Microelectronic Engineering 50 (2000) 375–381www.elsevier.nl / locate /mee

Influence of diffusion barriers on the nucleation and growth ofCVD Cu for interconnect applications

a , a b b b b*¨R. Kroger , M. Eizenberg , D. Cong , N. Yoshida , L.Y. Chen , S. Ramaswami ,bD. Carl

aDepartment of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, IsraelbApplied Materials, Santa Clara, CA 95054, USA

Abstract

Nucleation and growth behavior of Cu influence strongly the macroscopic properties of the resultant films. In this work thenucleation of CVD Cu on different underlayer materials is studied. It is found that nucleation on bare diffusion barriersurfaces leads to island growth and, therefore, bad wetting and adhesion. An enrichment of F, O and carbon was found at the

˚interface between the CVD Cu film and the diffusion barrier. However CVD Cu deposited on top of Ta with a 200-A PVDCu layer on top results in good wetting. CVD Cu films grown on a PVD Cu layer expose a highly preferred k111lorientation. In this case SIMS analysis reveals a comparably low concentration of oxygen, carbon and flourine at theinterface region between the CVD Cu and the barrier. These observations shed light on relevance of surface conditions forthe CVD Cu deposition process. They significantly affect both film adhesion and crystal orientation, which are crucial for theuse of CVD Cu as interconnect material. 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

The Interconnect delay time is the speed-limiting factor of very advanced microelectronic devices[1,2]. Therefore, the Al(Cu) material — on which the current interconnect technology is based —must be replaced by the low resistivity Cu. The deposition of Cu by CVD enables to meet therequirements for very stringent geometries and , 0.25-mm linewidth desired for future micro-electronic devices [3]. To control the properties of the interconnect material it is necessary tounderstand the nucleation and film growth of CVD Cu on different underlayer and its effect on themacroscopic properties of the resulting Cu films. This work focuses on the dependence of CVD Cu(using hfacCu(I)tmvs as precursor) nucleation and film growth on different PVD and CVD substrateunderlayers (PVD Ta, PVD TaN , CVD TiN and PVD Cu on top of Ta).x

*Corresponding author. Fax: 1 972-4-8291-978.¨E-mail address: mtrkro@vmsa.technion.ac.il (R. Kroger)

0167-9317/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0167-9317( 99 )00305-6

¨376 R. Kroger et al. / Microelectronic Engineering 50 (2000) 375 –381

2. Experimental

The CVD and PVD Cu deposition as well as the deposition of the barrier layers were performed in˚high vacuum cluster tools (EnduraE and CenturaE based). The PVD barriers ( | 200 A thickness)

were deposited using an ionized metal plasma (IMP) system, whereas the CVD TiN barrier wasproduced by a combination of CVD TiN deposition (using TDMAT as precursor) and subsequentcrystallization by a N /H plasma. The CVD TiN was produced in two steps leading to an2 2

˚approximate barrier thickness of 2 3 50 A. Morphology and microstructure were studied by means ofhigh resolution scanning and transmission electron microscopy (HRSEM/HRTEM). Film orientationwas investigated by XRD measurements. The concentration profiles of the elements were studied bysecondary ion mass spectrometry (SIMS). Atomic force microscopy was used to investigate thesurface topography.

3. Results and discussion

Nucleation on bare Ta, TaN and TiN surfaces is significantly more difficult than on Ta on which ax˚200-A PVD Cu ‘flash’ layer was deposited prior to the CVD Cu film growth. This can be seen in Fig.

1 showing the HRSEM images of the substrate surfaces after a CVD Cu deposition time of 90s.Comparably low nucleation densities are found on Ta and TaN , whereas on TiN a higher nucleationx

density can be observed. The CVD Cu layer on top of the ‘flash’ layer does not show distinguishableislands. Results of SIMS analysis of CVD Cu films grown for 420 s on top of bare Ta (a) and on topof a PVD Cu ‘flash’ layer on Ta (b) are shown in Fig. 2. The focus was set on the F, O and C signals

1and the measurements were carried out in the CsM -mode. A significant enrichment in F, O and Ccould be found at the interface between the Cu film and the barrier for (a) and two orders ofmagnitude lower signals for these elements were found for (b). SIMS measurements performed onbare TaN and TiN diffusion barrier (not shown here) reveal the same qualitative distribution of F, Ox

and C. This corresponds to observations of a less than 3-nm thick interfacial layer between the CVDCu film and the barrier reported previously [4]. Adhesion measurements performed by a simple scotchtape test showed a very poor adhesion behavior of CVD Cu films deposited directly on the diffusionbarrier. The adhesion of the CVD Cu film on a PVD Cu layer was significantly better.

XRD measurements of these films are shown in Fig. 3. The CVD Cu on top of bare Ta gives anintensity ratio for the 200 and 111 reflections I /I of about 0.47 which is close to the value of200 111

˚0.46 characterizing a randomly oriented film [5]. The CVD Cu film grown on a 200-A PVD Cu layergives a ratio value of less than 0.03 indicating a very strong k111l texture which was also reportedelsewhere [6].

A high resolution cross-sectional image of the nucleation stage (after 90 s of CVD Cu deposition) isshown in Fig. 4. It reveals island growth on TiN. Between the TiN barrier and the Cu grains a narrowinterlayer of less than 3 nm can be observed, obviously resulting in a poor wetting of the Cu andresponsible for the poor adhesion of the CVD Cu film. A TEM cross-sectional image of a CVD Cu

˚layer on Ta with a 200-A PVD Cu layer on top is displayed in Fig. 5. The CVD Cu grown on top ofthe PVD Cu layer cannot be distinguished from the underlying PVD Cu film. This gives rise to theconclusion that the CVD Cu film grows epitaxially on preexisting Cu grains. To study the interfacebetween the PVD Cu and Ta barrier HRTEM cross-sectional imaging was performed on a PVD Cufilm on top of Ta. The results are shown in Fig. 6. Both the Cu and the Ta films have a polycrystalline

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˚Fig. 1. HRSEM images of CVD Cu on top of Ta (a), TaN (b), Ta with a 200-A PVD Cu layer on top (c), and TiN (d) afterx

a deposition time of 90 s.

structure. The h111j and h110j planes of the Cu and the Ta grains can be distinguished. This showsthat there is no epitaxial growth of the Cu grains on top of the Ta. A narrow ‘intermediate’ disorderedarea (of less than 1 nm thickness) can be identified between the Cu and Ta. This gives rise to theinterpretation of an improvement of adhesion of these films by ‘anchoring’ which is caused by mixingof Cu and Ta at the surface during PVD Cu ion bombardment. The formation of an amorphousinterlayer and thus the poor wetting and adhesion of CVD Cu films deposited on bare barriers is verylikely a result of the surface chemistry in the early stages of film growth. The nature of the barriermaterial influences the disproportionation reaction and thus affects the CVD Cu formation reactionand the adsorption /desorption of F, O and C containing byproducts of the precursor. The mechanismof the surface reaction is still not clear [7,8]. A deeper understanding of the surface chemistry is,therefore, necessary to control the properties of CVD Cu films.

4. Conclusion

Our results emphasize the important role of the surface conditions for the Cu formation processduring CVD Cu growth. Nucleation density and grain orientation are strongly dependent on the

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˚ ˚ ˚Fig. 2. SIMS spectra of a CVD Cu film of a 1200 A on Ta (a) and a 1000 A CVD Cu on top of 200 A PVD Cu on Ta (b).

¨R. Kroger et al. / Microelectronic Engineering 50 (2000) 375 –381 379

˚ ˚ ˚Fig. 3. XRD spectra of a CVD Cu film of 1200 A on Ta (a) and a 1000 A CVD Cu on top of 200 A PVD Cu on Ta (b).

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Fig. 4. HRTEM cross-sectional image of a CVD Cu grain after 90 s deposition time on top of TiN.

substrate material, i.e., the used barrier material and the presence or absence of a PVD Cu layer.These observations can be attributed to the formation of an amorphous interlayer in the early stages ofgrowth. This significantly affects both film adhesion and crystal orientation, which are crucial for theuse of CVD Cu as interconnect material.

Acknowledgements

The authors express their thanks to C. Cytermann for performing SIMS measurements and A. Sapir¨for TEM preparation. R. Kroger acknowledges the support of the Minerva Fellowship Foundation.

This work was supported by Applied Materials, Santa Clara, CA.

Fig. 5. TEM cross-sectional image of CVD Cu (90 s deposition time) on a PVD Cu layer on top of Ta.

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Fig. 6. HRTEM cross-sectional image of the interface between PVD Cu and Ta.

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