Over the past few years, Materion has worked to greatly expand our fluoride portfolio which spans the optics, photovoltaic, phosphor and battery materials industries. Just as with ytterbium fluoride (YbF3) - a viable alternative to yttrium fluoride (YF3) and thorium fluoride (ThF4) for the infrared (IR) market - increasingly customized processes are required to meet different performance challenges. An example of that is magnesium fluoride (MgF2), which is critical in designs from the deep ultra violet (DUV) to the near infrared (NIR), and for that reason different production techniques have led to different products.
To understand why this is the case, we must look beyond its rather simple chemical reaction and into the raw material itself and the processing steps that follow. Often times these aspects are overlooked, or are buried so deep in the history of a product that they are taken for granted. Such factors are not considered when new products are proposed, or vendors are changed, which can jeopardize both time and development costs. The following will hopefully serve as a common reference point in differentiating MgF2 materials and explaining how process refinements ultimately determine which product is best for an application.
There are very few materials that are transparent in the UV – and even fewer at wavelengths less than 225nm (lower limit of oxides). For MgF2 to be successful at these wavelengths, absorption from any source must be eliminated. Controlling the lattice energy-reducing impurities drives the purity of the grade upwards of 5N and reduces spit from contaminated lower melting point regions. MgF2 is produced via aqueous reaction and residual oxygen and moisture must be essentially eliminated or risk spit from degassing, surface oxides or exploding particles. Materion developed special drying and melting technology to ensure UV grade MgF2 minimizes all traces of water and legacy reaction chemicals in the cooled crystalline evaporation material. The combination of large stoichiometric grains and high purity does limit the deposition rate - which is why this material is less common in VIS Specialty Optics Designs. In the UV however, the low QWOT (Quarter Wave Optical Thickness) design requirements allow for balancing spit and yield rate without the same cost consequences as those that drive costs in VIS/Specialty Optics grade products.
Beyond price and purity, there are two key esoteric features that dominate the VIS/Specialty Optics grade products: 1) residual oxide and 2) absorption/melting impurities. In the VIS, which uses considerably more material, cost is a primary concern. The role that rate, yield and scaling of the reaction/melting/sizing process tools play is analogous to specifying a production coating machine. While more oxide and some impurities are permissible in VIS, the critical factor is homogeneity of the coating material. Larger bulk vacuum kettles stabilize oxide levels and are used to freeze out detrimental melting impurities lost to fines in the sizing process. MgF2 has the largest variation around the world and locating a consistent and stable source – including raw material source and impurity controls – is crucial for high value and high yield products.
Over time, the different reaction chemistries have moved to the background and cost/kg is usually dominated by the melting approach. Hydrofluoric acid (HF) reactions offer enhanced protection from sulfur and chlorine impurity compounds. Throughout the visible region, the difference between oxyfluoride and full oxide impurities is most evident at high rates or long runs. That is where the heavier MgO compound will separate from the molten MgF2 in the pocket and cause spit. Certain processes may gain from the impact key lattice disruptive impurities have on the melt pool, but collaboration is essential when trying to test a lower cost variety in an optical design.
MgF2 remains critical for the UV, VIS and IR industries. Over the past 15 years, optics customers have sought the best combination of product cost and performance. Materion maintains three different grades of MgF2 and is committed to ensuring our customers understand their differences and identify the right material for their specific job. For questions, please contact David Sanchez, Applications Engineer at: David.Sanchez@Materion.com