Phase formation behavior and diffusion barrier property of reactively sputtered tantalum-based thin films used in semiconductor metallization

Abstract

Tantalum (Ta) and nitrogen-contained tantalum (Ta–N) thin films are sputter deposited at different argon/nitrogen flow ratios onto (001) silicon-based substrates with and without a titanium adhesion layer. The impact of varying the nitrogen flow rate and the underlying titanium on the phase formation process is also investigated using X-ray diffractometry, resistivity measurement and scanning electron microscopy. In contrast to previous works on bare silicon and thermally oxidized silicon wafers, our results indicate that a thin titanium adhesion layer inhibits the formation of high-resistivity (200 μΩ cm) tetragonal Ta over a wide range of realistic deposition conditions. The titanium layer leads to the deposition of a low-resistivity (29 μΩ cm) body-centered cubic α-Ta arising from its epitaxial orientation on the underlying titanium. The thresholds of nitrogen flow rates for depositing nitrogen-saturated α-Ta, amorphous Ta₂N (a-Ta₂N) and stoichiometric NaCl-type TaN on silicon are 0.25, 1.0 and 2.0 sccm, respectively. However, the underlying titanium can increase the thresholds for forming nitrogen-saturated α-Ta, a-Ta₂N and stoichiometric TaN to 1.0, 1.5 and 2.5 sccm, respectively. Consequently, the electrical properties and microstructures for Ta and Ta–N thin films on Ti are significantly changed. Moreover, the barrier properties of 40-nm-thick stoichiometric a-Ta₂N (Ta₆₇N₃₃) and nitrogen over-saturated a-Ta₂N thin films are evaluated. According to X-ray diffraction analyses and sheet resistance measurements, all of the a-Ta₂N barrier layers degrade in a similar manner, triggered mainly by an entire crystallization of the amorphous barrier layers. This is followed by a phase transformation process, sequentially forming Cu₃Si and TaSi₂. Cross-sectional transmission electron microscopy reveals that copper can penetrate through the crystallized films either along grain boundaries or thermal-induced crevices to react with silicon, subsequently forming Cu₃Si precipitates. As adequately doping nitrogen into stoichiometric a-Ta₂N can dramatically increase the crystallization temperature by approximately 150°C, the effectiveness of the nitrogen over-doped a-Ta₂N barrier layers can be greatly improved, subsequently elevating the degrading temperature by at least 100°C.

Introduction

Electrical and structural properties of tantalum (Ta) thin films have received considerable attention owing to their widespread applications in electronic devices and X-ray optics such as thin film resistors and absorbers for X-ray lithography [1], [2], [3]. Since the advent of copper interconnects for deep submicron multilevel integrated circuits (ICs), tantalum is a highly promising polish stop and adhesion layer for chemical-mechanical polishing of damascence process [4]. Furthermore, as generally accepted, tantalum and nitrogen-contained tantalum (Ta–N) thin films are the most promising diffusion barriers to prevent the highly diffusing copper from reacting with the underlying silicon and surrounding SiO₂ dielectric [5], [6], [7], [8], [9], [10], [11]. Thus, the feasibility of growing Ta and Ta–N thin films has been extensively studied, particularly in terms of controlling the phase, electrical property and microstructure of the Ta and Ta–N films so that they can be used as reliable barriers between Si/SiO₂ and copper [5], [6], [7], [8], [9], [10], [11], [12]. For example, related investigations have demonstrated that (1) tantalum sputtered in pure argon ambient on native oxide-forming substrates such as silicon wafers or thermally-oxidized silicon (SiO₂) layers is deposited as the high-resistivity tetragonal metastable phase (β-Ta) instead of the low-resistivity body-centered cubic phase (α-Ta) [6], [7], [8], [9], [13], [14], [15], [16], and (2) the phases sequentially formed by sputtering tantalum under increasing amounts of nitrogen partial flows include nitrogen-incorporated cubic Ta [α–Ta(N)], hexagonal Ta₂N and NaCl-type TaN [6], [7], [8], [9], [15], [16]. The effectiveness of these barrier layers in terms of chemical inertness for Cu metallization appears to follow a direction reverse to the phase formation order, that is from TaN, Ta₂N, α-Ta(N) to β-Ta [6], [7], [8], [9], [10]. However, from the perspective of electrical properties, thin films of α-Ta are preferred because their resistivity (typically 30 μΩ cm) is substantially smaller than those of TaN, Ta₂N and β-Ta (all exceeding 200 μΩ cm). Therefore, many methods have been developed to deposit α-Ta instead of β-Ta on silicon-based substrates [17], [18], [19], [20], [21]. However, to our knowledge, no study has reported on the growth of Ta and Ta–N films on titanium. As generally known, titanium is an important material used in ICs as either an adhesion layer or a source for forming salicide with the underlying silicon. In this work, we demonstrate that, over a wide range of realistic deposition conditions, a titanium layer on silicon or thermally oxidized silicon easily favors the formation of low-resistivity α-Ta. In addition, as the nitrogen content rises, the titanium layer also activates a new phase-transition process, subsequently forming α-Ta(N), amorphous Ta₂N (a-Ta₂N) and NaCl-type TaN in succession at relatively higher nitrogen critical flow rates than those required for films deposited on Si or SiO₂. Consequently, the electrical properties and microstructures associated with such films are significantly altered.

As tantalum is a highly stable refractory metal which is immiscible with copper and does not form copper compounds, the mechanism by which a Ta, Ta₂N or TaN barrier layer fails to protect copper against reacting/intermixing with silicon significantly differs from that presented by another widely used metallization system, Al/TiN. Although aluminum does not react with silicon to form silicides, its high reactivity with the TiN normally induces failure by dissociating the TiN, forming TiSi₂ and Ti-related aluminides [22]. Conversely, copper readily reacts with silicon at temperatures as low as 300°C [23]; however, it does not react with tantalum to form Cu-Ta compounds. Therefore, degradation of the tantalum-related nitride barrier layers is largely caused by penetration of copper through the barrier layers to react with silicon, subsequently forming Cu₃Si precipitates, and (or) interfacial reaction between silicon and the barrier layers; this ultimately forms tantalum-related silicides, such as TaSi₂ or Ta₅Si₃ [5], [7], [8], [9], [10], [11], [12]. Obviously, forming localized Cu₃Si precipitates at temperatures typically exceeding 750°C, instead of the formation of tantalum silicides, is the dominating mechanism for the failure of the highly stable TaN barrier layers [7], [9], [10], [12]. Tantalum behaves similarly by initially forming Cu₃Si at 600°C, which is usually accompanied by the formation of TaSi₂ late between 600 and 650°C [8], [9], [12]. However, whether or not this is responsible for the failure of Ta₂N barrier layers still remains unclear. Both amorphous and crystalline Ta₂N barrier layers could be degraded in a manner similar to crystalline TaN barrier layers through a penetration of copper into silicon to form Cu₃Si and TaSi₂ [8], [9]. According to a previous investigation, degradation of the Ta₂N barriers is attributed mainly to an interfacial reaction between silicon and elemental tantalum released from nitrogen-deficient crystalline Ta₂N [7]. In light of previous works, this work examines the property of Ta₂N thin films as diffusion barriers between silicon and copper to provide further insight into the failure mechanism of this material.