Parallel fabrication of silicon concave microlens array by femtosecond laser irradiation and mixed acid etching

A method of multi-beam femtosecond laser irradiation combined with modified HF-HNO₃-CH₃COOH etching is used for the parallel fabrication of all-silicon plano-concave microlens arrays (MLAs). The laser beam is split by a diffractive optical element and focused by a lens to drill microholes parallely on silicon. An HF-HNO₃-H₂SO₄-CH₃COOH solution is used to expand and polish laser-ablated microholes to form microlenses. Compared with the HF-HNO₃-CH₃COOH solution, the solution with H₂SO₄ can effectively reduce the etched surface roughness. The morphologies of MLAs at different laser powers and pulse numbers are observed. The image array formed by the silicon microlenses is also demonstrated.