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Optical Coatings_Gas Detection

Issues for Gas Detection Manufacturing

Materion has been manufacturing high performance gas detection filters for the last three decades.  Customers predominantly use thermopiles, pyroelectric, and lead selenide detectors to detect carbon dioxide, methane, and many more gases. These filters usually range from 1um up to 11um with the most common filter being around 4.3um for carbon dioxide detection. These filters can be applied to many different substrate types but generally the customer will utilize silicon, sapphire, or germanium. Signal-to-noise ratio, filter shift with temperature and angle, and repeatability are three major issues that detector manufacturers face daily. Materion creates custom solutions based on the needs of each detector manufacture in order to ensure that these risks are mitigated prior to high volume production.

One of the most critical issues that detector manufacturers deal with is differentiating between signal and noise. This is a major function of the optical filter. If the filter does not block out noise and transmit the signal properly to the detector then the detector may send a false detection signal to the system or not detect the signal at all. Materion designs the filter to ensure that the detector will have a high signal-to-noise ratio. Materion creates thin film coating designs with at least three cavities in order to create the optimal filter shape while keeping cost down for high volume manufacturing.

Figure 1 below shows the shape of filters compared to the cavity numbers. As the cavities increase, the band shape is more refined and helps mitigate the risk of noise entering through the filter. Materion also produces much higher cavity filters for high end applications if needed. Out-of-band blocking can also introduce noise that will interfere with detection. Some detector materials detect at wavelengths that are not near the required detection filter. In order to reduce the transmission of these wavelengths, Materion chooses specific deposition or substrate materials that do not transmit well at those wavelengths thus decreasing the transmission of the noise. Figure 1 - Multiple Cavity Designs                                                             
Figure 1: Shows the shape of filters compared to the cavity numbers.

Detectors are rarely in ideal environments. As temperature increases or decreases, the filter will shift its center wavelength which can cause false positives or potentially not detect the gas at all. The same problem occurs when the angle of incidence changes. Materion understands that the gas needing to be detected is not always at a 0 degree of incidence, and that the detector is not always operating at room temperature. Therefore, Materion has built three extremely refined processes and designs that will not fail in critical situations. The first design minimizes shift with angle.

Figure 2 compares a Materion design that is developed to minimize angle-of-incidence shift to a design that is not. The second design and process minimizes shift with temperature in the same way as seen in Figure 2 for the angle shift design. Depending on the other requirements, final shift per degree Celsius can be determined. The final solution is an option that most other thin film coaters do not provide which is referred to as the “Temperature Invariant Filter.”  The “Temperature Invariant Filters” shift magnitudes less than the typical optical filter due to temperature, and can be used in extremely critical medical, defense, and safety applications.  In order to design the filter properly, highly experienced application engineers will discuss the final application with the customer prior to composition. Figure 2 - Angles Designs

Figure 2:  Compares a Materion design that is developed to minimize angle-of-incidence shift to a design that is not.

Achieving repeatability from one optical filter lot to the next also causes major issues with detector manufacturers. If filters do not have proper tolerances for the optical filter, then customers may encounter limited transmission or no detection at all. Materion provides an option for limited run-to-run optical filter shift and also a low-cost option that permits more optical filter shift if the application allows for it. For filters that require tight tolerances, Materion uses optical monitoring as opposed to crystal monitoring. Optical monitoring requires a very experienced technician to monitor the entire coating process and deposit layers, depending on the filter response during the coating process. Crystal monitoring employs a crystal oscillator to determine the thickness of layers to be deposited in order to control the filter shape and location.  Materion provides both these options in order to supply the high-end gas detection market as well as the lower-end gas detection market.

Whatever your need, Materion has developed specialized expertise in gas sensors and provides a variety of products and services for the fast-growing safety sensor market. We have built our reputation for reliability, repeatability and price by delivering value to our customers for over 40 years. For more information, please contact:  Robert.Legg@Materion.com.