How Substrate Choice Affects a Thin-Film Optical Filter Design

When an infrared filter fails to meet specifications, it is often the coating that is blamed. However, it is just as likely that the substrate has shifted the curve, made the aperture unusable, or failed during mounting. Choice of substrate should be made at the same time as choice of wavelength and of coating material and be written on the first drawing.

BoDian Optical produces thin-film optical products for the ultraviolet, visible and infrared range. The workflow for production of thin-film filters is documented from substrate preparation through coating to spectral measurement and application. This allows filters to be produced to fit existing detectors and housings.

Key takeaways

• The substrate and film stack should be treated as one optical component.
• Verify the substrate’s transmission window prior to coating design fixation.
• Polish, thermal behavior, mounting, cleaning and test conditions to be included in the RFQ.

Why does the substrate matter in a thin-film optical filter?

Optical thin-film filters consist of a dielectric or metal layer which is tuned to the required spectral properties. The substrate of the filter, however, represents a mechanical boundary which cannot be changed by modifying the number of layers if it absorbs in the desired wavelength range.

The base affects how a part is going to behave in the workshop. Key properties are the hardness, the thickness, the edge quality, surface finish and thermal expansion. If you move a part to a different material, you have to re-check your calculations.

Which substrate properties change the optical result?

Start with the material’s transmission window

Each optical material has its own window through which good amounts of certain wavelengths are transmitted while others are absorbed. Select your films based on the window of the source, detector, and required passband. A broad infrared path will likely require a different substrate than a narrow band in a small sensor.

Temperature can change your mind. A part that works well for transmitting light on a room-temperature bench may need to be re-evaluated in a heated housing. Be sure to list the operating temperature range very early on in the part selection process to allow the supplier to properly evaluate the part for the entire design.

Refractive index affects the stack

The first film layer interacts with the substrate. By changing the refractive index of this layer you change the boundary conditions, the contrast of the layer, the edge of the layer and the transmission vs. reflection of the layer. Recalculate the layer recipe when you change the material. A simple curve that looks good enough often consists of copying the old thickness values and missings the specification.

Surface finish remains visible in the final part

Scratches, digs, pits, edge chips and poor flatness can scatter light and reduce the effective aperture of a coating. Since the coating adheres to the surface below, it cannot mask defects in a beam. Surface quality and inspection for applications like imaging, spectroscopy and machine vision must be specified along with the spectral target.

What do temperature and mounting add to the decision?

As the film stack and the substrate are made of different materials, they might expand with temperature differently. This stress can even shift the filter’s characteristics or even tear an edge when heating or cooling the filter or when bonding it into a housing. Thin filter parts are even affected by clamping force. Furthermore, humidity, vibration, cleaning cycles and the adhesive used all have to be tested and reviewed as well.

Design inputs not ratings. Specify the operating and storage temperature, humidity, mounting, vibration and cleaning in the RFQ. If the filter is to be situated near a strong optical source, specify the anticipated optical load and ask if absorbance testing or thermal testing is required.

How does substrate choice affect coating and testing?

The substrate should arrive at the coater clean, polished and fixtured consistently. The shape and thermal behavior of the substrate will also affect the fixturing and the coating’s uniformity across the aperture. The best route is dependent on the optical requirement, the type of substrate being used, the durability of the coating required and the production plan.

It’s also important to check out the transmission, reflectance and absorbance of your system over the relevant spectral range. A change of substrate can shift an edge, decrease the blocking or even increase the loss, whereas the center wavelength looks fine.

How should engineers screen common substrate choices?

Start with the material list as a first discussion. The silicones can be used for certain infrared applications, i.e. when the working band and the finish of the coating match. For Zinc sulfide, broad infrared transmission, good polish, good adhesion and low exposure are all important. Calcium fluoride has low absorption but is fragile and can be stressed by mounting. Sapphire is hard and resistant but the target band and coating must still match. Optical glass is used for visible and near-infrared work and one must check the full band and thermal conditions before selecting.

The material is chosen by supplier curves and the customer’s assembly. Even a cheap substrate with a complex holder can end up costing more in the complete system.

What belongs in a custom filter request?

Provide wavelength range of interest, desired transmission or reflection, preferred substrate, dimensions, thickness, clear aperture, surface quality, angle of incidence, polarization, operating temperature, humidity, durability requirements, quantity and desired report output. If the part needs to fit into an existing holder, please supply drawing or sample.

For a broad transmission requirement, the IAR2000-16000 infrared anti-reflection filter is a useful official reference. Its product page describes enhanced transmission from 2000 to 16000 nm. Treat that as a starting point and confirm the substrate, angle, and environment for the actual assembly.

The optical filter product center lists infrared and other thin-film families. If no standard part fits the wavelength, geometry, or substrate, send the design inputs for a custom review. The technical contact team can review a drawing or sample before production.

Frequently asked questions

Can the same coating be moved to another substrate?

No. Refractive index, transmission window, thermal behavior, and surface finish all change with a new substrate. Check the stack out and measure your completed part after making a change to a substrate.

Which substrate is best for an infrared filter?

A suitable detector will cover the required spectral band and withstand the mechanical and environmental stresses to which it will be subjected. However, there is no universal detector substrate for infrared work.

Why do polish and surface quality affect coating performance?

This surface follows the substrate surface. Scratches, pits, edge damage and surface flatness errors cause scatter which reduces the clear aperture even when the spectral response is very close to target.

Conclusion

Choose the substrate with the wavelength, coating, mounting, and environment in view. Check the transparent window and index first, then confirm surface quality, thermal behavior, and measured performance. NASA’s infrared spectrum overview and the NIST Physical Measurement Laboratory provide neutral background for wavelength and measurement terms. The NOAA homepage is another official reference for environmental measurement work.