Laser absorption precursor research: microstructures in the subsurface of mono-crystalline silicon substrate

In high-energy laser systems, the energy absorption coefficient of silicon optical elements is one of the most critical performance indicators. The absorption coefficient of substrate limits the absorption of the overall elements. Since mono-crystalline silicon is transparent in working wavelength range, the subsurface absorption precursors also influence the entire absorption dramatically. In this paper, the subsurface of a super-polished silicon substrate is exposed by ion beam etching (IBE) as deep as 4.6 μm. In different depth layers, morphology and energy absorption are measured with an atom force microscope and photothermal instrument, respectively. In the 100 nm layer, microstructures are found, and their heights decrease while widths increase with IBE. Finally, structures are diminished below the 1.12 μm layer. Absorption increases with the structures' appearance. When the structures are fully exposed, absorption reaches the peak value, 327.5% of the unremoved surface. Once structures are removed, the absorption value falls down to the lowest point, 67.5%, which verifies that structures influence the absorption significantly. According to the structure depth and energy dispersive spectrometer results, the structures are most likely the densificated micro zones, generated by fabrication processes. In practical fabrication, a subsurface layer of 1.12 μm thick needs to be removed by stress-less processes, to obtain a low-absorption element.