Silicon Laser Processing with Bursts

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Femtosecond laser precision micro-machining is nowadays widely applied using commercially available sources emitting at wavelengths around 800-1000 nm. However, in-volume and back-side modifications are restricted to dielectrics like glasses which are transparent at this wavelength range. Silicon, as the most utilized semiconductor material in microelectronics, is not accessible. The SILABUS project addresses the very challenging issue of achieving internal and backside modifications in silicon with ultrashort laser pulses. The ultrashort interaction time allows for obtaining an extremely high precision as the interaction is confined in a very small volume, and avoiding detrimental side-effects or damage of the surrounding material. As a narrow-gap semiconductor (band gap of 1.12 eV), silicon is only transparent at wavelengths above 1100 nm. However, it is not evident how to implement this technique to silicon due to its intrinsic properties. Specifically, the nonlinear refractive index of silicon is two orders of magnitude higher than the one of fused silica for instance. Therefore, ultrashort laser pulse propagation in silicon is prone to nonlinear distortions and the intensity is saturated at a level below the threshold of permanent modification of the material. We propose to resolve this fundamental problem by using a femtosecond laser emitting around 1300-2000 nm in a novel regime which is the GHz-burst mode. This approach will permit to distribute the delivered energy into many sub-pulses that are following each other at a GHz intra-burst repetition rate allowing for the production of permanent modifications at the focal point in an accumulative and controlled way, and thus, avoiding the aforementioned detrimental nonlinear effects.