Fine material processing using ultra-short pulsed 2 μm all-fiber lasers

Abstract

Precision processing of silicon (Si) and other semiconductors is becoming increasingly important following the rising demand for semiconductor-based devices. Laser-assisted material processing on and below the surface can enable improved precision as well as reduced material loss compared to the tools available today. The experimental results found in the literature are widely spread depending upon the used laser parameters at which processing is performed. Many explanations have been provided to these discrepancies, but no in-depth theoretical and experimental investigation describing the correlation between the laser parameters and the resulting spatiotemporal effects and their impact on material modification existed up to now.

This thesis introduces a novel nonlinear optical concept called the “nonlinear figure of merit (NFOM)”, supported by the results achieved with two independent numerical methods. It provides a theoretical guidance for controlling laser parameters to achieve sub-wavelength micro-structuring with high speed and precision. The concept is based on the interplay of several nonlinear effects such as multi-photon absorption and the Kerr effect, exhibiting a strong wavelength dependence.

The proposed 3D-NFOM exhibits a peak in the wavelength range roughly around 2.1 μm for Si from a local increase of the nonlinear refractive index and a dip of the multi-photon absorption. This peak leads to the possibility of efficient energy transfer from the ultra-short laser pulses to material and to precision material processing due to the high Kerr nonlinearity and the corresponding optimization of the laser parameters.

To experimentally complement the numerical investigations, an all-fiber Tm:doped large-mode area-based Master Oscillator - Power Amplifier (MOPA)-system electronically tunable between 1950 and 2450 nm was developed. With this laser system it was possible for the first time to generate Raman-shifted solitons in the active large-mode-area-fiber opening the way towards power scaling in large core solid-state waveguides. Using this system, it was possible to generate initial sub-surface Si modifications, for the first time in this wavelength range, with a pulse energy below the previously known damage threshold.

The obtained experimental results agree with the suggested NFOM, providing the guidance for controlling the wavelength and other laser parameters depending upon the targeted application. The concept can be applied to any semiconductor, which makes the results relevant for a broad range of processing applications in industry.