Dustin W. Janes

Ultrasmooth Polydopamine Modified Surfaces for Block Copolymer Nanopatterning on Flexible Substrates.

Nature has engineered universal, catechol-containing adhesives which can be synthetically mimicked in the form of polydopamine (PDA). In this study, PDA was exploited to enable the formation of block copolymer (BCP) nanopatterns on a variety of soft material surfaces. While conventional PDA coating times (1 h) produce a layer too rough for most applications of BCP nanopatterning, we found that these substrates could be polished by bath sonication in a weakly basic solution to form a conformal, smooth (root-mean-square roughness ∼0.4 nm), and thin (3 nm) layer free of large prominent granules. This chemically functionalized, biomimetic layer served as a reactive platform for subsequently grafting a surface neutral layer of poly(styrene-random-methyl methacrylate-random-glycidyl methacrylate) to perpendicularly orient lamellae-forming poly(styrene-block-methyl methacrylate) BCP. Moreover, scanning electron microscopy observations confirmed that a BCP nanopattern on a poly(ethylene terephthalate) substrate was not affected by bending with a radius of ∼0.5 cm. This procedure enables nondestructive, plasma-free surface modification of chemically inert, low-surface energy soft materials, thus overcoming many current chemical and physical limitations that may impede high-throughput, roll-to-roll nanomanufacturing.

Interactions between plasma and block copolymers used in directed self-assembly patterning

The directed self-assembly (DSA) of block copolymers offers a promising route for scaling feature sizes below 20 nm. At these small dimensions, plasmas are often used to define the initial patterns. It is imperative to understand how plasmas interact with each block in order to design processes with sufficient etch contrast and pattern fidelity. Symmetric lamella forming block copolymers including, polystyrene-b-poly(methyl methacrylate) and several high-χ silicon-containing and tin-containing block copolymers were synthesized, along with homopolymers of each block, and exposed to various oxidizing, reducing, and fluorine-based plasma processes. Etch rate kinetics were measured, and plasma modifications of the materials were characterized using XPS, AES, and FTIR. Mechanisms for achieving etch contrast were elucidated and were highly dependent on the block copolymer architecture. For several of the polymers, plasma photoemissions were observed to play an important role in modifying the materials and forming etch-resistant protective layers. Furthermore, it was observed for the silicon- and tin-containing polymers that an initial transient state exists, where the polymers exhibit an enhanced etch rate, prior to the formation of the etch-resistant protective layer. Plasma developed patterns were demonstrated for the differing block copolymer materials with feature sizes ranging from 20 nm down to approximately 5 nm.

Orthogonally Spin-Coated Bilayer Films for Photochemical Immobilization and Patterning of Sub-10-Nanometer Polymer Monolayers

Versatile and spatiotemporally controlled methods for decorating surfaces with monolayers of attached polymers are broadly impactful to many technological applications. However, current materials are usually designed for very specific polymer/surface chemistries and, as a consequence, are not very broadly applicable and/or do not rapidly respond to high-resolution stimuli such as light. We describe here the use of a polymeric adhesion layer, poly(styrene sulfonyl azide-alt-maleic anhydride) (PSSMA), which is capable of immobilizing a 1-7 nm thick monolayer of preformed, inert polymers via photochemical grafting reactions. Solubility of PSSMA in very polar solvents enables processing alongside hydrophobic polymers or solutions and by extension orthogonal spin-coating deposition strategies. Therefore, these materials and processes are fully compatible with photolithographic tools and can take advantage of the immense manufacturing scalability they afford. For example, the thicknesses of covalently grafted poly(styrene) obtained after seconds of exposure are quantitatively equivalent to those obtained by physical adsorption after hours of thermal equilibration. Sequential polymer grafting steps using photomasks were used to pattern different regions of surface energy on the same substrate. These patterns spatially controlled the self-assembled domain orientation of a block copolymer possessing 21 nm half-periodicity, demonstrating hierarchical synergy with leading-edge nanopatterning approaches.

Precision Marangoni-driven patterning

A Marangoni flow is shown to occur when a polymer film possessing a spatially-defined surface energy pattern is heated above its glass transition to the liquid state. This can be harnessed to rapidly manufacture polymer films possessing prescribed height profiles. To quantify and verify this phenomenon, a model is described here which accurately predicts the formation, growth, and eventual dissipation of topographical features. The model predictions, based on numerical solutions of equations governing thin film dynamics with a Marangoni stress, are quantitatively compared to experimental measurements of thin polystyrene films containing photochemically patterned surface energy gradients. Good agreement between the model and the data is achieved at temperatures between 120 and 140 °C for a comprehensive range of heating times using reasonable physical properties as parameter inputs. For example, thickness variations that measure 102% of the starting film thickness are achieved in only 12 minutes of heating at 140 °C, values that are predicted by the model are within 6% and 3 min, respectively. The photochemical pattern that directed this flow possessed only a 0.2 dyne cm(-1) variation in surface tension between exposed and unexposed regions. The physical insights from the validated model suggest promising strategies to maximize the aspect ratio of the topographical features and minimize the processing time necessary to develop them.

Plasma etch of block copolymers for lithography

To date, the semiconductor manufacturing industry has relied on optical lithography to enable the scaling of devices to ever smaller dimensions. Developers use extreme UV lithography to print very small features, but this technology has suffered numerous technical delays and is still not feasible for highvolume manufacturing. The industry is therefore pursuing new schemes, such as multiple patterning, that are intended to improve 193nm immersion lithography (the technique that uses a liquid with a relatively high refractive index to enhance resolution). These approaches have enabled successful fabrication of features smaller than the resolution limit of the 193nm immersion tools. However, their drawbacks are increased process complexities and higher costs. One potentially lower-cost alternative for generating smaller structures is the directed self-assembly (DSA) of block copolymers (BCPs),1, 2 where a BCP is deposited and aligned and one block is removed to form a pattern. There has been tremendous progress in advancing DSA patterning, yet significant hurdles remain to its commercial adoption. These include the need to reduce patterning defects and to integrate these materials into robust patterning schemes. The BCPs need to show resistance to the reactive ion etch (RIE) plasmas that are used to pattern the underlying materials. Furthermore, at such small dimensions, the initial pattern is often created using a dry RIE plasma (rather than a wet process in order to prevent collapse of the mask from capillary forces during drying). Therefore, it is necessary to have high etch contrast between the individual blocks of the BCP, and because welloriented BCP film thicknesses are typically less than 50nm. Figure 1. Scanning electron microscope (SEM) image of a crosssection of self-assembled poly(styrene-block-methyl methacrylate) (PSb-PMMA) with 21nm domains prior to PMMA removal (a) and post reactive ion etch (RIE) removal of PMMA domains (b). Minimal mask loss of polystyrene was observed.

Spatially-controllable and uniform photochemical transfer printing of block copolymer nanopatterns

Transfer printing processes with resolution approaching 10 nm are not very common because pattern fidelity is often lost due to translational motion of the patterning molecules. To overcome this challenge, we describe here a photochemical transfer printing (PTP) process in which covalent bonds are formed between a nanopatterned master film formed by self-assembled poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) and solution-deposited conformal layer poly(styrenesulfonylazide-alt-maleic anhydride) in the solid state, where the pattern is fully stable. The monolayer of grafted PS-b-PMMA is then transferred to an initially blank replica film using a photopolymerizable liquid conformal layer possessing very low viscosity (hexanedioldiacrylate, 9 cP), which is known to promote dimensional uniformity in UV nanoimprint technology. The chemical nanopatterns transferred to the replica substrate is continuous and robust enough to successfully direct the self-assembly of new PS-b-PMMA films cast upon it after thermal annealing. The experiments in this work demonstrate patterning resolution down to 14 nm half-periodicity. The PTP process is mechanistically controlled by light and therefore can be spatially controlled by photomasks.