The main etch is the primary phase of a plasma etch process responsible for bulk material removal — etching through the majority of the target film's thickness with the required anisotropy, selectivity, and uniformity. It is the step that defines the pattern in the target material.

Role of the Main Etch

• Removes the bulk of the target material — whether it's polysilicon, silicon oxide, metal, or dielectric.
• Defines the final feature profile — vertical sidewalls, controlled taper, or other target geometry.
• Must maintain selectivity to underlying layers (stop layer) and adjacent materials (resist, hard mask, spacers).
• Must achieve uniform etch depth across the wafer and within each die.

Key Parameters

• Etch Chemistry: The gas mixture is carefully chosen for the target material. Examples:
• Polysilicon: HBr/Cl₂/O₂ — provides high selectivity to SiO₂ gate oxide.
• SiO₂: CF₄/CHF₃/C₄F₈ + Ar — fluorine-based chemistry for oxide removal.
• Metal (Al, Cu): Cl₂/BCl₃-based for aluminum; copper uses dual-damascene (not directly etched).
• Si₃N₄: CH₂F₂/CHF₃ + O₂ — selective to oxide.

• Anisotropy: Achieved through ion bombardment (directional ions accelerated perpendicular to the wafer by the plasma bias) combined with sidewall passivation (polymer deposition on feature sidewalls protects them from lateral etching).

• Selectivity: The ratio of etch rates between the target material and adjacent materials. Critical selectivities:
• Target-to-stop-layer: Typically >20:1 required.
• Target-to-resist: Must etch the target before consuming the resist mask.

Process Windows

• Pressure: Lower pressure → more directional ions → better anisotropy but potentially more damage. Higher pressure → more chemical etching → faster but more isotropic.
• RF Power: Source power controls plasma density (etch rate). Bias power controls ion energy (anisotropy, selectivity).
• Temperature: Affects chemical reaction rates and polymer deposition. Wafer chuck temperature is typically controlled to ±0.5°C.

Endpoint Detection

• The main etch must stop at the right depth. Endpoint detection methods:
• Optical Emission Spectroscopy (OES): Monitors plasma light — when the target material is consumed, the emission spectrum changes.
• Laser Interferometry: Measures film thickness in real-time through interference of reflected light.
• Mass Spectrometry (RGA): Detects etch byproduct species in the chamber exhaust.

The main etch is the core value-creating step of the etch process — all other steps (breakthrough, over-etch, passivation) exist to support and refine the results of the main etch.