Tungsten CVD for Contact/Via Fill

Category: CVD

Process Overview

Tungsten Chemical Vapor Deposition (CVD) for Contact/Via Fill is a critical step in semiconductor manufacturing used to deposit conformal tungsten films that electrically connect transistors to interconnect layers. This process addresses the challenges of filling high-aspect-ratio features (e.g., 10:1 via/contacts) without voids, ensuring low-resistance pathways essential for advanced logic and memory devices. Tungsten’s low resistivity (≈ 5.6 μΩ·cm) and excellent step-coverage make it ideal for sub-7nm nodes, where copper electroplating is impractical for such small features. In data center chip production, reliable via fill is critical for maintaining signal integrity in high-performance compute and networking architectures.

The process typically uses tungsten hexafluoride (WF₆) as the precursor, silicon hydride (SiH₄) as a reducing agent, and hydrogen (H₂) as a carrier gas. Deposition occurs in a low-pressure CVD chamber, where precise control of gas flows, temperature, and pressure ensures uniform tungsten nucleation and growth.

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Key Process Parameters

| Parameter | Typical Value |
|-------------------|----------------------------------------|
| Temperature | 350°C ± 10°C (per SEMI C17) |
| Chamber Pressure | 5–10 Torr |
| WF₆ Flow Rate | 20–80 sccm |
| SiH₄ Flow Rate | 100–300 sccm |
| H₂ Flow Rate | 500–1000 sccm |
| Deposition Rate | 100–300 nm/min (aspect ratio > 8:1) |

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Equipment & Parts Required

1. Showerheads

   • Role: Uniform distribution of WF₆, SiH₄, and H₂ across the wafer surface to ensure consistent film thickness.
   • Caladan Link: Caladan’s fluorine-resistant showerheads (e.g., ALD-optimized models) prevent clogging from WF₆ decomposition byproducts.

2. Mass Flow Controllers (MFCs)

   • Role: Precise control of gas flows (±1% accuracy) to maintain stoichiometric ratios and avoid tungsten over-deposition or voids.
   • Caladan Link: High-precision MFCs for WF₆ (toxic, corrosive gas) and SiH₄ (pyrophoric) ensure safety and process stability.

3. Heater Assemblies

   • Role: Maintain ±2°C wafer temperature uniformity to prevent thermal gradients that cause non-uniform deposition.
   • Caladnan Link: Rapid thermal cycling heater modules (compliant with SEMI E142) enable tight process control.

4. Gas Purifiers

   • Role: Remove oxygen and moisture from WF₆ and SiH₄ lines to prevent oxide byproducts that degrade film quality.
   • Caladan Link: ISO 14644-1 Class 1 purifiers for ultra-high-purity gas delivery.

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Common Issues & Troubleshooting

1. Non-Uniform Deposition

   • Cause: Clogged showerhead orifice or uneven gas distribution.
   • Fix: Replace showerhead (every 10k–15k cycles) and perform O₂ plasma cleaning.

2. Particle Contamination

   • Cause: Degraded gas purifier filters or WF₆ line leaks.
   • Fix: Replace purifier filters (every 6 months) and check WF₆ line integrity with leak detectors.

3. Incomplete Via Fill

   • Cause: Suboptimal temperature (±5°C deviation) or low SiH₄ flow.
   • Fix: Calibrate heater assemblies and verify MFC SiH₄ output.

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Frequently Asked Questions

Q: What is the typical tungsten deposition rate for via fill?
A: "The deposition rate ranges from 100–300 nm/min, depending on chamber design and gas ratios, with 200 nm/min being standard for 5nm node via fill."

Q: Why is hydrogen used in tungsten CVD?
A: "Hydrogen acts as both a carrier gas and a reducing agent, ensuring WF₆ is fully converted to metallic tungsten while minimizing fluoride byproducts."

Q: How does temperature affect tungsten film quality?
A: "A ±2°C deviation from 350°C can increase void formation by 30%, per SEMI C17 guidelines, due to altered surface diffusion kinetics."

Q: What industry standards govern tungsten CVD gas purity?
A: "ISO 14644-1 and SEMI F22 specify < 0.1 ppm oxygen and moisture in WF₆/SiH₄ lines to ensure defect-free films."

Q: Can tungsten CVD be used for back-end-of-line (BEOL) interconnects?
A: "No—tungsten CVD is reserved for via/contacts; BEOL uses physical vapor deposition (PVD) for wider copper lines due to cost and throughput tradeoffs."

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Parts for This Process

Looking for parts to support this process? Caladan Semi stocks used and refurbished components including: Showerheads, MFCs (WF6, SiH4, H2), heater assemblies, gas purifiers.

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