Replacing Laminated Soft Coatings with Front Surface Hard Optical Coatings

Soft coatings, also called laminated coatings, are well known for having poor temperature stability, poor environmental durability, low blocking levels, high optical scatter and low transmission, yet they are extensively used due to their legacy status, ready availability, and low cost.

For the first time, Alluxa’s proprietary high speed plasma deposition technology delivers the optical performance and durability of hard coated thin film optical filters at laminated soft coating pricing.

Soft Coating Background

Soft coatings are multilayer thin film structures made up of thin layers of ZnS and Cryolyte. These coatings could also contain extremely thin silver layers. A typical soft coating is a simple band-pass structure on one substrate surface and a blocker on another (Figure 1).

The blocker may use thin silver layers and/or they may contain an organically died epoxy or an auxiliary blocking color glass. All components are laminated together in a vertical stack in an attempt to offer some degree of environmental protection.

Individual parts are cored or diced from a larger wafer. As the coatings are moisture sensitive and delicate, the edges of the coated substrate may be scribed away with the goal of extending the amount of time before the occurrence of noticeable and unsightly environmental degradation.

All soft filters fail eventually, where the lifetime is a function of the environment and the care in construction of the filter. A typical expected lifetime in normal office type environments is one to five years.

The low cost of soft coatings is obtained from a combination of the low capital cost of the deposition equipment, inexpensive and simple float glass substrates, and rapid cycle times. The cost vs. volume curve asymptotically flattens after only a few dozen 1 inch parts, or a handful of 2 inch or larger parts as the load sizes are comparatively low, and they contain vital handling costs due to the sealing and ring assemblies.

Figure 1. A typical soft coating has three structures laminated together to help provide moisture resistance. The structure is typically 2-5 mm in thickness and mounted in a ring to further try to restrict humidity ingress.

Soft coatings have large optical losses from both scatter and absorption in the visible and near infra-red, which further increases in the blue and UV. Figure 2 displays the spectral function of a series of typical commercially available dielectric (no silver) laminated soft coatings across the visible band.

Generally, soft coatings have severe ripple in the pass-band, which adds a degree of uncertainty to the instrument’s spectral function in addition to lowering average transmission. The uncertainty and ripple of the spectral functions of soft coatings are due to the manual and labor intensive nature of the deposition, which offers an enhanced degree of variation to performance compared to computer controlled hard coatings.

Figure 2. Soft coating designed for high transmission that contains no silver layers. The transmission can rise above 80% in the green/red, but drops to less than 50% in the UV blue. Also note the severe ripple in the pass-band attributable to the lack of computer controlled deposition control.

Alluxa Plasma Deposition of Hard Coatings

Hard dielectric films developed by Alluxa’s improved high speed plasma deposition process are nearly lossless, with extinction coefficients in the low ppm range from the UV to the IR.

These films are developed from sophisticated computer control algorithms that regulate the thickness of every single layer to an atomic monolayer or less, and consequentially they have dramatically flatter pass-bands and much higher transmission. Figure 3 depicts comparable fully blocked filters built using Alluxa’s hard coatings that are designed as drop in replacements of the soft coating filters shown in Figure 2.

Figure 3. Hard coatings by Alluxa on a single side of a glass substrate. Note that with an AR on the back the transmission would approach 100%. Also note the inherent improved “squareness” of spectral shape, which helps S/N by increasing transmission as well as blocking. Improved shape is direct result of the repeatability of automation.

Improved Temperature Stability

Alluxa hard coatings have a temperature stability of less than 5 part per million (ppm) of wavelength based on substrate selection. Soft coatings have a typical value of 50 ppm, meaning that the filter must be controlled to +/- 15 °C for a 1 nm filter tolerance for the instrument design.

Alluxa regularly builds filters to less than 1 picometer per degree Celsius at 1550 m, well less than 1 part per million change in wavelength for telecom applications.

Transmitted Wavefront Error (TWE)

Alluxa can provide filters with TWE values of less than ¼ wave peak to valley (Figure 4) based on user requirements. For several designs, the TWE of the finished filter is essentially identical to that of the uncoated substrate. Soft coatings with their laminated structure would have to go through a complicated post lamination grinding and polishing step to approach a single wave per inch of TWE.

Figure 4. Transmitted wave front error of fully blocked band-pass Alluxa hard coating on fused silica substrate. The substrate is 2 inches in diameter.

Ultra-Narrow Filters

Extraordinarily narrow films can be built and used in instruments without temperature control, due to the outstanding stability of Alluxa hard coatings. A multi-cavity filter of less than 0.4 nm at 1550 nm is shown in Figure 5, and a typical laser filter in the visible is depicted in Figure 6.

Figure 5. Ultra-narrow band filter built using Alluxa’s plasma deposition technology. The bandwidth is approximately 0.3 nm in bandwidth at 1570 nm.

Figure 6. Consecutive runs of an ultra-narrow band filter at 632.8 nm built using Alluxa’s plasma deposition technology. The bandwidth is approx. 2 nm wide.

Cost Comparisons

While soft coatings are developed in inherently lower cost bell jar evaporation chambers, they also have batch sizes that are much smaller and are almost entirely manual in operation, which adds cost and variability while decreasing yield. Alluxa processes are completely automated, guaranteeing repeatability, low direct labor cost, and continuous operation.

For OEM applications, Alluxa’s hard coatings have much bigger load sizes compared to standard soft coatings. As volumes approach 20 - 30 one inch parts, Alluxa is capable of beating or matching the cost structure of soft coatings, while providing the performance and reliability advantages of hard coating technology.

Table 1. Cost comparison of Alluxa hard coatings against soft coatings

Conclusion

Soft coatings are generally used due to low cost, although they possess high optical scatter, low transmission, and poor temperature stability and environmental durability.

Alluxa’s advanced high speed deposition technology has finally enabled hard coatings to compete on price with soft coatings, even in small batch sizes. The filters developed by Alluxa offer dramatically improved lifetime, spectral shape, and transmission.

This information has been sourced, reviewed and adapted from materials provided by Alluxa

For more information on this source, please visit Alluxa