ellipsometry

Ellipsometry is a non-destructive optical technique that measures thin film thickness and optical constants by analyzing how polarized light changes upon reflection from the sample. **Principle**: Linearly polarized light reflects from film surface. The reflected light becomes elliptically polarized. The change in polarization state (amplitude ratio psi, phase shift delta) relates to film properties. **Measurement**: Measures two parameters (psi, delta) per wavelength and angle. More information than simple reflectance. **Film properties**: Extracts thickness, refractive index (n), and extinction coefficient (k). Can measure multi-layer stacks. **Optical model**: Measured data fitted to optical model of film stack. Model includes layer thicknesses and optical constants. Goodness of fit validates model. **Non-contact**: Light-based measurement does not touch or damage wafer. Suitable for inline production monitoring. **Single-wavelength**: HeNe laser (632.8nm) for simple single-layer thickness measurement. Fast, inexpensive. **Accuracy**: Angstrom-level thickness accuracy for well-characterized films. Sensitive to sub-nanometer thickness changes. **Spot size**: Measurement spot typically 25-100 um. Small enough for in-die measurement on test structures. **Applications**: Gate oxide thickness, CVD film thickness, resist thickness, CMP removal monitoring, ALD cycle calibration. **Limitations**: Requires optical model. Ambiguous for very thick films without additional constraints. Transparent substrate complicates measurement.