CD & Overlay Measurement

Critical Dimension Measurement

Critical Dimension (CD) — the width of the smallest features — is the most important dimensional measurement in the fab. Two main approaches:

• CD-SEM: A scanning electron microscope optimized for measuring feature widths. Produces top-down images with ~1 nm measurement precision. Advantages: direct imaging, intuitive results. Limitations: slow (~10 wafers/hour for production measurements), can only measure top-down profiles.
• OCD (Optical CD / Scatterometry): Measures how light scatters from periodic structures (gratings). By fitting the measured spectrum to a physical model, it extracts CD, height, sidewall angle, and profile shape simultaneously. Advantages: fast (~100+ wafers/hour), non-destructive, provides 3D profile info. Limitations: requires periodic targets, model-based (not direct imaging).

Overlay Measurement

Overlay measures the alignment between the current lithography layer and the previous one. Even small misalignment (>2 nm) can cause shorts or opens between layers.

Overlay tools measure specially designed target structures printed on each layer:

• Image-based overlay (IBO): Optical microscope images box-in-box or frame-in-frame targets. The displacement between inner and outer boxes = overlay error.
• Diffraction-based overlay (DBO): Measures the difference in diffraction intensity from gratings that are intentionally offset by known amounts. More precise than IBO.

Overlay measurements feed directly into the lithography scanner's alignment corrections for subsequent wafers — a classic feedback control loop.