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 50X, 0.55 numerical aperture extra-long-working distance objective and new ly developed optics to obtain simultaneous images of the mask and wafe r. The images are nearly diffraction limited, have high brightness, un iform intensity to 5% full width at half-maximum, and are telecentric. They may be viewed either sequentially or superimposed on a charge co upled 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, 3 sigma. 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 conventio nally required to refocus the microscope objective. In addition, vibra tion induced offsets are eliminated by acquiring the mask and wafer im ages simultaneously. These offsets can be serious in systems in which the mask and wafer are locked together, even if the vibration is at a level which causes negligible deterioration of the resolution. We have observed that waveform errors of lambda/20, much better than required for diffraction limited performance, lead to image distortions of abo ut 25 nm. These distortions can be corrected either In software or by using very high-quality optics. We expect consideration of all these e ffects will be required to align far below the optical resolution. (C) 1995 American Vacuum Society.