total thickness variation

**TTV** (Total Thickness Variation) is a **wafer metrology parameter measuring the difference between the maximum and minimum thickness across a wafer** — quantifying wafer flatness as $TTV = t_{max} - t_{min}$, where thickness is measured at multiple points across the wafer surface. **TTV Measurement** - **Definition**: $TTV = max(t_i) - min(t_i)$ across all measurement sites on the wafer. - **Measurement**: Capacitive probes, interferometric thickness measurement, or ultrasonic methods. - **Sites**: Measured at standard SEMI-defined sites (typically 5, 9, or 25 sites per wafer). - **Specs**: Advanced node wafers typically require TTV < 2 µm (300mm wafers). **Why It Matters** - **Lithography**: TTV directly impacts lithographic depth of focus — non-flat wafers defocus during patterning. - **CMP**: Chemical-mechanical polishing uniformity is constrained by incoming TTV — higher TTV = harder to planarize. - **Yield**: Excessive TTV causes edge die yield loss — non-flat regions cannot be patterned accurately. **TTV** is **the flatness scorecard** — the single number that captures how much a wafer's thickness varies across its entire surface.