reflectometry

Reflectometry measures thin film thickness by analyzing interference patterns in light reflected from the film surface and underlying interfaces. **Principle**: Light reflects from both top surface and bottom interface of a transparent film. The two reflected beams interfere constructively or destructively depending on film thickness and wavelength. **Constructive/destructive**: When optical path difference = integer wavelengths, constructive interference (reflection peak). Half-integer wavelengths give destructive (reflection minimum). **Spectral reflectometry**: Measures reflectance vs wavelength. Oscillation pattern encodes film thickness. Thicker films show more oscillations. **Calculation**: Thickness = function of wavelength spacing between peaks, refractive index, and angle. **Advantages**: Fast, non-contact, non-destructive. Simple optical setup. Low cost compared to ellipsometry. **Spot size**: Can be very small (<5 um) for in-die measurements. **Multi-layer**: Can measure multi-layer stacks if layers have different refractive indices. Model fitting extracts individual layer thicknesses. **Endpoint detection**: Used for CMP endpoint (film thickness decreasing during polish) and etch endpoint (film thickness decreasing during etch). **Limitations**: Less information than ellipsometry (one parameter per wavelength vs two). Cannot independently determine n and thickness without prior knowledge. Requires optically transparent films. **Applications**: Oxide/nitride thickness monitoring, CMP uniformity mapping, etch depth measurement. **Equipment**: Standalone metrology tools (Nanometrics/Onto, KLA) or integrated sensors in process tools.