Suppression of grain boundary formation during laser recrystallization of thin Si-films using micro absorbers
Grain boundaries are one of the most significant defects in laser recystallized Si films. To obtain useable base materials for device fabrication these grain boundaries must be avoided, or at least localized, so that critical devices can be designed in defect free areas. A generally used method to localize the grain boundary formation is to modulate the temperature profile spatially during crystal growth. This can be achieved by integrated absorbers which are photolithographically defined optical anti-reflection stripes on the top of SOI or 3D structures. Using this technique Si films free form grain boundaries of typically 20 Mym width within the area between the absorbers are obtained. However, boundaries remain under the absorber stripes. With respect to process simulation for SOI and especially 3D applications we were involved in the modelling of grain boundary growth during laser recrystallization. Based on our numeric modelling we introduced a new concept called micro absorber to suppress the grain boundary formation even under the absorbers. Since the formation of grain boundaries may originate from stress due to the different densities of liquid and solid silicon and the geometrical non-uniformity of faceted recrystallizing film, reducing the size of facets down to some critical value may totally suppress the grain boundary formation. The first results obtained with an 18 W cw AR high + laser were satisfactory. Areas free from grain boundaries of up to 100 Mym x 50 Mym have been observed in 0,5 Mym thick seeded SOI film using integrated absorbers of a pitch of 4 microns.