THE EFFECT OF PROCESS COATINGS ON THE ALIGNMENT SIGNAL IN A PROXIMITY LITHOGRAPHY SYSTEM

The achievement of a high accuracy mask-to-wafer alignment system is one of the key steps in the development of submicron x-ray lithography technology. In this paper, we examine the effects of the depth of the wafer mark and of different process coatings (including photoresist) on the alignment signal in an alignment system that uses linear Fresnel zone plates. A mathematical model based on scalar diffraction theory has been developed to simulate the effect of various coatings on the diffracted signal. An experiment is also carried out to verify the model. From the prediction of the theoretical model and the experimental results, we discuss how the process coating may affect the alignment signals in our two state alignment system. In the light of these results, we deduce the optimal design of the wafer alignment marks in our alignment system.