April 6, 2006 – Researchers at Seiko Epson and JSR Corp. have created a high-quality silicon film formed with a spin coat method and inkjet patterning processes, with performance comparable with films developed through conventional CVD methods.
Typical manufacturing process for silicon film, used with low-temperature polysilicon thin-film transistors (TFTs), are formed in vacuum devices using photolithography equipment to transfer the pattern. Research has been conducted in recent years to create organic semiconductors formed from liquids, eliminating the need for vacuum devices as well as enable the use of printing technologies such as inkjet to create the pattern.
The new material, developed by Epson and JSR, is a high order silane compound of hydrogen and silicon (cyclopentasilane, Si5H10) dissolved in an organic solvent, spin-coated on a substrate and baked at 540 degrees Celsius in an inert atmosphere, forming a 50nm-thick amorphous silicon film. Irradiation by a 308nm XeCl excimer laser turns the film into a polycrystalline, with measured electron mobility of 74-108 cm2/Vs, approximately that achieved using the CVD method. A TFT prototype using a silicon film pattern created by inkjet method instead of spin-coating showed electron mobility of 6.5 cm2/Vs, lower than that achieved with spin coating.
More research is ongoing to controlling the oxygen levels in the organic dilutent (toluene in this case), and understand the “wettability” of the film during inkjet application, in order to control ink droplet sizing to create layers thin enough for laser crystallization
The research, aiming to find a low-cost, high-throughput process for fabricating high-performance TFTs, was commissioned by Japan’s New Energy and Industrial Technology Development Organization (NEDO).