Chlorine-based etch uses chlorine radicals and ions to etch metals, polysilicon, and compound semiconductors. Chemistries: Cl2 (chlorine), BCl3 (boron trichloride), HCl (hydrogen chloride), SiCl4 (silicon tetrachloride). What it etches: Aluminum, polysilicon, titanium, tungsten, compound semiconductors (GaAs, InP). Mechanism: Chlorine radicals react with material to form volatile chlorides. Ion bombardment assists. Aluminum etch: BCl3/Cl2 chemistry standard for Al etch. BCl3 scavenges oxide, enables native oxide breakthrough. Polysilicon gate etch: Cl2/HBr chemistry for poly gate with selectivity to oxide. Historical mainstream process. Selectivity: Chlorine does not readily etch silicon oxide. Good selectivity to oxide mask and substrate. Corrosion concern: Chlorine residues cause aluminum corrosion. Post-etch clean and passivation critical. Safety: Chlorine gases are toxic and corrosive. Proper exhaust and safety systems required. Comparison to fluorine: Fluorine for oxides, chlorine for conductors. Different reaction products.