Full length articleEffect of sputter pressure on microstructure and properties of β-Ta thin films
Graphical abstract
Introduction
Tantalum thin films are widely used in industry and therefore widely studied (e.g. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]). Until recently, however, there has been greater interest in the stable BCC α phase of tantalum than in the metastable β phase that appears only in thin films. The two phases are quite different and are therefore suited to different applications. While α-Ta films are used in a variety of applications such as wear-resistant coatings and diffusion barriers for Cu-Si interconnects, β-Ta has generally been desired only for Ta thin film resistors. However, the recent discovery of a giant spin Hall effect in β-Ta may be important for the development of next-generation magnetoresistive memory technologies [11], and has renewed interest in this phase.
Because α-Ta has been preferred in many applications for many years, relatively little is known about the structure and properties of β-Ta films. It is well-known that β-Ta has a high electrical resistivity compared to the conductive α phase, but reported values vary widely, from 112 µΩcm [9] to 1500 µΩcm [8]. The origins of these variations are unknown, but they have been attributed to included α-Ta [10], grain size [8], and oxygen content [12] in the films. Similarly, hardness has been reported to vary from ∼12 GPa [3] to ∼20 GPa [6] with variations attributed to stresses [3], strain rate [4,6] and grain size [5]. While there are few reports of elastic properties, Young's modulus has been reported to be as high as 194 GPa [5] or to vary with grain size and texture over the range 166–183 GPa [4]. Of course, properties are expected to depend intimately on structure and composition, but because systematic studies of pure β-Ta films are rare, the literature on the properties of β-Ta remains fragmentary and the actual correlations between composition, structure, and properties are unknown. Some reasons for this include that Ta films have been produced on a variety of different substrates, using a wide range of deposition parameters, and including a number of different impurities.
To explore synthesis-structure-properties relationships more systematically, we prepared a set of β-Ta thin films by sputtering in an ultra-high-vacuum system with sputter gas pressure, pAr, ranging from 0.3 to 2.2 Pa while holding other deposition parameters constant and taking steps to minimize impurities. In a previous article [13], we reported that the stresses varied dramatically, from –1360 to + 1140 MPa over this pressure range and that the resulting x-ray diffraction (XRD) peak shifts allowed us to show that the films were virtually 100% β-Ta with a single (002) fiber texture component that broadens significantly with pAr. These results, combined with an analysis of the distributions of energy and incident angle of species arriving at the substrate, allowed us to propose a model for phase selection in Ta films that explains virtually all findings to date.
In the present article, we report on the microstructure, morphology, and properties of these films. By carefully controlling the deposition environment to ensure that all films are pure β Ta, we are able to explain a growth phenomenon (increasing grain size with pAr) that has been reported but not explained, provide an explanation for the reported variations in hardness of the β phase, provide an estimate for the indentation modulus, and to provide, for the first time, a canonical value of the resistivity of β-Ta films with neither α-Ta content nor oxygen contamination.
Section snippets
Experiments and results
A series of seven β-Ta films were deposited under a range of sputter pressures from 0.3 to 2.2 Pa. Microstructure was characterized using scanning electron microscopy (SEM), mechanical properties were measured using nanoindentation, and resistivity was measured using a four-point probe.
Discussion
By varying pAr while carefully minimizing other sources of variation we obtain a series of virtually 100% β-Ta films with systematic variations in microstructure, allowing us to provide accurate and representative measurements of the properties of the pure β-Ta phase.
Summary and conclusions
A series of β-Ta films were deposited under sputter pressures from 0.3 to 2.2 Pa. By carefully controlling the deposition environment, we were able to produce a range of virtually 100% β-Ta films with negligible oxygen content and wide variations in microstructure and properties.
Grain size increases from 22.5 ± 3.5 nm to 49.2 ± 9.7 nm with increasing sputter pressure, which can be explained based on the energy and incident angle distributions of species incident on the growing film during
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Support for this work was provided by the National Science Foundation (DMR 1106223 and DMR 1810138). This work also made use of the facilities of the Cornell Center for Materials Research with support from the National Science Foundation Materials Research Science and Engineering Centers program (DMR 1120296 and DMR 1719875). We also thank Hysitron, Inc. (now Bruker Nano Surfaces) for support of the nanoindentation experiments.
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