Additive manufacturing advances: Printing electronics

Researchers from the Naval Research Laboratory in Washington, D.C. are doing their part to boost the burgeoning field, popularly known as 3-D printing, but more generally named additive manufacturing. The group has demonstrated that a combination of two technologies -- one to create a thin film and the second to "cut" designs out of the film -- could be a potentially powerful tool to create custom electronic components.

"I think of additive manufacturing as the democratization of manufacturing," said Eric Breckenfeld, a National Research Council Fellow at the Naval Research Laboratory who will present the team's findings at the AVS 62nd International Symposium and Exhibition, held Oct. 18-23 in San Jose, Calif. Breckenfeld said the lower equipment costs associated with additive manufacturing mean the technology is a great fit for rapid prototyping and can be used by small labs and start-up companies with limited funds.

One additive manufacturing technique that is gaining traction is called Laser Induced Forward Transfer, or LIFT. In LIFT a laser beam passes over a thin film of ink or paste. The ink absorbs the laser energy, which vaporizes a thin layer of solvent. The vaporized solvent gas rapidly expands, and the ink or paste is ejected from the film at very high speeds. One advantage of LIFT is that it can transfer high viscosity inks and pastes.

"LIFT can transfer pastes that are almost solid, like the consistency of toothpaste," Breckenfeld said. "An ink jet printer couldn't handle toothpaste."

LIFT was first developed in the 1980s as a way to eject molten droplets from thin copper films. Naval Research Lab scientists later developed the technique to print fluids and nano-powder suspensions. The team continues to push the technique to new materials, turning most recently to inks containing the transition-metal oxide vanadium dioxide (VO2). Vanadium dioxide undergoes a sharp semiconductor-to-metal phase transition near room temperature, making it an attractive material for a wide range of applications, including chemical sensors, ultra-fast electrical and optical switches, and coatings that change color with temperature.

To turn vanadium dioxide into a thin film compatible with LIFT, Breckenfeld and his colleagues turned to another newly developed technology, called polymer assisted deposition (PAD). The technique works by dissolving metal salts in a solution containing polymers. The metal ions bind to the polymer, forming a stable structure. The solution is placed on a spinning disk that spreads it into a thin film. The film is later cured in an oven to decompose the polymer.

"There has not been much overlap between groups that study PAD and those that study LIFT," Breckenfeld said. "We are one of the first groups to try combining the two techniques."

Breckenfeld and his colleagues explored a variety of solvents and heating steps to optimize the growth of vanadium dioxide films on glass and crystalline substrates. They then experimented using LIFT to print patterns with the PAD solutions.

"The transfer step proved to be the most challenging," Breckenfeld said. In order for LIFT to work, the thin film materials must absorb the wavelength of light of the LIFT laser. The researchers had to modify the vanadium dioxide PAD solutions to catch the energy of the laser light.

So far, the team has successfully printed simple patterns. Breckenfeld said the results show that LIFT and PAD technologies combined could directly print a wide range of commercially attractive electronic materials. The researchers plan to extend their own experiments to new materials soon.