Phase formation behavior and diffusion barrier property of reactively sputtered tantalum-based thin films used in semiconductor metallization
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.
Section snippets
Experimental details
A magnetron sputtering system, evacuated using a dry pumping system to reach a base pressure of 1×10−6 Pa, was used to deposit Ta and Ta–N films over a wide range of thicknesses from 0.1 to 2 μm on two types of substrates without intentional heating or biasing. One substrate type includes thermally oxidized (001)-oriented silicon wafers and bare (001) silicon wafers cleaned by the standard RCA method. Another substrate type is silicon-based wafer sputter-coated with a 10-nm-thick titanium
Results and discussion
Fig. 1 shows two typical X-ray diffraction patterns, recorded by θ-2θ scanning over a wide range of angles (30 to 100°) from 100-nm-thick tantalum films deposited on thermally-oxidized silicon substrates [pattern(a)] and on a titanium layer on silicon-based substrates [pattern(b)]. The indexed pattern (a), along with evidence that the resistivity of such tantalum films (200 μΩ cm) is approximately the same as the reported resistivity of β-Ta, suggests that the tantalum films sputter deposited
Conclusions
This work has demonstrated that a titanium thin layer alters the phase-forming behavior of tantalum-based films expected for silicon-based substrates by promoting the formation of low-resistivity α-Ta, ultimately inhibiting the formation of high-resistivity β-Ta, and increasing the critical nitrogen flow rates deemed necessary to form α-Ta(N), a-Ta2N and crystalline TaN. Although that the reason for why an underlying titanium layer widens the phase formation of Ta–N remains unclear, this work
Acknowledgements
We would like to thank Mr. C.S. Hsu of Hu-Wei Institute of Technology and Dr. H.Y. Lee of the Synchrotron Radiation Research Center (SRRC) for X-ray diffraction analysis, Dr. C.P. Liu for TEM assistance, and the National Science Council of the Republic of China for financially supporting this project under Contract nos. NSC 87-2215-E-035-005 and NSC 88-2215-E-035-004.
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