Ryo Kawajiri

Single-grain Si TFTs using spin-coated liquid-silicon

Liquid silicon offers us an opportunity of fabricating TFTs using printed silicon, instead of printed organic materials. We have fabricated single-grain Si TFTs on location-controlled Si grains with laser crystallization of a-Si formed from spin-coated liquid Si. Si grains with a maximum grain diameter of 3.5 µm were positioned at predetermined sites. Mobilities for electrons and holes are 391 cm2/Vs and 111 cm2/Vs, respectively.

Well-defined silicon patterns by imprinting of liquid silicon

Well-defined silicon patterns with a high aspect ratio and sharp edges were directly formed by imprinting of liquid silicon which was synthesized via the photopolymerization of cyclopentasilane (CPS: Si5H10) with UV light. After the patterns were formed, they were converted into amorphous silicon patterns by post-annealing at 400 °C. Both dotted and lined patterns whose size ranges from 1 μm to 100 nm were obtained, suggesting that their size could be further reduced. It is very noteworthy that all the obtained patterns demonstrated a good aspect ratio and sharp edges, despite a large shrinkage of 70–80% during the process. By conducting solid-phase crystallization at 800 °C, the pattern portion was converted into polycrystalline pure silicon, whereas the residual film region remained in an amorphous state containing large amounts of oxygen and carbon atoms. Based on the experimental results, the relationship between the decomposition and solidification processes of liquid silicon and its imprinting behavior was clarified and the mechanism of impurity condensation in a residual film region at the annealing temperature of 800 °C and that of well-defined shape formation were discussed. The developed method can be expected to be used in the fabrication of micro-silicon devices because of its production simplicity.

Formation of epitaxial cobalt disilicide thin film by solution processing

A precursor obtained by the reaction between cyclopentasilane and dicobaltoctacarbonyl in a toluene solution provides a method for the production of a low-resistivity thin film of cobalt disilicide CoSi2 (ca. 15 µΩ·cm) on a silicon (100) substrate by spin coating and annealing. Several types of analysis have revealed that the films formed on silicon substrates have a double-layer structure consisting of an upper layer (oxidized complex film) and an epitaxial CoSi2 layer with low resistivity along the (100) orientation of the silicon substrate. The former layer can be removed by dry etching or chemical mechanical planarization when used in actual applications.