Plasma Etch Mechanisms
Plasma Etch Mechanisms
Plasma etching combines chemical and physical mechanisms:
- Chemical etching: Reactive species from the plasma (F, Cl, Br radicals) react with the surface to form volatile products that pump away. High selectivity but isotropic.
- Ion bombardment: Energetic ions (Ar+, CF₃+) physically knock atoms off the surface. Anisotropic (directional) but poor selectivity.
- Ion-enhanced etching: The combination — ions damage the surface, making it more reactive to chemical species. This gives both anisotropy AND good selectivity.
Key plasma etch configurations:
- RIE (Reactive Ion Etching): Capacitively coupled plasma (CCP). Lower density, good for dielectric etch.
- ICP (Inductively Coupled Plasma): Higher plasma density, independent control of ion energy and density. Preferred for high-aspect-ratio features.
Etch Chemistry by Material
Etch Chemistry by Material
Different materials require different etch chemistries:
| Material | Etch Gases | Volatile Product |
|---|---|---|
| Silicon | SF₆, Cl₂, HBr | SiF₄, SiCl₄, SiBr₄ |
| SiO₂ | CF₄, C₄F₈, CHF₃ | SiF₄, CO, CO₂ |
| Si₃N₄ | CF₄/O₂, CHF₃ | SiF₄, N₂ |
| Tungsten | SF₆, CF₄ | WF₆ |
| Photoresist | O₂ | CO₂, H₂O |
Key Concept: Sidewall Passivation
To achieve vertical sidewalls, fluorocarbon gases (C₄F₈) deposit a thin polymer on sidewalls during etching. Ion bombardment removes this polymer from the bottom but not the sidewalls, so etching proceeds only downward. This passivation mechanism is key to anisotropic etching.
Knowledge Check
Knowledge Check
1 / 2What type of plasma source provides independent control of ion energy and density?