Electrodeposition of Ordered Copper Germanide from an Alkaline Tartrate-Complexed Electrolyte
Authors: Zhao Fu , Mibus Marcel , Tsui Lok-kun , Zangari Giovanni 
Published: February 2020

Copper is widely used for contacts and interconnects in Si-based semiconductor devices. Copper, however, is regarded as the fastest interstitial-diffusing element in Si with a diffusivity of ~ 10-8 cm2/s at room temperature [[i]]. The diffusion of copper into surrounding materials would significantly degrade their properties and the performance of semiconductor devices. Therefore, a barrier metal layer is necessarily required to completely surround all copper interconnects, which complicates the device fabrication. Recently, copper germanide (ɛ-Cu3Ge) has received much attention as a promising candidate material for contacts and interconnects due to its remarkably low resistivity of 5.5 μΩ cm at 300K [[ii]], which is only approximately three times the resistivity of pure Cu films. In particular, Cu3Ge is stable up to a temperature of 450°C, thus impeding the diffusion of Cu from the film into the surrounding materials, making it an excellent candidate for interconnects in Si-based devices [[iii]].

In the present work, the electrochemical deposition of ɛ-Cu3Ge thin films from an alkaline tartrate-complexed electrolyte [[iv]] was thoroughly investigated. Primarily, the partial (single element Cu and Ge deposition) and net (Cu-Ge alloy co-deposition) processes occurring in such electrolyte were studied by means of cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM) and electrochemical impedance spectroscopy (EIS) techniques. Subsequently, the electrodeposition of Cu-Ge thin films was realized in potentiostatic mode at various selected potentials. The morphology, composition and microstructure of as-deposited Cu-Ge thin films were characterized, and the results reveal that the Ge content in the deposits increases with decreasing applied potential in a sigmoidal manner, leading to a gradual disorder-order phase transformation of Cu-Ge alloy from a disordered face-centered-cubic solid solution (α-phase) to an ordered orthorhombic Cu3Ge intermetallic compound (ɛ-phase). Combining the morphological, compositional and structural characteristics of electrodeposited Cu-Ge thin films with the processes kinetics studies, a possible mechanism of Cu-Ge induced codeposition was postulated.

The dependence of resistivity of as-deposited Cu-Ge thin films on Ge content was also examined, which exhibits a non-monotonic behavior. A minimum value was found around stoichiometric Cu3Ge, which corresponds to the completion of the disorder-order transformation. The ɛ-Cu3Ge thin film displays a resistivity of 7.5 μΩ cm-1 (~50 nm) and 25 μΩ cm-1 (~1 μm), increasing with the deposition duration, which is believed due to the progressive incorporation of O into the films. In order to overcome this limitation, the effect of the bath temperature and pH on Cu-Ge electrodeposition kinetics, composition and microstructure were investigated, and optimal electrodeposition conditions for ɛ-Cu3Ge were identified.

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