Copper diffusion barrier performance of amorphous Ta–Ni thin films
Highlights
► Amorphous Ta–Ni thin films were deposited on Si substrate by magnetron sputtering. ► The amount of oxygen in the film was adjusted by controlling the substrate bias. ► Copper diffusion experiment was carried out by thermal heating at higher temperatures. ► Ta–Ni amorphous thin films was found to be an effective copper diffusion barrier. ► Incorporation of a few percent of oxygen into the film can retard copper diffusion.
Introduction
As the development of semiconductor industry is pacing to 45 nm technology node and below, the materials and integration scheme of both front-end-of-line (FEoL) and back-end-of-line (BEoL) has been going through revolutionary changes [1]. In BEoL integration, Cu metallization has been widely used because of its high electrical conductivity and good electromigration resistance. Continued improvement of the circuit performance requires the optimizaiton of the materials selection and process integration. Cu diffuses fast into dielectrics and Si at elevated temperatures, causing degradation of the semiconductor devices [2]. Thus a thin barrier has to be applied between the Cu and dielectrics to prevent Cu diffusion. Currently, a dual layer barrier Ta/TaN is used in the industry, with Ta as an adhesion layer and TaN the barrier to isolate Cu from reaching the dielectrics. Ta-based barrier has shown excellent performance in preventing Cu diffusion at elevated temperatures [3], [4]. An added advantage is that the high corrosion resistance of Ta eliminates corrosion related reliability concern [5]. The Ta/TaN bi-layer scheme has proven to be a very successful barrier up to 65 nm technology node. However, in the advanced semiconductor techniques with the increasing metallization layers and circuit density, the resistance–capacitance delay (RC delay) contributed from BEoL has become the controlling factor in limiting the device speed [6]. Thus to reduce the RC delay from BEoL, the resistance of the barrier has to be further reduced. The relatively high resistance of TaN has to be solved in the development of future generation of semiconductor device fabrication.
To reduce the electrical resistance while maintaining good Cu barrier performance, Ta based amorphous alloys, such as Ta–Ni, Ta–Ti and Ta–Cr have been proposed [7]. Amorphous alloy, or metallic glass, has been widely studied in bulk form (bulk metallic glass) [8]. However, its thin film counterparts have received little attention so far. Amorphous structure is free of grain boundaries and thus would eliminate the dominant diffusion channels at low temperatures. Crystalline thin films deposited at low temperatures usually display columnar texture, and the columnar boundaries act as fast diffusion routes, leading to even worse performance than polycrystalline thin films. Therefore employing the concept of amorphous metal alloy has the micro-structural advantage while preserving the good conductivity offered by the constitutive metals.
Amorphous alloys are thermodynamically unstable, thus one of the key concerns is their thermal stability at high temperatures, as it has to be stable above the semiconductor manufacturing peak temperature (400 to 450 °C). To improve the thermal stability of the metallic glass, two or more metals are generally incorporated [9]. A study by Fang et al. [10] showed that Ta–Ni films could prevent Cu reaction with Si substrate at temperatures up to 700 °C, which is comparable with TaN. In our preliminary work, we have also found that Ta based alloy thin films (Ta–Ni, Ta–Ti, and Ta–Cr) generally show lower electrical resistance than TaN. They also exhibit good adhesion with both Cu and dielectrics. Thus these amorphous metallic thin films are promising candidates to replace Ta/TaN barrier as Cu diffusion barrier for future generation of nano electronic devices.
During the study of Ti-based Cu diffusion barrier, it was observed that certain amount of oxygen was incorporated into the films during the plasma vapor deposition process [11], [12], [13], [14]. This is due to the strong affinity between Ti and the oxygen residue in the deposition chamber. Similar effect exists for Ta too, as demonstrated in our recent work [15]. Oxygen appears to stabilize Ta diffusion barrier as previously reported [16], and the beneficial effect might be derived from the so called stuffing effect, which is effectively the oxygen segregation at Ta grain boundaries [17] or other easy diffusion channels. Laurila et al. [18] reported formation of TaOx at Ta(O)/Cu interface at elevated temperature and attributed it as the reason for the enhanced barrier performance. However effect of oxygen on Si/Ta(O) interface was not discussed in the work. In our previous work using sputtered films on Si substrate, we have shown that oxygen addition to the amorphous metallic Ta–Ni films has improved their thermal stability [15]. The interfacial reaction between Ta and Si occurred at 750 °C for low oxygen-containing (<1 at%) Ta–Ni films, while for higher oxygen-containing (∼5 at%) samples, the interfacial reaction was delayed till 800 °C. The difference is related to a more stable Ta–O layer at Ta–Ni/Si interface in oxygen-rich Ta–Ni film. In higher oxygen-containing (∼5 at%) Ta–Ni film, the pre-existing Ta–O layer remained stable at 750 °C, while in low oxygen containing (<1 at%) Ta–Ni films, the Ta–O layer disappeared at 750 °C. The previous study was carried out to evaluate the thermal stability of the thin films only. Cu diffusion assessment was not performed [15]. As existence of oxygen changes the interfacial reaction between Ta–Ni and Si, diffusion of Cu through Ta–Ni barrier and subsequent reaction between Cu and Si could also be delayed with certain amount of oxygen. The previously proposed stuffing effect may also be applicable in the current study, as existence of oxygen in Ta–Ni grain boundary could result in higher Cu diffusion activation energy and lower Cu diffusion rate. Based on this hypothesis, beneficial effect could be obtained when introducing oxygen to Ta–Ni Cu diffusion barrier.
As amorphous Ta–Ni has promising potential as Cu diffusion barrier for BEoL applications, it is of great importance to further understand the Cu diffusion behavior leading to an appropriate assessment of its barrier performance. The failure mechanism of this class of materials will be of great interests too. In the current work, Ta–Ni thin films with different levels of oxygen concentration were deposited on Si substrate. A layer of Cu was then deposited as the top layer without breaking the vacuum. Cu barrier performance was evaluated and compared after annealing. The diffusion process and failure mechanism were studied based on advanced electron microscopic and spectroscopic analyses.
Section snippets
Experiment
Cu/Ta–Ni/Si stacks were prepared by depositing Ta–Ni and Cu sequentially on Si substrate using a magnetron sputtering machine. The chamber was pumped to 5.32 × 10−4 Pa before deposition. The Ta–Ni films were deposited using co-sputtering of Ta and Ni targets with impurity of 99.999%. Ar gas with pressure of 2.67 Pa was flowed through the chamber during the sputtering. The sputtering powers were RF 200 W for Ta and DC 80 W for Ni targets respectively. In order to obtain Ta–Ni films with different
Results and discussion
Ta–Ni films with zero bias (0 W) resulted in corporation of around 5.6 at% of oxygen, the sample is thus named as Ta67.34Ni27.06O5.60 based on the composition. Applying 100 W bias largely prevented oxygen incorporation, leading to the film composition Ta73.25Ni26.10O0.65. The effect of applying bias during deposition on the film composition has been discussed in previous work [15].
The sheet resistance of the Cu/Ta–Ni/Si samples at room temperature and after annealing at temperatures from 500 °C to
Conclusion
Ta–Ni amorphous thin films with 5.6 at% and 0.65 at% oxygen were formed by sputter deposition and evaluated for Cu diffusion barrier performance. The comparison between the two types of samples has revealed different barrier performance. While diffusion of Cu into Si was observed for both samples at 700 °C, formation of Cu3Si only occurred in the oxygen-deficient Ta73.25Ni26.10O0.65 sample, indicating that incorporation of a few percent of oxygen could effectively retard Cu diffusion and reaction
Acknowledgement
Financial support from Ministry of Education, Singapore (grant RG31/06) is gratefully acknowledged.
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