Effect of off-axis illumination on stepper overlay

Modified illumination schemes for wafer steppers, such as oblique and low partial coherence illumination, can improve the imaging performance for some types of microlithography patterns. Implementation of these methods by simple insertion of apertures into a stepper illuminator, without reoptimization of the illuminator, can lead to deterioration in illumination power and uniformity. The different apertures may also affect lens aberrations, lens distortion and illumination telecentricity and require adjustments for the effective control of dose, focus and reduction ratio. This study has examined the set-up of a modified illumination system, the effects of four different illuminator apertures on several lens parameters and the consequent effect on overlay between layers printed with different apertures. It is found that the lens performance is either not significantly changed or can be compensated adequately in the case of off-axis illumination apertures but deteriorates when using low partial coherence illumination. However, preliminary overlay tests between fields printed using different apertures show acceptable performance in all cases.