scatterometry

Scatterometry analyzes the diffraction pattern from periodic structures to extract detailed dimensional and profile information about the features. **Technique**: Essentially synonymous with OCD. Broadband light diffracts from grating structure. Specular (zeroth-order) reflection spectrum analyzed. **Physics**: Electromagnetic interaction between light and periodic structure creates wavelength-dependent reflectance that encodes structural information. **Measurement modes**: **Spectroscopic**: Vary wavelength at fixed angle. Most common for semiconductor. **Angular**: Vary angle at fixed wavelength. **Both**: Combine spectral and angular data for more parameters. **RCWA modeling**: Rigorous Coupled-Wave Analysis solves Maxwell's equations for parameterized grating profile. Library of spectra generated for parameter combinations. **Fitting**: Measured spectrum matched to library to find best-fit profile parameters. Regression or library-search algorithms. **Sensitivity**: Different wavelengths and polarizations sensitive to different profile parameters. UV sensitive to top CD, longer wavelengths to bottom. **Mueller matrix scatterometry**: Measures full polarization response for more information. Detects asymmetry and overlay. **Overlay measurement**: Scatterometry-based overlay (SCOL) measures alignment between layers using specially designed targets. Growing alternative to image-based overlay. **Limitations**: Requires periodic targets (cannot measure isolated features). Model assumptions affect accuracy. Complex profiles need many parameters. **Process control**: Fast, non-destructive measurement enables real-time APC (Advanced Process Control) feedback loops.