Formation of Fine Periodic Structures on Back Surface of Si Substrate by a Femtosecond Laser at 1552 nm

Si is a semiconductor with a band gap energy of 1.12 eV, which corresponds to a wavelength of 1127.2 mn Therefore, Si is considered as transparent for radiations longer than this wavelength. We have demonstrated that micromachining through an Si substrate by a femtosecond laser at 1552 nm is possible. We have found that periodic structures similar to the laser-induced periodic surface structure (LIPSS) with a periodicity of similar to 320 nm was formed on the back surface when the laser was focused on the back surface. The change of the surface was only observed on the back surface with no damage on the front surface. The LIPSS on the front surface were formed in perpendicular direction to the polarization plane of the laser while those on the back surface were always formed parallel to the polarization. The effects of laser irradiation conditions on the LIPSS have been studied. Furthermore, the LIPSS was found on the back surface after laser-assisted back-side wet etching by KOH solution, where the surface was in contact with water during irradiation. Both LSFL and HSFL were formed. Under some conditions, HFSL formed at center while LSFL formed on the edge of laser scan trace.