Single-Grain Silicon TFTs on a Plastic Substrate by Doctor Blade Coating of Cyclopentasilane

Abstract

Liquid silicon is found as the material combining both the advantages of high quality silicon devices and the low cost solution processing method. Single-Grain Thin-Film Transistors can be produced by Excimer Laser Annealing of the resulting film and grain location control by the µ-Czochralski process. Other works have used spin-coating and inkjet printing for liquid silicon based devices, however both processes are not roll-to-roll process compatible. In addition a high thermal annealing step (650°C), incompatible to plastics, is required for the reduction of hydrogen content before laser crystallization. In this work, both issues are focused on. A precursor of the gravure printing process, doctor blade coating, is used to imitate a roll-to-roll compatible solution process and is optimized to produce uniform films of liquid silicon. Excimer Laser Annealing is used as a low temperature pre-annealing method to decrease the hydrogen content for crystallization. Pure cyclopentasilane has been used as the liquid silicon material. Silicon dioxide surface modification by 0.55%HF dip results in a better wetting of the liquid together with an elevated temperature of 70°C. Higher temperatures lead to even better wetting properties, but more liquid silicon will evaporate. After UV polymerization of the CPS for 20 minutes and thermal annealing at 350°C for 1 hour, an a-Si layer has been formed. Excimer Laser pre-annealing of many low energy shots removes hydrogen without significant deterioration of the film. A maximum grain size of 5µm has been produced by using a long pulse configured laser recipe that decreases the number of shots linearly while increasing the laser energy density by 50mJ/cm². SG-TFTs on polyimide have been manufactured at the maximum processing temperature of 350°C. The mobility of the NMOS was ..., and the mobility of the PMOS was .... [to be obtained by mid June]. Finally, a next step towards gravure printing has been taken, by advancing the doctor blade coating method to the removal of the excess layer while keeping the cavity patterns in the film filled. Blade elasticity is a dominant factor in manual blading. An elastic blade can remove more excess than a rigid blade since the flexibility allows adjustment on the surface, but will also remove the liquid from inside the patterns. A combination of a rigid blade and the careful excess removal by the elastic blade gives the best results. This work shows the potential of liquid silicon, and brings us closer to the mass production on flexible substrates using this new material.