Tungsten Hexafluoride
Properties
| State | Gas (colorless, dense gas at room temperature; liquid below 17°C) |
| Color | Colorless |
| Solubility | Reacts with water (hydrolysis to tungstic acid and HF); soluble in organic solvents |
| Melting Point | 2.3°C |
| Boiling Point | 17.1°C |
About Tungsten Hexafluoride
Tungsten hexafluoride is a colorless gas at room temperature, but the modifier "colorless" undersells how strange this molecule is. WF6 is one of the heaviest gases known, with a vapor density around 12.9 g/L — about ten times that of air — so it pools in the bottom of any unswept enclosure and slugs out the bottom of a leaky line in visible streams when it hits humid air. The molecule itself is a textbook octahedron, six W-F bonds of 1.83 Angstrom each pointing to the corners with no Jahn-Teller distortion because the d^0 W(VI) center has no electrons to break symmetry. The boiling point at 17.1 °C means a cylinder sitting in a 25 °C semiconductor fab is pure gas, while one in an air-conditioned 15 °C clean subfab condenses to a clear liquid — fab piping has to be heat-traced to prevent slugging. WF6 is the workhorse precursor for tungsten chemical vapor deposition in semiconductor manufacturing: at 300 to 450 °C, WF6 reacts with H2 or SiH4 to deposit tungsten metal that fills contact holes and via bottoms in DRAM and 3D NAND stacks, where the high conductivity, low diffusivity, and excellent step coverage of CVD tungsten beat every alternative metal. Production sits around 200 to 300 tonnes per year globally, almost all of it consumed by Samsung, TSMC, Intel, SK Hynix, and Micron.
Where you'll encounter it
If you have ever toured a wafer fab or seen a cross-section TEM of a 3D NAND stack, the dark vertical pillars cutting through 100+ alternating layers of silicon and oxide are tungsten plugs deposited from WF6. The standard recipe is a thin nucleation layer grown from WF6 + SiH4 to seed the surface, followed by a bulk fill from WF6 + H2 at higher temperature for throughput. The byproducts — HF and SiF4 — get scrubbed in a wet bed and neutralized with KOH before the exhaust hits the abatement stack. The most common process problem is fluorine attack on the underlying barrier layer (TiN or TiAlN) when the nucleation step runs too thin, which causes voiding in the via and shows up later as open contacts on the wafer probe. Subfab gas cabinets for WF6 carry triple containment, automatic shutoff valves on HF leak detection, and an emergency dilution N2 purge — this is one of the few gases where a single failed fitting can take down a whole production line until decontamination crews finish.
Common Uses
- Precursor gas for tungsten CVD plug fill in DRAM contact and via stacks
- Precursor for word-line tungsten in 3D NAND flash memory stacks of 100+ layers
- Precursor for tungsten gate metal in advanced FinFET and gate-all-around CMOS nodes
- Source gas for ALD tungsten nucleation and barrier-layer deposition in advanced interconnect
- Calibration and process-development gas in nuclear enrichment cascades and centrifuge testing
- Specialty deposition gas for tungsten thin films on optical and X-ray mirror substrates
- Etchant for selective removal of tungsten in microelectromechanical systems fabrication
- Research reagent in actinide and transition-metal fluoride chemistry
Safety Information
GHS classification: Acute Toxicity Category 2 (inhalation), Skin Corrosion Category 1A, Eye Damage Category 1, and Specific Target Organ Toxicity Single Exposure Category 1 for the respiratory tract. WF6 hydrolyzes instantly with moisture to tungstic acid and HF, and the HF is the actual killer — the OSHA PEL for HF is 3 ppm and the IDLH is 30 ppm, with calcium gluconate gel staged at every WF6 cabinet for skin exposure. The W TLV is 1 mg/m^3 for soluble compounds. Density of about 10x air means leaks pool at floor level, so floor-mounted detectors at 5 ppm HF triggering automatic source isolation are standard. Decommissioning a contaminated tool requires N2 purge cycles to below 1 ppm fluoride before the panels come off. Never open a WF6 line in atmosphere — the tungstic acid plug will form, attack the stainless steel tubing, and continue releasing HF for hours.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.