Helium as an exceptional protective gas for femtosecond laser processing of crystalline silicon

Femtosecond laser processing in specific ambient atmospheres typically enables high-concentration doping of gas molecules into silicon, which significantly affects the material's properties. When the ambient gas serves merely as a protective atmosphere rather than an intentional dopant source, this effect can lead to the unintended doping of non-target elements, which may adversely affect subsequent results. However, this is ineffective with helium. Experiments reveal that silicon samples processed by a femtosecond laser in a helium atmosphere exhibit smooth micro/nano-structured surfaces, maintain a good monocrystalline phase, possess low helium concentration, and retain almost unaltered electrical properties. Subsequent thermal annealing can eliminate the incorporated helium in silicon. These properties are attributed to the chemical inertness, small atomic radius, and low atomic mass of helium. The experimental results demonstrate that helium acts as an exceptional protective gas during femtosecond laser processing of monocrystalline silicon, showing promising potential for broader applications in other ultrafast laser-based processes.