Elsevier

Thin Solid Films

Volume 645, 1 January 2018, Pages 193-202
Thin Solid Films

Microstructure and mechanical properties in the thin film system Cu-Zr

https://doi.org/10.1016/j.tsf.2017.10.030Get rights and content

Highlights

  • Combination of combinatorial materials science approach and nanoindentation

  • Synthesis of binary thin film material library with small thickness variation

  • Films with increasing Zr content exhibit a significantly increased hardness.

Abstract

A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library.

Introduction

Copper (Cu) is widely utilized in electronic devices as a metallization layer due to its excellent electrical and thermal conductivity. However, the reliability and lifetime of microelectronic devices are often determined by the mechanical properties of the metallization layer since plastic deformation can lead to failure by fatigue damage, necking or fracture. In electronic devices Cu is typically deposited through electroplating, where impurities originating from the utilized chemicals can severely deteriorate the mechanical performance by grain boundary embrittlement [1]. Therefore, it is of interest to develop high strength Cu alloys, while not compromising on the desirable properties of electrical and thermal conductivity. Whereas several studies focus on utilizing top-down approaches such as severe plastic deformation [2], [3], [4], [5], [6], others [7], [8], [9], [10], [11], [12] use bottom-up approaches such as thin film growth to engineer the nano-/microstructure and achieve both, high mechanical strength and electrical conductivity. The present study uses the latter approach to extend the current understanding of correlations between nano-/microstructure and electrical/mechanical properties of Cu-Zr alloy thin films, which promise an increased mechanical performance by segregation of Zr to the Cu grain boundaries without causing embrittlement [7].

Most studies have investigated Cu-Zr alloys with Zr contents > 20 at.%, which leads to the formation of amorphous, metallic glasses [13], [14], [15], [16]. In contrast, alloying of Cu with Zr contents < 20% can lead to the formation of CuxZrx precipitates, which improve the electromigration lifetime of Cu interconnects by up to two orders of magnitude [17], [18], as well as the mechanical properties [7].

In addition to being limited to a specific solute content, investigations of mechanical properties of Cu films alloyed with immiscible elements are rare, with some studies focusing primarily on the fabrication of multilayered materials [19], [20], [21], [22], [23], [24], [25], [26], [27]. In this study the advantages provided by the non-equilibrium synthesis process of sputtering and the combinatorial approach of synthesizing a well-defined composition spread thin film library are utilized to provide insights into correlations between the microstructure and mechanical, as well as electrical properties of hypoeutectic compositions of Cu(100  x)Zrx (2.5 < x < 6.5 at.%, 3.4 < x < 8.8 wt%) alloy thin films. For optimization of electrical properties Cu-rich alloys are preferred due to the superior conductivity of Cu in comparison to Zr and to prevent the formation of metallic glasses, which typically have an inferior electrical performance. As such, a composition range of hypoeutectic Cu-Zr alloys was selected and fabricated in a single combinatorial sputter deposition. The coverage of the compositional spread was defined by the confocal deposition geometry of the used sputter system (Table 1).

Section snippets

Synthesis of a Cu100  xZrx (2.5 < x < 6.5) thin film composition spread materials library

The binary composition spread thin film library Cu100  xZrx (2.5 < x < 6.5) was fabricated in a single co-deposition run under identical conditions by combinatorial sputtering on a thermally oxidized 4-inch diameter Si (100) wafer substrate with a 1.5 μm thick SiO2 diffusion barrier. A magnetron sputtering system with a confocal target setup was used for the deposition (AJA ATC 2200-V). The thin film library was grown by simultaneous sputtering of two diametrically opposite Cu targets (4-inch

Structural characterization

Fig. 1(a) shows a schematic view of a 4-inch Si wafer substrate on which the Cu-Zr thin film was deposited. SEM - EDS measurements of the films at intervals of 4.5 mm confirmed the opposing Cu and Zr compositional gradients across the substrate (Fig. 1(b)). While the Cu content decreased from ~ 98 to 94 at.%, the Zr content increased from ~ 2 to 6 at.% along the gradient. The mean thickness of the film was estimated to be 2.1 μm with a standard deviation of ± 0.26% using profilometry at intervals of ~ 5

Discussion

The equilibrium Cu-Zr phase diagram shows a variety of phases with complex crystal structures formed by composition and temperature specific reactions. However, the Cu-rich (> 90 at.% Cu) composition space exhibits only a single eutectic reaction at a temperature of ≈ 1003 °C [38], where the maximum solid solubility of Zr in the terminal fcc Cu solid solution was estimated to be ≈ 0.12 at.% [39]. However, a stoichiometric compound Cu9Zr2 containing ≈ 18 at.% Zr is also formed as a result of the

Conclusions

The synthesis of a Cu-Zr thin film composition spread was carried out using combinatorial sputtering to achieve a well-controlled binary compositional variation with low thickness variation. Through this preparation method, the measurement areas along the compositional variation were suitable for comparative mechanical, electrical and structural investigations, because these properties can be very sensitive to varying film thicknesses, or process conditions.

A notable influence of the Zr content

Acknowledgements

The authors would like to thank Mr. Benjamin Breitbach for assistance in conducting the XRD experiments and analysis. T. Oellers acknowledges a PhD fellowship from the International Max Planck Research School for Surface and Interface Engineering (IMPRS-SurMat).

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