Charles T. Black

Integration of self-assembled diblock copolymers for semiconductor capacitor fabrication

We combine a self-organizing diblock copolymer system with semiconductor processing to produce silicon capacitors with increased charge storage capacity over planar structures. Our process uses a diblock copolymer thin film as a mask for dry etching to roughen a silicon surface on a 30 nm length scale, which is well below photolithographic resolution limits. Electron microscopy correlates measured capacitance values with silicon etch depth, and the data agree well with a geometric estimate. This block copolymer nanotemplating process is compatible with standard semiconductor processing techniques and is scalable to large wafer dimensions.

Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells

Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells.

Efficient Organic Solar Cells with Helical Perylene Diimide Electron Acceptors

We report an efficiency of 6.1% for a solution-processed non-fullerene solar cell using a helical perylene diimide (PDI) dimer as the electron acceptor. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor-acceptor interfaces, indicating that charge carriers are created from photogenerated excitons in both the electron donor and acceptor phases. Light-intensity-dependent current-voltage measurements suggested different recombination rates under short-circuit and open-circuit conditions.

Self-aligned self assembly of multi-nanowire silicon field effect transistors

We demonstrate the efficacy of diblock copolymer self assembly for solving key fabrication challenges of aggressively scaled silicon field effect transistors. These materials spontaneously form nanometer-scale patterns that self-align to larger-scale lithography, enabling construction of sub-lithographic semiconducting transistor channels composed of arrays of parallel nanowires with critical dimensions (15 nm width, 40 nm pitch) defined by self assembly. The number of nanowires in the arrays is readily adjusted, greatly reducing the complexity associated with width-scaling of nanowire transistors. We measured Schottky source/drain multi-nanowire n-channel devices comprised of 6, 8, 10, and 16 nanowires, with current drives of ∼5μA∕wire and current on/off ratios of ∼105.

Nanometer-scale pattern registration and alignment by directed diblock copolymer self-assembly

We describe a fabrication method that combines the alignment capabilities of optical lithography with the sub-lithographic dimensions achievable using self-assembled diblock copolymer films. We use surface topography to direct the assembly of in-plane cylindrical copolymer domains so as to subdivide larger patterns defined using optical lithography, in the process registering the location of each 20-nm polymer domain to the lithographic pattern. Our approach provides an application for self-assembly in the fabrication of complex microelectronic circuits entailing alignment of multiple patterned layers. We detail the influence of such process parameters as lithographic pattern dimensions and density, copolymer film thickness, and anneal time on the quality of the resulting nanometer-scale-domain registration.

Electric-field penetration into metals: consequences for high-dielectric-constant capacitors

A consequence of the finite electronic screening length in metals is that electric fields penetrate short distances into the metal surface. Using a simple, semiclassical model of an idealized capacitor, we estimate the capacitance correction due to the distribution of displacement charge in the metal electrodes. We compare our result with experimental data from thin-film high-dielectric-constant capacitors, which are currently leading contenders for use in future high-density memory applications. This intrinsic mechanism contributes to the universally-seen decrease in measured dielectric constant with capacitor film thickness.

Polymer self-assembly as a novel extension to optical lithography.

The extreme technological complexity associated with continued dimensional scaling of the photolithographic patterning process to sub-50 nm dimensions has forced the semiconductor industry to seek increasingly innovative alternative approaches. One unconventional method under preliminary consideration involves using self-assembling block copolymer films as high-resolution patterning materials for defining integrated circuit (IC) elements. While these materials are attractive because of their ability to define nanometer-scale dimensions, their ultimate utility as a viable patterning method remains in question because of issues relating to pattern roughness and defectivity. In this issue, Prof. Paul Nealey and co-workers at the University of Wisconsin present compelling first demonstrations of experimental methods by which polymer self-assembly can generate the pattern elements essential for IC fabrication.

High-capacity, self-assembled metal-oxide-semiconductor decoupling capacitors

We combine nanometer-scale polymer self assembly with advanced semiconductor microfabrication to produce metal-oxide-semiconductor (MOS) capacitors with accumulation capacitance more than 400% higher than planar devices of the same lateral area. The self assembly technique achieves this degree of enhancement using only standard processing techniques, thereby obviating additional process complexity. These devices are suitable for use as on-chip power supply decoupling capacitors, particularly in high-performance silicon-on-insulator technology.

Control of Self-Assembly of Lithographically Patternable Block Copolymer Films

Poly(alpha-methylstyrene)-block-poly(4-hydroxystyrene) acts as both a lithographic deep UV photoresist and a self-assembling material, making it ideal for patterning simultaneously by both top-down and bottom-up fabrication methods. Solvent vapor annealing improves the quality of the self-assembled patterns in this material without compromising its ability to function as a photoresist. The choice of solvent used for annealing allows for control of the self-assembled pattern morphology. Annealing in a nonselective solvent (tetrahydrofuran) results in parallel orientation of cylindrical domains, while a selective solvent (acetone) leads to formation of a trapped spherical morphology. Finally, we have self-assembled both cylindrical and spherical phases within lithographically patterned features, demonstrating the ability to precisely control ordering. Observing the time evolution of switching from cylindrical to spherical morphology within these features provides clues to the mechanism of ordering by selective solvent.

Sub-50-nm self-assembled nanotextures for enhanced broadband antireflection in silicon solar cells

Materials providing broadband light antireflection have applications as highly transparent window coatings, military camouflage, and coatings for efficiently coupling light into solar cells and out of light-emitting diodes. In this work, densely packed silicon nanotextures with feature sizes smaller than 50 nm enhance the broadband antireflection compared with that predicted by their geometry alone. A significant fraction of the nanotexture volume comprises a surface layer whose optical properties differ substantially from those of the bulk, providing the key to improved performance. The nanotexture reflectivity is quantitatively well-modelled after accounting for both its profile and changes in refractive index at the surface. We employ block copolymer self-assembly for precise and tunable nanotexture design in the range of ~10-70 nm across macroscopic solar cell areas. Implementing this efficient antireflection approach in crystalline silicon solar cells significantly betters the performance gain compared with an optimized, planar antireflection coating.