Tungsten hexafluoride (WF₆) for ALD

Description

Tungsten hexafluoride (WF6) for ALD

WF6 is a widely used tungsten precursor for atomic layer deposition (ALD) and related thin-film processes. It offers high volatility and a strong reactivity with suitable reducing co-reactants to produce tungsten-containing films with good conformality, which is important for high aspect ratio structures in microelectronics.

Why WF6 is used for ALD

• Volatile and reactive: WF6 can be delivered in a gas phase and reacts readily on surfaces with appropriate co-reactants to form tungsten-containing films.
• Conformal deposition: In ALD, WF6 cycles with surface-limited reactions can yield uniform, conformal films even on complex topographies.
• Versatility with co-reactants: When paired with hydrogen-containing species or plasmas, WF6 can lead to metallic tungsten films or tungsten-containing compounds suitable for device applications.

How WF6 is used in ALD

A typical ALD cycle with WF6 follows a self-limiting sequence:

1. WF6 exposure: The substrate is pulsed with WF6, allowing adsorption on reactive surface sites.
2. Purge: Inert gas purges remove excess WF6 and volatile byproducts.
3. Co-reactant exposure: A reducing co-reactant (for example, H2, H2 plasma, or a silicon hydride such as SiH4) is pulsed to reduce surface fluorides and form tungsten on the surface.
4. Purge: A second purge clears out byproducts such as HF or fluorinated species.

• The cycle is repeated to build up the tungsten film monolayer by monolayer.

Common co-reactants and variants

• Hydrogen based routes: H2 gas or H2 plasma with WF6 to deposit metallic tungsten.
• Silane-based routes: SiH4 or related silane derivatives as reducing co-reactants.
• Plasma-enhanced ALD (PEALD): Using H2 or other plasmas can improve film quality and lower the deposition temperature.
• In some systems, careful control of fluorine-containing byproducts helps minimize fluoride incorporation in the film.

Typical process considerations (ranges and guidance)

• Temperature window: The exact window depends on reactor design and the chosen co-reactant. Typical ALD windows for WF6-based tungsten deposition are in the mid to high hundreds of degrees Celsius, with PEALD approaches enabling lower temperatures. Start with a conservative range (for example, some researchers operate around 200–350°C) and optimize for your equipment.
• Exposure and purge times:
  • WF6 exposure: short pulses (sub-second to a few seconds), depending on the delivery system.
  • Co-reactant exposure: similar timescales, often 0.5–2 seconds.
  • Purges: several seconds (often 5–15 seconds) to ensure surface and gas-phase species are removed before the next step.
• Cycle count: Build thickness by repeating cycles; calibrate growth per cycle (GPC) for your reactor to predict film thickness.
• Film properties: Films deposited with WF6 and reducing co-reactants are typically metallic tungsten with varying fluorine content depending on the process. Post-deposition annealing or specific purge strategies can help reduce fluorine incorporation.

Note: The exact parameters vary widely with reactor type (cold-wall vs hot-wall, flow tubes, showerhead), co-reactant choice (gas vs plasma), substrate, and desired film properties. Always start from literature-reported ranges for your reactor and perform systematic optimization.

Practical tips and safety

• Safety first: WF6 is highly toxic and reacts violently with moisture, forming corrosive HF and other fluorinated byproducts. Use dedicated gas handling systems, proper ventilation, and appropriate personal protective equipment. Consult the Material Safety Data Sheet (MSDS) and institutional safety guidelines.
• Dry environment: Maintain an anhydrous, inert atmosphere during preparation, delivery, and purging. Use moisture traps and HF scrubbers as required.
• Purity of precursors: Ensure high-purity WF6 and co-reactants to minimize impurities in the film.
• Substrate preparation: Clean and pre-treat substrates to provide reactive surface sites for WF6 adsorption.
• Characterization: Use X-ray reflectivity, X-ray diffraction, XPS, and resistivity measurements to monitor film thickness, crystallinity, composition, and fluorine content.
• Environmental and waste handling: Manage fluorine-containing byproducts properly and follow local regulations for hazardous waste.

Alternatives and when to consider them

• Other tungsten ALD precursors exist (for example, WCl6 with SiH4 or related silane-based routes). Some applications prefer these to minimize fluorine incorporation or to achieve different film qualities. Compare:
  • Film purity and fluorine content
  • Required substrate temperature
  • Film crystallinity and resistivity
  • Equipment compatibility and safety considerations
• If ultra-high purity tungsten or very low fluorine incorporation is essential, evaluate alternative precursors and co-reactants, possibly including plasma conditions that enhance reduction efficiency.

Quick-start checklist

• Review your reactor documentation and literature for WF6-based ALD recipes compatible with your setup.
• Ensure a strictly dry, inert environment and proper handling for WF6 and co-reactants.
• Start with a conservative set of pulses and purges, then systematically vary:
  • Substrate temperature within the recommended window
  • WF6 pulse duration and purge time
  • Co-reactant exposure time and plasma conditions (if using PEALD)
• Characterize films after a few cycles to gauge growth per cycle and adjust parameters accordingly.
• Monitor fluorine content and, if needed, apply post-deposition treatments or adjust purge strategies to minimize impurities.