Elsevier

Microelectronic Engineering

Volume 110, October 2013, Pages 29-34
Microelectronic Engineering

Enhanced growth and Cu diffusion barrier properties of thermal ALD TaNC films in Cu/low-k interconnects

https://doi.org/10.1016/j.mee.2013.03.176Get rights and content

Highlights

  • Applying a plasma treatment prior deposition  improved growth of thermal ALD TaxNyCz films.

  • This can be achieved in particular on low-k dielectrics.

  • Barrier testing = high temperature anneal + electric fields (BTS) + triangular voltage sweep.

  • Homogeneous ALD TaxNyCz films may exhibit a Cu barrier performance similar PVD TaN.

  • This finding gives rise to a re-discussion of thermal ALD TaxNyCz films.

Abstract

For thermal ALD TaxNyCz films improved growth behaviour and Cu diffusion barrier performance are demonstrated by applying a plasma treatment prior to film deposition, in particular on low-k dielectrics. Two different kinds of ALD processes for depositing thermal ALD TaxNyCz films are applied in this study, involving either TBTDET or PDMAT as a precursor. Ammonia is used as a reactant and Ar as a purging gas in both processes. Within the experiment, two types of pre-treatments prior to ALD are investigated: a wet-chemical pre-treatment using diluted (0.5%) HF, and plasma pre-treatments using Ar/H2 or N2 plasmas. It is examined by transmission electron microscopy (TEM) from a microstuctural perspective whether improved growth behaviour of thermal ALD TaxNyCz films can be achieved by applying a plasma treatment prior to film deposition. The Cu diffusion barrier properties of 10–15 nm ALD TaNC films are then evaluated by bias temperature stress (BTS) and triangular voltage sweep (TVS) measurements on metal-insulator-semiconductor (MIS) test structures, after annealing at up to 600 °C under H2/N2 atmosphere. The results imply that, from a process side, thermal ALD TaNC films can intrinsically achieve a Cu diffusion barrier performance similar to PVD TaN. However, if no treatment was applied, Cu drift occurred.

Introduction

The geometrical dimensions of Cu interconnects are reduced for every new technology node, implying that the Cu diffusion barrier, the adhesion promoter and the Cu seed layer must also be thinned, in order to avoid top feature pinch-off during the Cu electroplating. However, especially the downscaling of the Cu seed layer reaches a critical level below trench widths of ∼50 nm, depending on the aspect ratio, since a minimum Cu thickness of ∼30 nm as measured in the field may be required to ensure a continuous seed layer for Cu plating and to avoid oxidation of an adhesion promoter like Ta or Co, while a Cu thickness of max. ∼30 nm may be required to avoid top feature pinch-off during the Cu ECD [1].

Thus, further scaling of Cu/low-k interconnects requires ultra-thin and highly conformal TaN barrier films. To obtain high step coverage it is common sense that thermal atomic layer deposition (ALD) is a suitable technique, because the ALD-typical self limiting growth behaviour ensures a structure independent saturation coverage if cycle times are sufficiently long – in contrast to plasma enhanced techniques, where a directed particle flow exists. Furthermore, thermal ALD potentially allows batch processing of wafers which typically leads to high throughput in volume manufacturing.

Thermal ALD of TaxNyCz has been widely studied for conformal barrier deposition in Cu damascene structures [2], [3], [4], [5], [6], [7]. A comprehensive characterization of a pentakis-dimethylamido-tantalum (PDMAT) based commercially available ALD TaxNyCz process was exemplarily published already by Wu et al. [6]. According to Wu et al. these films crystallize in a face-centered cubic structure for thicknesses higher than 1.5 nm, and contain Ta and N at a ratio of about 1:2, i.e. it was found that these films were N-rich and highly resistive. From theory, one might expect a good Cu diffusion barrier performance of these ALD TaxNyCz thin films based on the hypothesis that grain boundaries are sufficiently stuffed by N. Besides, impurities like H, O and C were incorporated into the films, usually a few atom-percent. It has been reported earlier that an anneal of PDMAT based TaxNyCz thin films at 600–900 °C in vacuum lead to an out-diffusion of H, O and C, as well as a densification of TaxNy and consequently a substantial suppression of oxygen penetration into the films [2]. However, these temperatures are rather high for BEoL processing. Park et al. have published a comprehensive description of ALD TaxNyCz film properties involving tertiary butylimido-tri(dimethylamino)-tantalum (TBTDET) as a precursor [4]. The density of their amorphous, highly resistive (107 μΩcm) films was 3.6 g/cm3, with large amounts of carbon as impurities, i.e. not chemically bound to Ta–C according to XPS, and oxygen was found to easily penetrate into the films. Besides Cu and oxygen penetration or interdiffusion, one major drawback of thermal ALD TaxNyCz films was considered the fact that thermal ALD is complicated in particular on low-k substrates, leading to discontinuous films and in turn to a low density and a poor barrier performance against Cu diffusion [5]. It is unclear though, whether these findings were an intrinsic property of the ALD process itself or whether they were caused by a hindrance of initial film growth, which continued to influence the film growth throughout the whole film thickness, respectively. Since thermal ALD primarily is a chemical process (precursor decomposition is negligible within the ALD temperature window, no plasma is applied), initial film growth at ALD is strongly depending on the chemical state of the substrate surface, which is characterized by the density of reactive sites.

The aim of this work, thus, was to enhance initial growth of thermal ALD TaxNyCz films on dielectric substrates by a pre-treatment, and to enable continuous growth throughout the ALD film thereby. Second, it is investigated whether such a pre-treatment can increase the Cu diffusion barrier performance of thermal ALD TaxNyCz films.

Even though a relevant barrier thickness would be about 1–3 nm, and ALD barriers would probably be applied only for this purpose, we investigate a standard barrier thickness of 15 nm (10 nm for ALD on ULK) in this paper in order to exclude potential test structure preparation defects (we found earlier that a weak barrier may be even 30 nm in thickness, but will still show to fail). Also, we would not focus on the integration and scaling challenges here, but rather address the mere material properties of ALD TaNC films again.

Section snippets

Experimental

Thermal ALD TaxNyCz films were deposited using two different kinds of ALD processes, see Table 1. A first ALD process (“ALD I”) involved TBTDET as a precursor and ammonia as a reactant, as well as Ar as a purging gas, typically within a process window ranging between 250 and 350 °C, at pressures of approximately 200 Pa. Our thermal ALD process for growing a TaxNyCz thin film by means of TBTDET was described earlier [2].

In a second ALD process (“ALD II”) thermal ALD TaxNyCz thin films were

Results and discussion

Fig. 1a and b shows the TEM images of a thermal ALD TaxNyCz film without any pre-treatment on SiCOH and porous ULK, respectively, resulting in an inhomogeneous film in either case. It is reasonable to relate this inhomogeneous microstructure to the earlier finding that ALD TaNC films did not act as a reliable Cu diffusion barrier. In turn this observation leads to the question, though, whether a uniformly grown ALD TaxNyCz film potentially could be a reliable Cu diffusion barrier, and how this

Conclusion

It is concluded that also thermal ALD TaxNyCz films can serve as conformal Cu diffusion barriers for Cu/low-k interconnects, provided that plasma pre-treatment is performed prior to the deposition. This result implies that the intrinsic Cu diffusion barrier properties of ALD TaxNyCz can be similar to PVD TaN, if not initial growth hindrance acts as a limiting factor for film homogeneity and density on dielectric surfaces.

Acknowledgments

This project has been funded in line with the technology funding for regional development (ERDF) of the European Union and by funds of the Free State of Saxony (SAB Fördervorhaben. Grant No. 66047/1135).

References (12)

  • H. Wojcik et al.

    Microelectron. Eng.

    (2012)
  • C.C. Yang et al.

    IEEE Electron Device Lett.

    (2010)
  • C. Hossbach et al.

    J. Electrochem. Soc.

    (2009)
  • H. Kim et al.

    J. Appl. Phys.

    (2005)
  • Park. Jin-Seong et al.

    J. Electrochem. Soc.

    (2002)
  • H. Wojcik et al.

    Int. Interconnect Technol. Conf. IEEE

    (2007)
There are more references available in the full text version of this article.

Cited by (6)

View full text