global flatness

**Global Flatness** is a **wafer metrology parameter that characterizes the overall shape and planarity of the entire wafer** — measuring how well the wafer surface conforms to an ideal flat plane, typically expressed as GBIR (Global Back-surface Ideal Range) or TTV. **Global Flatness Metrics** - **GBIR**: Global Back-surface Ideal Range — front surface deviation range when the back surface is chucked ideally flat. - **TTV**: Total Thickness Variation — the maximum minus minimum thickness across all measurement sites. - **Warp**: Maximum deviation of the median surface from a reference plane — measures wafer bowing. - **Bow**: Deviation of the center point from a plane defined by the wafer edge — concave vs. convex shape. **Why It Matters** - **Chucking**: Wafer chucks must be able to flatten the wafer — excessive warp prevents proper wafer hold-down. - **Lithography**: Global flatness affects alignment and overlay — the stepper assumes a flat wafer. - **Incoming Quality**: Incoming wafer global flatness specs are critical for subsequent process quality. **Global Flatness** is **the big picture of wafer shape** — characterizing overall wafer planarity for process compatibility and lithography performance.