ã€Packaging Network News】 Delft University of Technology and the Hokuriku University of Science and Technology in Japan announced on April 21st, 2015 that it has developed a technology to “print†polysilicon layers on paper. The related papers were published in the academic journal Applied Physics Letters (Applied Physics Letters).
The TFTs fabricated using this technique have a carrier mobility of 23.5 cm2/Vs at the maximum. At present, the mainstream of semiconductor materials for flexible electronic circuits is organic semiconductors and oxide semiconductors, but polysilicon has also been competing.
The printing methods developed this time are: (1) In the absence of oxygen and moisture, the “silicon ink†is coated on a substrate heated to 80°C. The silicon ink is cyclopentasilane (CPS, hydrogen bonding). Silicon is formed on a five-membered ring) dissolved in a solvent; (2) ultraviolet rays (UV) having a wavelength of 365 nm are irradiated at a temperature of 100°C, and then CPS is polymerized to form a polysilane; (3) The polysilane layer is irradiated with tens of nanometers of excimer laser light to become polysilicon. According to reports, after the above process, the process temperature including the TFT formation process will not exceed 150°C.
In the TFT fabricated by this method, the carrier mobility of the NMOS fabricated by irradiation of the higher energy laser only once was 2.6 cm 2 /Vs, and the carrier mobility of the PMOS was 3.9 cm 2 /Vs. The carrier mobility of an NMOS made by repeatedly irradiating multiple weaker lasers is 21.0 cm 2 /Vs, and the carrier mobility of PMOS is 23.5 cm 2 /Vs. The current on-off ratios of the NMOS and PMOS produced by one shot irradiation were 104 or higher, and the current ON/OFF ratios of the NMOS and PMOS produced by multiple shots were low and 103 and 102 respectively.
The Hokuriku University of Science and Technology of the University of Science and Technology has long been researching the use of CPS-based silicon inks to make electronic circuits, and has developed products such as polysilicon TFTs and amorphous silicon solar cells. However, the previous process temperature was as high as 350 to 400°C and it was not possible to use resin substrates such as PET. This process temperature is below 150°C, so more flexible substrates can be used.