Paul F. Nealey

Epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates

Parallel processes for patterning densely packed nanometre-scale structures are critical for many diverse areas of nanotechnology. Thin films of diblock copolymers can self-assemble into ordered periodic structures at the molecular scale (∼5 to 50 nm), and have been used as templates to fabricate quantum dots, nanowires, magnetic storage media, nanopores and silicon capacitors. Unfortunately, perfect periodic domain ordering can only be achieved over micrometre-scale areas at best and defects exist at the edges of grain boundaries. These limitations preclude the use of block-copolymer lithography for many advanced applications. Graphoepitaxy, in-plane electric fields, temperature gradients, and directional solidification have also been demonstrated to induce orientation or long-range order with varying degrees of success. Here we demonstrate the integration of thin films of block copolymer with advanced lithographic techniques to induce epitaxial self-assembly of domains. The resulting patterns are defect-free, are oriented and registered with the underlying substrate and can be created over arbitrarily large areas. These structures are determined by the size and quality of the lithographically defined surface pattern rather than by the inherent limitations of the self-assembly process. Our results illustrate how hybrid strategies to nanofabrication allow for molecular level control in existing manufacturing processes.

Effects of synthetic micro- and nano-structured surfaces on cell behavior

Topographical cues, independent of biochemistry, generated by the extracellular matrix may have significant effects upon cellular behavior. Studies have documented that substratum topography has direct effects on the ability of cells to orient themselves, migrate, and produce organized cytoskeletal arrangements. Basement membranes are composed of extracellular matrix proteins and found throughout the vertebrate body, serving as substrata for overlying cellular structures. The topography of basement membranes is a complex meshwork of pores, fibers, ridges, and other features of nanometer sized dimensions. Synthetic surfaces with topographical features have been shown to influence cell behavior. These facts lead to the hypothesis that the topography of the basement membrane plays an important role in regulating cellular behavior in a manner distinct from that of the chemistry of the basement membrane. This paper describes the topography of the basement membrane and reviews the fabrication of synthetic micro- and nano-structured surfaces and the effects of such textured surfaces on cell behavior.

Density multiplication and improved lithography by directed block copolymer assembly

Self-assembling materials spontaneously form structures at length scales of interest in nanotechnology. In the particular case of block copolymers, the thermodynamic driving forces for self-assembly are small, and low-energy defects can get easily trapped. We directed the assembly of defect-free arrays of isolated block copolymer domains at densities up to 1 terabit per square inch on chemically patterned surfaces. In comparing the assembled structures to the chemical pattern, the density is increased by a factor of four, the size is reduced by a factor of two, and the dimensional uniformity is vastly improved.

Epithelial contact guidance on well-defined micro- and nanostructured substrates

The human corneal basement membrane has a rich felt-like surface topography with feature dimensions between 20 nm and 200 nm. On the basis of these findings, we designed lithographically defined substrates to investigate whether nanotopography is a relevant stimulus for human corneal epithelial cells. We found that cells elongated and aligned along patterns of grooves and ridges with feature dimensions as small as 70 nm, whereas on smooth substrates, cells were mostly round. The percentage of aligned cells was constant on substrate tomographies with lateral dimensions ranging from the nano- to the micronscale, and increased with groove depth. The presence of serum in the culture medium resulted in a larger percentage of cells aligning along the topographic patterns than when no serum was added to the basal medium. When present, actin microfilaments and focal adhesions were aligned along the substrate topographies. The width of the focal adhesions was determined by the width of the ridges in the underlying substrate. This work documents that biologic length-scale topographic features that model features encountered in the native basement membrane can profoundly affect epithelial cell behavior.

Directed Self-Assembly of Block Copolymers for Nanolithography: Fabrication of Isolated Features and Essential Integrated Circuit Geometries

Self-assembling block copolymers are of interest for nanomanufacturing due to the ability to realize sub-100 nm dimensions, thermodynamic control over the size and uniformity and density of features, and inexpensive processing. The insertion point of these materials in the production of integrated circuits, however, is often conceptualized in the short term for niche applications using the dense periodic arrays of spots or lines that characterize bulk block copolymer morphologies, or in the long term for device layouts completely redesigned into periodic arrays. Here we show that the domain structure of block copolymers in thin films can be directed to assemble into nearly the complete set of essential dense and isolated patterns as currently defined by the semiconductor industry. These results suggest that block copolymer materials, with their intrinsically advantageous self-assembling properties, may be amenable for broad application in advanced lithography, including device layouts used in existing nanomanufacturing processes.

Block copolymers and conventional lithography

The lithographic process is arguably the key enabling technology for the digital age. Hundreds of millions of devices can be fabricated on a single chip because patterns with features as small as 50 nm can be written with a remarkable level of perfection, in registration with the underlying substrate, and with complex geometries. As the drive to pattern at ever shrinking length scales continues, however, new imaging materials may be required to meet manufacturing constraints. We highlight some of the recent advances in integrating self-assembling block copolymers into the conventional lithographic process to address issues of resolution and process control.

Biological length scale topography enhances cell-substratum adhesion of human corneal epithelial cells

The basement membrane possesses a rich 3-dimensional nanoscale topography that provides a physical stimulus, which may modulate cell-substratum adhesion. We have investigated the strength of cell-substratum adhesion on nanoscale topographic features of a similar scale to that of the native basement membrane. SV40 human corneal epithelial cells were challenged by well-defined fluid shear, and cell detachment was monitored. We created silicon substrata with uniform grooves and ridges having pitch dimensions of 400-4000 nm using X-ray lithography. F-actin labeling of cells that had been incubated for 24 hours revealed that the percentage of aligned and elongated cells on the patterned surfaces was the same regardless of pitch dimension. In contrast, at the highest fluid shear, a biphasic trend in cell adhesion was observed with cells being most adherent to the smaller features. The 400 nm pitch had the highest percentage of adherent cells at the end of the adhesion assay. The effect of substratum topography was lost for the largest features evaluated, the 4000 nm pitch. Qualitative and quantitative analyses of the cells during and after flow indicated that the aligned and elongated cells on the 400 nm pitch were more tightly adhered compared to aligned cells on the larger patterns. Selected experiments with primary cultured human corneal epithelial cells produced similar results to the SV40 human corneal epithelial cells. These findings have relevance to interpretation of cell-biomaterial interactions in tissue engineering and prosthetic design.

Graphoepitaxy of cylinder-forming block copolymers for use as templates to pattern magnetic metal dot arrays

We report a method to fabricate high-quality patterned magnetic dot arrays using block copolymer lithography, metal deposition, and a dry lift-off technique. Long-range order of cylindrical domains oriented perpendicular to the substrate and in hexagonal arrays was induced in the block copolymer films by prepatterning the substrate with topographic features and chemically modifying the surface to exhibit neutral wetting behaviour towards the blocks of the copolymer. The uniformity of the domain size and row spacing of block copolymer templates created in this way was improved compared to those reported in previous studies that used graphoepitaxy of sphere-forming block copolymers. The pattern of block copolymer domains was transferred to a pattern of magnetic metal dots, demonstrating the potential of this technology for the fabrication of patterned magnetic recording media.

Sub-50 nm period patterns with EUV interference lithography

We have used transmission diffraction gratings in an interferometric setup to pattern one-and two-dimensional periodic patterns with periods near 50 nm. The diffraction gratings were written with e-beam lithography. The exposures were made at 13.4 nm wavelength with undulator radiation, which provides spatially coherent radiation. This technique offered a multiplication of pattern frequency by a factor of 2 and √2 in the one-and two-dimensional cases, respectively. Interference lithography with gratings offers a number of advantages, including achromaticity and insensitivity to misalignment. The demonstrated structures include line/space patterns with 45 nm period and a square array of holes with 56 nm period.

Nanoscale topography of the corneal epithelial basement membrane and Descemet's membrane of the human

PURPOSE Quantitatively define and compare the nanoscale topography of the corneal epithelial basement membrane (anterior basement membrane) and Descemets membrane (posterior basement membrane) of the human. METHODS Human corneas not suitable for transplantation were obtained from the Wisconsin Eye Bank. The corneas were placed in 2.5 mM EDTA for 2.5 h or 30 min. for removal of the epithelium or endothelium, respectively. After removal of the overlying cells, specimens were fixed in 2% glutaraldehyde and either examined in this state by atomic force microscopy only or dehydrated through an ethanol series and prepared for transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). RESULTS The subepithelial and subendothelial basement membrane surfaces have a similar appearance that consists of an interwoven meshwork of fibers and pores. Topographic feature sizes were found to be in the nanometer size range with the epithelial basement membrane features larger and less densely packed than Descemets membrane features. The topographic features are fractile in nature and increase surface area for cell contact. CONCLUSION With the use of the TEM, SEM, and AFM, a detailed description of the surface topography of corneal epithelial basement membrane and Descemets membrane of the human cornea are provided. The significance of differences in corneal basement membrane topography may reflect differences in function of the overlying cells or may be related to differences in cell migration and turnover patterns between the epithelium and endothelium.