diffraction-based overlay

**DBO** (Diffraction-Based Overlay) is an **overlay metrology technique that measures the registration error between two patterned layers using diffraction from overlay targets** — the intensity of +1st and -1st diffraction orders shifts with overlay error, enabling sub-nanometer overlay measurement. **DBO Measurement** - **Targets**: Gratings with intentional offsets — two gratings with +d and -d programmed shifts. - **Principle**: Overlay error breaks the symmetry between +1st and -1st diffraction orders: $Delta I = I_{+1} - I_{-1} propto OV$. - **µDBO**: Micro-DBO uses small (~10×10 µm) targets with multiple pads for X and Y overlay — fits in scribe line. - **Swing Curve**: The signal-to-overlay relationship follows a sinusoidal curve — calibration required. **Why It Matters** - **Accuracy**: DBO achieves sub-0.5nm accuracy — essential for <5nm node overlay requirements. - **Small Targets**: µDBO targets are small enough for in-die placement — no scribe line limitation. - **Tool-Induced Shift**: DBO is susceptible to optical TIS (Tool-Induced Shift) — correction is critical. **DBO** is **measuring misalignment with light** — using diffraction order intensity asymmetry for sub-nanometer overlay metrology.