Removal of turning marks from aluminum alloy mirror surfaces using ion beam planarization

In theory, mirror-like optical surfaces made of aluminum alloys provide exceptional reflectivity across a wide range of wavelengths, from the deep ultraviolet to the far infrared. However, waviness resulting from single-point diamond turning tool marks and other manufacturing defects can significantly reduce actual reflectivity, particularly in the visible and ultraviolet spectra. In order to use aluminum alloy mirrors for applications requiring wavelengths below the infrared range, the machining marks on these mirrors must be removed. Ion beam planarization efficiently smooths surfaces by transferring the flat surface of a sacrificial layer into the underlying base material using ion beam etching. In this study, the ma-N2400 series photoresist was characterized as a planarization layer. The photoresist was shown to effectively embed the characteristic waviness structures originating from ultra-precise diamond turning. No surface roughening was observed on the photoresist or the rapidly solidified aluminum (RSA) 501 base substrate during ion beam etching using CF 4 process gas. Substantial reduction of the tool mark amplitude was observed for diamond-turned RSA 501 mirrors with tool mark periods ranging from 3 μ m to 6 μ m, leading to a roughness decrease from approximately 3 nm to around 1 nm rms.