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.