Defocus-resolved construction of process windows in nanosecond laser irradiation of oxidized silicon

We present a defocus-dependent, quantitatively validated framework for nanosecond fiber-laser irradiation of thermally oxidized silicon. By systematically varying the beam defocus, the on-axis peak power density i and spot diameter d are correlated with distinct surface morphologies on SiO 2 (150 nm)/Si (111). Controlled scanning experiments made with nanosecond fiber laser (IRE-Polyus ILI-1-50, wave length λ = 1062 nm, pulse width τ ≈ 120 ns, Pulse Repetition Frequency 50 kHz, and scan velocity v = 100 mm·s –1 ) reveal a continuous morphological sequence — wrinkling, blistering, and premelting — governed by thermo-mechanical stress and interfacial adhesion. Surface features were quantified using two-dimensional Fourier analysis for wrinkle wavelength (λ ≈ 8–10 µm) and automated image segmentation for blister size distributions (1–3 µm). Point-exposure measurements yield a blister-onset energy of 0.09 J, corresponding to a per-pulse fluence of 0.17 J·cm –2 . Logistic regression with 95 % bootstrap confidence intervals with bootstrap replications B = 2000 defines the wrinkle-to-premelting transition, producing a statistically bounded ( I , d ) phase diagram. Control experiments confirm that both dynamic scanning and the oxide cap are indispensable: point irradiation on SiO 2 /Si produces only shallow ⟨110⟩ slip lines from substrate relaxation, while bare Si remains featureless under identical conditions. Hydrofluoric (HF) etching verifies that the observed blisters are true interfacial delaminations. This framework extends classical buckling–delamination mechanics to cyclic nanosecond heating and provides predictive guidelines for precision, sub-melting laser microfabrication of dielectric–semiconductor systems.