Choose The Wrong Substrate And The Coating Is Wasted: How To Select Germanium, Silicon, Zinc Selenide, Or Chalcogenide Glass

(MENAFN- GetNews) Hangzhou, Zhejiang, China - November 0 5 , 2026

Choose the Wrong Substrate and the Coating Is Wasted: How to Select Germanium, Silicon, Zinc Selenide, or Chalcogenide Glass

For the same infrared filter, the coating design can be completely identical, but change the substrate and the performance, cost, and lead time all change. The substrate is not a passive carrier. It determines which wavelength band the filter can work in, what environment it can withstand, and what aperture it can support.

There are only a few commonly used substrates in infrared optics, and their applicable boundaries are quite clear. Understanding these boundaries can save a lot of detours during selection.

1. Start with the transmission band

The first threshold for a substrate is its transmission band. If this requirement is not met, none of the other parameters matter.

Germanium's transmission range covers roughly 2 to 14 μm, making it the workhorse material for mid-wave and long-wave infrared. Its refractive index is very high, about 4.0. This value has a side effect: single-surface reflection loss is close to 36%, and bare substrate transmittance is very low, so an anti-reflection coating is mandatory before use.

Silicon's transmission range is 1.2 to 7 μm, covering near-infrared to mid-wave. Its refractive index is about 3.4, and it also requires AR treatment. Silicon's mechanical strength and hardness are far higher than germanium's, and its density is lower, giving it an advantage in weight. The drawback is strong absorption beyond 9 μm, so it cannot be used in the long-wave band.

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The cause of the adhesion issue must also be stated accurately. The main cause is not that the surface is dirty or easily oxidized, but that chalcogenide glass surfaces struggle to form effective chemical bonding with the hard coating, compounded by the aforementioned thermal mismatch stress. Surface cleaning and activation can improve the interface, but controlling thermal mismatch stress is the core difficulty of this process route and the reason it has not been widely overcome by the industry for a long time.

In terms of chemical stability, zinc selenide is sensitive to acids and bases, so care must be taken in selecting cleaning agents. Germanium has a tendency to oxidize at high temperatures. The chemical stability of chalcogenide glass depends on the specific formulation, with high-selenium formulations being relatively more stable. These characteristics determine the boundaries of packaging methods and operating environments.

5. Cost and availability: an often overlooked dimension

Germanium prices are significantly affected by resource and export policies. Global refined germanium production is highly concentrated, and infrared optics is one of the main consumption sectors, so price fluctuations directly impact cost. Supply chain uncertainty is the primary motivation for many OEMs to consider alternatives.

Silicon costs much less, large-size wafer supply is ample, and the processing supply chain is mature. This is the economic basis for the rising penetration of silicon-based solutions in mid-wave infrared.

Zinc selenide is medium-cost; the main difficulty lies in producing large-size, high-quality blanks, and lead times for large-aperture products are often long.

The cost advantage of chalcogenide glass is reflected in the molding step. Traditional crystalline materials must be cut, ground, and polished into lenses, with each piece processed individually-especially time-consuming for large apertures and aspheres. Chalcogenide glass can be molded in a single step, enabling high-volume production efficiency. Therefore, in high-volume, small-to-medium aperture aspheric lens scenarios, chalcogenide glass has a clear cost advantage.

But this advantage has boundaries. Molded shape error can already reach the sub-micron level in conventional aspheric scenarios, with PV around 0.5 μm, and the cost advantage is obvious. The shortcomings are surface roughness, high-order surface form errors, and the ability to directly mold diffractive surfaces and freeform surfaces.

These aspects still fall short of single-point diamond turning, which can achieve surface roughness below 5 nm. Therefore, for scenarios involving microstructures, the common practice is to mold first and then turn the diffractive surface or polish to compensate-not simply replace turning with molding.

6. The practical order of selection

Putting these dimensions into a decision sequence is more effective than comparing item by item.

Step one: determine the band. Where the operating wavelength falls immediately eliminates more than half of the materials. For mid-wave 3 to 5 μm, germanium, silicon, zinc selenide, and chalcogenide glass are all options. For long-wave 8 to 12 μm, the main choices are germanium and zinc selenide; silicon has already dropped out of range. For far-infrared above 15 μm, zinc selenide is the mainstay; germanium still has engineering usability near 14 μm, but absorption rises with wavelength, and longer bands usually give way to zinc selenide.

Step two: determine temperature stability requirements. For equipment with a wide operating temperature range and high-precision temperature measurement requirements, the temperature drift coefficient is a hard threshold, and silicon and low-dn/dT chalcogenide glass have advantages here. For temperature-stable indoor equipment, the weight of this item can be lowered.

Step three: determine mechanical and environmental requirements. For outdoor, coastal, and wipe-cleaning scenarios, hardness and chemical stability take priority. Soft substrates need enhanced protective coatings, or the material should be changed directly.

Step four: determine cost and lead time. After screening with the first three steps, choose the most cost-effective among the remaining options. Note that cost is not just the unit price; it also includes AR coatings, protective coatings, processing losses, and calibration labor caused by batch consistency.

After these four steps, the choice in most scenarios converges to one or two options. What truly needs weighing is usually the segment between performance and cost.

MULTI IR has turned this process into a mass-production capability for hard coatings on chalcogenide glass substrates. The value of this direction lies in solving two problems simultaneously: the cost advantage brought by molding, and the shortcomings of soft substrates in mechanical and environmental resistance.

Specifically, the hardness of this type of DLC hard coating exceeds 18 GPa, cross-hatch adhesion reaches grade 5B, and average transmittance in the infrared band remains above 93%. These three metrics correspond exactly to the three difficulties mentioned earlier: hardness addresses scratch resistance, adhesion addresses cracking risk, and transmittance addresses the optical performance penalty. This process remains a threshold that only a few companies in the industry can mass-produce stably.

The substitution relationship must also be stated clearly. Chalcogenide glass still falls short of germanium in long-wave transmittance and large-aperture lenses, and its refractive index is lower, so the same optical power requires steeper curvature, adding difficulty to large-aperture designs. For high-end, high-precision scenarios, germanium will still be used in the short term; glass will first capture the mid-to-low-end volume market.

The essence of selection is not finding a universal material, but finding the optimal solution under the specific constraints of a given scenario. Among the four items-band, temperature drift, mechanical environment, and cost-whichever is the hard constraint determines which way the answer leans.

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.

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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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