Alignment and overlay accuracy of an advanced x-ray stepper using an improved alignment system (Poster Paper)

Demands on advanced exposure tools for ULSI applications have increased rapidly during the last few years. Overlay accuracy, one of the key subjects, has been under continuous development. In X-ray lithography the overlay is mainly determined by pattern displacement on X-ray masks, by process-induced wafer distortions and by stepper-related contributions, generated by a limited alignment accuracy and displacements during the pattern replication. It is the aim of this paper to characterize the stepper-related contributions to the overlay budget. For this purpose we investigated the overlay performance of vertical XRS-200 type X-ray steppers equipped with an advanced optical alignment system. Using a multiple exposure technique, a series of alignment experiments were performed on typical CMOS layers. The results show that the wafer chuck flatness has, beside the alignment accuracy, a major influence on the overlay accuracy.