Graphene-enabled block copolymer lithography transfer to arbitrary substrates

Block copolymer (BCP) films exhibit nanoscale phase separation on the order of 10–100 nm [1, 2]. This property has attracted a great deal of interest because it opens up the possibility of top-down patterning of materials at the nanoscale. In particular, a technique known as BCP lithography has been developed to take advantage of this possibility [3]. BCP lithography involves coating a surface with BCP, effecting the nanoscale phase separation, and using the variable-thickness phase-separated polymer as a mask for etching the underlying substrate [4]. Depending on conditions of polymer preparation, phase separation, and substrate surface functionalization, the patterns can exhibit a range of morphologies, with circular lattices, lines, and more complex shapes emerging as possibilities [5, 6]. BCP lithography therefore represents a powerful tool for facile generation of nanoscale patterned structures on surfaces. These surfaces may find use in microelectronics fabrication, membrane fabrication for nanofiltration, and photonic materials design and synthesis, among other applications [6, 7].

Despite its promise, a significant roadblock for BCP lithography is the sensitivity of the BCP nanoscale phase separation to the chemical makeup of the surface to be etched. This often necessitates preparation of the surface, methods of which include chemical and photolithographic pre-patterning and covalent grafting of random copolymers to ensure that the BCP wetting properties of the surface are optimal [8, 9]. In light of the wide variety of materials used in modern electronics, the need to develop surface preparation procedures that are specific to individual surface chemistries creates a bottleneck in developing BCP lithography into a microfabrication technique that can be applied generically across a range of materials.

In earlier work, we developed method for transferring chemically functionalized graphene from one substrate to another that is mild and compatible with a very broad range of substrates. Briefly, we showed that chemical hydrogenation of single layer graphene weakened the adhesion of the graphene to its substrate, allowing delamination of the material in water and relamination onto a new substrate [10]. Chemical functionalities, including alkyl and amino groups, that were initially present on the graphene were preserved during the transfer process; thus this technique enabled surface chemical functionalization of a substrate without actually having to perform chemistry on that substrate. Subsequent studies showed that the single layer hydrogenated graphene (SLHG) was mechanically strong enough to support small electronic and polymeric structures as well [11, 12].

Earlier work has shown that graphene and other materials can be effectively etched into nanopatterns using BCP lithography [13, 14]. In addition, the phase separation of BCPs on graphene has been examined for patterning purposes, [15, 16] and at least one group has reported transferring these BCP patterns to a substrate for subsequent lithographic processing [17]. In that report, the researchers used a wet etch method to remove a graphene oxide (GO) layer from a SiO₂ preparative substrate by etching with hydrofluoric acid, a caustic and poisonous chemical. Here we present an alternative method of BCP lithography transfer, based on our mild water lift-off of SLHG. This method is compatible with essentially any substrate, the only requirement being that the new substrate can be immersed in water. We also report a modification of graphene oxide that allows for a similar water-based delamination and relamination. Graphene oxide is water-dispersible and abundant; therefore, its use as a BCP lithography platform can be extended to very large area applications where SLHG is impractical and cost-prohibitive.

2.1. Hydrogenated graphene preparation

2.2. Graphene oxide preparation

2.3. BCP thin film preparation

2.4. Porous silicon nitride preparation with BCP mask

2.5. Data collection and measurments

We present a schematic of our workflow in figure 1. Briefly, we prepare SLHG or RGO on a convenient surface, usually SiO_x. We then deposit a BCP thin film onto the SLHG or RGO and phase-separate the BCP using solvent annealing. This phase separation segregates the BCP into two separate nanoscale blocks of each polymer. We then delaminate the BCP/graphene stack in water and relaminate onto an arbitrary substrate. The BCP remains phase-separated throughout the process since each block of BCP is essentially not soluble in water. Once the BCP/SLHG is relaminated, further selective swelling may generate patterns such as nanopores, and the material is then etched using conventional oxygen plasma or reactive ion etching. The etching proceeds more quickly through the thin regions of the BCP and into the underlying substrate. This fact accounts for the ability to use the BCP as a lithographic mask to transfer the phase-separated nanopatterns to the underlying substrate.

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We observed well-separated nanoscale phases using polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) as a representative BCP. The PS-b-P2VP was spun onto SLHG in a thin layer (See Materials and Methods) and phase separated using solvent annealing in THF for 5 min (figure 2(a)). This exploits the different solubilities of PS and P2VP in THF vapor; the phase separation is relatively fast [25]. This phase-separated BCP on SLHG is the patterned mask that could be subsequently delaminated in water for transfer onto new substrates. Figure 2(b) shows the surface morphology after relamination of the BCP/SLHG on the new SiO₂ substrate. We observed that the phase separation is retained to a high degree of order after the transfer process. The sample was then soaked in ethanol for 30 min to form pores. In ethanol, the P2VP blocks will be collapsed onto the wall of the PS blocks and the underlying SLHG to generate porous PS films as shown in figure 2(c). Subsequent oxygen plasma etching was undertaken to degrade the phase-separated P2VP inside the pores as well as the SLHG underneath [25]. The final structures contain the open-pore PS film as shown in figures 2(d) and (e). The pores were on the order of 20–40 nm in diameter with a pitch of 24.6 ± 2.1 nm. We characterized the thickness of the final film after 2 min oxygen plasma etching (figure 2(d)) and the film thickness was approximately 25 nm, which is comparable to the measured pore depth. The final structure is typically hexagonal close packed P2VP pores, as shown in figure 2(e). We also observed a somewhat disordered pore network consisting of misaligned patches of hexagonal close packed P2VP pores (figure 2(e)). That misalignment is present both before and after delamination and relamination and may arise from either the heterogeneity of the graphene hydrogenation causing inhomogeneous adhesion of the polymer blocks across the surface of the material.

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Figure 3 shows results from the complete procedure carried out with SiN_x as the target substrate of interest. We phase separated the BCP layer into two different patterns (pores and parallel cylinder 'fingerprints'). The fact that this transfer method allows us to etch two different BCP nanopatterns onto a substrate simultaneously illustrates the utility of the method. After phase separation and transfer to SiN_x (figure 3(a)), the material was treated with a CF₄ reactive ion etch (RIE) for 30 s. We observed a thin residue from the BCP mask after RIE, so we performed additional oxygen plasma etching followed by a sonication in acetone for 5 min to completely remove the residue (figure 3(b)). We used scanning electron microscopy (SEM) to image the different regions that had been covered with the BCP mask (figures 3(c)–(d)). The resulting images confirm that the characteristic BCP pore pattern had been etched into the SiN_x substrate to a pore depth of 20 nm (see Supplementary Information, SI figure S2. For comparison, AFM performed on the region outside the BCP mask showed a very rough surface characteristic of treatment with aggressive CF₄ RIE; therefore, the area shows no sign of regular pore formation (SI figures S2(b) and S2(c)). In addition, XPS analysis of the region with the mask clearly showed that the C 1 s peak weakens dramatically after RIE and subsequent O₂ plasma cleaning, indicating that the carbon-rich mask was removed and the porous surface generated by the etching was composed primarily of SiN_x (SI figure S1 (available online at stacks.iop.org/NANOX/2/014009/mmedia)). The XPS-measured atomic ratios of Si/C after transfer but before RIE and O₂ plasma cleaning were 0.0034 and 6.174 in the masked (SI figure S1(b)) and unmasked (SI figure S1(c)) areas, respectively. By contrast, the atomic ratios of Si/C after RIE and O₂ plasma cleaning were 17.42 and 12.48 in the masked (SI figure S1(e)) and unmasked (SI figure S1(f)) areas, respectively. The much more consistent elemental composition after plasma cleaning is reflective of the fact that the vast majority of the polymer mask has been eliminated during the cleaning process.

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To further examine the applicability of the process, we applied our method to a number of polymer substrates. Polyethylene (PE) and polystyrene (PS) are prototypical flexible, inert plastics. The BCP mask was successfully relaminated onto PE with the phase separation as shown in figure 4(a). The sample was then soaked in ethanol for 30 min to form pores (figure 4(b)), after which the BCP mask can be used for surface modification of PE with a dry etching process. Notably, we extended our approach to PS substrates. Both the PS and the BCP used here (PS-b-P2VP) are soluble in toluene, which makes simple application and phase separation of BCP on PS impossible. However, interposing the RGO layer enables integration of PS and nanopatterned BCP. Figure 4(c) shows relaminated BCP/RGO mask on 35 mm cell culture PS petri dish. The mask is phase-separated in a fingerprint structure (figure 4(d)) and ready for dry etching.

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Single-layer HG is only one atomic layer thick; therefore, any holes or tears in the SLHG expose the substrate underneath the graphene layer. Thus, wafer-scale delamination and relamination of phase-separated BCP (PS-b-P2VP) is infeasible with SLHG as a transfer layer since the polymer tends to adhere strongly to the exposed substrate through imperfections in the SLHG layer. This adhesion tends to cause substantial damage to the SLHG/BCP stack when delamination on water is attempted, as the BCP adhered to the substrate sticks and causes more tearing in the overlayers. To circumvent this issue, we employed spin-coated or dropcast films of reduced graphene oxide (RGO) as the transfer layer (thickness ∼10–100 nm). These films, while still thin, are thicker than SLHG and cover the substrate more completely, obviating the adhesion problem and enabling more robust transfer of BCP films greater than several inches across. We and others have shown elsewhere that dropcast RGO films can enable water-based delamination of sheets on the order of 1 ft² (∼600 cm²) with or without etchants [24, 30]. Importantly, another significant advantage over SLHG is that BCP masks can be delaminated after the open pores have been collapsed; therefore, additional ethanol soak and oxygen plasma etch processing steps after the mask has been moved to the target substrate are not necessary. In other words, all processes to generate BCP masks could be completed on the initial RGO-coated substrate, and once relaminated onto a new substrate, the mask will be immediately accessible for subsequent post-processing.

To illustrate the utility of RGO in large-area BCP lithography applications, we deposited GO across a 2-inch Si wafer and thermally annealed (in air at 250 °C for 30 s) to form mildly reduced GO, which is weakly adherent to its substrate [24]. We then spin-coated PS-b-P2VP onto the RGO film and phase separated the material using THF solvent annealing before transferring via delamination/relamination as previously described for SLHG above. Figure 5(a) shows the delamination of the entire 2-inch area of the open-pore PS-b-P2VP/RGO mask from SiO_x and relamination onto another wafer with minimal damage to the membrane. We tested two different phase separation conditions by varying the solvent annealing time. We obtained pores (figure 5(b)) annealed for 5–10 min and parallel cylinder structures (figure 5(c)) annealed for 45 min and successfully transferred the phase-separated BCP masks onto new large wafers. Therefore, this approach is applicable to wafer-scale surface modifications.

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As a final demonstration, we highlight that this approach can modify surfaces with complex patterns by overlaying multiple PS-b-P2VP/RGO masks. Figure 6(a) shows an image of two PS-b-P2VP/RGO layers sequentially phase separated, delaminated and stacked onto a SiN substrate. Each layer was phase separated with fingerprint structures before transfer to SiN, the second layer being relaminated onto a dried first layer. Figure 6(b) shows an AFM image of the etched SiN surface after CF₄ RIE for 2 min and subsequent cleaning with oxygen plasma for 5 min. The resulting overlapped fingerprint grooves—in two different fingerprint patterns—is apparent from the image. The appearance of amorphous residues in figure 6(b) is likely due to contaminants which are trapped between the two BCP/RGO layers during transfer. With delamination and relamination in an ultraclean environment, these residues might be less prominent. This result suggests that emergent complexity could be generated through overlapping lattices (e.g., moiré structures) by rationally designing multiple BCP/RGO layers. RGO is robust and can be tailored in any size and shape, as shown in figure 5, to modify surfaces with special patterns. Although the current result in figure 6 is from a non-oriented fingerprint BCP mask, potentially multiple layers of pre-defined masks can be developed by using our approach.

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We demonstrated the use of single-layer hydrogenated graphene or reduced graphene oxide (RGO) as thin support layers for phase-separated block copolymer (BCPs) and for nanoscale pattern transfer. We use weakly-bound graphene-based films to delaminate subsequent adlayers via a simple water lift-off. The mildness of the transfer technique allows phase-separated BCPs to be deposited on almost any substrate without concerns about contamination from harsh chemical etchants. We have shown the utility of this technique for employing phase-separated BCPs as lithographic masks for the substrates SiN and polyethylene, and we have shown that this technique enables transfer of BCP patterns to polystyrene, a result that is impossible without an interposing graphene layer. We also demonstrated that thin layers of RGO can be employed in wafer-scale BCP lithography on Si and SiO_x. Finally, we have shown that we can achieve complex hierarchical patterns by stacking multiple BCP mask layers, a technique that cannot be performed using standard BCP phase separation on a single substrate. Future development of this process will focus on thin-film techniques that are more adaptable to large areas, such as doctor blading. These techniques enable lithography of regular nanoscale patterns over large areas, and may find application in nanofiltration membranes, antireflective coatings, and photonically active materials.

This work was funded by the Office of Naval Research through Base programs at NRL.

The data that support the findings of this study are available upon reasonable request from the authors.