Advanced Excimer-Laser Recrystallization Technology for Crystal-Si Thin-Film Devices

This paper reviews an excimer-laser annealing (ELA) method of Si thin-films on glass studied in the Tokyo Institute of Technology, aiming at position-controlled ultra-large grain growth with high packing density by a single shot irradiation of a laser-light pulse. Key concepts are (I) 2-D modulation of the laser light intensity on the sample surface, (II) reduction of the heat removal rate from the molten Si thin layer of high temperature, and (III) enlargement of the effective specific-heat while keeping the effective thermal-conductivity low for annealed layers. There were two possible solutions for the condition (I). The first solution is of an application of a cross-coupled phase-shifter formed on a transparent quartz plate. The second solution is of a half-tone phase-modulation method (PAMELA method) using semi-transparent thin films on quartz substrate with phase-shifter. The condition (II) is satisfied by changing the SiO2 underlayer to the porous or organic silica underlayer for reducing vertical heat flow flux to the cool underlayer, and by thinning the Si layer for reducing heat flow flux along the highly conductive Si layer. The condition (III) is satisfied, by the SiON capping layer for a KrF excimer laser, and by the SiOC capping layer for a XeCl excimer laser, since they have a reasonable light absorption coefficient, low thermal conductivity, large specific-heat and sufficient heat tolerance.