Mark P. Stoykovich

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.

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.

Self-assembling resists for nanolithography

In this paper we present our approach for integrating block copolymers into the lithographic process so as to enable molecular-level control over the dimensions and shapes of nanoscale patterned resist features and simultaneously retain essential process attributes such as pattern perfection, registration, and the ability to create non-regular device-oriented structures. Combining self-assembling materials with advanced lithographic tools may allow current manufacturing techniques to be extended to the scale of 10 nm and below and meet the long-term requirements detailed in the International Technology Roadmap for Semiconductors (2004)

A two-dimensional model of the deformation of photoresist structures using elastoplastic polymer properties

A model was developed for predicting the collapse behavior of photoresist structures due to the drying of rinse liquids during wet chemical processing. The magnitude of the capillary forces was estimated using the classical thermodynamics of surface tension, and the deformation of the structure was modeled using beam bending mechanics that accounts for both elastic and plastic modes of deformation. The two-dimensional model can predict the critical beam height of collapse as a function of the wetting behavior of the rinse liquid on the beam, the elastic and plastic mechanical properties of the polymeric photoresist, and the beam dimensions. Collapse behavior was predicted for polymer nanostructures with elastoplastic mechanical properties similar to those of bulk poly(methyl methacrylate). We have compared the collapse predictions from our model with the results of models that account only for elastic or plastic deformation behavior. Regimes in the elastic-plastic mechanical property space for which it is...

Seed-mediated growth of shape-controlled wurtzite CdSe nanocrystals: platelets, cubes, and rods.

Prior investigations into the synthesis of colloidal CdSe nanocrystals with a wurtzite crystal structure (wz-CdSe) have given rise to well-developed methods for producing particles with anisotropic shapes such as rods, tetrapods, and wires; however, the synthesis of other shapes has proved challenging. Here we present a seed-mediated approach for the growth of colloidal, shape-controlled wz-CdSe nanoparticles with previously unobserved morphologies. The synthesis, which makes use of small (2-3 nm) wz-CdSe nanocrystals as nucleation sites for subsequent growth, can be tuned to selectively yield colloidal wz-CdSe nanocubes and hexagonal nanoplatelets in addition to nanorod and bullet-shaped particles. We thoroughly characterize the morphology and crystal structures of these new shapes, as well as discuss possible growth mechanisms in the context of control over surface chemistry and the nucleation stage.

Binary blends of diblock copolymers as an effective route to multiple length scales in perfect directed self-assembly of diblock copolymer thin films

The directed assembly of binary blends of diblock copolymers on chemically nanopatterned substrates was investigated as a means to pattern features of controlled dimensions at the nanoscale. The lamella-forming blends assembled without defects and in registration with underlying chemical surface patterns that had periods LS that were commensurate with the bulk lamellar period of the blends LB. LB was tuned between the bulk lamellar periods of the block copolymers LO1 and LO2.

Hydrogen silsesquioxane as a high resolution negative-tone resist for extreme ultraviolet lithography

Hydrogen silsesquioxane (HSQ) was evaluated as a high resolution negative-tone photoresist for extreme ultraviolet (EUV) lithography. The following imaging properties of HSQ were evaluated in EUV exposure: sensitivity, contrast, resolution, and line edge roughness (LER). In this article we report that HSQ has a sensitivity of 11.5mJ∕cm2 with a contrast of 1.64 in EUV exposure and is able to resolve 26nm dense lines (70nm thick film) with a LER of 5.1nm (3σ). These results, especially with regard to the sensitivity and low line edge roughness, imply that this class of materials may hold distinct advantages over traditional chemically amplified resists and should be further explored for application in EUV lithography.

Percolating transport and the conductive scaling relationship in lamellar block copolymers under confinement.

The topology and transport behavior of the lamellar morphology self-assembled by block copolymers in thin films are shown to depend on the length scale over which they are characterized and can be described by percolation in a network under confinement. Gold nanowires replicating the lamellar morphology were fabricated via self-assembled poly(styrene-block-methyl methacrylate) thin films and a lift-off pattern transfer process. The lamellar morphology exhibits long-range connectivity (macroscopic scale); however, characterization of electrical conduction over confined areas (5-500 μm) demonstrates a discrete probability of disconnection that arises due to the underlying network structure and a lack of self-similarity at these microscale dimensions. In particular, it is proved that the lamellar network morphology under confinement has a conductance that is nonlinear with channel length or width. The experimental results are discussed in terms of percolation theory, and a simple, two-dimensional Monte Carlo model is shown to predict the key trends in the network topology and conductance in lamellar block copolymers, including the dependencies on composition, extent of spatial confinement, and confinement geometry. These results highlight the need to exquisitely control or engineer the self-assembled nanostructured pathways formed by block copolymers to ensure consistent device performance for any application that depends upon percolating material, ionic, or electrical transport, especially when confined in any dimension. It is also concluded that the two most promising approaches for enhancing conductivity in block copolymer materials may be achieved either at the limits of (1) perfectly oriented, single-crystalline or (2) high defect density, polycrystalline microphase separated morphologies and that nanostructured systems with intermediate defect densities would be detrimental to transport in confined systems.

Measurement of the x-ray dose-dependent glass transition temperature of structured polymer films by x-ray diffraction

The glass transition temperature (Tg) of polymer nanostructures was measured using a technique based on synchrotron x-ray diffraction from periodic grating structures. Poly(methyl methacrylate) (PMMA) nanostructures consisting of 1:1 lines:spaces with a 100 nm period and 100 nm height were characterized to have a Tg of 118 °C, which is comparable to the Tg of PMMA in bulk systems. The Tg of the PMMA structures also was measured as a function of absorbed x-ray dose. Doses ranging from 0 to 2400 mJ/mm3 were delivered to the PMMA structures prior to the Tg measurements; the Tg of the structures was found to decrease from 118 °C to 95 °C, respectively. The dose dependence of the PMMA glass transition temperature can be attributed to changes in the polymer molecular weight under exposure to x rays.