24 Apr 2015
Dutch-Japanese team develops low-temperature method to layer silicon on substrate from "ink" using laser pulse.Delft University of Technology, in the Netherlands, has pioneered a method to produce silicon on substrate from "ink" with single laser pulse. The technique allows silicon in the polycrystalline form used in electronics circuitry, to be produced directly on a substrate. The work was this week first reported in Applied Physics Letters.
In seeking to develop the next generation of micro-electronic transistors, researchers have long sought to find a suitable replacement for brittle silicon-based electronics. To this end, a wealth of recent research into fully flexible electronic circuitry has focused on various organic and metal-oxide ink materials, which often lack all the favorable electronic properties of silicon but offer superior "printability."
The capacity for printing silicon ink onto substrates has existed for some time, but it has required a 350°C thermal annealing step -- far too hot for many of the flexible surfaces that made such a production process potentially appealing. The Delft researchers’ new method completely bypasses this step, transforming the liquid silicon directly into polysilicon.
"It was very simple," said Ryoichi Ishihara, the professor who led the Delft team along with collaborators at the Japan Advanced Institute of Science and Technology in Ishikawa, Japan. "We coated liquid polysilane directly on paper by doctor-blading, or skimming it by a blade directly in oxygen free environment. Then we annealed the layer with an excimer-laser.”
The laser blast only lasted a few tens of nanoseconds, leaving the paper completely intact. In testing its conductive performance, Ishihara and his colleagues found that thin-film transistors using the laser-printed layer exhibited mobilities as high as those of conventional poly-silicon conductors.
The team says that the most immediate application of the new printing capability is in wearable electronics, because it allows for the production of fast, low-power and flexible transistors “at a remarkably low cost”. Ishihara believes the future of the project, which involves improving the production process of the thin-film transistors to include additional non-silicon layers, will hold a wealth of possible further applications: "This process can be expanded to biomedical sensor and solar-cell areas and will also realize stretchable - and even edible – electronics," he suggested.
The article in Applied Physics gives more details of the Delft technique and its potential: “Printing electronics has led to application areas which were formerly impossible with conventional electronic processes. Solutions are used as inks on top of large areas at room temperatures, allowing the production of fully flexible circuitry.
“Using this method, poly-silicon transistors were created at a maximum temperature of only 150 °C. This method allows silicon device formation on inexpensive, temperature sensitive substrates such as polyethylene terephthalate, polyethylene naphthalate or paper, which leads to applications that require low-cost but high-speed electronics.
“This method has been used in the creation of NMOS and PMOS TFTs with mobilities of 21.0 and 23.5 cm2/Vs, respectively, at a process temperature of only 150 °C. This technique also allows silicon to be directly integrated on inexpensive substrates for applications such as low-cost UHF-RFID tags.”
About the Author
Matthew Peach is a contributing editor to optics.org.