scatterometry
**Scatterometry (OCD Metrology)** is the **non-destructive optical technique that measures critical dimensions, film thickness, and profile shape of patterned features by analyzing how light scatters from periodic structures** — providing the primary inline dimensional metrology for semiconductor manufacturing with sub-angstrom sensitivity and seconds-per-site measurement speed.
**How Scatterometry Works**
1. **Illumination**: Broadband light (DUV to NIR, 190–900 nm) strikes a periodic grating target on the wafer.
2. **Measurement**: Reflected/diffracted light spectrum captured (intensity vs. wavelength and/or angle).
3. **Library Matching**: Measured spectrum compared against a pre-computed library of simulated spectra using rigorous coupled-wave analysis (RCWA).
4. **Parameter Extraction**: Best-matching simulation yields the geometric parameters (CD, height, sidewall angle, overlay).
**Measurement Types**
| Technique | Measurement | Equipment |
|-----------|------------|----------|
| Spectroscopic Ellipsometry (SE) | Film thickness, optical constants, CD | KLA Aleris, NOVA PRISM |
| Normal-Incidence Reflectometry | Film thickness, CD | Nanometrics Atlas |
| Mueller Matrix Ellipsometry | Asymmetric profiles, overlay | KLA SpectraFilm |
**What OCD Measures**
- **CD (Critical Dimension)**: Line width, space width — sub-nm precision.
- **Height/Depth**: Trench depth, fin height, resist thickness.
- **Sidewall Angle**: Profile shape — 88° vs. 90° matters for contact fill.
- **Overlay**: Misalignment between layers — replacing traditional imaging overlay.
- **Film Thickness**: Under-layer and multi-film stack characterization.
**OCD vs. CD-SEM**
| Aspect | OCD/Scatterometry | CD-SEM |
|--------|-------------------|--------|
| Speed | 2–5 seconds/site | 10–30 seconds/site |
| Throughput | 50–100 wafers/hour | 10–20 wafers/hour |
| Damage | Non-destructive (photons) | Potential resist shrinkage (electrons) |
| 3D Profile | Full profile (height, angle) | Top-down only |
| Targets | Dedicated gratings | Device structures |
| Model | Requires physical model | Direct measurement |
**Advanced Applications**
- **FinFET profile monitoring**: Fin height, top CD, bottom CD, sidewall angle — all from one measurement.
- **EUV process control**: Resist CD and profile for dose/focus optimization.
- **3D NAND channel hole shape**: Deep hole diameter vs. depth profile.
Scatterometry is **the backbone of inline dimensional metrology in modern fabs** — its non-destructive nature, speed, and 3D profile sensitivity make it indispensable for process control at every lithography and etch step in advanced semiconductor manufacturing.