main etch

# Main Etch (ME)

Main etch performs most of the intended bulk removal.

## Place the step in context

Identify the incoming surface, the intended transformation and the surface delivered to the next module. A local abbreviation does not define chemistry, exposure time or acceptance limits.

## Integration context

A main step balances removal rate, dimensional control, mask survival and uniformity. It may stop on a qualified endpoint or a timed criterion. A rate measured on an unpatterned monitor does not account for all loading and aspect-ratio effects.

## What to verify

Verify feature depth and profile across the relevant layouts before selecting the landing transition.

Before selecting removal conditions, identify all exposed neighboring materials and the amount of loss each can tolerate. Verify the result using the measurements appropriate to the structure, then check that the surface is suitable for the next deposition, contact or clean step.

## Related terminology

main etch, ME. Short abbreviations are local process vocabulary and can have other meanings; use the full module name when searching.

## Source and scope

[Etch process resources](https://www.cleanroom.byu.edu/processes). The diagram is an original teaching schematic. Layer choices and process families describe an example rather than a universal manufacturing recipe.

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## Additional existing reference

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.