chlorine-based etch

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.