Shengxiang Ji

Molecular transfer printing using block copolymers

Soft lithographic techniques augment or enhance the capabilities of traditional patterning processes and expand the diversity of materials that can be patterned. Realization of robust parallel techniques for creating chemical patterns at the nanoscale has been challenging. Here we present a method for creating and replicating chemical patterns that uses functionalized homopolymer inks that are preferentially segregated into the nanodomains of phase-separated diblock copolymer films. The inks are transferred by reaction to substrates that are brought into contact with block copolymer films, creating chemical patterns on the substrate that mirror the domain structure present at the film surface with high fidelity and resolution. In addition to printing from self-assembled domain structures, we can also direct the assembly of the block copolymer films from which transfer occurs using lithographically defined masters so as to replicate and transfer patterns of inks with controlled and well-defined geometries. The transferred patterns may be at higher resolution than the lithographically defined master, and the process can be repeated to create multiple copies of identical replicas. Transfer of one ink from one block of the copolymer is also possible, and filling the interspatial regions of the pattern with a second ink provides a pathway toward creating patterns with diverse chemical functionalities.

Metal Nanodot Memory by Self-Assembled Block Copolymer Lift-Off

As information technology demands for larger capability in data storage continue, ultrahigh bit density memory devices have been extensively investigated. To produce an ultrahigh bit density memory device, multilevel cell operations that require several states in one cell have been proposed as one solution, which can also alleviate the scaling issues in the current state-of-the-art complementary metal oxide semiconductor technology. Here, we report the first demonstration of metal nanodot memory using a self-assembled block copolymer lift-off. This metal nanodot memory with simple low temperature processes produced an ultrawide memory window of 15 V at the +/-18 V voltage sweep. Such a large window can be adopted for multilevel cell operations. Scanning electron microscopy and transmission electron microscopy studies showed a periodic metal nanodot array with uniform distribution defined by the block copolymer pattern. Consequently, this metal nanodot memory has high potential to reduce the variability issues that metal nanocrystal memories previously had and multilevel cells with ultrawide memory windows can be fabricated with high reliability and manufacturability.

Preparation of poly(ethylene glycol) protected nanoparticles with variable bioconjugate ligand density

Maleimide-functional poly(ethylene glycol)-b-poly(epsilon-caprolactone) nanoparticles (NPs) were prepared via the Flash NanoPrecipitation technique. Subsequent reaction with a model ligand, bovine serum albumin (BSA), was conducted using thiol-maleimide conjugation. Reaction of up to 22% of NP surface maleimide-PEG tethers was obtained, with the percent conversion being essentially independent of the ratio of maleimide-PEG to methyl-PEG over the range 30-100%, respectively. At the highest surface coverage, BSA is calculated to essentially cover the NP surface area. Reaction parameters (reaction order and docking constant) describing the extent of ligand conjugation were determined. The reaction order is applicable to the conjugation of ligands presenting free thiol functionalities, while the value of the docking constant is ligand-dependent and accounts for physical and dynamic properties of the ligand-PEG interaction. Jointly, the particle formation process, using block copolymer-directed kinetically controlled assembly and surface functionalization represent a versatile new platform for the preparation of bioconjugated NPs with accurate control of ligand density and minimal processing steps.

Three‐dimensional Directed Assembly of Block Copolymers together with Two‐dimensional Square and Rectangular Nanolithography

Self-assembling sphere-forming block copolymers that normally adopt a hexagonal packing in thin films are directed to assemble on chemically patterned surfaces into their three-dimensional bulk-like body-centered cubic morphology; two-dimensional manufacturing-relevant patterns with square and rectangular symmetry can be derived from the films using molecular transfer printing at a resolution beyond the limits of current lithographic tools and materials.

Directed Assembly of Non-equilibrium ABA Triblock Copolymer Morphologies on Nanopatterned Substrates

The majority of past work on directed assembly of block copolymers on chemically nanopatterned surfaces (or chemical patterns) has focused on AB diblock copolymers, and the resulting morphologies have generally corresponded to equilibrium states. Here we report a study on directed assembly of ABA triblock copolymers. Directed assembly of thin films of symmetric poly(methyl methacrylate-b-styrene-b-methyl methacrylate) (PMMA-b-PS-b-PMMA) triblock copolymers is shown to be capable of achieving a high degree of perfection, registration, and accuracy on striped patterns having periods, L(s), commensurate with the bulk period of the copolymer, L(o). When L(s) is incommensurate with L(o), the triblock copolymer domains can reach dimensions up to 55% larger or 13% smaller than L(o). The range over which triblock copolymers tolerate departures from a commensurate L(s) is significantly larger than that accessible with the corresponding diblock copolymer material on analogous directed assembly systems. The assembly kinetics of the triblock copolymer is approximately 3 orders of magnitude slower than observed in the diblock system. Theoretically informed simulations are used to interpret our experimental observations; a thermodynamic analysis reveals that triblocks can form highly ordered, non-equilibrium metastable structures that do not arise in the diblock.

Modification of a polystyrene brush layer by insertion of poly(methyl methacrylate) molecules

The ability to tune the wetting behavior of poly(styrene-block-methyl methacrylate) on a polystyrene (PS) brush by insertion of hydroxyl-terminated poly(methyl methacrylate) (PMMA-OH) was demonstrated. The brush properties before and after insertion of PMMA-OH were studied with goniometry, ellipsometry, near edge x-ray absorption fine structure spectroscopy, and scanning electron microscopy. The initial PS brush served as a barrier to the grafting of PMMA onto the substrate. The amount of PMMA that could penetrate through the PS brush barrier and graft onto the substrate depended on the initial PS brush thickness. As a result, the PS:PMMA ratio in the final composite brush, and therefore the brush wetting properties, could be carefully controlled. The final composition also depended on the grafting sequence of the brush molecules. This may offer a generalized approach for fabricating neutral brush surfaces without requiring the synthesis of specific random copolymers.

Directed Self-Assembly of Polystyrene-b-poly(propylene carbonate) on Chemical Patterns via Thermal Annealing for Next-Generation Lithography.

Directed self-assembly (DSA) of block copolymers (BCPs) combines advantages of conventional photolithography and polymeric materials and shows competence in semiconductors and data storage applications. Driven by the more integrated, much smaller and higher performance of the electronics, however, the industry standard polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) in DSA strategy cannot meet the rapid development of lithography technology because its intrinsic limited Flory-Huggins interaction parameter (χ). Despite hundreds of block copolymers have been developed, these BCPs systems are usually subject to a trade-off between high χ and thermal treatment, resulting in incompatibility with the current nanomanufacturing fab processes. Here we discover that polystyrene-b-poly(propylene carbonate) (PS-b-PPC) is well qualified to fill key positions on DSA strategy for the next-generation lithography. The estimated χ-value for PS-b-PPC is 0.079, that is, two times greater than PS-b-PMMA (χ = 0.029 at 150 °C), while processing the ability to form perpendicular sub-10 nm morphologies (cylinder and lamellae) via the industry preferred thermal-treatment. DSA of lamellae forming PS-b-PPC on chemoepitaxial density multiplication demonstrates successful sub-10 nm long-range order features on large-area patterning for nanofabrication. Pattern transfer to the silicon substrate through industrial sequential infiltration synthesis is also implemented successfully. Compared with the previously reported methods to orientation control BCPs with high χ-value (including solvent annealing, neutral top-coats, and chemical modification), the easy preparation, high χ value, and etch selectivity while enduring thermal treatment demonstrates PS-b-PPC as a rare and valuable candidate for advancing the field of nanolithography.

Mechanical properties of polymeric nanostructures fabricated through directed self-assembly of symmetric diblock and triblock copolymers

Grating arrays of polystyrenic nanostructures were fabricated by directed assembly of lamellae-forming poly(styrene-b-methyl methacrylate) diblock and poly(methyl methacrylate-b-styrene-b-methyl methacrylate) triblock copolymer films on chemical patterns and subsequent removal of polyacrylic regions by soft x-ray blanket exposure and fluid development. The collapse of gratings induced by capillary forces in a fluid rinse was observed when the aspect ratio of gratings was above a critical value or the critical aspect ratio of collapse (CARC). In stark contrast to the performance of traditional polymer photoresists, the CARC of gratings fabricated from block copolymers decreased monotonically with increasing LS. For a given pattern period (LS), the CARC of polystyrenic gratings fabricated from diblock copolymers was larger than that of gratings fabricated from an analogous triblock copolymer. The apparent elastic moduli of gratings that were calculated from CARC data using an elastic cantilever beam bending...