Comparison of Electrochemical Characteristics of in Film on Cu and Ru Surfaces

Abstract

Indium is a potential superconducting interconnect material in a quantum computer, becoming superconducting at 3.37 K [1]. In particular, Indium has good ductility in cryogenic environments, making Indium bonding advantageous for 3D quantum systems. Many researchers have used the ECD method to fabricate Indium micro-bumps because of the requirement of low temperature and achieving a high aspect ratio with a high deposition rate. There have been some studies on the formation of micro-bump of Indium on Copper surfaces using the ECD technique, however, Copper is not a superconducting material. Cobalt and Nickel are materials with electromagnetic properties that affect the conductivity of the quantum system. Ruthenium is considered a suitable underlayer for direct deposition owing to its low resistivity (bulk resistivity of 7.1 μΩ.cm) [2, 3] and transition temperature of 0.47 K [4]. To the best of our knowledge, the deposition of an In film on the Ru surface has not been studied. Therefore, this study aims to evaluate the formation of In thin films on the Ru surface as well as compare the characteristics of In on Cu and Ru surfaces to develop the superconducting Indium micro-bump technology.Indium deposition was studied in a standard three-electrode cell using a potentiostat-galvanostat system. Before deposition, the ruthenium surface was cleaned for 180s at a fixed current of -10 mA/cm2 in 1.2 mol/L H2SO4 [5]. The primary electrolyte was an indium solution (MicroFAB In4950) with an additive (MicroFAB Uniformer).In Fig. 1, the electrochemical behavior of In solution on Cu and Ru surfaces was investigated. Indium metal begins to deposit at -0.87 V and dissolve at -0.25 V on the Cu surface. Meanwhile, the peak currents of In on the Ru surface at the cathode and anode are -1.75 V and 1.17 V, respectively. The electrochemical reaction of Indium is on Cu and Ru surfaces by the following reaction (1):In3+ + 3e- ↔ In (1)SEM micrographs observations of both surfaces at each stage show that island growth occurs when the potential is applied, specifically that the nanoparticles coated on the Ru surface have a porous morphology. The formation of the porous structure upon increasing the voltage on the Ru surface is due to the generation of H2 bubbles, so this study further clarifies the mechanism of In film formation on the Ru substrate. In addition, the change in current is evaluated to determine the effect of current on the structure of the In film. The roughness, thickness, and crystal structure of In film layer have all been thoroughly investigated. To evaluate the bonding ability in low temperatures, the micro-bump formation on both Cu and Ru substrates is demonstrated.In conclusion, the properties and deposition mechanism of In films on Cu and Ru are investigated using the ECD technique. The surface properties of Ru influenced nucleation and In film properties, suggesting a potential study to elucidate the mechanism of In deposition on Ru. Figure 1