fluorine-based etch

Fluorine-based etching uses fluorine-containing gases and their plasma-generated radical species to remove silicon, silicon dioxide, silicon nitride, and other materials in semiconductor manufacturing. Common fluorine source gases include carbon tetrafluoride (CF4), sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), and various fluorocarbon gases such as C4F8, C4F6, CHF3, and CH2F2. In plasma, these gases dissociate to produce atomic fluorine (F*) radicals along with CFx fragment species, each playing distinct roles in the etch process. Atomic fluorine is a powerful spontaneous etchant of silicon — it reacts with Si at room temperature without ion bombardment to form volatile SiF4, enabling high etch rates exceeding 500 nm/min. This spontaneous chemical etching is largely isotropic, making it useful for applications like silicon trench release in MEMS but problematic for anisotropic pattern transfer. For directional etching, fluorocarbon gases are preferred because CFx radicals deposit a thin fluorocarbon polymer passivation layer on surfaces while fluorine species etch the substrate. Ion bombardment selectively removes the polymer from horizontal surfaces while sidewalls remain passivated, enabling highly anisotropic profiles. The balance between etching (F radicals) and passivation (CFx polymer) is controlled by gas chemistry, source power, bias power, and pressure. Carbon-rich gases like C4F8 and C4F6 produce more polymer and are used for selective oxide-over-silicon etching, as the polymer inhibits silicon etching but is continuously removed from oxide surfaces through CO formation. SF6-based plasmas are widely used for high-rate silicon etching, particularly in the Bosch process for deep reactive ion etching (DRIE) where alternating SF6 etch and C4F8 passivation steps create deep, high-aspect-ratio trenches. Fluorine chemistry offers excellent selectivity of SiO2 to Si (>40:1) and SiN to Si under optimized conditions.