IPL Sintering Advance Could Improve Electronics Manufacturing

Research into the use of Intensed Pulsed Light (IPL) to rapidly fuse conductive nanoparticles has revealed that densification in IPL increases the density of a nanoparticle thin-film or pattern, with greater density leading to functional improvements such as greater electrical conductivity. This discovery could open the door to the development of faster, less expensive production processes for electronics manufacturing.

Researchers at Oregon State University characterized IPL of silver nanoparticle films. They observed a temperature turning point in IPL, and correlated the turning point in the evolution of film temperature during IPL with the additional observation that film densification leveled off beyond a critical pulse fluence and number of pulses. They developed a computational model to predict the evolution of film temperature and density during IPL and to capture the experimentally observed turning point in temperature during IPL.

Unsintered, (left), and sintered nanoparticles. Courtesy of Rajiv Malhotra.

The research showed that the temperature turning point occurred when the densification in IPL reduced the nanoparticles’ ability to absorb further energy from the light. Researchers further found that the optical fluence per pulse had a greater effect on the achievable film density in IPL, as compared to the number of pulses.

The interaction observed between optical absorption and densification led to an understanding of why densification levels off after the temperature turning point in IPL. Earlier research has shown that nanoparticle densification begins above a critical optical fluence per pulse, but that it does not change significantly beyond a certain number of pulses. The OSU study explains why, for a constant fluence, there is a critical number of pulses beyond which the densification levels off.

“The leveling off in density occurs even though there’s been no change in the optical energy and even though densification is not complete,” said professor Rajiv Malhotra. “It occurs because of the temperature history of the nanoparticle film, i.e. the temperature turning point. The combination of fluence and pulses needs to be carefully considered to make sure you get the film density you want.”

A smaller number of high-fluence pulses quickly produces high density. For greater density control, a larger number of low-fluence pulses would be required.

“We were sintering in around 20 seconds with a maximum temperature of around 250 degrees Celsius in this work,” said Malhotra. “More recent work we have done can sinter within less than two seconds and at much lower temperatures, down to around 120 degrees Celsius. Lower temperature is critical to flexible electronics manufacturing. To lower costs, we want to print these flexible electronics on substrates like paper and plastic, which would burn or melt at higher temperatures. By using IPL, we should be able to create production processes that are both faster and cheaper, without a loss in product quality.”

IPL sintering allows for faster densification — in a matter of seconds — over larger areas compared to conventional sintering processes such as oven-based and laser-based.

“For some applications we want to have maximum density possible,” Malhotra said. “For some we don’t. Thus, it becomes important to control the densification of the material. Since densification in IPL depends significantly on the temperature, it is important to understand and control temperature evolution during the process. This research can lead to much better process control and equipment design in IPL.”

The research will help large-area, high-speed IPL to realize its full potential as a scalable and efficient manufacturing process. IPL could potentially be used to sinter nanoparticles for applications in printed electronics, solar cells, gas sensing and photocatalysis. Products that could evolve from the research, Malhotra said, are radiofrequency identification tags, a wide range of flexible electronics, wearable biomedical sensors, and sensing devices for environmental applications.

The research was published in Nanotechnology (doi: 10.1088/0957-4484/27/49/495602).