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Stepper Overlay Calibration Using Alignment To A Latent Image

We describe a method which minimizes the time required to set up and sustain optimum overlay performance of the DSW step-and-repeat system throughout a product wafer run. Improvement in both system performance and productivity is realized. Fundamental to the technique are the latent image formed by the exposure of photoresist, the AWA' global alignment system and the laser-metered stages of the DSW® system. We eliminate the need to develop wafers and to read verniers during the DSW® system set-up. Furthermore, we demonstrate the potential for on-the-fly calibration using product reticles and product wafers. The approach is generally applicable to the overlay calibration of optical exposure tools.