Internal structuring of silicon with ultrafast lasers
Résumé
An important challenge in the field of three-dimensional (3D) ultrafast laser processing is to achieve permanent modifications in the bulk of silicon (Si) and narrow-gap materials. High-energy infrared femtosecond lasers pulses fail when conventional laser machining configurations are used. By comparisons between ultrafast plasma images, 3D energy density maps inside the samples and nonlinear propagation simulations, we show the strong nonlinear and plasma effects causing a strict
clamping of the intensity that can be delivered inside Si with tightly focused infrared pulses. To circumvent this limitation, we will describe a solution inspired by solid-immersion microscopy to achieve hyper-focusing of the pulses. From a proof-of-concept experiment, we demonstrate the first micro-modifications achieved inside Si with sub-100fs pulses.
For more practical alternatives, we will describe today’s effort on optimizations in the time domain. We will discuss the picosecond regime limiting the detrimental nonlinearities and provoking progressive thermal band gap closure to assist energy deposition. We also perform multi-pulse irradiation experiments where femtosecond, picosecond and nanosecond pulses are synchronized. This identifies breakdown channels seeded by pre-ionization or local thermal pre-stimulation. Another approach is to rely on accumulation strategies. To this aim, we rely on ultrafast trains of pulses at the highest achievable repetition-rates (up to THz). From these approaches, we will introduce multi-timescale control parameters exploited for improved energy deposition and for demonstrations of reliable 3D laser writing deep inside silicon chips that would not be possible otherwise.