wafer acceptance criteria

**Wafer Acceptance Criteria (WAC)** are the **set of measurable thresholds that a semiconductor wafer must pass at various stages of fabrication** — to be accepted for the next process step or for final shipment to the customer. **What Are Wafer Acceptance Criteria?** - **Definition**: Quantitative pass/fail limits on critical parameters. - **Parameters Tested**: - **Defect Density**: Max defects per cm² (inspected by KLA tools). - **Film Thickness**: Uniformity within $pm$ specification (measured by ellipsometry). - **Critical Dimension (CD)**: Line width within tolerance (SEM measurements). - **Electrical**: Sheet resistance, threshold voltage ($V_t$), leakage current. - **Disposition**: Pass (ship), Fail (scrap), Hold (review by engineer). **Why It Matters** - **Yield Protection**: Catching bad wafers early prevents wasting downstream processing costs. - **Customer SLA**: Contractual obligations for defect levels (e.g., < 0.1 defects/cm²). - **Continuous Improvement**: Trending WAC data drives process optimization and equipment maintenance. **Wafer Acceptance Criteria** are **the quality gates of semiconductor manufacturing** — ensuring every wafer meets the exacting standards required for reliable chips.