Diffractive-refractive hybrid microscope objective for 193 nm inspection systems

The challenge in designing a complex optical system for the deep-UV regime is a consequence of the limited material selection combined with the demand of cement free optical groups. Especially for optical mask inspection where the presence of a protecting pellicle requires a long working distance an all-refractive solution for a high NA objective seems to be very critical. The combination of diffractive and refractive components to a hybrid optical system offers the advantageous possibility to overcome the addressed limitations. Here we present the realization of a hybrid microscope objective with a working distance of 7.8 mm and a numerical aperture of 0.65 for 193 nm mask evaluation. Despite the relative large bandwidth of 0.5 nm the use of calcium fluoride is not necessary but all components are based on fused silica. The small number of employed optical elements leads to a compact volume concept. The realized objective fits in a conventional mask evaluation tool. Classical refractive approaches didn't succeed in the simultaneous realization of all these critical specifications. For the realization of the diffractive optical element as the most determining element, a sophisticated holographic lithography process with a subsequent ion-etching technique was introduced.