September 26, 2007 – Researchers at the National Institute of Standards and Technology (NIST) say they’ve come up with a new “cold zone” annealing technique that processes block copolymers and other thin films to align into regular patterns of nanocylinder lines, which when chemically removed offer a pattern template for building advanced microstructures.
Typical oven annealing for block copolymer thin films causes one of the two polymers to segregate into the nanocylinder lines as little as 5nm apart, or with equally regular arrays of nanoscale dots. Traditional oven annealing can’t produce high-enough quality films, and a “hot-zone” anneal — where thin films are slowly moved through a heated region to reach a temperature just beyond the point where the cylinders become disordered — offers a low-defect approach but little orientation control, and the temperature threshold is too high to prevent degradation for some copolymers, NIST explains in a statement.
Instead, the team developed a “cold zone” annealing system that not only processes the copolymers well below the order-disorder transition temperature, but it also “repeatedly produces a highly ordered thin film in a matter of minutes,” and they also found that tweaking the annealing conditions modifies the cylinder alignment. The orientational correlation lengths measured after approximately 5hrs above the glass-transition temperature (≈2µm) were an order of magnitude greater than that obtained under equivalent static annealing, according to their report in the Sept. 12 edition of Nano Letters.
Being able to process polymers much more quickly at these transition temperatures “suggests zone annealing [may be] a route toward more robust nanomanufacturing methods based on block copolymer self-assembly,” the NIST researchers wrote. Yields have been shown to be “product quality,” and there are “virtually no limitations on sample dimensions.” The method is being evaluated for fabrication of highly ordered sub-30nm features, though more work is needed to better understand exactly why the cold zone annealing works so much better, and to refine the measurements for evaluating results.
IMAGE CAPTION: Schematic of the NIST ‘cold zone’ annealing process for polymer thin films on a semiconductor wafer (left). Experiment images are color-coded to show regions with different cylinder orientations, as measured by atomic force microscopy. Relatively rapid transit times (middle) leave a jumble of different regions that become largely homogeneous at slower speeds (right).