Modification and Machining on Back Surface of a Silicon Substrate by Femtosecond Laser Pulses at 1552 nm

Silicon (Si) is widely used material in microelectronics, MEMS, photonics and more. Although Si is not transparent for commonly used processing lasers in near infrared to ultraviolet spectral range, the light at low energy than its band gap energy should be transmit through Si substrate. Using a short pulse laser in this wavelength range, non-linear machining of Si should be performed just like non-linear processing of transparent materials by conventional lasers. In this study, we have been studied a laser grooving on back surface of Si substrate by an infrared femtosecond laser. The laser was focused at the back of the Si substrate. The top surface of the Si showed no change while morphology of back surface changes into aggregation of small particles, the size of which is about 500 nm. From cross sectional view of the Si substrate, a structural change was observed in regions close to the focus position. Raman spectra indicate that the single crystal Si was transformed into amorphous and poly-crystal Si in irradiated region. Although machining for the back surface of the Si is quite difficult, machining of the grooves of few micrometers was achieved in the laser-assisted wet etching using KOH solution.