Impact of prebonding plasma treatment on indium

Bonding between materials is a critical step in fabricating advanced microelectronic components. Bonding typically requires processing steps, such as plasma treatment, to ensure high yield success. While plasma treatment may be functionally necessary, the plasma effects leading to bonding improvement are poorly understood, which hinders development of new protocols. We have addressed this gap using x-ray and ultraviolet photoelectron spectroscopies to probe changes in surface chemical composition. Here, we explore the plasma gas parameter space containing hydrogen, helium, and nitrogen on indium thin films as a model system for the indium contacts used in cryogenic electronic applications. We find that, for all plasma treatments, carbon-containing contaminants are effectively removed, especially those containing C—C bonds. Conversely, indium is found to be more highly oxidized after plasma treatment and subsequent brief exposure to air during transfer to the analysis chamber. Nitrogen is observed on all indium surfaces including control and reference samples, for which likely nitrogen origins are discussed. Nitrogen-containing plasma treatments on the indium films cause the appearance of an oxidized nitrate component. Considering it has been found that nitrogen-containing plasmas produce poorer bond strength and uniformity, we postulate that the presence of nitrate species inhibits uniform bonding. We also discuss the consistent trends in oxide, indium metal, and other component intensities observed for plasmas containing only hydrogen/helium, only nitrogen/helium, or a mix of both hydrogen and nitrogen with helium. Improved understanding of plasma impact on surface chemistry will facilitate informed development of new plasma treatment protocols.