LASER-ABLATION OF MULTILAYER POLYMER-FILMS

We study the efficiency of using multilayer structures as an etch-stop mechanism in the ablation of polyimide films by ultraviolet lasers. T he study is done using a photothermal model that includes the light ab sorption by the decomposed fragments, which shield the polymer from th e laser beam, an intermediate zone in which the polymer is suffering a phase transition and the underlying unburned material. The layers are differentiated from each other through the optical properties. Variat ion in the optical properties of polyimide has been achieved by a prop er selection of impurities. From our modeling work, we conclude that o ptically thin foils may be used as etch stop in the ablation process w hen the penetration depth of the middle layer is around three times la rger than the penetration depth of the surrounding layers, this for fl uences below 200 mJ/cm2. We also present some experimental results.