ratio nanopatterning at high sensitivity. Contrast curves of six developers: MIBK, MIBK/IPA (1:3), IPA/water (7:3), n-amyl acetate, xylene, and xylene/methanol (3:1), were compared for the highest contrast and sensitivity. SML’s pattern density limits and lift-off capability were also evaluated.
SML was found to be a capable and versatile EBL resist. Aspect ratios of at least 9:1 are possible at 30 keV, suggesting over 100% improvement as compared to PMMA or ZEP. IPA/water (7:3) was found to be the most suitable developer for high-contrast and high-sensitivity nanopatterning. Using IPA/water (7:3) developer, SML’s sensitivity is close to PMMA and therefore represents a 40% improvement in sensitivity over existing SML results. Metal lift-off was found to be easy and efficient.
Based on the experiences gained through this research, the following recommendations are offered for further work with SML: (a) to find a stronger developer (stronger than MIBK) and combine it with a small molecule non-solvent such as methanol, (b) to develop pattern collapse prevention techniques such as supercritical drying  with exchange liquid other than IPA and/or use of surfactants , and (c) to invest efforts to find damage-free electron microscopy imaging conditions.
electron beam lithography
methyl isobutyl ketone
scanning electron microscope
The authors would like to acknowledge Daniel Royston from EM Resist Ltd. for providing the SML resist samples used in this work and Scott Lewis from the University of Manchester and Peter McGovern from EM Resist Ltd. for the helpful discussions. In addition, the support of the University of Alberta nanoFAB, NRC-NINT, NSERC, Alberta Innovates, and iCORE is also gratefully acknowledged.
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