Performance characteristics of a dual focus x‐ray alignment microscope

We report a dual focus alignment microscope which we have implemented on our Anorad x‐ray exposure tool. The microscope uses an Olympus 50×, 0.55 numerical aperture extra‐long‐working distance objective and newly developed optics to obtain simultaneous images of the mask and wafer. The images are nearly diffraction limited, have high brightness, uniform intensity to 5% full width at half‐maximum, and are telecentric. They may be viewed either sequentially or superimposed on a charge coupled device video camera. We have used conventional image processing techniques to locate individual vertical and horizontal x‐ray mask and oxide level wafer features to about 15 nm, 3σ. This is much less than the 500 nm optical resolution of the microscope. The dual focus imaging avoids the very precise high‐speed mechanical motion conventionally required to refocus the microscope objective. In addition, vibration induced offsets are eliminated by acquiring the mask and wafer images simultaneously. These offsets can be ser...