Stability of Al and Ag metallic thin film mirrors in a space environment under the implantation of low energy helium ions

Abstract

The stability of aluminum (Al) and silver (Ag) metallic thin films (MTFs) under helium ion bombardment has been investigated in the laboratory to replicate the effect of alpha particle bombardment on spacecrafts and satellites in a space environment. The implanted helium ions have varying fluence and energies ranging from 0.5 - 3 keV. The helium ion fluence in the present study has been chosen according to 4 and 6 years journey of a solar orbiter. The reflectivity of Al and Ag MTFs is investigated over a wide range of electromagnetic radiation covering ultraviolet to near infrared (200 - 2500 nm), prior and post helium ion implantation. It is observed that the degradation in the reflectivity of the above-mentioned MTFs is reasonably low for helium ion implantation and no significant impact is observed on reflectivity of both (Al and Ag) MTFs in the investigation. This opens a channel of utilization of these MTFs to provide better protection for the optical components used in spacecrafts. Surface characterization such as surface roughness is carried out to investigate the surface morphology of MTFs prior and post implantation using atomic force microscopy (AFM). It is observed that the effect of implantation on surface morphology is in accordance with the experimental results of reflectivity. SRIM/TRIM simulations help to obtain the distribution profile and penetration depth of helium ions inside the Al and Ag MTFs.

Optical Thin Films in Space Environment: Investigation of Proton Irradiation Damage

“…The experimental results were used to determine the guard threshold fluence for each type of coating when experiencing some damage following proton irradiation. It is well-known that for fluences above of 10 17 cm –2 , low-energy ions implanting in metals, such as W, Au, and Cu, can result in bubble formation. − A sponge-like morphology was observed in the case of gold films, associated with bubbles whose size increases with the fluence and forming large blisters for fluences of the order of 5 × 10 17 cm –2 . , Various experiments of H- and He-ion irradiation on metal thin-film multilayers used in nuclear physics were carried out, − as well as for components used in lithographic apparatus. − The irradiation of charged particles can induce charge accumulation and alter the optical performance of extreme-ultraviolet multilayers via changes in the surface morphology and optical and structural characteristics, which, in the worst cases, can result in major damages, such as delamination and blistering. − More data come from recent experiments that test Al and Ag metallic coatings for space applications and in particular with He-ion irradiation, which however do not define the damage thresholds for which the formation of these bubbles determines a degradation of the optical properties. In particular, common metal coatings, such as Al and Au thin films, have not been systematically studied so far.…”

“… 25 − 28 The irradiation of charged particles can induce charge accumulation and alter the optical performance of extreme-ultraviolet multilayers via changes in the surface morphology and optical and structural characteristics, which, in the worst cases, can result in major damages, such as delamination and blistering. 29 − 32 More data come from recent experiments that test Al and Ag metallic coatings for space applications 33 and in particular with He-ion irradiation, 34 which however do not define the damage thresholds for which the formation of these bubbles determines a degradation of the optical properties. In particular, common metal coatings, such as Al and Au thin films, have not been systematically studied so far.…”