Laser ablation patterning of dielectric layer stacks for 193 nm mask fabrication

Fabrication of dielectric optical masks by ablation patterning using a 193 nm laser is demonstrated. Ablation of layer systems that are highly reflective at 193 rim (HR 193mn) is accomplished by using an absorbing layer to initiate ablation. This absorbing layer is deposited on the substrate under the HR system. Thus it is possible to induce spatially defined ablation of the whole stack by irradiating the absorbing layer through the substrate (rear side ablation). Using the system as a mask, i.e. irradiating it from the front side, the reflective layer system prevents penetration of high fluence radiation to the absorbing layer, so that the function of the reflective mask is not restricted. A HR 193 nm system, composed of 42 layers, alternating Al2O3 and SiO2, was prepared. The substrate was fused silica. For the absorbing layer (a) HfO2 and (b) SiOx (x < 2) of about 30 run thickness were used. In the case of HfO2, residual fragments of the absorbing layer diminish the transmission in the ablated areas. In the case of SiOx, which is absorbing at 193 nm for x < 2, these residuals can be oxidized to SiO2 by thermal treatment or by additional low fluence laser pulses after the patterning process. SiO2 has high transmission at 193 nm improving the mask performance. For the ablation process single pulse irradiation with a fluence of about 800 mJ/cm(2) is Sufficient. The mask has a damage threshold of > 1 J/cm(2).