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.