Chemical Vapor Deposition (CVD)
LPCVD, PECVD, and the chemistry of depositing films from gases
CVD Fundamentals
CVD Fundamentals
In Chemical Vapor Deposition, gaseous precursors react on or near the wafer surface to form a solid film. The basic steps:
- Gas precursors are delivered to the chamber
- Precursors transport to the wafer surface (diffusion through a boundary layer)
- Precursors adsorb onto the surface
- Chemical reactions occur, forming the desired film
- Byproducts desorb and are pumped away
Key CVD variants:
- LPCVD (Low Pressure): 0.1–1 Torr, 600–900°C. Excellent uniformity and step coverage. Used for silicon nitride, polysilicon.
- PECVD (Plasma Enhanced): Uses plasma to enable reactions at lower temperatures (200–400°C). Critical for back-end processing where metals are already on the wafer.
- MOCVD (Metal-Organic): Uses metal-organic precursors. Key for III-V semiconductors (GaN, GaAs) in LEDs and power devices.
Analogy: Spray Painting
CVD is like spray painting, but at the molecular level. Instead of paint droplets, gas molecules arrive at the surface and chemically bond to it, building up a uniform coating layer by layer.
CVD Precursors and Film Chemistry
CVD Precursors and Film Chemistry
The choice of precursor gases determines the film composition, deposition rate, and required temperature. A few canonical recipes:
| Film | Precursors | Reaction | Variant |
|---|---|---|---|
| Polysilicon | SiH₄ | SiH₄ → Si + 2H₂ | LPCVD ~620 °C |
| Si₃N₄ | SiH₂Cl₂ + NH₃ | 3 SiH₂Cl₂ + 4 NH₃ → Si₃N₄ + 6 HCl + 6 H₂ | LPCVD ~780 °C |
| SiO₂ (TEOS) | Si(OC₂H₅)₄ + O₂ | TEOS → SiO₂ + organic byproducts | PECVD ~400 °C |
| Tungsten plug | WF₆ + H₂ | WF₆ + 3 H₂ → W + 6 HF | CVD ~400 °C |
Two regimes determine the deposition rate:
- Surface-reaction limited (low T): Reaction kinetics on the wafer set the rate. Very temperature-sensitive (rate doubles every ~10 °C) but insensitive to gas flow — gives excellent uniformity.
- Mass-transport limited (high T): Precursor diffusion through the boundary layer sets the rate. Less temperature-sensitive but very flow-sensitive — used in high-throughput epi reactors.
Key Concept: Step Coverage
Step coverage is the ratio of film thickness on the bottom of a trench to that on the field. Good LPCVD reaches >90% step coverage because the surface-reaction-limited regime gives precursors time to reach every surface. PECVD is more directional and often gives 40–70%.
Knowledge Check
Knowledge Check
1 / 2Why is PECVD preferred over LPCVD for back-end-of-line processing?