Extreme ultraviolet resist materials for sub-7 nm patterning

Li Li *a, Xuan Liua, Shyam Pala, Shulan Wangb, Christopher K. Ober *c and Emmanuel P. Giannelis *c
aDepartment of Advanced Technology Development, GlobalFoundries, Malta, NY 12020, USA. E-mail: li.li1@globalfoundries.com
bDepartment of Chemistry, Northeastern University, Shenyang, 110819, China
cDepartment of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA. E-mail: cko3@cornell.edu; epg2@cornell.edu; Tel: +1-607-255-8417

Received 31st January 2017

First published on 26th June 2017

Continuous ongoing development of dense integrated circuits requires significant advancements in nanoscale patterning technology. As a key process in semiconductor high volume manufacturing (HVM), high resolution lithography is crucial in keeping with Moore's law. Currently, lithography technology for the sub-7 nm node and beyond has been actively investigated approaching atomic level patterning. EUV technology is now considered to be a potential alternative to HVM for replacing in some cases ArF immersion technology combined with multi-patterning. Development of innovative resist materials will be required to improve advanced fabrication strategies. In this article, advancements in novel resist materials are reviewed to identify design criteria for establishment of a next generation resist platform. Development strategies and the challenges in next generation resist materials are summarized and discussed.

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Li Li

Li Li is now the Advisory Scientist in the Department of Advanced Technology Development in GlobalFoundries, USA. He received his PhD degree from the Department of Materials Science and Engineering at Carnegie Mellon University in 2012. After finishing the postdoc work in Cornell University, he moved to the semiconductor research development center (SRDC) at IBM in 2015 and then took the current position with the acquisition of SRDC by GlobalFoundries. His research interests include advanced patterning technology and novel resist materials development, electronic devices fabrication and energy storage/conversion. He was awarded with 2015 TMS Yong Leader Professional Development Award and serves in the editorial board of Metallurgical and Materials Transaction and Community Board of Nanoscale Horizons, RSC.

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Christopher K. Ober

Christopher K. Ober is the Francis Bard Professor of Materials Engineering and director of Cornell Nanoscale Facility at Cornell University. Ober arrived at Cornell as an Assistant Professor in 1986 where he has served as department chair, Associate Dean and as interim Dean of the College of Engineering. Ober is a Fellow of ACS, JSPS, APS and AAAS. His research is focused on lithography, patterning, the biology materials interface and control of surface structure in thin films. As a reflection of his contributions to lithography, Ober in 2015 was honored with the Photopolymer Science & Technology Outstanding Contribution Award. He is the winner of 2013 SPSJ International Award and 2006 American Chemical Society Award in Applied Polymer Science, and received a Humboldt Research Prize in 2007.

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Emmanuel P. Giannelis

Emmanuel P. Giannelis is the Walter R. Read Professor of Engineering in the Department of Materials Science and Engineering and the Associate Dean for Research and Graduate Education in the College of Engineering at Cornell University. He is also the co-director of the KAUST-CU Center for Energy and Sustainability. He received his PhD degree from Michigan State University in 1985 and moved to Cornell University as an Assistant Professor in 1987. He is a fellow of ACS and member of the European Academy of Sciences. His research interests include nanomaterials for energy, electronic devices, biomedical, and environmental applications. He has won the 2014 Cooperative Research Award from the American Chemical Society and has served on the editorial boards of Small, Chemistry of Materials and Macromolecules.

Key learning points

(1) The fundamental principles of lithography and photoresist materials chemistry

(2) Materials and processing requirements for EUV lithography in sub-7 nm patterning technologies

(3) The potential candidate materials for next generation resist platforms

1. Development of EUV technology

As the foundation for the development of modern semiconductor industry, advances in lithography technology have fueled the advancement of information technology. Scaling of integrated circuit manufacturing evolved from the initial small-scale integration (SSL) with only 1–10 transistors on one chip to large-scale integration (LSI), very large scale integration (VLSI) and eventually ultra-large-scale integration (ULSI) with more than 1 million patterning transistors on individual chips. Ongoing development in the semiconductor industry calls for denser fabrication technology to meet expectations set by the Moore's law, which predicts that the number of transistors on integrated circuits doubles every two years. In the past 50 years, this golden rule has been followed by the semiconductor community with the scaling of semiconductor device nodes decreasing from 10 μm in 1971 to 14 nm in 2014. Currently, leading semiconductor corporations announced the goal to accomplish the fabrication of 10 nm and 7 nm nodes before 2018. To achieve this goal, advancement in small scale lithography technology is considered as one of the key factors.

Lithography is a patterning process to form desired shapes in micron/nanometer dimensions by illuminating photosensitive or electron-sensitive materials with ultraviolet light (photolithography) or by exposing them to an electron/ion beam (electron-beam lithography/focused ion beam lithography).1 Other than the traditional patterning technology, novel methods such as soft lithography,2 nanoimprint lithography,3 and dip-pen lithography4 have been introduced to fabricate soft materials for applications such as bio-electronics, gas sensors, and nanowires. The current review is focused on photolithography technology with high throughout and resolution for semiconductor device fabrication.

Optical lithography is inherently limited by two factors, namely resolution and depth of focus, as described by the following equations:

image file: c7cs00080d-t1.tif(1)
image file: c7cs00080d-t2.tif(2)
where R is the resolution of the critical dimension (or the minimum half-pitch), DOF the depth of focus, k1 the Rayleigh coefficient, λ the optical wavelength, and NA the numerical aperture of the imaging system. Among these factors, k1 is determined by the processing conditions, such as illumination and the resist properties. NA is determined by the optical lens and media used in the illumination system, as shown in eqn (3):
NA = n × sin[thin space (1/6-em)]θ (3)
where n is the refractive index of the medium and θ the angular aperture of the lens. According to the Rayleigh equations shown above, the two primary strategies for small scale patterning are to (1) decrease the optical wavelength and (2) increase the NA value with appropriate DOF. The optical wavelength for photolithography used in semiconductor high volume manufacturing (HVM) evolved from visible g-line (436 nm), ultraviolet i-line (365 nm) to deep UV 248 nm KrF and 193 nm ArF excimer lasers. Shorter wavelength lithography, such as 157 nm F2, extreme ultraviolet (EUV) with 13.5 nm wavelength and X-ray (0.4 nm) or even shorter wavelength electron and ion beams have also been considered as promising next-generation lithography technologies. The motivation for increasing the NA value led to immersion technology. Because of its higher refractive index (n = 1.44) than air, high transparency/purity, and low cost, water is selected as the primary medium for the 193 nm immersion technology. Meanwhile, the immersion technology also increases DOF for lower NA imaging and provides a sufficient processing margin for chip manufacture. High-resolution patterning can also be achieved by decreasing the k1 value. Multi-patterning technology based on this strategy was introduced for resolution enhancement to fabricate the 32 nm node and beyond.5 Without a short wavelength exposure source or a high refractive index medium, a small feature size with half or even smaller optical resolution limit can be achieved by combining multi-cycles of exposure, patterning and etching to enhance the feature density. Double-patterning is the foundation of the multi-patterning technology while dual-tone photoresist/development, self-aligned double patterning (SADP) and dual/multi exposure/etch were introduced to shrink the feature size, which in turn increased the mask design complexity, resist sensitivity and thus the process control cost for HVM. Meanwhile, with the decrease in scaling, new issues such as the layout and overlay restrictions emerged. For the current state-of-the-art 10 nm technology, multi-patterning technology still provides an effective strategy to increase the patterning density without the necessity to introduce EUV technology. However, with further shrinkage of the feature sizes to 7 nm and beyond, the semiconductor industry gradually realized the necessity to step forward to the EUV technology by considering the process complexity and manufacturing expenses introduced by the multi-patterning technologies.

Fig. 1 presents the prospective technology roadmap of semiconductor key components from 22 nm to sub-5 nm.6 The wafer size is now migrating from 300 mm to 450 mm to minimize the manufacturing cost and to leverage advancements in recent patterning technology. Shrinkage of feature size on chips can be combined with new transistor designs using state-of-the-art 3D Fin-FET architecture, leading to weak electrostatic control with decreased gate size and FIN dimensions below the 5 nm node, resulting in vertically stacked gate-all-around (GAA) transistors or nanowire structures with development in new channel materials.7 As the photolithography technology of choice in the current semiconductor technology node, ArF with immersion and multi-patterning technology has an apparent disadvantage in terms of cost and complexity, which requires optimization of multiple patterning procedures with resist stack materials. In addition, increasing challenges from defect control lead to unstable and unpredictable device yield. Other challenges come from the insufficient pattern fidelity as well as the necessity for new integration designs. These limitations inspired development of the EUV technology for the sub-7 nm node, though the technology itself has been proposed to increase the patterning resolution a long time ago. Compared with ArF multi-patterning, EUV technology is advantageous since it requires a relatively simpler process based on direct patterning. However, the complexity in mask design and selection of resist materials and the exposure source is increased. For middle of line patterning, EUV technology has decreased the layout complexity with 2D metal intra-cell routing and reduced number of vias. This feature can potentially increase the device yield and provide more flexible area options for the design of the tight pitch with dense/iso line and space under 10 nm. In addition to EUV, other technologies, such as directed self-assembly (DSA) of block copolymers with highly ordered precise nanopatterns in the typical size range of 3–50 nm8 and e-beam with high-sensitivity resist materials,9 are also being explored for possible solutions of semiconductor device HVM. As shown in Fig. 2, a polymer topcoat deposited by initiated chemical vapor deposition (iCVD) was used to immobilize the block copolymer (BCP) for successfully achieving the patterning of sub-10 nm feature size.8 There still remains much to do regarding the use of DSA for large scale production, such as decreasing the cost and lowering the defects. Taking into account the cost/yield ratio, manufacturing feasibility and technical availability/maturity, top-down lithographic technology EUV patterning is currently considered the technique of choice for microelectronics HVM for sub-7 nm node and beyond.

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Fig. 1 Prospective technology roadmap of semiconductor key components beyond 22 nm. Reprinted with permission from ref. 6. Copyright 2013, IEEE.

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Fig. 2 An illustration of sub-10 nm patterning achieved by DSA with iCVD topcoat. (a) The patterning route with the fourfold density multiplication DSA of P2VP-b-PS-b-P2VP (VSV). The Al2O3 pattern with a half pitch of 9.3 nm is formed by sequential infiltration synthesis (SIS); (b) top-down SEM image and schematic representation of the e-beam resist pattern after trim etch with a full pitch of 74 nm; (c) the patterning formation of DSA after SIS with etching of the topcoat by breakthrough BCl3/Cl2 RIE; and (d) alumina line/space pattern with a pitch of 18.5 nm after O2 RIE (full pitch, 18.5 nm). Reprinted with permission from ref. 8. Copyright 2017, Nature Publishing Group.

Currently, the primary challenges of EUV lithography for HVM involve the source/scanner, the mask, and the photoresist, as shown in Fig. 3.10 The EUV source should have a stable high power exposure with single wavelength and narrow bandwidth, as well as a long lifespan of optics combinations. The current EUV sources can be divided into two types: tin-discharge-produced plasma (DPP) light source and laser-produced plasma (LPP).11 Both sources create an extremely high temperature to produce the short wavelength of 13.5 nm. Considering the large amount of heat released from the light source, thermal stability is an important factor in choosing the optics. Currently, the primary material used for optical mirrors is multi Mo/Si layers. Moreover, radiation emission of the EUV light source is generated in all directions, calling for an optical system that can collect the radiation power and redirect it through the illumination system (represented as the collector mirror). Another challenge is that the debris from the exposure region may contaminate the mirrors, which can decrease the lifetime of the source and thus reduce power. Currently, the leading EUV scanner supplier ASML has begun to provide commercial EUV lithography tools to semiconductor manufacturers for trial development of some critical layers of chip processing. Based on their tool specification, the 200 W EUV source at an intermediate focus can have a significant impact in HVM. In terms of the EUV mask, the primary challenge is to control the defects that will influence UV transmittance during exposure and lead to patterning issues. The phase defect from the surface of the substrate and the bottom of the multilayers, and the amplitude defect from the top multilayers are considered as two primary types of defects in the mask.11 The size of the defects scales down with decreasing node size, leading to more-demanding requirements for EUV photomask fabrication as well as defect inspection tools.

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Fig. 3 Schematic representation summarizing the challenges in the current EUV lithography. Reprinted with permission from ref. 10. Copyright 2012, De Gruyter.

In addition to the requirements from imaging, overlay and control of defects, another concern for the manufacturing feasibility of EUV tools is its wafer throughput, which significantly influences the manufacturing cost and the utilization ratio, though not all modules for sub-7 nm technology require EUV patterning. As reported by ASML, the proposed plan for the real volume manufacturing is to reach 1500 wafers per day. EUV productivity is influenced by several factors, such as source power, system availability and scanner. Another particularly important parameter influencing the wafer throughput is resist sensitivity. The EUV resist system includes the resist materials, auxiliary materials and resist-related processing. In the following section, potential resist candidates for sub-7 nm lithography will be primarily reviewed followed by discussion of high performance resist design.

2. Resist materials for sub-7 nm lithography

The basic process flow for optical lithography that is shown in Fig. 4a primarily includes two sections, namely track related processes and exposure. After dissolving in the organic solvents (for example, propylene glycol monomethyl ether acetate (PGMEA)), the resist is spin coated on the surface of the substrate followed by prebaking (called post-apply bake (PAB) or soft bake) to remove the excessive organic solvent and stabilize the photoresist film. The wafer is then exposed to UV light with specific regions shielded in the mask to define the patterning information. Post exposure bake (PEB) for chemically amplified resists (CAR) is used to trigger chemical reactions for solubility changes between exposed and unexposed regions, while this step is not necessary for some metal based photoresists with different patterning formation mechanisms. The wafer is then developed by different developers to remove either the unexposed region or the exposed region. The development process is critical to determine the patterning profile and control critical dimension uniformity. Tetramethyl ammonium hydroxide (TMAH) solution is the most common positive tone developer while some organic solvents are used as negative tone developers. Resist performance is evaluated by a number of factors, including resolution, line width roughness (LWR), line edge roughness (LER), sensitivity, outgassing, cross-section profile, defectivity control, optical absorbance, and etch resistance. A trade-off relationship between resolution, LER/LWR, and sensitivity is used to balance resist parameters and the corresponding performance, as shown in Fig. 4b.12 As the feature size decreases with increasing resolution, high-sensitivity resist materials are required leading to large LER values. The Z-value was introduced to simplify resist evaluation and to describe the RLS trade-off relationship, as shown in the following equation:13
Z = R3 × L2 × D (4)
In eqn (4), R is the resolution (nm), L the LWR or LER (nm), and d the dose or sensitivity (mJ cm−2). This parameter provides an effective and ready reference for selection of resist materials in semiconductor fabrication. A smaller Z value indicates a better overall performance of the resist material. Note that the Z-factor does not take into account the normalized image log slope and the value between different exposure tools may vary.14 Other than the Z factor, a similar parameter called KLUP (LUP: Lithographic Uncertainty Principle) was also used to evaluate the performances of chemically amplified resists (CAR) and to describe the triangular relationship involving input from photon energy, film thickness and blur induced from acid diffusion in the resist chemistry.15

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Fig. 4 (a) Schematic representation of the basic lithography process for positive tone development and negative tone development; (b) schematic representation of trade-off relationships between resolution, LER and sensitivity for photoresists. Reprinted with permission from ref. 12. Copyright 2015, Nature Publishing Group; and (c) schematic representation of radiation interactions between EUV photons and the chemically amplified resist: (i) photon absorption and ionization; (ii) secondary electrons generation; (iii) electron reaction with PAG; and (iv) acid diffusion for catalyzing deprotection. Reprinted with permission from ref. 20. Copyright 2010, IOP Science.

Shrinkage in the patterning size with high resolution and aspect ratio requires a resist material with high etch resistance and excellent profile during patterning transfer. Pattern collapse and floating patterns after development will cause serious issues during fabrication. Film thickness, though thinner for EUV absorption, should be carefully controlled within the required limit to ensure that etching will not damage the substrate. A bilayer/trilayer structure or a sensitive layer combined with the hard mask is used to control pattern collapse. Meanwhile, development of low viscosity and high density solvents that lack surface tension, such as supercritical CO2, was proposed to decrease the capillary force during drying of the developer. The dry development rinse process (DDRP) and the corresponding dry development rinse material (DDRM) were comprehensively studied to minimize pattern collapse with increasing resolution.16 Some initial effort to decrease the pattern collapse ratio has also been performed by combining metal-film coating with conventional films.17 LWR/LER is the general representation of random local variability for line/space patterns during process control. Local tip to tip critical dimension uniformity (CDU) of line ends and of contacts should also be taken into consideration. There are three primary contributors to CDU: optical image quality from the scanner, photon shot noise during exposure, and resist/process conditions. For example, LWR for CAR based photoresist materials is attributed to the inhomogeneity distribution of resist molecules in the intermediate regions where both soluble and insoluble molecules exist.18 Resist sensitivity, or photospeed, is strongly dependent on the photoacid generator (PAG) load in CAR while a high concentration of PAG usually leads to high sensitivity in resist materials. However, a high load of PAG will also cause issues, such as a negative influence on the dissolution kinetics of the resist, compatibility problems between PAG and polymers, and more outgassing.11 Critical challenges for novel EUV resist design are listed in Table 1.19

Table 1 Critical challenges for design of novel EUV resists for sub-7 nm patterning. Reprinted with permission from ref. 19. Copyright 2011, SPIE
Challenge Areas of focus
Fundamental EUV interaction with the resist Electron blur, acid yield, electron affinity, EUV quantum yield
Resolution Polymer-bound PAG, low activation LG, non-CA resist, swelling control, acid diffusion
LWR Polymer-bound PAG, etch trim, rinse, polymer homogeneity
Photospeed EUV sensitization, higher PAG loading
Pattern collapse Lower aspect ratio, UL matched for adhesion, surfactant rinse
Outgassing PAG byproducts from ionization, LG and solvent effects
Out-of-band radiation 193 and 248 nm sensitivity, reduce longer wavelength absorption
Defects PAG aggregation mitigation, hydrophobic byproducts
Quality control EUV photospeed test

Selection of different tone resists will influence the lithographic imaging quality, processing window, and patterning bias between dense and iso lines. Negative and positive tone resists should be carefully selected based on the mask used during processing. Positive tone resists are dominant in high resolution patterning technology based on extensive exploration and optimization. Optical absorption can lead to a different impact on negative and positive tone resists. While positive tone resists tend to show an overcut profile, negative tone resists are not as prone to it. Negative tone resists are also susceptible to patterning bridging defects. On the other hand, negative tone resists also show patterning advantages such as reduction in pattern collapse, poorly exposed resist residual, proximity effect, and LWR in gate processes. As a result, dual-tone resists offer a promising solution in high resolution patterning and have attracted some attention.

2.1 Polymer based chemically amplified resists (CAR)

As the most commonly used photoresist material in KrF and ArF patterning, polymer based CAR is generally composed of a backbone matrix polymer, a photoacid generator for acid catalyst formation, and a dissolution inhibitor that can affect the solubility difference between unexposed and exposed regions. PAG generates acid during exposure that diffuses into the matrix polymer and catalyzes the deprotection of pendant groups in the polymer during PEB, thus changing the solubility of the photoresist in the developer. Photogenerated acid is not consumed during the deprotection reaction and thus can catalyze repeated reactions, leading to the term “chemically amplified”. Depending on the form of interconnection of PAG and the polymer matrix, polymer based CARs can be divided into the PAG blend type and the PAG bound type. Poly(4-hydroxystyrene) (PHS), styrene-derivatives and acrylate copolymers are the three most commonly used KrF and ArF backbone resist materials with applications extended to the EUV resist platform.18 Currently, application of polymer based CARs has been confirmed for 12 nm half pitch patterning with LER of 3.9 nm and exposure dose of 36.1 mJ cm−2 for line/space patterning, and ×16 nm contact with a dose of 30 mJ cm−2.21 JSR showed that their new CAR with high Tg resin and short diffusion length of PAG resolved a 13 nm half pitch line/space patterning with a dose of 35.5 mJ cm−2. Meanwhile, addition of new sensitizers to conventional CARs shows 9–16% increase in sensitivity without loss of LWR or resolution.

CAR has been shown through simulation to be feasible for patterning of the 7 nm half-pitch node with optimized resist chemistry and processing parameters.22 However, novel non-chemically amplified resists are considered to be a long-term solution for achieving sub-5 nm patterning and beyond. One barrier for further development of polymer based CARs for EUV patterning is their low photon absorption because of the poor EUV absorbance cross-section from carbon and oxygen, and high photon shot noise from the high photon energy. The next-generation CARs should be based on high-absorption polymers for sensitivity improvement. Different from 193 nm ArF lithography, the interaction between EUV photon and CAR is primarily based on radiation chemistry while secondary photoelectrons are generated for further ionization and electronic excitation, as shown in Fig. 4c.20 The number of generated secondary electrons can be estimated from the W-value, the average energy required for production of an ion pair.

The photon density in 13.5 nm photolithography is much lower than the density used in 193 nm ArF patterning with the same dosage. The thinner films also lead to tremendous decrease in total optical absorption.23 Therefore, development of the next-generation polymer CARs for EUV applications should specifically focus on two aspects: first, to increase the number of photogenerated secondary electrons per photon to optimize quantum yield based on polymer composition and chemistry, and second, to increase the amount of photoacid generated per photon and to enhance the quantum efficiency with more electrons captured.23 Meanwhile, efforts should be made towards improvements in stochastic control of the homogeneous distribution of components.

Non-metal elements with high EUV absorption cross-section, such as fluorine, can be incorporated into the base polymer to increase the absorption coefficient. Yamamoto et al. have shown that the absorption coefficient of the backbone polymer can be increased by a factor of more than 2 to a value of 8.6 μm−1 with a fluorine concentration of 38 wt% by incorporating a side group of fluorine atoms into poly(4-hydroxystyrene) (PHS).24 The negative impact of fluorine incorporation includes reduction in the acid generation efficiency due to interference of fluorine with acid generators under the low energy electrons. An increase in the absorbance coefficient of the backbone polymer upon incorporation of fluorine atoms is also observed in some non-CAR resists, such as PMMA. The trifluoromethyl derivative of PMMA shows an increase in the photospeed by 1.7× with an absorption coefficient of 20 μm−1 compared with the control resist. In addition, the polymer composition can also be tuned to increase the glass transition temperature and to decrease LWR/LER.25

The next-generation PAG with new chemistry and reduced acid diffusion-path should be further studied to control the local CDU. Some initial research demonstrated that introduction of substituent groups, such as iodine, into PAG results in an increase in the decomposition of PAG due to secondary electrons.26 Different from the ArF resists, the quantum efficiency in EUV resists is highly dependent on the concentration of acid generators sensitized by secondary electrons.27 However, the PAG concentration is limited by the LWR/LER requirements, since the dissolution kinetics of the resist is negatively impacted by an increase in the PAG concentration.18 The inhomogeneous distribution of the acid generator should also be minimized. In addition, compatibility between PAG and the backbone polymer is another concern for optimization of the PAG concentration.18 Compared with PAG-blended polymer CARs, the PAG-bound type showed better resolution and sensitivity and attracted more attention as the EUV technology extended to sub-10 nm patterning.18 For application of polymer based CARs in EUV, etch resistance and resist pattern collapse need to be carefully reviewed with an increase in the patterning aspect ratio. On top of further resist development, enhancement in material processing, such as selection of underlayer materials, development of PAB/PEB, and optimization of the rinse process, should also be investigated.

Negative tone imaging (NTI) resist materials with organic solvent developers are currently attracting considerable attention. Despite the relatively higher cost over TMAH, they have advantages in their low swelling and smooth-dissolving behavior, which help in improving pattern collapse and local CDU. NTI has shown improved lithographic performance for special patterning such as contacts and narrow trenches because of the high optical contrast and large photon density with bright mask and low EUV flare.28 CAR materials are considered as promising candidates for the development of NTI resists under EUV, though non-CAR NTI materials have also been actively investigated. A significant change in the solubility parameter at de-protection is required for the design of NTI-CAR materials and this change can be introduced by hydrophobicity variations.29 Tarutani et al. synthesized a new polymer material with low Ea by the de-protection reaction to maintain short acid diffusion and large hydrophobicity change to achieve excellent dissolution rate contrast in the organic developers. This new NTI-CAR material exhibited great lithographic performance for the 20 nm isolated trench at a dose of 6.25 mJ cm−2 and for the dense contact hole at 32.4 mJ cm−2.29 Cost-effective strategies have been investigated to simplify the resist stack while retaining the excellent performance for small scale patterning. Shirakawa reported the synthesis of Si-containing resists by dissolving a methacrylate copolymer with Si-containing groups and acid-decomposable groups.28 This material showed a similar lithographic performance for both patterning of contact holes and line/space to that shown by the conventional NTI resist. SiO2 was generated during processing and served as the etching hard mask. As a result, no additional spin on glass (SOG) or hard mask was needed in the resist stack. The exploration of NTI resist also promoted development of new organic developers to improve the process. Fujifilm proposed FN-DP301 to replace nBA as the optimal developer for 14 half pitch (HP) line/space patterning with better retention in film thickness.30

2.2 Molecular glass resists

Molecular glass resists form a class of functional materials composed of small and monodisperse organic molecules that form a stable amorphous glass at room temperature. Compared with conventional polymer photoresists, molecular glass resists have monodisperse building blocks in much smaller dimensions, which is beneficial for achieving high resolution and low LWR/LER patterning. The repeatable structure with well-defined control of molecular weight and stereochemistry allows a precise synthetic material.31 Furthermore, their small free volume units inhibit the photo-generated acid distribution (or control atom interaction in non-CAR material) to influence the dissolution kinetics of resist materials, leading to a decrease in local CDU. The molecular glass resists also show the benefit in thin film formation properties and high thermal stability compared with polymer materials. Their high glass transition temperature inhibits crystallization of the resist materials. In addition, they show little internal stress or swelling attributed to the intermolecular chain entanglement of conventional polymer resists.

The design guidelines for the next-generation molecular glass resists (CAR based) are shown in Fig. 5 and two new generation architectures for EUV resist materials design as well as the illustration of the corresponding chemical structure are shown in Table 2.31 The glass transition temperature is a key parameter for the design of molecular glass core materials, since the resist should have sufficient thermal stability within the amorphous state during subsequent processing after exposure. PEB is required to provide the thermal energy to activate the deprotection reaction of CAR. Molecular core materials with high glass transition temperatures are required to form stable thin films for patterning. Meanwhile, hard materials with high etch resistance and absorbance are also good candidates. Phenolic compounds including hydroxyl groups are considered as promising candidates for EUV resists, since oxygen is a high absorbance element at 13.5 nm. Positive tone molecular glass resists derived from the polyphenolic core can be extended to different new resist materials with modification of functional groups, such as dendrimers, polyphenols, calyx[4]resorcinarene, truxenes, calixarenes, etc.32 Acid labile protecting groups, such as tert-butoxycarbonyl (t-BOC), in the positive tone resist (cross-linking groups in the negative tone resist) are used to improve adhesion to underlayer materials and solubility in the developers.33 The protecting groups can also be used to tune solubility, sensitivity, etch resistance and the glass transition temperature.

image file: c7cs00080d-f5.tif
Fig. 5 Design guidelines for components of molecular glass resists: (1) a molecular glass core with a tetrahedral, planar, or ring structure; and (2) acidic functional groups for tuning the solubility change with acid-labile protecting groups in positive-tone resists or crosslinkable functional groups in negative tone resists. Reprinted with permission from ref. 31. Copyright 2008, Wiley-VCH.
Table 2 Two new-generation architectures for EUV resist materials design and the corresponding chemical structures
Resist architecture Photoresist Chemical structure example
Branched MG resist Phenolic and phenylbenzene derivatives image file: c7cs00080d-u1.tif
Ring MG resist Calix[n]arene and calyx[4]resorcinarene derivatives image file: c7cs00080d-u2.tif

Calixarene derivatives are one of the most commonly studied molecular glass resists for their potential HVM use in EUV. As shown by Dow calixarene with 2-naphthoylethylvinyl ether as the protecting group can be used for 28 nm line/space patterning and sensitivity of 31.1 mJ cm−2.34 In addition, a new class of resists with high glass transition temperatures from 80 to 140 °C were synthesized by combing bulkier aldehydes with different protecting groups. For example, the Noria molecule is based on calyx[4]resorcinarene that has a double-cyclic oligomer with ladder-like links between the two rings to form a more rigid structure than calixarene. It has been shown to achieve high resolution patterning. The optimized Noria derivatives with pendant adamantyl ether groups have been used for patterning of 25 nm line/space with a sensitivity of 9.0 mJ cm−2.35 To achieve high resolution patterning, some non-chemically amplified molecular resists have also been investigated. The Ober group reported the synthesis of a p-tert-butyl calyx[4]resorcinarene core functionalized with photoactive diazonaphthoquinone groups (DNQ-CR) as a positive tone resist, which has been used for patterning of 50 nm features under E-beam irradiation.36 Cross-linking and the Wolff rearrangement mechanism were proposed to explain patterning formation. The Henderson group synthesized novel non-chemically amplified positive tone resists by blending NBnDCh, an aliphatic molecular resist based on deoxycholic acid, and NBnHPF, based on an aromatic molecular resist containing two phenol groups, with a calixarene dissolution promoter.37 The dissolution inhibitors that use photosensitive protecting groups became dissolution promoters after photolysis without water.

2.3 Inorganic based photoresists

Polymer based resists suffer from the pattern collapse issue with the small scale patterning, as shown in Fig. 6a. Due to the stringent material requirements including high-etch resistance, sufficient sensitivity and resolution for sub-7 nm EUV lithography,11 inorganic based photoresist materials offer several advantages in terms of mechanical strength and thus high-etch resistance compared with neat polymers. However, radiation absorbance is critical and should be considered in selecting inorganic photoresists for EUV lithography.
image file: c7cs00080d-f6.tif
Fig. 6 (a) The necessity for introducing the high etch-resistant photoresist with the increasing demand of small scaling; and (b) photo-absorption cross-section as a function of atomic number from Z = 1 to 86 at the EUV wavelength. Reprinted with permission from ref. 23. Copyright 2016, SPIE.

Ideal photoresist materials should have a balanced amount of radiation absorbance to induce the necessary photochemical reaction for patterning formation. Absorption should also be low enough to avoid the penetration of energy throughout the film, causing pattern deformation. Theoretically, the photo-absorption of materials in the soft X-ray region can be calculated using known atomic scattering factors, f = f1 + if2, as long as the energy of the photons is outside the absorption threshold regions. The atomic photo-absorption cross section, μa, may be readily obtained from the tabulated values of f2 using the relation

μa = 2 × r0 × λ × f2 (5)
where r0 is the classical electron radius, λ the X-ray wavelength, and f2 the imaginary part of the atomic scattering factor. Given the atomic photoabsorption cross section, the linear absorption coefficient (μ) of a specific material can be calculated by
image file: c7cs00080d-t3.tif(6)
where ρ is the density, xi the number fraction of element i and Ai the atomic weight of element i. As long as the density is known, absorption at the EUV wavelength can be calculated. This is an extremely useful approach in the design of resist platforms for EUV lithography. The EUV photo-absorption cross section of elements from atomic number 1 to 86 is shown in Fig. 6b. Note that elements commonly used in photoresists at other wavelengths, such as fluorine, are highly absorbing at ∼13 nm, rendering them problematic for EUV applications. Other elements including carbon, silicon, zirconium and hafnium have very high transmission, allowing EUV photons to pass through the entire resist film.

Selection of an appropriate solvent for a specific application, such as for photoresist spin coating or development, is usually determined on a trial-and-error basis. However, a deeper understanding of the solubility properties is needed in order to design and optimize high resolution EUV resists. Especially for the case of a negative tone resist the solubility characteristics of exposed and unexposed areas are often similar, making selection of an appropriate developer very challenging. An excellent tool for predicting the properties of various solvents is the Hildebrand solubility parameter which is defined as

δ = (E/Vm)1/2 (7)
where E represents the cohesive (vaporization) energy and Vm the molar volume. The Hildebrand solubility parameter can be broken down into three components that represent the dispersive, polar and hydrogen bonding forces within the solvent. These components are the Hansen solubility parameters and are related to the Hildebrand parameter by the following equation:
δt2 = δd2 + δp2 + δh2 (8)
Using Hansen solubility parameters it is possible to fully describe the solubility behavior of any organic or inorganic material. This is accomplished by plotting a variety of solvents in a three dimensional graph with each Hansen parameter in each axis. All good or moderately good solvents for a given material fall within an approximately spherical volume of solubility, which is defined by the Hansen parameters of the material and the interaction radius R (Fig. 7). By defining the volume of solubility for a photoresist thin film before and after exposure it is possible to identify the solvents that will selectively dissolve the exposed or unexposed film in order to generate positive or negative tone patterns respectively. The Giannelis and Ober groups performed a systematic study on the dispersion of hybrid inorganic/organic Hf- and Zr-based photoresists under UV exposure and suggested design strategies for high performance hybrid inorganic resist materials.38

image file: c7cs00080d-f7.tif
Fig. 7 (a) Three dimensional plot of Hansen solubility parameters for the hybrid EUV resist HfIBA film in 17 different solvents; and (b) Teas graph of the HfIBA film in the solvents with ternary fractional solubility parameters. Different colors represent different solubility of the film in the corresponding solvents (represented as a dot or a square): red refers to insoluble film within 3 minutes; green refers to soluble film within 5 seconds. Reprinted with permission from ref. 38. Copyright 2016, Royal Society of Chemistry.

Hydrogen silsesquioxane (HSQ) is commonly used as a type of non-chemically amplified inorganic negative tone resist for E-beam lithography. It shows high etch resistance and small local CDU.9 However, the primary barrier preventing further commercial use of HSQ as a EUV HVM resist is its low sensitivity and short shelf-life. Currently, tremendous attention has been devoted to metal oxide based resist materials, such as TiO2, SnO2, HfO2 and ZrO2. These materials have been proposed as promising next-generation resist candidates. TiO2 has been used for direct imprinting over large scales with nanoimprint lithography from a mixture of polymerizable liquid titanium methacrylate.39 Ganesan et al. achieved 30 nm TiO2 line patterning with a resist formulation based on the combination of an allyl-functionalized titanium complex and acrylate-based monomers under step-and-flash imprint lithography.40 The as-prepared TiO2-based resist has low viscosity for dispersion and low sensitivity to variation in patterning density. The resist formulation can be extended to fabricate wide classes of metal oxide based resist materials, such as ZrO2 and Ta2O5.

The Ober and Giannelis group at Cornell proposed a novel dual-tone resist system with materials of balanced EUV absorption (ZrO2, HfO2) as the inorganic core and photosensitive materials as organic ligands.38,41 Both positive and negative tone patterning can be achieved in the same material with different developers. A schematic representation of the molecular structure of the hybrid nanoparticle resist is shown in Fig. 8. Other than providing high mechanical strength and etch resistance to the hybrid resists, the core materials are also responsible for adjusting EUV absorption to guarantee sufficient energy to activate the photochemical reactions and to prevent diffraction of excessive photons throughout the film which may cause patterning deformation. The organic ligand shell is used to control dispersion of the inorganic core in solvent and to induce the solubility switch before and after exposure. Meanwhile, selection of ligands strongly influences the sensitivity of the resists. By choosing trans-2,3-dimethylacrylic acid (DMA) as the organic ligand coupled with HfO2 and ZrO2, 20 nm line/space has been successfully patterned at 2.4 and 1.6 mJ cm−2, respectively. Addition of photoactive compounds, although not mandatory for EUV patterning, can increase resist sensitivity and patterning resolution by adjusting internal photochemistry. The hydrodynamic size of the hybrid nanoparticles in organic solvents and TMAH can be controlled within sub-5 nm,38,41 showing structural advantages as the next generation photoresists for sub-7 nm patterning. In addition to HfO2 and ZrO2 nanoparticles, other metal oxide nanoparticles, such as Ti, Sn, In, and Zn based nanoparticles, are also being investigated for high resolution patterning.

image file: c7cs00080d-f8.tif
Fig. 8 Hybrid inorganic/organic nanoparticles with the inorganic core providing high etch resistance and EUV absorption while the organic shell improves solubility and increases photochemical sensitivity.

Understanding the patterning mechanism for this dual tone non-chemically amplified metal-oxide based nanoparticle resist is complicated. The current understanding of negative tone resists is based on the combined effect of ligand exchange and inorganic core condensation. Based on the ligand exchange mechanism, the PAG or photoradical generator releases ligands that displace surface ligands with smaller binding affinity to nanoparticle cores. Meanwhile, the size of the nanoparticles increases after UV exposure without observed cross-linking, leading to solubility change in organic developers.41 A schematic representation of these two proposed mechanisms is shown in Fig. 9. The patterning capability of positive tone resist materials is attributed to hydrolysis of the inter-particle oxo-bridges in the base solution after exposure, which leads to more soluble nanoparticles in TMAH in regions unprotected by carboxylic acid. A Zn oxo cluster photoresist was also synthesized by hydrolysis-condensation reactions with ZnMAA and ZnO patterns were formed by DUV exposure to fabricate the field-effect transistors, as shown in Fig. 10.42

image file: c7cs00080d-f9.tif
Fig. 9 Schematic representation of the proposed patterning mechanisms for the hybrid inorganic/organic nanoparticle resist: (a) ligand exchange; and (b) particle condensation. Reprinted with permission from ref. 41. Copyright 2015, American Chemical Society.

image file: c7cs00080d-f10.tif
Fig. 10 (A) Process flow of ZnO line/space patterning with Zn-oxo-cluster (ZnOCs) as the inorganic based photoresist; and (B) the structural characterization of the as-prepared ZnOCs resist film. The hybrid inorganic–organic based ZnOCs photoresist was formed via hydrolysis-condensation reactions with ZnMAA and H2O by a sol–gel based method. The cluster size is measured to be around 2.5 nm, as shown in TEM images. The cross-linking aggregation induced by the replacement of MAA ligands on the surface of ZnOCs with –OH ligands during the DUV exposure is attributed to the patterning formation. Reprinted with permission from ref. 42. Copyright 2017, Royal Society of Chemistry.

Metal nano clusters can also be added into CAR or non-CAR materials as the sensitizer to absorb EUV electrons and to increase the interaction efficiency of secondary electrons with the resist materials. The high mechanical strength introduced from metal addition leads to an increase in the etch resistance and a decrease in necessity of the hard mask underlayer. The Keszler group and Inpria proposed a new class of non-chemically amplified metal oxide sulfate (MSOx) based resists with the peroxo group bound to the metal oxide atoms for achieving the sub-10 nm patterning under EUV.43 The presence of peroxo inhibits the complete polymerization of the network during processing before exposure. Meanwhile, secondary electrons generated from the interaction between resist or other low energy electrons and EUV photons decompose the peroxo group, leading to the formation of oxygen radicals and eventually emission of O2 between neighboring molecules. A three dimensional framework is then formed for coordination of metals in the network. The size of particles changes significantly from small clusters to large units after exposure, leading to solubility change in the developer and patterning formation.43 A schematic representation of the detailed mechanism is shown in Fig. 11A with the plan-view SEM image and cross-section TEM image of the 9 nm line shown in Fig. 11B.44 In addition, another HfO2 based hybrid non-chemically amplified polymer with the synthetic route shown in Fig. 11C was reported to show the reasonable EUV patterning performance of 40 nm line/space features, while the patterning mechanism is attributed to the polarity change triggered by photons for the unexposed and exposed region.45

image file: c7cs00080d-f11.tif
Fig. 11 (A) Patterning mechanism of HfOx under EUV exposure. Reprinted with permission from ref. 43. Copyright 2011, SPIE; (B) plan-view SEM and TEM cross-sectional images of the patterned HafSOx film with e-beam and 25% TMAH as the developer. Reprinted with permission from ref. 44, Copyright 2014, American Chemical Society; and (C) fabrication route for the HfO2 based hybrid polymer as the EUV photoresist. Reprinted with permission from ref. 45. Copyright 2016, Royal Society of Chemistry.

Organometallic resists have also been actively developed due to their high EUV optical density to maximize the use of photons and to reduce the electron blur of secondary electrons with high mass density and short mean free path. While HfOx based resists began to show improvement in areas such as EUV photosensitivity, shelf-stability and equipment compatibility, tin based organometallic resists (Y-series materials) have also been developed by Inpria as their second generation patterning platform with high EUV absorption (absorption coefficient = 20 μm−1) and excellent etch resistance selectivity.46 The as-prepared materials achieved patterning of 13 nm HP at 35 mJ cm−2 under EUV exposure. In addition, antimony, tellurium and bismuth based organometallic carboxylate resists and platinum/palladium oxalates for EUV patterning have also been reported.47

2.4 Other resist materials and auxiliary materials

Other types of resists have also been actively studied for sub-10 nm small-scale patterning. Some non-chemically amplified polymer-based resists have been reported with potential functionality for EUV HVM. Satyanarayana used (4-(methacryloyloxy)phenyl)-dimethylsulfoniumtriflate (MAPDST) homopolymer and poly(MAPDST-co-MMA) for 20 nm line/space patterning under EUV with the E0 center dose of 10 and 30 mJ cm−2.48 Formulation of the negative tone resist consists of some EUV absorption enhancing moieties and a sulfonium group that serves as the radiation-sensitive component to change the polarity of the molecules to more-hydrophobic sulfide. Takei et al. developed a new cellulose-based biomass resist and characterized its potential lithographic performance under electron beam and EUV exposure.49 This material is derived from woody biomass while water is used for both spin coating and development. Line patterns with 50–100 nm spacing were achieved by electron beam lithography with reasonable etch selectivity compared with PHS and HMPA polymer. This green resist may provide potential functionality for small scale fabrication of bio-compatible devices, solar cell devices, and nanoelectromechanical systems.

To mitigate patterning collapse and to improve the lithographic performance of resist materials for the RLS trade-off relationship, auxiliary materials for topcoat, rinse and underlayer materials are also actively studied. The topcoat materials are used to control the out-of-band (OoB) sensitivity and outgassing protection under EUV exposure. Meanwhile, topcoat materials can reduce loss of the resist film and thus bridging between patterns during processing. The ideal topcoat material should have high EUV transmittance and absorption for OoB photons while remaining dissolvable in the developer to avoid the repeated rinse processing. High hydrophobicity is useful for the design of top-coat materials to decrease defects and increase scan speed. Increasing the fluorine content in the polymer is one choice. However, processing simplification and cost requirements for HVM necessitate topcoat-less resist materials. Another topic focuses on rinse materials to mitigate pattern collapse. Dry development rinse materials (DDRM) were introduced recently to prevent the capillary force and decrease the pattern collapse for high aspect ratio features to extend the resolution limit of EUV resist materials.16 The detailed processing flow for the dry development rinse process (DDRP) is shown in Fig. 12. Compared with the conventional process, the encapsulating DDRM is spin-coated on the wafer rather than spin-drying directly after the development and rinse. The wafer is then spin-dried and baked followed by two dry etching steps to remove the resist layer. The water drying action is avoided during processing. As a result pattern collapse due to the asymmetrical surface tension from capillary forces is minimized. The requirement for DDRM includes high dry etching selectivity, non-mixing with resist patterns and excellent planarization properties. Si-based polymer resists are used as common DDRM that show good compatibility with organic solvents and water. Kulmala proposed a new pattern collapse mitigation strategy called the “polymer freeze” technique for inorganic resist materials by encapsulating the resist in a polymer film to remove the wafer that is not dried after development and rinsing. The method increased the aspect ratio of the freestanding resist structures and improved the process window under e-beam and EUV interference lithography.50

image file: c7cs00080d-f12.tif
Fig. 12 Flow process of the dry development rinse process (DDRP). Reprinted with permission from ref. 16. Copyright 2015, SPIE.

3. Resist materials: challenges and prospective

EUV lithography is now considered as one of the most feasible choices for HVM of integrated circuits for sub-7 nm node and beyond. The advancement of EUV resists should be considered to break the RLS trade-off relationship to achieve the necessary breakthrough of patterning technology. In addition to the consideration from high EUV sensitivity, low local CD uniformity and high patterning resolution, the next generation resist systems should also efficiently solve the issues of pattern collapse, resist homogeneity, etch resistance, UV out of band, outgassing, HVM compatibility, defect, and shelf-life. Highly ordered structures are required as building blocks for the resist platform to fabricate the future ultra-small features with high precision and reliability. The disordered polymer based CARs combined with the development of EUV enablers and new processing still can temporarily satisfy the current need for resist development. Based on the current progress of the resist materials, metal based resists show better functionality and potential for the long term solution of sub-7 nm patterning and beyond. The rapid progress of resist technology promotes concurrently the development of auxiliary materials, such as topcoat and DDRM, and resist processing, such as PAB/PEB, development and drying. The Moore's law might be approaching its end and the development of new resist materials is on the verge of a new era with the accompanied challenges and opportunities for the coming atomic patterning.


The authors gratefully acknowledge the support from GlobalFoundries and Cornell Nanoscale Science and Technology (CNF), the Cornell Center for Materials Research (CCMR) and the KAUST-Cornell Center of Energy and Sustainability (KAUST-CU). Dr L. Li and Dr X. Liu contributed equally to this work.


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