PECVD Dielectric
Plasma-Enhanced CVD (PECVD) is a thin film deposition technique that uses plasma to activate chemical reactions at lower temperatures than thermal CVD — enabling dielectric deposition on temperature-sensitive structures and achieving tunable film properties through plasma conditions.
How PECVD Works
1. Precursor gases flow into chamber (e.g., SiH4 + N2O for SiO2; SiH4 + NH3 + N2 for SiN).
2. RF plasma (13.56 MHz or 2.45 GHz) dissociates gases into reactive radicals and ions.
3. Radicals adsorb and react on heated wafer surface (200–400°C).
4. Film grows — by-products pumped away.
vs. Thermal CVD (LPCVD)
| Parameter | Thermal LPCVD | PECVD |
|---|---|---|
| Temperature | 650–900°C | 200–400°C |
| Film quality | High density | More porous |
| Conformality | Better | Moderate |
| Stress control | Limited | Wide range |
| Throughput | Low | High |
| BEOL compatible | No (Al melts at 660°C) | Yes |
Common PECVD Films
- PECVD SiO2: ILD dielectric, passivation. Deposited with SiH4 + N2O or TEOS + O2.
- PECVD SiN (Si3N4): Passivation, diffusion barrier, etch stop. SiH4 + NH3 + N2.
- PECVD SiON: Tunable refractive index between SiO2 and Si3N4. ARC layer.
- PECVD a-Si: Polysilicon precursor, TFT backplanes.
- PECVD Low-k (SiCOH): Ultra-low-k (k~2.7) ILD for Cu interconnects.
Stress Tuning
- LF power (380 kHz) increases ion bombardment → compressive stress.
- HF power (13.56 MHz) reduces bombardment → tensile stress.
- Dual-frequency PECVD: Independent stress tuning from -500 MPa to +500 MPa.
- Application: Tensile SiN capping over NMOS for electron mobility enhancement.
Key Equipment
- Applied Materials Producer, Novellus Sequel (now Lam Research): Batch PECVD.
- Tokyo Electron Livas: Single-wafer cluster PECVD for tight uniformity.
PECVD is indispensable in back-end-of-line processing — its low-temperature operation makes it the only practical method for depositing dielectrics over completed transistors and metal interconnects.