Process · 8 min read
Magnetron Sputtering Explained: How LUNOX Window Film Is Made
Most architects spec sputter window film without knowing what magnetron sputtering actually is. That's fine — the spec sheet tells the story. But if you're choosing between two suppliers and the IR rejection numbers are within a few points, what's happening inside the chamber matters more than the brochure copy. Here's the process, in production-floor detail.
The vacuum chamber
Sputter film is produced in long horizontal vacuum chambers. The chamber is pumped down to roughly 10⁻⁵ torr — about a hundred-millionth of atmospheric pressure. That vacuum matters: any residual gas reacts with the metal targets and corrupts the film optical properties. At LUNOX, the pump-down cycle alone takes 6–8 hours before each production run, and the chamber is leak-tested to a moisture residual below 0.1 ppm.
The substrate — optical-grade polyethylene terephthalate (PET) — enters one end of the chamber on a continuous roll, passes under the sputter targets at controlled speed, and exits the other end with the coating already deposited. A typical chamber processes 152 cm wide PET at 1.5–3 m/min depending on the stack thickness being deposited.
The magnetron target
Inside the chamber, the metal "target" is a slab of pure silver, titanium, or copper (depending on the layer being deposited). Above the target sit permanent magnets that confine the plasma — that's the "magnetron" part of "magnetron sputtering."
When a high-voltage DC current is applied (typically 400–700 V at LUNOX), it ionizes the argon gas in the chamber. The Ar⁺ ions accelerate toward the target, smash into it, and physically eject (sputter) individual metal atoms. The magnetic field above the target keeps the plasma dense and localized, which:
- Improves deposition efficiency (more atoms per Ar⁺ ion).
- Lets you run at lower chamber pressure — finer film quality.
- Keeps the target erosion uniform, so target lifetime is longer.
The layer stack
A sputter window film is not a single metal layer. It's a designed optical stack — typically 5–9 layers across 100–200 nanometers total thickness. A representative LUNOX sputter stack:
- Base layer (Ti or TiOₓ): 20–30 nm. Adhesion-promoting transition layer between the PET substrate and the metal stack.
- Silver layer 1: 12–15 nm. The primary infrared-reflecting layer.
- Spacer (TiOₓ): 30–50 nm. Optical-tuning layer that controls reflected color and the position of the IR rejection peak.
- Silver layer 2: 8–12 nm. Reinforces IR rejection at longer wavelengths.
- Copper barrier: 2–3 nm. Prevents silver migration over decades of UV exposure.
- Top spacer (TiOₓ): 30–40 nm. Final optical tuning + scratch protection.
- Sealing layer (TiOₓ dense): 5–10 nm. Hermetic encapsulation against humidity and oxidation.
The IR rejection performance and the visible color (exterior reflection, interior transmission) are governed entirely by the layer thicknesses and the spacer dielectric constants. A 2 nm error in the silver thickness shifts the reflected color a noticeable amount. This is why ISO Class-7 cleanrooms and tight process control matter so much.
The "low-emissivity" effect, explained
The reason sputter rejects infrared so well comes down to the silver. Silver has the highest infrared reflectance of any metal — about 98% across the 3–50 μm thermal-IR range. By placing two silver layers with a tuned dielectric spacer, you build an interference filter that reflects the IR band while transmitting most of the visible band. That's how LUNOX sputter Max series hits 83.4% IR rejection at 30% VLT.
The "Low-E" coatings used on insulating glass units (IGUs) work on the same principle, but they're deposited directly on the glass surface during float-glass manufacture. Sputter film achieves comparable performance on the polymer substrate, which means you can retrofit existing single-pane or non-Low-E IGU glass without replacing the glazing.
Quality control on the production line
At LUNOX, every sputter run is monitored at three points:
- In-situ optical monitoring measures the reflected/transmitted spectrum continuously as the PET passes through the chamber. Deviations of more than ±2% trigger an automatic line-speed adjustment.
- Post-deposition VLT/IR scan at the chamber exit. Any roll outside spec is flagged and either reworked or scrapped.
- End-of-line accelerated aging: a 1-meter sample from every roll is subjected to 1,000 hours of accelerated weathering (UV + humidity + temperature cycling) before the parent roll ships.
Why this matters for buyers
When you're comparing sputter film suppliers, the numbers on the brochure are downstream of three things: target material purity, chamber pressure control, and post-deposition QC. Suppliers who skip any of those produce films that look fine on day 1 but degrade visibly in 5–7 years. The LUNOX field record on the Lotte World Tower is 9 years and counting at the as-spec'd color — that's a direct product of the process discipline above.