The impact to image quality and overlay by using different pupil fills in a DUV scanner of 0.6 NA studied by using overlay metrology. primary optical aberrations such as spherical aberration, field curvature, astigmatism were studied using the Phase Shift Focus (PSF) monitor, while coma was studied by using coma monitor structure. Distortion was studied by using telecentricity and overlay-through- focus test. It was found that different illumination mode contributes to different extent of these aberrations, confirming that illumination aberrations couple with projection lens aberrations to affect image quality. We also observed partial coherence variation across field as well the orientational dependency of partial coherence. Coma was found to introduce a second order curvature in the displacement curve and it affects distortion by the effect of field magnification error, as indicated by the telecentricity and overlay-through-focus test. It is concluded that optical aberration is the worst for low sigma illumination however, it provides the least variation in partial coherence across field. Off Axis Illumination also impacts distortion pattern and overlay results, so any application using mixed pupil fills needs to be addressed carefully, accuracy of the PSF measurement has to be improved to apply this method to augment the advantage of speed and convenience in collecting a lot of across field data. It is also found that PSF monitor is not applicable for quadrupole illumination.
The alignment and overlay metrology issues for various damascene process architectures were studied and optimized. The Buried Hard Mask, Via First and Trench First approaches are studied comparatively for the Via to Metal1 and Metal2 to Via alignments. Alignment capability was studied by the alignment signal strength and alignment repeatability. Overlay metrology capability was studied by the overlay target appearance, static repeatability, target correlation and Tool Induced Shift. Final overlay measurement and long term overlay stability were used as a mean to verify the result. A Design of Experiment was done with splits in the hard mask material/thickness and the degree of copper CMP. It was found that for Metal2 alignment to Via, the Buried Hard Mask approach possess a showstopper unless one align Metal2 to Metal1 instead. The effect of CMP to the alignment to Metal1 level seems to have less trouble than the Tungsten CMP counterpart except the overlay target acquisition might be difficult for underpolish case. The choice of which damascene approach to take depends also on the trade off between overlay and CD control and other process performance and should be customized by individual's requirement.