Evaluation of advanced cleaning processes for sensitive surfaces in optical instrumentation

Abstract

Contamination has become a major issue in all high-tech industries, with increasingly stringent cleanliness requirements. It is the case for sensitive parts of space flight hardware, such as optics or other embedded equipment for which surface cleanliness is a critical topic, since even a thin layer of contaminant can adversely affect performance. Sometimes, cleaning may be needed to reach a specified cleanliness surface level at different integration phases of instrument or satellite, or as a corrective action in case of anomaly. Thus it is recommended to consider cleaning techniques without direct contact with the sensitive or fragile surface to prevent any damage after treatment. Five processes or techniques among those described for instance in the ECSS standard dedicated to ultracleaning of flight hardware (laser, vacuum bakeout, carbon dioxide (CO2) snow, supercritical CO2 and plasma) have been chosen for a test campaign to assess their efficacy on representative samples of sensitive optical parts (e.g.: glasses, mirrors, treatments, coatings, etc.). The study has been focused on the removal of molecular residues from substrates intentionally contaminated by a mixture of typical chemical contaminants. After a description of the selected cleaning techniques and their pros and cons, this paper will present the methodology adopted for the process validation using analytical chemistry and the comparative results of the performance of each cleaning technique. The impact on materials and the ease of implementation have been evaluated as well. In fine, the objective of this study is to provide some guidance to choose one or several non-contact processes suitable for cleaning sensitive space hardware, in order to guarantee contamination-free surfaces before flight and limit any risk of failure.