Polishing Iridium Coatings for Zero-Net-Stress

Research on the topic of coating silicon mirrors with iridium has shown that the task of reducing coating stress to minimize mirror distortion remains a subject of continuous study and an area of ongoing improvement. Many efforts have been pursued on the deposition of iridium in a manner that aims to reduce and/or balances coating stress, including bi-layer deposition with chromium and iridium, front and back coating, heat treating, annealing, ion beam figure correcting and atomic layer deposition. It has been determined through our investigation of this topic that one of the simplest and most effective ways to coat these optics in a distortion-free manner is by using the atomic layer deposition ALD coating method. ALD offers 3 main advantages including lower operating temperatures, simultaneous front and back coating and accurate thickness. These advantages allow the iridium coating to be deposited without distorting the mirrors, but the trade off is an increase in surface roughness that can be twice as high as the desired 0.4-0.5 nm RMS specification required for X-Ray mirrors. OptiPro is proposing that their Elastic Emission Machining (EEM) process be used as a solution for smoothing these ALD iridium coated thin mirrors and consequently solving the issue of thin mirror distortion. EEM is capable of atomic-level machining that does not inject heat into the surface, nor does it damage the atomic lattice structure, which as a result leaves a very smooth surface and prevents sub-surface damage. These attributes of EEM also include a precision polishing tool that greatly reduces the risk of inducing mid-spatial frequencies onto the surface, making it an ideal method for accurately smoothing the rougher iridium surface that results from ALD coating.