Hollow microstructures fabricated by combination of nonlinear laser lithography and wet etching deep inside silicon

Silicon is the most widely used material in microelectronics and integrated photonics. Despite the different methods available, they generally do not allow the fabrication of 3D microstructures deep inside silicon. We have recently demonstrated laser-based two-step fabrication of complex 3D structures deep inside silicon (Tokel et al., Nat. Photon., 2017). Laser modification of desired points is based on permanent disruption of the crystalline structure as a result of the laser beam collapsing and momentarily creating conditions far from thermal equilibrium, leading to heat- and pressure-induced stresses. This disruption causes the Si-Si bonding to be much weakened in comparison to conventional crystalline Si, and the weakened structure can be chemically etched with high selectivity using an optimized HF, HNO₃, CH₃COOH, Cu(NO₃)₂, H₂O. While the laser-induced disruption can be limited to a region with a diameter of ~1 µm, limited by the laser wavelength and can be induced at any point within the Si chip, the etched structures have minimal features sizes in the range of 5-10 µm. Here, we report a systematic study on this two-step process, including minimum achievable feature size after etching, optimization of the surface quality and complexity of the 3D structures.