site flatness

**Site Flatness** is a **wafer metrology parameter measuring the flatness (or thickness variation) within a small, localized area (site) on the wafer** — typically measured as SFQR (Site Flatness Quality Reference), which is the range of the surface within a site relative to a local reference plane. **Site Flatness Metrics** - **SFQR**: Site Flatness Quality Region — the range of the front surface deviation from a best-fit reference plane within the site. - **SFQD**: Site Flatness Quality Deviation — the maximum deviation from the reference plane within the site. - **Site Size**: Typically 25mm × 25mm or 26mm × 33mm — matching die sizes for relevance to lithography. - **Edge Exclusion**: Typically 2mm or 3mm edge exclusion — edge sites are measured but may have relaxed specs. **Why It Matters** - **Lithography**: Steppers expose one site (die) at a time — site flatness determines the local focus budget. - **Tighter Than TTV**: Even if global TTV is good, individual sites may have poor flatness. - **Yield**: Each site's flatness directly affects that die's patterning quality — site flatness predicts die-level yield. **Site Flatness** is **flatness where it matters most** — measuring wafer planarity within die-sized regions for lithography-relevant quality control.