Microstructure and mechanical properties in the thin film system Cu-Zr
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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2022, Surface and Coatings TechnologyCitation Excerpt :Furthermore, patches 1 and 3, containing high amounts of Ag and subjected to Ag segregation, can be attributed to zone T for all the studied pulse widths. All obtained diffractograms were characterised by a right peak shift with respect to the Ag (111) line, explained by the presence of solid solutions, with possible contribution from tensile stresses, as already observed for CuZr alloys [34–36]. The broadening and asymmetry of peaks observed on textured patches are associated with the presence of chemical gradients, causing crystal lattice strain.
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2022, Journal of Alloys and CompoundsCitation Excerpt :On the other hand, Oellers et al. recently reported complete solid solubility between Cu and Zr atoms up to 5.5 at% Zr in room temperature magnetron sputtered Cu-Zr thin films without any evidence of Zr segregation [24]. Interestingly, Zr addition to nanocrystalline Cu produced significant microstructural changes resulting in improved mechanical properties of nanocrystalline Cu-Zr alloys [16,21,24]. In addition, mechanically alloyed nanocrystalline Cu-Zr powder with 1 at%, 2 at% and 5 at% Zr exhibited excellent grain size stability up to 800 °C for 1 h [11].
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2019, Thin Solid FilmsCitation Excerpt :However, these (Me1, Me2) alloy films are soft, exhibiting a low hardness H < 10 GPa, a high effective Young's modulus E⁎ and a low resistance to cracking. As examples, (Ni,Ti) [1] (Zr,Ti) [2,4], (Ti,Nb) [3], (Ti,Cr) [5], (Cu,Zr) [6], (Cu,Mo) [7], (Al,Mo) [8], (Ta,Ag) [9], NiZr [10] alloy films can be given. The low ratio H/E⁎ ≤ 0.1 of these films is one of the key reasons why the (Me1, Me2) alloy films easily crack [11,12].