Seed-guided high-repetition-rate femtosecond laser oxidation for functional three-dimensional silicon structure fabrication

Abstract

Laser oxidation provides a flexible maskless functional three-dimensional silicon structure fabrication strategy. Here, oxide layers are fabricated on silicon substrates by high-repetition-rate femtosecond laser irradiation. Formation mechanisms of the femtosecond laser oxidation are analyzed to tune the oxide layer morphology. When the laser fluence is high, the oxidation process is accompanied by laser ablation, so the oxide layer is always on top of laser ablated groove. To obtain the flat oxide layer, defect is firstly generated on silicon substrate by the laser irradiation at a high laser fluence of 126 mJ/cm2, function as a high optical absorption. It is called seed as it can assist to achieve lower fluence laser oxidation at 38 mJ/cm2 and result in a flat oxide layer. Full width at half-maximum of the oxide layer is also reduced. The laser fabricated oxide layers are used as etching masks. Three-dimensional silicon gratings are fabricated by deep reactive ion etching, which demonstrate desirable beam diffraction properties.