Recognizing that the interest in printed electronics is growing because of the potential to create cheaper circuits more efficiently than conventional methods, researchers at Soonchunhyang University (South Korea) investigated the use of copper nanoparticle ink with green laser light in the laser sintering process.
Kye-Si Kwon and his colleagues at Soonchunhyang University previously worked with silver nanoparticle ink, but they turned to copper (derived from copper oxide) as a possible low-cost alternative. Metallic inks composed of nanoparticles hold an advantage over bulk metals because of their lower melting points. Although the melting point of copper is about 1083°C in bulk, according to Kwon, copper nanoparticles can be brought to their melting point at just 150–500°C through laser sintering. Then, they can be merged and bound together.
Kwon's group concentrates on photonic approaches for heating nanoparticles by the absorption of light. "A laser beam can be focused on a very small area, down to the micrometer level," explains Kwon and doctorate student Md. Khalilur Rahman. Heat from the laser serves two main purposes: converting copper oxide into copper and promoting the conjoining of copper particles through melting.
A green laser was selected for these tasks because its light (in the 500–800 nm wavelength absorption rate range) was deemed best suited to the application. In the experiment, Kwon and his team used commercially available copper oxide nanoparticle ink, which was spin-coated onto glass at two speeds to obtain two thicknesses. Then, they prebaked the material to dry out most of the solvent prior to sintering. This is necessary to reduce the copper oxide film thickness and to prevent air bubble explosions that might occur from the solvent suddenly boiling during irradiation. After a series of tests, Kwon's team concluded that the prebaking temperature should be slightly lower than 200°C.
The researchers also investigated the optimal settings of laser power and scanning speed during sintering to enhance the conductivity of the copper circuits. They discovered that the best sintered results were produced when the laser power ranged from 0.3 to 0.5 W. They also found that to reach the desired conductivity, the laser scanning speed should not be faster than 100 mm/s, or slower than 10 mm/s.
Additionally, Kwon and his group investigated the thickness of the film—before and after sintering—and its impact on conductivity. The research team concluded that sintering reduces thickness by as much as 74%.
In future experiments, Kwon's team will examine the substrate effects on laser sintering. Taken together, these studies can provide answers to some of the uncertainties hindering printed electronics.
Full details of the work appear in the journal AIP Advances.