Elsevier

Acta Materialia

Volume 150, 15 May 2018, Pages 317-326
Acta Materialia

Full length article
Effect of sputter pressure on Ta thin films: Beta phase formation, texture, and stresses

https://doi.org/10.1016/j.actamat.2018.02.050Get rights and content

Abstract

The metastable tetragonal beta phase of tantalum can be made in thin film form by sputter deposition and has very different properties from the stable BCC alpha phase. Both phases are important in thin film technologies. However, despite fifty years of study, the mechanism of phase selection remains unknown. To evaluate the role of energetic deposition, we prepared a series of films under varying Ar sputter pressures. Measurements of film stress as a function of sputter gas pressure allow us to unambiguously index diffraction peaks to determine phase and texture. This peak indexing allows us to confirm that β-Ta has a distorted Frank-Kasper sigma structure (P4¯21m), rather than the β-U structure (P42/mnm) that is usually assumed. We find only the beta phase in our films in the form of a dominant (002) β-Ta fiber component that becomes broader as the pressure increases. Based on calculations of the energy of incident Ta atoms and Ar neutrals, we show that resputtering could account for the changes in texture distribution. By comparing these results with a detailed review of the literature, we are able to propose a phase selection mechanism that is consistent with the vast majority of published results, namely that β-Ta grows epitaxially on a TaOx layer, possibly TaO2, that forms during the initial phase of deposition. Oxygen is not required in the growing film to maintain the beta structure.

Introduction

For most of the fifty years since its discovery [1], the metastable β phase in tantalum thin films has been treated primarily as a nuisance (see, e.g. Refs. [[2], [3], [4], [5], [6], [7], [8], [9], [10]]). The stable α phase is widely used in thin film form; for example, as a diffusion barrier for Cu interconnects due to its low resistivity and insolubility with Cu, and as wear- and corrosion-resistant coatings due to its high ductility and passivating oxide. In contrast, the only significant technological application of the β phase has been in thin film resistors, thanks to its high resistivity and low temperature coefficient of resistivity. Accordingly, the majority of research on phase formation in Ta films has, to date, focused on obtaining the stable α phase [2,3,5,[7], [8], [9], [11], [12], [13], [14],[11], [12], [13], [14]]. However, discovery of a giant spin Hall effect in β-Ta, which may enable development of a new generation of magnetoresistive random access memory technologies, has revived significant interest in this phase (e.g. see citations to Liu et al. [15]).

In light of these different applications, an ability to understand and control phase formation and properties in sputtered β-Ta films is needed. However, a consensus on the conditions under which β-Ta forms, and the mechanism by which it forms, has yet to emerge. Indeed, studies of phase formation in Ta films have produced many contradictory results. We have conducted a careful review of the literature and have identified several parameters that have been reported to be important in phase selection: substrate temperature, substrate crystal structure and composition (including impurities), impurities in the sputter gas, and the energies of the species impinging on the substrate. Of these, only the effect of substrate temperature is well known: α-Ta forms readily when deposited at high temperatures [[16], [17], [18]], although the temperatures required have been reported to be as low as 300 °C [18] and as high as 800 °C [17].

The substrate on which Ta is deposited is said to affect the phase that appears [[19], [20], [21]]. Substrates such as Nb [22,23] and Ti [3] can promote α-Ta via epitaxial growth. Ta deposited on oxides and readily-oxidized materials usually forms beta, while Ta deposited on oxide-free materials usually forms alpha [19,20,24]. Impurities may also be important, although unifying trends have not been reported. For example, α-Ta can be promoted by either preheating a substrate to remove adsorbed water [16,25], or by deliberately adding organic impurity layers [26].

The effect of impurities in the sputter gas environment has also been investigated. Some researchers concluded that oxygen in the sputter gas is required to form β-Ta [19,20]; indeed it has been suggested that β-Ta is an impurity-stabilized phase that forms when the oxygen concentration exceeds the solubility limit in α-Ta [25,27,28]. However, there is at least one report of β-Ta on oxide-free substrates in a nominally oxygen-free environment [21], while some claim that adding oxygen to the sputtering gas actually promotes α-Ta growth [[29], [30], [31]]. Nitrogen in the sputtering gas is associated with the formation of α-Ta [3,17,25,31,32], but the presence of unknown gas impurities, either from worse vacuum [33] or failing to presputter [16], has been said to promote the beta phase.

Still others have focused on incident particle energy during film growth, including variations in sputter power [33], bias voltage [7], sputter gas species [2], sputtering type (RF [7], HiPIMS [9], etc.), and sputter gas pressure [4]. It might be expected that α-Ta would form in higher-energy environments since it is the equilibrium phase, and some researchers do observe this [9,34,35]. However, others observe a “window”-like behavior, where films sputtered within some range of energies form α-Ta and those with either higher or lower energies form β-Ta [2,4,7,8,33,36]. The kinetic energy of the bombarding particles [33,37] and the particle momentum [8] have both been proposed as parameters that control phase selection. One study claimed that both low bombarding energy and a high flux of Ar+ ions are required for the formation of α-Ta during ion-assisted deposition [2].

Despite these efforts, no unifying theory that can explain phase selection in sputtered Ta films has been proposed. Furthermore, relatively little is known about the microstructure and properties of the β-Ta films that are produced. Indeed, even the crystal structure of β-Ta is not well established. A structure proposed in 1973 with space group P42/mnm [38] is still widely cited, though recent work [6] suggests that a structure proposed in 2002, with space group P4¯21m [39] is more likely.

The working gas pressure and substrate temperature are often considered to be the primary process variables in sputter deposition [40]. Since the effects of temperature on phase formation in Ta films have been explored [16,18,25], we investigated the effects of varying Ar sputter gas pressure, pAr, while holding temperature (and all other variables) constant and minimizing the effects of impurities arising from the base pressure gas, the sputter gas, the substrate surface, and the target. Under these conditions, pAr strongly affects the film stress and texture. However, unlike in previous studies [4,33,34,[41], [42], [43]], it does not affect phase formation—only β-Ta is seen. We show how pAr determines stresses and how understanding the stresses allows us to unambiguously interpret the phase, crystal structure, and texture. Understanding the texture, in turn, allows us to propose a model for β-Ta phase formation that accounts for virtually all previously published observations.

Section snippets

Experiments

Tantalum thin films were prepared by DC magnetron sputter deposition in an ultra-high vacuum (UHV) system [44] with a base pressure of 2.7 × 10−6 Pa (2 × 10−8 Torr) or lower, taking steps to minimize impurities that might affect phase formation. The deposition system comprises separate load-lock, transfer, deposition, and stress-measurement chambers. Substrates were (001) Si with a diameter of 100 mm and thickness of 525 μm with a native oxide layer. As-received substrates were handled

Stress

Fig. 1 shows the equal-biaxial in-plane stress as well as the position of the (002) β-Ta peak vs. sputter pressure for all samples. The stress changes dramatically across the range, from strongly compressive (−1360 MPa at 0.3 Pa) to strongly tensile (1140 MPa at 2.2 Pa), and the peak shifts are consistent with the measured stresses.

Phase analysis

XRD measurements (Fig. 2) revealed that all films show only β-Ta peaks and have (002) texture. Fig. 2a shows θ-2θ scans at the same scale for all films. Only β-Ta

Discussion

Surprisingly, pAr has a dramatic effect on nearly all aspects of film structure except phase. Below, we analyze how pAr determines the energy and incident angle of the species arriving at the substrate during deposition, and how these, in turn, determine stresses and texture. Finally, we use these results to propose a model for β-Ta phase formation that is consistent with our results and with the majority of the published literature.

Summary and conclusions

Many factors affect phase selection in Ta thin films. We eliminated substrate effects (all films are deposited on (100) Si with native oxide), impurity effects (all films are deposited under UHV in UHP Ar with additional O2 filtration), and heating effects (all substrates were unheated) so that we could examine the effects of sputter pressure (from 0.3 to 2.2 Pa) in isolation.

Stresses were found to vary from −1360 to 1140 MPa, consistent with existing stress generation models. XRD returned many

Acknowledgments

Support for this work was provided by the National Science Foundation (DMR 0706507). 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). We also thank Yong Qiang Wang at Los Alamos National Laboratory for assistance with RBS measurements.

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