Dry Etch Process
Dry Etch (Reactive Ion Etching) is the primary pattern transfer technique in semiconductor manufacturing that uses chemically reactive plasma to selectively remove material — providing the anisotropic (vertical) etch profiles essential for sub-10nm feature patterning, where the interplay between chemical etching (reactive species) and physical bombardment (ion energy) determines the etch rate, selectivity, and profile quality.
Dry Etch Mechanisms
| Mechanism | Directionality | Selectivity | Example |
|---|---|---|---|
| Chemical (isotropic) | None — etches all directions | High | Downstream ashing |
| Physical (sputtering) | Highly directional | Low | Ion milling |
| Ion-Enhanced Chemical (RIE) | Directional | Moderate-High | Standard RIE |
- RIE synergy: Ion bombardment enhances chemical reaction rate on horizontal surfaces (where ions strike) → vertical etching 10-50x faster than lateral → anisotropic profile.
Etch Tool Types
| Tool | Plasma Source | Frequency | Use |
|---|---|---|---|
| CCP (Capacitively Coupled) | Parallel plate | 13.56 MHz + 2-60 MHz | Dielectric etch, low energy |
| ICP (Inductively Coupled) | Coil above chamber | 13.56 MHz source + RF bias | Metal, Si, high-density plasma |
| ECR (Electron Cyclotron) | Microwave + magnetic | 2.45 GHz | Specialized thin films |
| ALE (Atomic Layer Etch) | Pulsed plasma | Various | Atomic precision etching |
Common Etch Chemistries
| Material | Chemistry | Byproducts |
|---|---|---|
| Silicon | SF6, CF4/O2, Cl2/HBr | SiF4, SiCl4, SiBr4 |
| SiO2 | CF4/CHF3/C4F8 + O2/Ar | SiF4, CO, CO2 |
| Si3N4 | CHF3/CH2F2 + O2 | SiF4, N2, HCN |
| W (tungsten) | SF6/CF4 | WF6 |
| Organic (resist) | O2, N2/H2 | CO2, H2O |
| Cu (etch-back) | Not easily etched — use CMP instead | — |
Key Etch Parameters
- Etch Rate: nm/min of material removed.
- Selectivity: Ratio of target etch rate to mask/underlayer etch rate. Target: > 10:1.
- Uniformity: Etch rate variation across wafer. Target: < 2% 3σ.
- CD Bias: Difference between mask CD and etched feature CD.
- Profile Angle: 88-90° = vertical (ideal anisotropic). < 85° = tapered.
Etch Endpoint Detection
- Optical Emission Spectroscopy (OES): Monitor plasma emission wavelengths — intensity change signals layer transition.
- Interferometry: Monitor reflected laser intensity — periodic oscillations track film thickness.
- Mass Spectrometry: Detect etch byproduct species in exhaust.
Dry etching is the critical pattern transfer step that defines every feature on a chip — from transistor gates at 3nm width to via holes with 50:1 aspect ratio, the precision of the etch process directly determines whether the designed patterns are faithfully reproduced in silicon.