Novel x-ray mask concept for mix&match lithography fabrication of MOS devices by synchrotron radiation lithography

The investigated X-ray steppers Karl Suss XRS 200/1 and XRS 200/3 use an optical alignment system for adjusting the parallelism, for gap setting and alignment between mask and wafer. In general the optical signal, when mask and wafer are at gap distance, is dependent on the optical characteristics (transparency of the mask, reflectivity of the wafer, resist film interference etc.) of both mask and wafer. In this paper, different methods to decouple the optical signal of the mask by that of the wafer and to effectively improve the optical signal contrast, are presented. The methods used allow to reach in all examined cases an alignment accuracy down to 5 nm. This value can be regarded as the resolution limit of the alignment system itself.