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.