Jae-Byum Chang

Complex self-assembled patterns using sparse commensurate templates with locally varying motifs

Templated self-assembly of block copolymer thin films can generate periodic arrays of microdomains within a sparse template, or complex patterns using 1:1 templates. However, arbitrary pattern generation directed by sparse templates remains elusive. Here, we show that an array of carefully spaced and shaped posts, prepared by electron-beam patterning of an inorganic resist, can be used to template complex patterns in a cylindrical-morphology block copolymer. We use two distinct methods: making the post spacing commensurate with the equilibrium periodicity of the polymer, which controls the orientation of the linear features, and making local changes to the shape or distribution of the posts, which direct the formation of bends, junctions and other aperiodic features in specific locations. The first of these methods permits linear patterns to be directed by a sparse template that occupies only a few percent of the area of the final self-assembled pattern, while the second method can be used to selectively and locally template complex linear patterns.

A path to ultranarrow patterns using self-assembled lithography.

The templated self-assembly of block copolymer (BCP) thin films can generate regular arrays of 10-50 nm scale features with good positional and orientational accuracy, but the ordering, registration and pattern transfer of sub-10-nm feature sizes is not well established. Here, we report solvent-annealing and templating methods that enable the formation of highly ordered grating patterns with a line width of 8 nm and period 17 nm from a self-assembled poly(styrene-b-dimethylsiloxane) (PS-PDMS) diblock copolymer. The BCP patterns can be registered hierarchically on a larger-period BCP pattern, which can potentially diversify the available pattern geometries and enables precise pattern registration at small feature sizes. Sub-10-nm-wide tungsten nanowires with excellent order and uniformity were fabricated from the self-assembled patterns using a reactive ion etching process.

Assembly of sub-10-nm block copolymer patterns with mixed morphology and period using electron irradiation and solvent annealing.

Block copolymer self-assembly generates patterns with periodicity in the ∼10-100 nm range and is increasingly recognized as a route to lithographic patterning beyond the resolution of photolithography. Block copolymers naturally produce periodic patterns with a morphology and length-scale determined by the molecular architecture, and considerable research has been carried out to extend the range of patterns that can be produced from a given block copolymer, but the ability to control the period of the pattern over a wide range and to achieve complex structures with mixed morphologies from a given block copolymer is limited. Here we show how patterns consisting of coexisting sub-10-nm spheres and cylinders and sphere patterns with a range of periods can be created using a combination of serial solvent anneal processes and electron-beam irradiation of selected areas of a film of poly(styrene-block-dimethylsiloxane). These techniques extend the capabilities of block copolymer lithography, enabling complex aperiodic nanoscale patterns to be formed from a single block copolymer thin film.

Three‐Dimensional Nanofabrication by Block Copolymer Self‐Assembly

Thin films of block copolymers are widely seen as enablers for nanoscale fabrication of semiconductor devices, membranes, and other structures, taking advantage of microphase separation to produce well-organized nanostructures with periods of a few nm and above. However, the inherently three-dimensional structure of block copolymer microdomains could enable them to make 3D devices and structures directly, which could lead to efficient fabrication of complex heterogeneous structures. This article reviews recent progress in developing 3D nanofabrication processes based on block copolymers.

Nanoscale imaging of RNA with expansion microscopy

The ability to image RNA identity and location with nanoscale precision in intact tissues is of great interest for defining cell types and states in normal and pathological biological settings. Here, we present a strategy for expansion microscopy of RNA. We developed a small-molecule linker that enables RNA to be covalently attached to a swellable polyelectrolyte gel synthesized throughout a biological specimen. Then, postexpansion, fluorescent in situ hybridization (FISH) imaging of RNA can be performed with high yield and specificity as well as single-molecule precision in both cultured cells and intact brain tissue. Expansion FISH (ExFISH) separates RNAs and supports amplification of single-molecule signals (i.e., via hybridization chain reaction) as well as multiplexed RNA FISH readout. ExFISH thus enables super-resolution imaging of RNA structure and location with diffraction-limited microscopes in thick specimens, such as intact brain tissue and other tissues of importance to biology and medicine.

Aligned Sub-10-nm Block Copolymer Patterns Templated by Post Arrays

Self-assembly of block copolymer films can generate useful periodic nanopatterns, but the self-assembly needs to be templated to impose long-range order and to control pattern registration with other substrate features. We demonstrate here the fabrication of aligned sub-10-nm line width patterns with a controlled orientation by using lithographically formed post arrays as templates for a 16 kg/mol poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) diblock copolymer. The in-plane orientation of the block copolymer cylinders was controlled by varying the spacing and geometry of the posts, and the results were modeled using 3D self-consistent field theory. This work illustrates how arrays of narrow lines with specific in-plane orientation can be produced, and how the post height and diameter affect the self-assembly.

Design rules for self-assembled block copolymer patterns using tiled templates

Directed self-assembly of block copolymers has been used for fabricating various nanoscale patterns, ranging from periodic lines to simple bends. However, assemblies of dense bends, junctions and line segments in a single pattern have not been achieved by using sparse templates, because no systematic template design methods for achieving such complex patterns existed. To direct a complex pattern by using a sparse template, the template needs to encode the key information contained in the final pattern, without being a simple copy of the pattern. Here we develop a set of topographic template tiles consisting of square lattices of posts with a restricted range of geometric features. The block copolymer patterns resulting from all tile arrangements are determined. By combining tiles in different ways, it is possible to predict a relatively simple template that will direct the formation of non-trivial block copolymer patterns, providing a new template design method for a complex block copolymer pattern.

Sub-10 nm structures on silicon by thermal dewetting of platinum

A study of the dewetting behavior of platinum-thin-films on silicon was carried out to determine how variation of dewetting parameters affects the evolution of film morphology and to pinpoint which parameters yielded the smallest, most circular features. Platinum film thickness as well as dewetting time and temperature were varied and the film morphology characterized by means of scanning electron microscopy (SEM) analysis. Two different pathways of dewetting predicted in the literature (Vrij 1966 Discuss. Faraday Soc. 42 23, Becker et al 2003 Nat. Mater. 2 59-63) were observed. Depending on the initial criteria, restructuring of the film occurred via hole or droplet formation. With increased annealing time, a transition from an intermediate network structure to separated islands occurred. In addition, the formation of multilayered films, silicide crystals and nanowires occurred for certain parameters. Nevertheless, the dewetting behavior witnessed could be related to physical processes. Droplets with a mean diameter of 9 nm were formed by using a 1.5 nm thick platinum film annealed at 800 °C for 30 s. To demonstrate the suitability of the annealed films for further processing, we then used the dewetted films as masks for reactive ion etching to transfer the pattern into the silicon substrate, forming tapered nanopillars.

Highly ordered square arrays from a templated ABC triblock terpolymer.

Square-symmetry patterns are of interest in nanolithography but are not easily obtained from self-assembly of a diblock copolymer. Instead, we demonstrate highly ordered 44 nm period square patterns formed in a thin film of polyisoprene-block-polystyrene-block-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer blended with 15% PS homopolymer by controlling the film thickness, solvent anneal conditions, the surface chemistry and topography of the substrates. The square patterns consist of PFS pillars that remained after removal of the PI and PS with an oxygen plasma. On an unpatterned smooth substrate, the average grain size of the square pattern was increased dramatically to several micrometers by the use of brush layers and specific solvent anneal conditions. Templated self-assembly of well-ordered square patterns was demonstrated on substrates containing nanoscale topographical sidewalls and posts, written by electron beam lithography, in which the sidewalls and base of the substrate were independently chemically functionalized.

Hybrid Microscopy: Enabling Inexpensive High-Performance Imaging through Combined Physical and Optical Magnifications

To date, much effort has been expended on making high-performance microscopes through better instrumentation. Recently, it was discovered that physical magnification of specimens was possible, through a technique called expansion microscopy (ExM), raising the question of whether physical magnification, coupled to inexpensive optics, could together match the performance of high-end optical equipment, at a tiny fraction of the price. Here we show that such “hybrid microscopy” methods—combining physical and optical magnifications—can indeed achieve high performance at low cost. By physically magnifying objects, then imaging them on cheap miniature fluorescence microscopes (“mini-microscopes”), it is possible to image at a resolution comparable to that previously attainable only with benchtop microscopes that present costs orders of magnitude higher. We believe that this unprecedented hybrid technology that combines expansion microscopy, based on physical magnification, and mini-microscopy, relying on conventional optics—a process we refer to as Expansion Mini-Microscopy (ExMM)—is a highly promising alternative method for performing cost-effective, high-resolution imaging of biological samples. With further advancement of the technology, we believe that ExMM will find widespread applications for high-resolution imaging particularly in research and healthcare scenarios in undeveloped countries or remote places.