Effect of a Cu seed layer on electroplated Cu film

https://doi.org/10.1016/j.mee.2012.12.004Get rights and content

Abstract

Cu films with different thicknesses were electroplated onto Cu seed layers. Cu seed layers with different thicknesses were sputtered on a Ta barrier layer; the thickness of Cu seed layer was varied by changing the sputtering time. To investigate the influence of the Cu seed layers on the performance of the electroplated Cu films, the morphology, grain size, crystallographic orientation, and mechanical and chemical properties of the electroplated Cu films are presented in this paper. As the thickness of the Cu seed layer increases, the grain size increases, and the surface morphology changes from flat to rough to smooth. The adhesion of the Cu seed layers to the substrate increases with increasing thickness of the Cu seed layers but eventually decreases. After the electroplated Cu films are deposited, the morphology, grain size, and crystallographic orientation of the electroplated Cu films are significantly influenced by the seed layers. The hardness of the electroplated Cu film increases with the thickness of the Cu seed layer, and finally reaches a constant value. The adhesion between the Cu film and the substrates is influenced by the Cu seed layer as indicated by the morphology of the film. The Cu film is less corrosion-resistant when its roughness value is higher. Better mechanical and chemical properties are obtained when the thickness of the Cu seed layer is 150 nm.

Highlights

► The morphology of the electroplated Cu films is influenced by the Cu layers. ► The hardness of the electroplated Cu film is influenced by Cu seed layer. ► The Cu film adhesion is influenced by Cu seed layer. ► The Cu film is less corrosion-resistant when its roughness value is higher.

Introduction

Cu is widely used in the manufacturing of electronic devices because of its excellent electrical conductivity, resistance to migration and amenability to dual damascene processing [1], [2], [3]. The manufacturing of microelectronics presents significant challenges for Cu interconnects with the increase in microelectronic interconnect layers during the integrated circuit (IC) manufacturing process [4]. In the process of IC manufacturing, Cu interconnects are electroplated onto a Cu seed layer that has been sputtered on the barrier layer because this process is less expensive and straight-forward than other metal deposition techniques [5], [6].

The surface/interface effects that determine the reliability of Cu interconnects [7] have recently become more critically important. The properties of sputtered Cu layers and the interactions between Cu, Ta/TaN and SiO2 have been widely investigated in recent years [8], [9], [10], [11], [12], [13]. It has been shown that the out-diffusion of Ta atoms toward Cu layers, which leads to the formation of Cu, Ta and Cu–Ta oxides, may cause the failure of Cu/Ta/SiO2/Si multilayer structures during thermal annealing [14]. Moreover, the Ta/TaN bi-layer structure has much better diffusion barrier properties toward Cu than pure Ta or pure TaN films [15]. Early breakdowns of 100 nm Cu damascene lines can usually be observed with reductions in the thickness of the diffusion barrier Ta and Cu seed layers [16], which may be one of the most important obstacles to overcome in the IC manufacturing process.

Other researchers have focused on the electrochemical deposition of Cu films [17], [18], [19]. The crystal size, texture, and electrical properties of the electroplated Cu produced by optimizing the additives of the electroplating solution, the electroplating conditions and the heat-treatment process have been discussed extensively, both experimentally and theoretically [20], [21]. For example, polyethylene glycol (PEG) can improve the wettability of the plating electrolyte on the Cu seed layer and electroplated Cu film, which can influence the Cu interconnect resistance [22]. The types of additives also affect the submicrometer trench-filling process during the Cu-electrodeposition process [23]. Some studies have shown that bis-(3-sulfopropyl) disulfide (SPS) accelerates the Cu bottom-up growth process and that Janus green B (JGB) can inhibit Cu deposition at the end of the filling stages, leading to the suppression phenomenon usually caused by high JGB concentrations.

Previous research has been less focused on the interaction between the Cu seed layers and the electroplated Cu films. There is little information about the influence of the sputtered Cu seed layer on the microstructure and mechanical properties of the electroplated Cu films. Cu seed layers with different morphologies and grain sizes can be realized by controlling the sputtering time. In this paper, the relationship between the properties of the Cu seed layer and the performance of the electroplated Cu film is investigated in terms of the morphology, grain size, crystallographic orientation, and other properties of electroplated Cu films with different thicknesses.

Section snippets

Cu seed layer preparation

A middle-frequency magnetron sputtering system was used to deposit a Ta barrier layer and a Cu seed layer on a SiO2 layer produced by the thermal oxidation of a Si (100) substrate. Impurities on the SiO2 layer were removed by Ar+ ion bombardment. Under controlled conditions (pressure: 2 Pa, bias voltage: 150 V), a 50 nm Ta barrier layer was first sputtered onto the SiO2 layer. Subsequently, Cu seed layers with different thicknesses were sputtered onto the Ta layer. The sputtering process

Crystallographic orientation and grain size

The grain sizes of the Cu seed layers sputtered on the oxidized Si substrate with a Ta barrier layer are calculated based on the Scherrer equation [24], [25], [26], [27]:L=KλβcosθHere, K is a dimensionless constant and can generally be set to unity, λ is the X-ray wavelength, θ is the Bragg angle, L is the grain size, and β is the width at half of the peak corrected for the instrumentation broadening. Fig. 1(1) shows that the intensity of Cu(1 1 1) increases with the seed layer thickness. The

Conclusions

The influences of the Cu seed layer on the performance of electroplated Cu films are investigated in this paper. The surface morphology, adhesion, and roughness of the copper seed layer have a strong dependence on the thickness of the Cu seed layer. The morphology, grain size, and crystallographic orientation of the electroplated Cu films are significantly influenced by the seed layers. Consequently, the stress and morphology of the film has an influence on the hardness of the electroplated Cu

Acknowledgements

The authors acknowledged the financial support from the National Natural Science Funds for Distinguished Young Scholars (Grant no. 50825501) and the National Natural Science Foundation of China (Grant no. 50775122).

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