Infrared Filter Coating Requires Precise Control of Materials and Processes

Hangzhou, Zhejiang, China – October 9, 2026

Where Is the Real Difficulty in Infrared Filter Coating?

An infrared filter looks like just a thin disc. But its performance metrics—center wavelength accuracy, bandwidth control, blocking depth, and temperature stability—all depend on the coating on its surface. The coating design determines the theoretical upper limit; the coating process determines what can actually be achieved. The gap between the two is where process value lies.

The coating stack of an infrared filter typically consists of dozens to over a hundred alternating layers of high- and low-index materials. Take a 4260 nm narrowband filter for CO₂ detection as an example. Its design is based on the Fabry–Pérot interference principle, and the stack is often formed by alternately depositing high- and low-index materials such as Ge and ZnS. The total physical thickness is generally on the order of several micrometers to a dozen-plus micrometers. When a single-layer thickness deviation reaches about 1% of the design value, the accumulated error across multiple layers can cause the actual spectrum to deviate from the designed spectrum—manifesting as center wavelength shift, passband broadening or deformation, and reduced peak transmittance.

The first step in coating is substrate cleaning and pre-treatment. Optical-grade substrates such as Si and Ge are typically ultrasonically cleaned in sequence with acetone, isopropyl alcohol, and deionized water to remove surface organics, particles, and ionic residues, then dried with high-purity nitrogen or by spin drying. For applications requiring higher film adhesion, oxygen plasma treatment can be used when material compatibility allows, to remove residual organics, activate the surface, and increase surface energy. If the substrate is not properly treated, no amount of subsequent coating precision will help—poor adhesion or film delamination can ruin the entire effort.

The choice of coating method directly affects film quality. Electron beam evaporation combined with ion beam assisted deposition (IAD) is a commonly used combination in infrared filter production. The electron beam heats the coating material in a crucible, causing it to evaporate and deposit onto the substrate, while an ion beam generated by an ion source bombards the growing film to improve its density and refractive index stability. IAD offers relatively controllable cost and high deposition efficiency, making it suitable for volume production.

Ion beam sputtering (IBS) is a higher-precision option. The ion beam directly bombards a target, and the sputtered atoms deposit onto the substrate to form the film. IBS produces sharper film interfaces and more precise thickness control, making it suitable for narrowband filters with extremely demanding spectral performance. However, its deposition rate is slower, equipment cost is higher, and single-piece production cycles are longer. For gas detection filters requiring volume production, cost is a practical factor that must be considered.

In terms of material selection, visible and near-infrared bands commonly use SiO₂ paired with TiO₂, Ta₂O₅, or HfO₂. For infrared bands, the choice depends on the operating wavelength: Ge is transparent from about 2–14 μm, ZnS from about 3–5 μm and 8–12 μm, and YF₃ from about 0.2–12 μm—though specific ranges vary with material grade, thickness, and process. Material purity significantly affects absorption and scattering losses in the film, so optical-grade purity is usually a basic requirement.

Real-time monitoring during coating is key to ensuring precision. Quartz crystal monitoring uses a quartz crystal microbalance to monitor deposited mass and convert it into film thickness, with resolution on the order of 0.1%. However, absolute accuracy is affected by factors such as tooling factor, temperature, and stress. For complex stacks with over a hundred layers, the cumulative error of quartz crystal monitoring can still cause final performance to deviate from design. High-end filters often add optical monitoring, using a monochromator/detector or spectrometer system to monitor the spectral response of the film and make corrections during deposition or after each layer. The dual monitoring approach of “quartz crystal + optical monitoring” has become a common configuration in high-precision infrared filter production.

Post-treatment after coating is equally important. Annealing—typically at 200–300°C after coating, lasting several hours, with specifics optimized for the material system—can release internal stress in the film and improve environmental stability. This is followed by spectral testing, adhesion testing such as cross-hatch/tape testing, and temperature-humidity cycling tests to verify film reliability in real operating environments.

Infrared filter coating is a systems engineering process. From substrate treatment to final inspection, every step affects final performance. MULTI IR has accumulated years of process expertise in infrared optical coating, from electron beam evaporation to ion beam sputtering, from standard products to custom coating stacks, covering applications including gas detection, thermal imaging, and industrial thermometry.

MULTI IR – National-level “Little Giant” Specialized and Sophisticated Enterprise. Over 10,000 types of infrared sensitive components in stock. Top 3 globally in comprehensive strength. Product portfolio covers infrared filters, optical coatings, infrared sensors, and other core categories. Lead drafter of the infrared filter industry standard.

Website: www.mirhz.com | Global Site: www.miroptech.com

About Us

Founded in 2007, Hangzhou MULTI IR Technology Co., Ltd. is an optoelectronic technology enterprise integrating R&D, production, and sales. Its products are widely applied in aerospace, medical care, AR/VR, display imaging, photography, and other fields, steadily holding the position of the world’s largest spot supplier of optical components.

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