focus-exposure matrix

**FEM** (Focus-Exposure Matrix) is a **lithographic characterization technique where a test wafer is exposed with systematically varying focus and dose across the wafer** — each field (or sub-field) receives a different focus/dose combination, creating a matrix that maps the patterning response across the two-dimensional parameter space. **FEM Layout** - **Rows**: Different focus settings (e.g., -100nm to +100nm in 10nm steps) — one focus per row of fields. - **Columns**: Different exposure doses (e.g., ±10% around nominal in 1% steps) — one dose per column. - **Matrix Size**: Typically 10-20 focus settings × 10-20 dose settings — covering the entire wafer. - **Measurement**: After develop, measure CD at each field — plot CD vs. focus and dose. **Why It Matters** - **Process Window**: FEM data is used to construct Bossung curves and determine the process window (depth of focus × exposure latitude). - **Optimization**: Find the optimal focus and dose that centers the process within the window. - **Qualification**: FEM is the standard method for qualifying new lithography processes and mask designs. **FEM** is **the lithographic experiment** — systematically varying focus and dose to map the complete patterning response space.