Interpretation and Modeling of Laser-Induced Backside Wet Etching Procedure

Laser-induced backside wet etched fused silica surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and ellipsomerty, and a new numerical model was developed to interpret the etching procedure on the basis of our recent results. ArF and KrF lasers were used for etching, while the applied liquid absorbers were naphthalene/methyl-methacrylate and pyrene/acetone solutions. The XPS measurements showed that the completely cleaned, etched fused silica surface layers were contaminated by carbon (which originated from the organic absorber molecules). The optical parameters of the modified layers were measured by spectroscopic ellipsometer. The thicknesses of these carbon contaminated layers were very thin (between 10 and 30 nm), while their absorption coefficient and the refractive index at 248 nm were between 100 000 and 180 000 cm, and 1.85, respectively. Our previous numerical model was completed on the basis of the determined properties of this modified layer, and our results proved that this layer plays key role in the etching procedure.