Micromachining of the back surface and interior of Si using infrared femtosecond laser pulses

This paper reports the 3D microprocessing of Si using a femtosecond laser at a wavelength of 1552.5 nm. As Si is optically transparent at this wavelength, the authors attempted to machine the back surface and interior of a Si substrate by a nonlinear absorption process, similar to the nonlinear process used to treat dielectric materials using visible and near-infrared ultrashort lasers. The femtosecond laser impinged on the front surface while focusing at or near the back surface. The authors scanned the laser beam linearly at several focus positions across the back surface at different scan speeds and repetition rates. Changes occurring in the interior of Si were observable only by infrared microscopy, whereas those on the back surface were observable by visible optical microscopy and scanning electron microscopy. Meanwhile, no change was detected on the front surface where the laser impinged. After a certain period, the point of irradiation showed changes; afterward, changes in the interior of Si began to occur continuously. However, the changes on the back surface occurred in a rather discrete manner, observed intermittently. This may be attributed to the heat accumulation due to multiple pulse irradiations, which increased the local temperature. This resulted in increased absorption along the incident laser path and prevented the delivery of a sufficient amount of energy to induce ablation on the back surface. The morphologies observed on the altered back surface were a granular band and a laser-induced periodic surface structure.