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.

Lamellae-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films, with bulk period L 0, were directed to assemble on lithographically nanopatterned surfaces. The chemical pattern was comprised of “guiding” stripes of cross-linked polystyrene (X-PS) or poly(methyl methacrylate) (X-PMMA) mats, with width W, and interspatial “background” regions of a random copolymer brush of styrene and methyl methacrylate (P(S-r-MMA)). The fraction of styrene (f) in the brush was varied to control the chemistry of the background regions. The period of the pattern was L s. After assembly, the density of the features (domains) in the block copolymer film was an integer multiple (n) of the density of features of the chemical pattern, where n = L s/L 0. The quality of the assembled PS-b-PMMA films into patterns of dense lines as a function of n, W/L 0, and f was analyzed with top-down scanning electron microscopy. The most effective background chemistry for directed assembly with density multiplication corresponded to a brush chemistry (f) that minimized the interfacial energy between the background regions and the composition of the film overlying the background regions. The three-dimensional structure of the domains within the film was investigated using cross-sectional SEM and Monte Carlo simulations of a coarse-grained model and was found most closely to resemble perpendicularly oriented lamellae when W/L 0 ∼ 0.5–0.6. Directed self-assembly with density multiplication (n = 4) and W/L 0 = 1 or 1.5 yields pattern of high quality, parallel linear structures on the top surface of the assembled films, but complex, three-dimensional structures within the film.

We report the induction of perpendicularly oriented cylindrical domains in PS-b-PMMA block copolymer (BCP) films thicker than 100 nm by thermally annealing on a substrate modified with a random copolymer. The effects of annealing temperature, composition of the substrate-modifying random copolymer, and BCP film thickness on the morphology of PMMA cylinder forming PS-b-PMMA were studied. For BCP films thicker than 100 nm, the fabrication of perpendicular PMMA cylinders is highly dependent on both the substrate-modifying random copolymer and the annealing temperature as these two parameters control the interactions of the BCP with the substrate and the free surface, respectively. We found the best perpendicular structures to be created by using a random copolymer brush with a styrene fraction (F St) near 0.70 and an annealing temperature near 230 °C. Perpendicular cylinder structures were achieved in ∼300 nm thick films using these conditions. When the BCP film was thicker than 300 nm, nucleation and growth of the microdomains proceeded independently from each interface. We present scanning electron microscope (SEM) and cross-sectional transmission electron microscope (TEM) images of these perpendicular structures and explain the results on the basis of previous simulation reports.

ABSTRACT:We provide evidence that banks smooth income by managing provisions for loan losses and loan charge-offs in a coordinated fashion that varies across the bust and boom phases of the business cycle and across homogeneous and heterogeneous loan types. In particular, during the 1990s boom, we predict and find that banks accelerated provisioning for loan losses and made this less obvious by accelerating loan charge-offs, especially for homogenous loans for which charge-offs are determined using number-of-days-past-due rules. We also provide evidence that the valuation implications of banks' provisions for loan losses and loan charge-offs vary across the phases of the business cycle and loan types reflecting the effect of these factors on banks' income smoothing. In particular, during the 1990s boom, we predict and find that charge-offs of homogenous loans have a positive association with current returns and future cash flows, because these charge-offs are recorded primarily by healthy banks with good future prospects reducing over-stated allowances for loan losses. We also predict and find that these charge-offs have a positive association with future returns that is explained by their positive association with future net income and recoveries. Our results are consistent with the market only partially appreciating healthy banks' overstatement of charge-offs of homogeneous loans based on number-of-days-past-due rules during the 1990s boom, because of the perceived nondiscretionary nature of these charge-offs.

Chemically patterned surfaces comprised of polymer mats and brushes of well-defined chemistry were fabricated at the length scale of 10 nm. A key concept is the integration of new materials, cross-linked polymer mats, with traditional lithographic processing. Resist was patterned on top of cross-linked polystyrene mats. After etching, regions of the remaining mat with dimensions ranging from 10 to 35 nm were separated by interspatial openings to the underlying substrate. End-grafted polymer brushes, in this case hydroxyl-terminated poly(2-vinylpyridine) or polystyrene−poly(methyl methacrylate) random copolymer, were grafted into the exposed, interspatial regions from films spin-coated over the patterned mat. Both block copolymer wetting studies, with polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), and near-edge X-ray fine structure spectroscopy showed that with sufficient cross-linking the polymer mat chemistry was unaffected by the subsequent grafting of the polymer brush. The precise definition of both the chemistry and the geometry was demostrated with two sensitive applications of nanoscale chemical patterns: the site-specific immobilization of Au nanoparticles and the directed assembly of overlying PS-b-PMMA films.