Secondary electron yield of air-exposed ALD-Al₂O₃ coating on Ag-plated aluminum alloy

Secondary electron yield (SEY) of air-exposed metals tends to be increased because of air-formed oxide, hydrocarbon, and other contaminants. This enhances the possibility of secondary electron multipacting in high-power microwave systems, resulting in undesirable occurrence of discharge damage. Al₂O₃ coatings have been utilized as passive and protective layers on device packages to provide good environmental stability. We employed atomic layer deposition (ALD) to produce a series of uniform Al₂O₃ coatings with appropriate thickness on Ag-plated aluminum alloy. The secondary electron emission characteristics and their variations during air exposure were observed. The escape depth of secondary electron needs to exceed the coating thickness to some extent in order to demonstrate SEY of metallic substrates. Based on experimental and calculated results, the maximum SEY of Ag-plated aluminum alloy had been maintained at 2.45 over 90 days of exposure without obvious degradation by applying 1 nm Al₂O₃ coatings. In comparison, the peak SEY of untreated Ag-plated aluminum alloy grew from an initial 2.33 to 2.53, exceeding that of the 1 nm Al₂O₃ sample. The ultra-thin ALD-Al₂O₃ coating substantially enhanced the SEY stability of metal materials, with good implications for the environmental dependability of spacecraft microwave components.