Development of femtosecond infrared fiber laser for multiphoton silicon micromachining

Abstract

Femtosecond laser is widely used in material processing. Application of ultrashort lasers makes it possible to process with higher precision compared to picosecond and nanosecond lasers. Moreover, a major challenge in picosecond and nanosecond laser processing is providing enough power for ablation. In the femtosecond regime, the peak power required for ablation can be achieved at lower pulse energies compared to picosecond and nanosecond pulses. Additionally, high peak intensity of femtosecond laser allows 3D material processing through multiphoton absorption by focusing the laser beam inside the bulk of material, for which the linear absorption is low (The bandgap of the material is wider than the photon energy). The same approach can be used for multiphoton surface processing, which would increase the processing precision. Such lasers could be useful for both surface and subsurface processing depending on where we focus the beam. For the past 50 years, silicon has been one of the most widely used materials in electronics technology including micro- and nanoelectronics, solar cell technology, telecommunications, etc. To the best of our knowledge, there is no existing technology up to now, which allows both surface and subsurface processing of silicon with the same laser. Er-doped fiber laser is operating at 1.55 µm wavelength, where the photon energy of the laser is less than the silicon bandgap energy. We designed and built an Er-doped all-fiber-integrated pulsed laser for multiphoton surface processing of silicon. The pulse duration of the compressed pulse is 390 fs. The laser system is capable of supplying up to 1.3 W output power at 905 kHz repetition rate, namely 1.5 µJ energy per pulse. The output beam is nearly diffraction limited with high beam quality. The laser beam is applied to process the silicon surface at different pulse energies. The depth of the trenches generated by the laser beam at various power levels is measured to investigate how the ablation depth varies with power. Subsurface silicon processing with the same laser will be investigated in our future work.