Electrochemical Properties of Indium CMP for Quantum Applications
Authors: La Mai Thi Ngoc , Nakayama Kohei , Tanaka Fabiana Lie , Inoue Fumihiro 
Published: December 2024

Indium is emerging as a promising material in the semiconductor and quantum device integration technology, particularly as an interconnect material. Its application is extensively studied for fabricating microbumps and for filling into Through-Silicon Vias, thereby facilitating hybrid bonding connections crucial for quantum computing advancements. Achieving such connections, especially hybrid bonding, demands precision at the submicron level. Chemical Mechanical Planarization is a critical process in this context, which plays an important role in achieving the desired flatness and required surface quality. It achieves this through a combination of mechanical abrasion and electrochemical dissolution of the metal. Despite its importance, research on Indium CMP is relatively nascent, indicating a significant opportunity for exploration in this area. This study aims to delve into the specifics of Indium CMP, particularly focusing on understanding the electrochemical characteristics during the polishing process. By examining the impact of slurry composition and various influencing factors on the polishing efficacy, we aim to offer a comprehensive insight into the Indium CMP process.
We investigate Indium films electroplated onto Cu substrates with a thickness of approximately 1 µm. The films' electrochemical behaviors were evaluated in both acidic (pH 3) and alkaline (pH 10) slurry, utilizing dynamic electrochemical evaluation equipment. The abrasive type for both slurries is a Silica base with less than 50 nm diameter. To explore the effects of oxidants, varying concentrations of Hydrogen Peroxide were introduced. The experimental parameters included an applied force of 1 psi and a testing area of 1.3 cm².
Figures 1a and 1b show in situ open-circuit potential (OCP) measurements of Indium over time with and without polishing. In both acidic and alkaline slurries, OCP values for Indium experienced a sharp decrease during polishing, followed by a rapid increase upon stopping polishing, indicating the quick formation and subsequent removal of a passivation layer. With the addition of H2O2, the passivation layer's removal, growth, and subsequent removal continue under dynamic and static conditions in the pH 3 slurry. However, in the pH 10 slurry, the potential jump was minimal, and the passivation layer was only partially removed upon restarting polishing. The rate of passivation layer removal roughly corresponded to its formation rate, suggesting the presence of a thin oxide film throughout polishing. This phenomenon may be due to the excessive thickness of the passivation layer at a 0.4% H2O2 concentration, forming too thick layer to remove by CMP.
Figures 1c and 1d illustrate the corrosion current and potential of Indium at various concentrations of H2O2. With increasing H2O2 concentration, the corrosion potential increases in both slurries. In acidic slurry without H2O2, the corrosion potential of Indium was -0.54 V corresponding to its presence as Indium metal in the Pourbaix diagram. When H2O2 was added, the corrosion potential increased, indicating the formation of Indium (III) ions in an acidic environment and the strong oxidizing agent H2O2. In the pH 3 slurry, despite the addition of an oxidant, the passivation layer exits, attributed to the presence of a corrosion inhibitor within the slurry. This phenomenon contributes to the reduction in the corrosion current of the Indium film with increasing H2O2 concentration, as corrosion inhibitor mitigates active corrosion by blocking it. Inhibitors enhance the resistance area between the metal surface and the electrolyte, thus impeding or stopping galvanic reactions and the loss of electrons from the metal surface.
In alkaline solutions, Indium metal remains stable in the absence of oxidants. However, upon the addition of H2O2, the corrosion potential value shifted from -0.714 V without H2O2 to -0.38 V for 0.2% H2O2. This alteration aligns with the emergence of the In2O3 passive layer, as indicated by the Pourbaix diagram. The rise in corrosion potential with H2O2
stemmed from the restricted anodic reaction due to the concurrent growth of the cathodic reaction, leading to the formation of a thick passivation layer. This layer impedes ion transport, so necessitating energy (potential) for the polishing process to proceed. Simultaneously, the corrosion current of Indium increased in the alkaline slurry, signifying active dissolution. According to Haedo Jeong et al., the active zone corresponds to the oxidation reaction's dissolution area, indicating the ongoing oxidation of the metal.
The investigation into Indium CMP revealed a notable dependence on the composition of the abrasive slurry. Indium metal demonstrates stability in both acidic and alkaline slurry in the absence of oxidants. Notably, the introduction of H2O2
to the slurry distinctly induces a phase transition from Indium ions in acidic slurry to In2O3 in alkaline environments, suggesting a potential study to elucidate the mechanism of Indium CMP.

Figure 1