image-based overlay

**IBO** (Image-Based Overlay) is the **traditional overlay metrology technique that measures alignment between layers by imaging overlay targets** — a microscope images box-in-box or bar-in-bar targets, and image processing extracts the registration error from the relative positions of the target features. **IBO Measurement** - **Targets**: Box-in-box (BiB) or bar-in-bar (AIM marks) — inner box from current layer, outer box from reference layer. - **Imaging**: High-magnification brightfield microscopy with optimized illumination wavelength and focus. - **Algorithm**: Image processing determines the center of each target element — overlay = center difference. - **Multi-Wavelength**: Measure at multiple wavelengths — optimize for signal quality and accuracy. **Why It Matters** - **Mature**: IBO is the most established overlay technique — decades of calibration and characterization data. - **Large Targets**: Traditional BiB targets are large (20-30 µm) — consume valuable scribe line space. - **TIS**: Tool-Induced Shift from optical asymmetries — must be calibrated out using 0°/180° measurement. **IBO** is **measuring alignment with a microscope** — the classic overlay metrology technique using optical imaging of registration targets.