depth of focus

**Depth of Focus (DOF)** is the **range of focus positions within which the aerial image maintains sufficient contrast and the patterned CD stays within specification** — the lithographic focus budget available to accommodate wafer non-flatness, stage errors, and lens aberrations. **DOF Factors** - **Rayleigh DOF**: $DOF = k_2 frac{lambda}{NA^2}$ where $k_2 approx 0.5-1.0$ — fundamental physics limit. - **Wavelength ($lambda$)**: Shorter wavelength reduces DOF — EUV (13.5nm) has very tight DOF. - **NA**: Higher NA reduces DOF quadratically — high-NA EUV halves DOF further. - **Feature Dependent**: Dense features, isolated features, and contacts each have different DOF. **Why It Matters** - **Budget**: DOF must accommodate wafer flatness (TTV, nanotopography), chuck accuracy, leveling errors, and lens field curvature. - **EUV**: EUV DOF is ~50-80nm — extremely tight, requiring excellent wafer flatness and stage control. - **Scaling**: As features shrink and NA increases, DOF decreases — the most critical lithographic challenge at advanced nodes. **DOF** is **the focus tolerance** — the razor-thin range of focus positions where lithographic patterning produces acceptable features.