Detection of actual focus variations by focus automatic measurement

We developed a new focus monitoring method that is simple yet highly accurate. We used simple measurement tools: a conventional binary mask and an optical overlay inspection machine. Our method was sufficiently precise to detect sub-100nm focus errors, and we demonstrated that it had high focus sensitivity under various illuminations (NA, σ, and illumination aperture). We measured actual focus errors by KrF scanner processing and determined the extent of each, i.e., field curvature/astigmatism, wafer topography, and best focus stability.