Electrical and SEM testing of absorber defectivity in the plated-gold process for x-ray masks

It is widely recognized that the 1:1 x-ray mask is the most technically challenging aspect of proximity x-ray lithography, since high resolution and precise pattern placement must be achieved completely free of defects. SEM investigation is an excellent tool for x-ray mask inspection. However, it is sometimes assumed that only the SEM has sufficient resolution to perform meaningful defect detection on x-ray masks. An electrically probed test chip pattern for x-ray mask defectivity measurement and improvement has been designed and implemented. This pattern is printed with an optical stepper on silicon wafers with plating base. These are then processed like an x-ray mask through x-ray absorber definition. Since the absorber is a high-conductivity metal and the substrate is insulating, electrical shorts and opens correspond to extra and missing absorber. This paper describes a series of defect types revealed by these two rather different methodologies: SEM inspection of completed masks using the KLA SEMSpec, as well as the electrically probed test chip pattern. The two methods are compared and contrasted. The main nuisance and genuine defects in our x-ray mask process are catalogued.