ocd (optical critical dimension)

OCD (Optical Critical Dimension) uses optical scatterometry to extract detailed 3D profile information of periodic structures by analyzing diffracted light. **Principle**: Broadband light illuminates periodic grating structure. Diffraction pattern (zeroth-order reflectance spectrum) depends on grating profile - CD, height, sidewall angle, footing, rounding. **Model-based**: Measured spectrum compared to library of simulated spectra from RCWA (Rigorous Coupled-Wave Analysis) electromagnetic models. Best-matching model yields profile parameters. **Parameters extracted**: CD (top, middle, bottom), height, sidewall angle, footing, profile asymmetry, film thicknesses. Multiple parameters from single measurement. **Speed**: Very fast measurement (~1 second per site). High throughput for inline production monitoring. **Non-destructive**: Optical measurement does not damage features. Can measure production wafers. **Accuracy**: When properly calibrated to TEM reference, OCD achieves sub-nm precision. Model accuracy depends on quality of assumed profile shape. **Targets**: Requires periodic grating structures (lines/spaces, hole arrays) in scribe line or designated metrology areas. **Applications**: Gate CD and profile, FinFET fin profile, spacer thickness, etch profile monitoring, litho CD and resist profile. **Complementary to CD-SEM**: OCD provides 3D profile information that top-down CD-SEM cannot. CD-SEM provides real-structure imaging. **Vendors**: KLA (SpectraFilm/Shape), Nova (NOVA T600), Onto Innovation.