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.