Hyperspectral Coating Challenges
Challenges to Critical Fluorides
In this edition of CMN we are going to take another look at the challenges specific to hyperspectral coatings. In a previous article1 we talked about the differences between various grades of SiO2 where the remnants from chemical reactions can follow the material through deposition and influence thin film performance. In this article we will look into several critical fluoride compounds for UV, VIS & IR films. In the cases of MgF2, YF3 and YbF3, legacy water or mineral impurities can have dramatic consequences. Different processing steps and product evolutions are needed to minimize the risk to customers using these materials for hyperspectral applications.
The challenges with MgF2 are numerous, and after years of VIS AR dominance some manufacturing approaches have lost favor due to low cost alternatives and the relative ease of supply from sources outside of Europe and the USA. However, the unanticipated result is that sometimes those extra steps and techniques can be critical at different parts of the spectrum. During principle reactions like precipitation, recipes that do not use HF acid for fluorination are susceptible to sulfur, sodium and calcium impurities, which can have negative consequences on melting behavior and spitting. Acidic reactions must be carefully controlled to avoid “gel” states, which can harbor water during the manufacturing melting stage and lead to detrimental oxide formation along with spitting and during coating. During manufacturing, melt behavior is controlled to minimize the surface area and maximize crucible packing-density while optimizing yield. As a result, hyperspectral designs2 may require a mix of products differentiated by particle size distribution or overall morphology more intimately linked to the recipe and tool than in the past.
The YF3 and YbF3 cases have all the same difficulties as MgF2 with the added challenge of growing up in the IR where thicker layers, mixed index stacks and longer runs are commonplace. For years there has been discussion regarding both of these materials as ThF4 replacements3. In this instance long coating times and careful control of condensation temperature rise are key attributes of the material expectations. In the case of YF3 the granule and crucible packing was already very high, meaning that for hyperspectral success the purity needs to be improved to eliminate all UV and VIS sensitive impurities. Some multispectral or barrier designs may contain a combination of very thin and thick layers (of the same material) in the same stack, driving up the scale of cast YF3 cones and a lower density YbF3 polycrystalline evaporation material.
In this article we looked at how materials traditionally developed and used for one part of the spectrum can require novel thinking and engineering to achieve their potential in a different part of the spectrum. While high purity, excellent drying and melting technology and good packing density still dominate, the more daunting challenge to true hyperspectral materials is providing high spectral coverage in the forms and at the volumes necessary for a growing market. Materion strives to provide timely technological solutions and products that enable our customers to offer the next generation of optical devices.
Materion offers a complete line of targets and evaporation materials for hyperspectral applications. Contact your Materion Advanced Materials group Sales Representative or for concerns/questions, David A. Sanchez, Sr. Materials & Applications Scientist, David.Sanchez@Materion.com.