Viewpoint pubs.acs.org/macroletters
Photopatterning of Block Copolymer Thin Films Austin P. Lane,‡ Michael J. Maher,† C. Grant Willson,†,‡ and Christopher J. Ellison*,‡ †
Department of Chemistry and ‡McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: Block copolymers are potentially useful materials for large-area 2-D patterning applications due to their spontaneous selfassembly into sub-50 nm domains. However, most thin film engineering applications require patterns of prescribed size, shape, and organization. Photopatterning is a logical choice for manipulating block copolymer features since advanced lithography tools can pattern areas as small as a single block copolymer domain. By exposing either the block copolymer or a responsive interfacial surface to patterned radiation, precise control over placement, orientation, alignment, and selective development of block copolymer domains can be achieved. This Viewpoint highlights some of the recent research in photopatterning block copolymer thin films and identifies areas of future opportunity.
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designs.17 Examples of top-down patterning techniques include localized thermal annealing,18 raster solvent vapor annealing,19,20 and small-area BCP deposition.21 The most common approach is to photopattern a physical or chemical template on the substrate interface, which acts as a guide for directed selfassembly (DSA).22,23 By carefully controlling the size, shape, and organization of these guides, BCPs can form patterns that are potentially useful for defining circuit features and magnetic domains for bit-patterned media.24 This Viewpoint is focused on photopatterning BCPs with a special emphasis on lithographic applications. The goal of this work is not to provide a comprehensive review of the subject; we direct the reader to those works here.25,26 Rather, this Viewpoint highlights recent research and identifies areas of future work and opportunity. The first section focuses on different strategies for patterning adjoining interfaces that confine a BCP thin film. The second section focuses on BCP materials that are inherently photosensitive and can act as negative or positive tone resists. Photopatternable interfaces: While the BCP domain orientation in thin films can be influenced by factors such as the molecular architecture of the BCP and the commensurability of the film thickness with the natural domain periodicity, it is largely determined by the interfacial interactions between the BCP and both the substrate and free interface.27 These interactions (and, thus, BCP orientation) can be controlled by precisely tuning the chemistry of the interfaces. The substrate is usually modified by immobilizing polymers on the surface, either through grafting or cross-linking reactions.28−30 The free interface can be selectively tuned by applying polymeric top coats over the BCP film.31−34 Many applications require a
lock copolymer (BCP) self-assembly has gained significant attention in a wide variety of fields, including lithography,1 nanofiltration,2 and electrolyte membranes.2,3 In thin films, BCPs are capable of self-assembling into different morphologies with feature sizes on the order of several to hundreds of nanometers.4 The self-assembled patterns and morphologies can be used to template nanoparticle growth5 and surface functionalization6 or to transfer patterns into an underlying substrate.7,8 BCPs are especially well-suited for lithography because they combine large-area patterning capability with the critical reduction in feature size that is difficult to achieve with photolithography alone due to optical resolution limits. Perhaps the most promising future applications for BCP patterning are the production of next-generation storage media,9 FinFET architectures,10 and contact holes for current integrated circuit designs.11 Other high resolution lithography techniques, such as extreme ultraviolet (EUV) lithography12 and electron beam lithography (EBL),13 also provide a route to smaller feature sizes required for these applications. However, these processes currently do not have the throughput to meet manufacturing standards and require new exposure tools that could be prohibitively expensive. BCP lithography integrates well with current production infrastructure and is mainly limited by the time required for self-assembly, which can be on the order of seconds.14 However, BCP lithography still has many challenges to face before it can be used in commercial manufacturing. Arguably, the most important challenge is manipulating the natural, repeating structures of BCPs to form patterns with prescribed size, shape, and organization. For example, the servo zone of a hard disk drive must have a distinct design that is impossible to achieve using BCP self-assembly alone.15,16 Additional top-down processes, often referred to as “nanosculpting” or “photostructuring”, are key to defining the myriad of irregular shapes needed in most semiconductor device © XXXX American Chemical Society
Received: January 26, 2016 Accepted: March 9, 2016
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Future opportunities: In addition to orientation control, photopatternable interfaces have been critical in the development of DSA techniques. For example, photolithography has been used to define trenches that guide self-assembly via grapho-epitaxy.47,48 Additionally, lithography in tandem with other processes has been used to make directing guidelines for chemo-epitaxy.49−54 While some of these approaches are amenable for high volume manufacturing,55−57 they also require additional processing steps, such as trim etching, to define chemical guidelines with the correct dimensions. Chemical guidelines need to have a critical dimension between 0.5 and 1.5 L0 and a pitch of n × L0,58,59 where L0 is the periodic domain spacing of the BCP. As the L0 of the BCP approaches 10 nm, photopatterning guidelines becomes infeasible due to the resolution limits of photolithography. To circumvent this issue, a trim etch step is often used to shrink the critical dimension of the guidelines. However, trim etching processes can be difficult to control. We envision a process that uses directly photopatternable interfaces to simplify the current DSA schemes and avoid additional processing steps. A hypothetical DSA scheme that could be very promising is shown in Figure 2. Here, a P2N surface treatment is irradiated so that the exposed area is rendered neutral. During this transformation, the photoacid diffuses laterally in the film and reacts with areas that were initially not exposed, thereby shrinking the unexposed areas. If the preferential, unexposed areas can be shortened in width to the order of 0.5−1.5L0, they can function as guidelines for an overlying BCP film. Whereas most DSA schemes focus on patterning the guideline directly, this scheme focuses on patterning the interstitial region between the unexposed guidelines. This introduces two advantages: (1) It significantly relaxes the photopatterning resolution requirements, and (2) it eliminates the trim etch process step. While material libraries for this type of process already exist, DSA using this technique has yet to be demonstrated. The main obstacle for this process is controlling the alignment of the domains to form features that are parallel to the edge of the patterned interface. Often, the BCP aligns to form the perpendicular “ladder” structure seen in the N2P image of Figure 1. An understanding of the process variables that control this alignment is crucial. Opportunities for further research also include studying the kinetics of acid diffusion through the interfacial layer and the shape of the resulting diffusion profiles. Another interesting technique for photostructuring BCP films is molecular transfer printing (MTP). Similar to the processes described previously, MTP modifies the chemistry of an interface to precisely control the orientation of BCP domains. Most photostructuring processes rely on a masktemplated radiation source to define a guiding pattern for BCP alignment. However, MTP uses interfacial reactions between a BCP and an adjoining interface to graft a small portion of the BCP film to another surface. After separating the “master” BCP film from the chemically functionalized replica, a new BCP film can be deposited on top of the replica. Annealing the new BCP film reproduces the pattern of the original master template. The first example of MTP used BCP films saturated with low molecular weight homopolymers as transfer inks.60 Later works demonstrated that transfer could be accomplished using a photopatternable conformal layer,61−63 which physically grafts to the BCP film and removes a small portion of the film providing the chemical template for daughter BCP films. MTP has not yet been used explicitly for defining locally patterned
custom arrangement of BCP domains with parallel or perpendicular orientations, and it may be desirable to transfer BCP patterns in some but not all areas of a substrate. This is particularly important for transistor design,10 making plasmonic waveguides,35 and patterning servo zones on magnetic media in the data storage industry.36,37 “Block out” strategies have been developed to enable pattern transfer in select areas. However, these processes require several complex steps, including e-beam patterning, which can be prohibitive for large scale manufacturing.38 Alternatively, spatial control over the orientation of lamellar domains could also enable the design of custom patterns, because only the perpendicular domains are capable of pattern transfer by etch processes. Therefore, controlling the surface chemistry of specific regions to change the orientation of BCP domains locally is very valuable. Several strategies for controlling areas of BCP domain orientation using radiation have been reported. The selective cross-linking of substrate surface treatments via UV light,30,39 X-ray sensitive monolayers,40 e-beam sensitive surface treatments,41 and area-selected oxidized surfaces42−44 have all been explored. Work in our lab has focused on leveraging traditional photolithographic techniques to locally modify the interaction of the surface treatment and top coats with the BCP. These patternable surface treatments and topcoats were designed to be inherently sensitive to catalytic amounts of photoacid, which cause a drastic change in the chemistry of the polymers and thus their wetting characteristics.45,46 Figure 1 showcases the
Figure 1. BCP self-assembly on materials that become neutral upon exposure (P2N) and preferential upon exposure (N2P).
self-assembly of poly(styrene-block-4-trimethylsilylstyrene) on two different photoacid-sensitive surface treatments that were patterned by 193 nm lithography. In the top image, the surface is inherently neutral (i.e., both BCP domains have the same affinity to wet or contact the interface). Upon exposure to strong acid, the surface becomes highly preferential for one of the BCP domains (N2P). This typically induces a BCP domain orientation that is parallel to that interface. In the bottom image, the surface treatment is inherently preferential but becomes neutral upon exposure to photoacid (P2N). These two strategies confer control over where the perpendicular domains are formed. 461
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Figure 2. Theoretical directed self-assembly technique. A photoacid-labile surface treatment containing photoacid generator is exposed and photoacid is generated. Photoacid reacts with the surface treatment to produce neutral regions. Lateral acid diffusion shrinks the unexposed areas. BCP is applied and the unexposed areas act as guidelines and direct the self-assembly during thermal annealing.
areas on a substrate. However, it may be enabling for imprint lithography, since large area exposures are required in this process and generating new imprint templates is often a limiting step. Another attractive aspect of MTP is that the master BCP film is typically directed with a sparse chemical guide pattern, while the replica formed by grafting BCP chains must necessarily possess a dense guide pattern (i.e., where every domain of the subsequently applied BCP film is directed by exactly one guide stripe). This suggests MTP could be useful in multiplying the density of guide patterns, which could assist in further reducing defects. Photopatternable BCPs: BCPs with photoreactive functional groups can be directly patterned to induce either chemical or physical changes. Some photochemical transformations cause changes in thin film morphology;64,65 however, most reported photopatterning processes involve changing the solubility of one BCP domain.66−68 When irradiation breaks covalent bonds and causes one domain to become more soluble, the remaining film can function as a template for subsequent pattern transfer.69−72 This is similar to the role of a positive tone resist for traditional photolithographic processes. Conversely, a negative tone BCP cross-links upon exposure to light, allowing the unexposed regions of the film to be developed.73−76 Below, we highlight the advantages and disadvantages of each system, as well as some recent examples of each. Negative tone: The same strategy used to immobilize selective portions of BCP surface treatments can also be used to cross-link one or both domains of a BCP. Azides,77 benzocyclobutenes,78 epoxides,68,75 and other small molecule additives73,74,76 can be incorporated into a BCP, then activated to induce a cross-linking reaction. Once a portion of the BCP film is rendered insoluble, the rest of the film can be removed and replaced with a different BCP,75 or subjected to a treatment that causes a change in morphology,79 as seen in Figure 3. Both processes increase the diversity of feature sizes and shapes available in a single patterning step. However, using a negative tone BCP for patterning also necessitates a second etch or development step to remove the un-cross-linked domain. In addition, increasing the complexity of the polymer
Figure 3. Scheme and SEM images showing two cross-linkable PSPMMA BCPs with different periodicities deposited on the same wafer. Reproduced with permission from ref 74. Copyright 2008 ACS.
system by incorporating more functional units can make BCP synthesis more challenging. Positive tone: Positive tone photopatterning is accomplished by either degrading one domain of the BCP66,80,81 or by cleaving a photolabile linker that tethers the blocks together.67,69−72,82−84 These materials are effective for applications where BCP features need to be transferred into an underlying substrate, since only one development step is required to create a sacrificial template for pattern transfer. Seminal work by Thurn-Albrecht et al.66 demonstrated that PMMA domains could be removed from a PS-PMMA BCP film by exposure to light followed by development in a selective solvent. Degradation can also be achieved by cleaving a pendant group from the polymer backbone.85,86 The residue left in the patterned block can be used as a functionalized platform for deposition/reaction of other small molecules, including those that may enhance etch contrast. While this strategy is effective for certain chemical moieties, it cannot be used as a generic strategy for positive tone photopatterning. A more general 462
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ACS Macro Letters method involves attaching both blocks to a photocleavable junction. The linker is often based on o-nitrobenzyl rearrangement photochemistry, but other photosensitive moieties have been used as well.87,88 When triggered, the linker decomposes, liberating one block from the other. The more soluble block is then dissolved by a selective solvent. While wet development processes often have perfect chemical selectivity versus a physical etch process, they can be disadvantageous for high aspect ratio features due to pattern collapse.89,90 As the size of BCP features continue to scale downward, the aspect ratio of the BCP template must increase for effective pattern transfer. Therefore, a different method for developing photopatterned BCP films should be developed to avoid damaging the template with liquid developers. Future opportunities: We believe that polymers capable of “photochemically-induced dry development” could be an important area for future work. In this process, a polymer is blended with a photoacid generator, exposed to light (patterned or flood exposures) and subjected to a postexposure bake. The combination of light and heat generates a strong acid catalyst that causes the polymer to completely depolymerize into low molecular weight byproducts, which are volatile enough to evaporate from the surface. Incorporated into a BCP architecture, the adjacent polymer domain would remain intact in the film to serve as the etch mask for subsequent pattern transfer. Since this type of development process relies on chemical reactions to create a relief pattern but uses no wet development step, dry developing materials could be an elegant solution for generating high aspect ratio BCP structures without pattern collapse. Several dry developing resists were developed in the 1980s for conventional positive tone photoresist applications, including poly(olefin sulfones),91 nitrocellulose,92 and various polyethers and polyesters.93 Arguably, the most sensitive of these resists is polyphthalaldehyde (PHA),94,95 which can be synthesized at low temperatures by the ring-closing polymerization of o-phthalaldehyde.96,97 Due to its low ceiling temperature, PHA readily depolymerizes by scission of the acetal bonds in the polymer backbone in the presence of minute quantities of acid catalyst.98 Unfortunately, this polymer’s extreme sensitivity makes it difficult to handle without inadvertent degradation, and it is too unstable for even the mildest thermal treatments. Unsubstituted PHA homopolymers are also prone to degradation in solution and have short shelf lives. However, PHA can be stabilized in solution by functionalizing the aromatic ring with electron-withdrawing substituents.99,100 Several attempts have been made to incorporate this polymer into a BCP structure,101,102 but the synthesis is challenging. The individual blocks must be polymerized separately and coupled together using, for example, a click chemistry-type reaction. Other stable dry developing resists have been developed as well, based on either polycarbonate103−106 or polyformal107,108 chemistries. Upon reacting with strong acid, these polymers decompose by eliminating small molecules from the polymer backbone. The structures of some examples of these polymers and their degradation products are shown in Scheme 1. The sensitivity and speed of these resists are typically enhanced by incorporating benzylic or allylic functionalities. The aromatic products of these reactions provide a strong driving force for decomposition. These resists are very sensitive to radiation when combined with a photoacid generator. These polymers are also thermally stable with accessible glass
Scheme 1. Example Structures of (a) Polyphthalaldehyde, (b) an Allylic Polycarbonate, and (c) a Benzylic Polyformal and the Degradation Products Produced by Each upon Reacting with Strong Acid and Heat
transitions for thermal annealing, making them attractive platforms for photopatternable BCPs. The main obstacle for these materials is the synthetic challenge associated with creating BCPs from benzylic or allylic polycarbonates and polyformals. These polymers are mainly synthesized by stepgrowth polymerization, which can be difficult to incorporate into a BCP synthesis scheme. For polymer chemists, an opportunity exists to invent a new synthetic pathway to these dry developing polymers based on chain-growth polymerization, which is more amenable toward low dispersity BCP synthesis. In summary, photopatternable BCPs and interfaces provide a route to defining a wide range of tailored patterns in a BCP film. A plethora of top-down strategies are available to polymer chemists for defining the size, shape, orientation, and alignment of BCP features. This field is primed for the introduction of new chemistries and processes that could help advance BCP lithography and make it an even more attractive technology for future microelectronics applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Nissan Chemical Industries, Lam Research, the ASTC, and the National Science Foundation (Grants EECS-1120823 and EEC-1160494) for financial support. M.J.M. thanks National Science Foundation Graduate Research Fellowship (Grant No. DGE-1110007) for financial support. C.G.W. thanks the Rashid Engineering Regents Chair and the Welch Foundation (Grant #F-1830) for partial financial support. C.J.E. thanks the Welch Foundation (Grant #F1709) for partial financial support. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the sponsors.
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ACS Macro Letters
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DOI: 10.1021/acsmacrolett.6b00075 ACS Macro Lett. 2016, 5, 460−465
