Mask Error Enhancement Factor (MEEF) quantifies how much a dimensional error on the photomask is amplified (or reduced) when transferred to the wafer. It is the ratio of the wafer CD change to the mask CD change (after accounting for magnification), and it is a critical metric for understanding mask quality requirements.

MEEF Definition

Where:

• $\Delta CD_{\text{wafer}}$ = Change in critical dimension on the wafer.
• $\Delta CD_{\text{mask}}$ = Change in critical dimension on the mask.
• $M$ = Mask magnification (typically 4× for DUV/EUV — meaning mask features are 4× larger than wafer features).

Interpreting MEEF

• MEEF = 1: A mask error transfers 1:1 to the wafer (after magnification correction). Linear behavior — ideal.
• MEEF > 1: Mask errors are amplified on the wafer. A 1 nm mask error (0.25 nm at wafer scale for 4× mask) causes more than 0.25 nm of wafer CD change.
• MEEF < 1: Mask errors are attenuated — the wafer is less sensitive to mask imperfections. This is favorable.
• MEEF >> 1 (e.g., 3–5): Dangerous territory. Small mask errors cause large wafer errors, making mask quality requirements extremely stringent.

What Affects MEEF

• Feature Size vs. Resolution: As features approach the resolution limit, MEEF increases dramatically. Near the resolution limit, MEEF can reach 3–5× or higher.
• Pattern Type: Dense lines typically have lower MEEF than isolated features or contacts.
• Assist Features: SRAFs can reduce MEEF by improving aerial image robustness.
• Illumination: Off-axis illumination schemes affect MEEF differently for different feature types.
• Phase-Shift Masks: AttPSM and AltPSM generally achieve lower MEEF than binary masks.

Practical Impact

• If MEEF = 3 and the wafer CD tolerance is ±1.5 nm, then the mask CD must be controlled to ±0.5 nm at wafer scale — or ±2 nm at mask scale (for 4× mask).
• At advanced nodes with MEEF = 4–5, mask CD control requirements become sub-nanometer at mask scale — pushing the limits of mask metrology and fabrication.

MEEF directly determines how good the mask must be — it is one of the key metrics linking mask manufacturing specifications to wafer patterning performance.