Femtosecond laser single step, full depth cutting of thick silicon sheets with low surface roughness

Separation of silicon wafers is an integral part of semiconductor device manufacture. The widely used mechanical separation methods produce large kerf widths and micro-cracks. Existing laser separation, singulation and dicing methods can reduce cutting kerf widths, but usually involve multiple steps. Although single step laser through cutting by ablation has been reported before, surface roughness is usually poor. This paper presents a method for single step, full-depth cutting of silicon wafer sheets of 200 µm in thickness with an 800 nm wavelength femtosecond-pulse laser focused using a microscope objective lens under ambient condition. By introducing parallel scan lines, through cut is realized with an excellent sidewall full area roughness of 191 nm Ra, which is much better than that of reported previously in laser through-cutting of thick silicon sheets of similar thickness. It is found that by increasing the number of overscan, pulse energy, or number of parallel scan lines, the sidewall surface roughness reduces and the kerf edge straightness at the bottom surface is improved. The effects of p- and s-polarization on cutting quality are compared. Sidewall quality can be improved by appropriate selection of beam polarization and cutting speed. The mechanisms involved are discussed.