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 the LUNOX Sputter Supreme series hits 97% 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 in sputter production
Good sputter production checks the film at three points:
- In-situ optical monitoring measures the reflected and transmitted spectrum continuously as the PET passes through the chamber, so drift is corrected during the run.
- Post-deposition VLT/IR scan at the chamber exit, so any roll outside spec is caught before it ships.
- Accelerated aging: samples are put through accelerated weathering (UV, humidity and temperature cycling) to confirm the coating stays stable.
For buyers, the proof is in the paperwork: every LUNOX roll is inspected before it ships, and each shipment includes an independent lab certificate.
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.