main etch
**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.