Macromolecular Self‑Assembly a Promising Alternative to Photolithography
A novel technique for controlling the orientation of nanostructures (red and blue) is to use disordered, roughened substrates. Silica nanoparticles (orange), cast onto silicon substrates (grey), create “tunable” substrates which can control self-assembly, despite inherent disorder.
Macromolecular self‑assembly is emerging as an alternative to conventional photolithography, a mainstay of the semiconductor industry, according to speakers at the APS March Meeting session who reported on the latest research in this area. As photolithography edges closer to fundamental physical limits, physicists are looking to create microchips and data storage devices from novel materials such as organic molecules and polymers.
Photolithography is a powerful technique for etching surfaces with light and designing such patterned structures as microprocessors. However, the technique is limited by the wavelength of light used, and the current state‑of‑the‑art can only precisely etch details on a scale of 30 nanometers or larger. In contrast, macromolecular self‑assembly uses polymer building blocks–which self‑assemble with very little energy–to construct nanoscale patterned surfaces with great precision.
Paul Nealey and his colleagues at the University of Wisconsin are investigating techniques to integrate self‑assembling block‑copolymers into the lithographic process, with the goal of achieving sub‑15 nanometer resolution while retaining such essential lithographic benefits as pattern perfection and high‑volume manufacturing.
NIST’s Alamgir Karim has developed what be believes could be a robust, high‑throughput nanomanufacturing technique for self‑assembling block copolymers. It employs two‑dimensional physical and chemical patterns (templates lined with troughs separated by crests) that can direct, in three dimensions, the orientation of “block copolymers.” Block copolymers are materials consisting of a long chain of one type of building block strongly bonded to a chain consisting of another type of monomer.
Karim uses a temporal zone (cold‑hot‑cold) annealing of block copolymer films. Computer simulations demonstrated that when a heated zone sweeps across the template, the polymer molecules that have been deposited on the template self‑assemble into well‑aligned, almost defect‑free lines. Using this technique, the block copolymers can form arrays of tiny dots that, in turn, could be used as the basis for electric components capable of cramming 1000 gigabytes of memory into a device the size of a pack of gum.
A major challenge in realizing the potential of polymer nanotechnology is controlling the self‑assembly process. Karim and his NIST colleagues have developed techniques for accurately measuring thin film polymetric nanostructures in 3D, drawing on tomographic small‑angle scattering methods. For instance, they combine many 2D neutron scattering images into a single composite imaging pattern that reveals the thin film’s 3D internal structure, thus enabling them to determine if the nanoscale polymer structures are in the correct positions and free of defects.
Being able to measure these nanoscale structures is just part of the challenge; one still needs to control molecular function with nanometer‑scale precision. To that end, Christopher Ober of Cornell University reported on his use of block copolymers to deliver chemical functions to the near‑surface region with precise control of surface functionality. His team has found that by using block copolymers alone and in combination, it is possible to tailor not just surface properties, but also the mechanical behavior of the polymer surface region.
In an intriguing twist, NIST scientist Kevin Yeager discovered that deliberately roughening his templates with a sprinkling of nanoparticle silica forces block copolymers into a perpendicular standing position relative to the template–a critical feature for nanotech applications. The internal structure remains disordered using this technique, but it could prove to be a useful, inexpensive way to achieve those vertical structures for applications that require just the surface to be smooth.
For Daniel Savin of the University of Vermont, investigating the process of self‑assembly is less about lithography and electronics and more about biological interactions. For instance, nature uses the same building blocks (amino acids) embedded in block copolymers to self‑assemble proteins. He is interested in mimicking nature’s process, incorporating biological elements into directed self‑assembly of block copolymers to form large‑order structures that can respond to different solution conditions.
At the March Meeting, Savin described how he uses polypeptide‑based polymers that are “tunable” and have potential applications as viscosity modifiers and gels for application in cosmetic products such as shampoos, as well as for liquid crystals. The bulk properties of these materials depend on the morphology of these polymers, which can be finely tuned by altering the acidity or the temperature of the solution.
Elsewhere on the self‑assembly research front, scientists at Brookhaven National Laboratory have developed a new method for controlling the self‑assembly of nanometer and micrometer‑sized particles based on designed DNA shells that coat a particle’s surface. According to Dmytro Nykypanchuk, who described this work at the meeting, the method is unique because it employs two types of DNA attached to the particles’ surfaces. The first forms a double helix, while the second is non‑complementary, neutral DNA that provides a repulsive force. This enables the scientists to regulate the size of particle clusters and the speed of self‑assembly with greater precision.
In subsequent experiments, the Brookhaven scientists used the attractive forces between complementary strands of DNA to create 3D, ordered crystalline structures of nanoparticles with unique properties such as enhanced magnetism and improved catalytic activity. They also added “thermal processing,” heating the DNA‑linked particles, then cooling them back down to room temperature, thereby allowing the nanoparticles to unbind, reshuffle, and find more stable binding arrangements.