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Replica Molding at the Subnanometer Scale Using Elastomeric Polymers

Written by: 
Jeff Morse, Ph.D
Replica molding of patterns having <10 nm features remains a significant nanomanufacturing challenge since the feature sizes at these length scales approach those of the monomers typically used for replication. In this investigation, Elhadj and colleagues demonstrate that polymers having an average radius of ~1 nm, average bond length of ~0.2 nm, and average distance between cross links of ~1 nm are capable of replicating vertical features from a solid substrate having dimensions significantly smaller than the average monomer size.


Reviewed by Jeff Morse, Ph.D, National Nanomanufacturing Network

Replica molding is the transfer of topographical patterns from a master substrate onto a polymer or other material forming an inverse mold, followed by the formation of a replica by solidifying a liquid precursor over the inverse mold. In nanomanufacturing, nanoimprint lithography (NIL) is an excellent example of this technique, which has been used to create patterns down to ~10 nm quite reproducibly with good pattern fidelity. Yet replication of patterns having <10 nm features remains a significant challenge since the feature sizes at these length scales approach those of the monomers used for replication. Ultimately, the limits for replication will be determined by the interfacial energy of the polymer-surface contact, which will dictate whether a polymer can accommodate the curvature required to conform to molecular scale variations in surface topography. While replication of sub-10 nm features has been achieved previously, replication of molecular scale features remains less understood and significantly more difficult to achieve.

Elhadj Figure 2
(A) Schematic of the replication procedure of a crystal surface on the {100} and {101} oriented surfaces of KDP using a PDMS mold and PU replica. (B) Schematic of macroscopic KDP crystal showing location of the {100} and {101} faces. The elementary and macrosteps replicated on the KDP surface are located on the {100} face. (C) Photograph of the original KDP crystal supported on a stainless steel disk (15 mm diameter). The hole seen in the image is on the underside of the crystal and represents the original location of the seed crystal used during crystal growth. (D) A PU replica of the original crystal in (C) supported on a stainless steel disk. The hole of the seed crystal is now missing from the PU replica because the {100} and {101} surfaces are replicated in part (A).
Recently, Elhadj et. al. explored the use of regular arrays of steps on the faces of ionic crystals of single-molecule height as a surface with which to demonstrate mold replication using hard-polydimethylsiloxane (h-PDMS), an elastomeric material that exhibits very low interfacial free energy, flexibility, and resistance to contamination. Since the lower limit on replica molded features is ultimately determined by additional factors including atomic radii, molecular shapes, van der Waals interactions, and thermal and entropic effects, the authors chose a surface having regular, well-defined features at the molecular scale. Crystal surfaces used for this investigation included the {100} face of KH2PO4 (KDP), and {104} face of calcite (CaCO3), each having step heights of 0.37 and 0.31 nm respectively. Additionally, the authors implemented siloxane formulations in this study that had been previously optimized for high-resolution molding based on viscosity, modulus, and surface hardness. As a result, the h-PDMS enables replication of shallow features with higher fidelity than is possible with normal PDMS (n-PDMS) since the latter has higher compressibility that often leads to deformation of the molded feature. Additionally, h-PDMS has an inert surface that eliminates contamination and reduces adhesion thereby providing ease in release from both the crystal surface and polyurethane replicas formed.

For the replication process, the ionic crystals were mounted onto a stainless steel disc for ease of handling. H-PDMS was spin-coated onto the crystal after which the crystal was flipped upside down and brought into contact with an uncured n-PDMS substrate which was then cured. The ionic crystal was then pulled away from the h-PDMS after which polyurethane (PU) was poured into the h-PDMS mold and cured by UV light exposure. The PU replica was then pulled away from the h-PDMS mold. Atomic force microscopy was conducted to measure the features of the ionic crystal surface, first to ensure that no damage occurred during the process and second for characterization of the PU replica. For the KDP crystal, PU replica step heights of 0.38 nm were measured, in close agreement with the calibrated step for the master material, indicating no appreciable shrinkage of the polymers during curing which can result in deformation of the pattern.

In this investigation, the authors demonstrated that polymers having an average radius of ~1 nm, average bond length of ~0.2 nm, and average distance between cross links of ~1 nm are capable of replicating vertical features from a solid substrate having dimensions significantly smaller than the average monomer size. The angstrom scale features achieved by replication in this study represent a significant finding to the nanomanufacturing community, particularly from the standpoint of identifying new limits on achievable feature sizes possible using stamp based methods, most notably NIL techniques which are already included in the Semiconductor Industry roadmap. Further work in this area must additionally confirm effects of lateral dimension on replication processes as the characterization techniques typically used tips having radius of ~2 nm. Additionally, the impact of surface roughness of the original master mold will ultimately create additional challenges to quantify the limits of this technique.

Image reproduced with permission from Elhadj S, et.al. Nano Letters 2010 10(10):4140–4145. DOI: 10.1021/nl102409d. Copyright 2010 American Chemical Society.