Patterning Under Pressure: Researchers Test a New Way to Manufacture Nanoscale Metal Patterns without the Need for a Vacuum.

Computers, cell phones, and many other electronic devices are made possible by tiny chips inscribed with nanoscale circuit patterns. Nanoscale patterns can also help scientists speed up chemical reactions and manipulate light, but the costs of producing the patterns can be high. Now a team of researchers from Case Western Reserve University, in Cleveland, Ohio, has demonstrated a new way to create tiny patterns that eliminates an expensive requirement of some other manufacturing processes: the vacuum. The team's new techniques could help pave the way to low-cost mass production of nanoscale patterned films at atmospheric pressure.

As a first step in the new process, the Ohio researchers created thin films of polymer laced with positively charged metal ions. By supplying free electrons in the gas phase to reduce the ions in select locations, the team could then create a pattern of crystalline metal in the film. Pure electron beams might seem like a natural source for the necessary negatives charges, but because specialized and expensive vacuum systems are required to generate such beams, the team turned its attention to plasma discharges instead. Plasmas contain ions and other highly reactive particles, in addition to electrons, but the researchers found the jumbled mix could successfully reduce the metal ions in the polymer film without oxidizing the film. Perhaps more importantly, the plasma could be generated at ambient temperature and pressure, greatly reducing the processing costs.

Initially, the research team tested their idea by exposing the films directly to the plasma. The direct approach produced patterns with line widths as small as 30 micrometers, but also caused unwanted heating and sputtering of the film. In order to separate the film from the punishing heat of the full plasma beam, the team developed a way to extract just a small portion of the plasma current. "The new process is gentler and cleaner," says Mohan Sankaran, a professor of chemical engineering at Case Western who helped lead the project.

Another aspect of the team's new approach was to use a mask to create patterns, as opposed to moving the plasma beam to write directly on the film. To get help with the masking techniques, Sankaran's group partnered with the lab of Philip Feng, a professor in the Electrical Engineering and Computer Science Department at Case Western, and an expert in mask fabrication. "We combined Professor Feng's masking techniques with a novel plasma source that my group developed to produce the nanoscale patterns," says Sankaran. So far, the team has created patterns of silver particles approximately 150 nanometers in size. They describe the new approach in a paper appearing in the Jan/Feb edition of the Journal of Vacuum Science and Technology B.

The key advantages of the process are that it takes place at ambient temperature and pressure, is a single step whereby metal is reduced directly as a film, and is scalable for large area fabrication, notes Sankaran. As a next step, the team plans to experiment with ways to optimize the mask geometry to ensure quality pattern transfer and reduce the width of the pattern even further.