Passivation of aluminum mirrors with SF6- or NF3-based plasmas

Abstract

This research compares the use of NF3 and SF6 process gases for the removal of the native oxide from Al mirrors and their subsequent fluorination using low-temperature electron-beam generated plasmas. This single step process produces a stoichiometric AlF3 layer of controllable thickness which provides an excellent passivation layer for Al mirrors for far-ultraviolet optics applications. We show that NF3 produces more atomic F content within the plasma than SF6 for similar process conditions, allowing faster oxide removal and fluoride film growth. Al mirrors passivated in either SF6 or NF3 were subsequently studied as a function of gas flow concentration. We found that above a threshold value of atomic F content in the plasma (≈2 × 1018 m−3) samples processed with SF6 or NF3 exhibit similar optical and structural properties regardless of process parameters.