Focus control

摘要

Focus control for modern lithographic apparatus is per field and based on available focus measurement data, the focus measurement data derived from focus markers on scribe and inner-scribe lanes (intra-field). A polynomial fit X~nY~m (n≤l, m≤4) is made to focus data to determine appropriate focus correction. The problem is that for 3D NAND processes significant height differences exist between (inner) scribe lanes and device features (scribe to cell offset). The fit parameters will deviate strongly in case focus data points are missing (for example for edge dies where no complete field dala is available). In figure 1 a situation is sketched giving rise to the described problem.