anti-reflective coating (arc)

Anti-Reflective Coatings (ARC) are thin layers below or above resist that control reflections and improve CD uniformity. **Bottom ARC (BARC)**: Applied before resist. Absorbs light that would reflect from substrate. Most common. **Top ARC (TARC)**: Applied above resist. Reduces reflections at resist-air interface. Less common. **Why needed**: Substrate reflections cause standing waves in resist, CD variation with topography. **Swing curve**: Without ARC, CD varies sinusoidally with resist thickness. ARC minimizes swing. **Materials**: Organic polymers (spin-on) or inorganic (CVD silicon oxynitride). **BARC requirements**: Refractive index matched to minimize reflection. Absorbing at exposure wavelength. **BARC etching**: BARC must be opened (etched through) before main etch. Adds process step. **Thickness**: Optimized for exposure wavelength and resist system. Typically 20-80nm. **At advanced nodes**: BARC essential for CD control. Multi-layer ARCs sometimes used. **Inorganic vs organic**: Inorganic more process robust, organic easier to remove.