Microelectronic Engineering 88 (2011) 635–640

Contents lists available at ScienceDirect

Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

Sputtered Ru–Ti, Ru–N and Ru–Ti–N films as Cu diffusion barrier

Ji Li, Hai-Sheng Lu, Yong-Wei Wang, Xin-Ping Qu *

State Key Lab of ASIC and System, Department of Microelectronics, Fudan University, Shanghai 200433, China

a r t i c l e i n f o

Article history:Available online 23 June 2010

Keywords:Copper interconnectDiffusion barrierRuThermal stability

0167-9317/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.mee.2010.06.031

* Corresponding author.E-mail address: xpqu@fudan.edu.cn (X.-P. Qu).

a b s t r a c t

Ultrathin Ru–Ti alloy, Ru–N and Ru–Ti–N films were fabricated as diffusion barriers to Cu metallization.The thermal stability, phase formation, surface morphology and atomic depth profile of the Cu/Ru–Ti(10 nm)/Si, Cu/Ru–N(10 nm)/Si and Cu/Ru–Ti–N(10 nm)/Si structures after annealing at different tem-peratures were investigated. Comparing to the single Ru layer, both N doping and Ti alloying improve thethermal stability and diffusion barrier properties to Cu. The Cu on the Ru–Ti layer has better morphologythan Cu on the Ru–N layer, while the Ru–Ti–N layer has the best thermal stability and has great potentialto be applied as a single layer diffusion barrier.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, Ta/TaN bilayer deposited by physical vapor deposi-tion (PVD) are currently used as diffusion barriers in the copperinterconnect [1]. However, the Cu/Ta/TaN tri-layer stack encoun-ters difficulties in scaling to the 32 nm or 22 nm node where ultrathin barrier is needed to maintain a low effective interconnectresistivity (qeff = 2.2 lX cm) [2].

Ruthenium (Ru), with nearly twice the electrical conductivity ofTa, shows good adhesion ability to Cu [3] and capability of directlyelectroplating of Cu on it [4]. However, Ru, with a columnar struc-ture, is not a good diffusion barrier to Cu, and always need to com-bine with a good diffusion barrier, such as TaN [5,6]. Recently,some work has been done to incorporate Ta, N into Ru to forman amorphous single barrier layer [7,8].

In this paper, Ti or N or Ti–N incorporated Ru films were inves-tigated as a promising single diffusion barrier layer to copper. Thethermal stability results show the good properties of Ru–Ti andRu–Ti–N as Cu diffusion barriers.

2. Experimental

The n-Si(1 0 0) wafers were cleaned using a standard chemicalclean process followed by dipping in diluted HF before deposition.The base chamber pressure was below 6 � 10�5 Pa. The Ru–Ti filmswere deposited on the Si substrates by co-sputtering of the Ru, Titargets with purity of 99.99%. Different volume ratios of Ru:Ti wereachieved through adjusting the sputtering power of the Ru and Titargets. During the Ru–Ti deposition, the Ar gas flow rate was30 sccm under the pressure of 0.65 pa. The Ru–N films were depos-

ll rights reserved.

ited with reactive sputtering using ratio of Ar:N2 of 30:3 (namedRuN A), 27:3 (named RuN B) and 20:4 (named RuN C). The Ru–Ti–N films were deposited using co-sputtering of Ru, Ti withAr:N2 of 30:3. The thickness of the three kinds of barriers was keptat 10 nm. Cu film was then deposited without breaking the vac-uum. After deposition, the samples were annealed at the tempera-tures from 300 �C to 550 �C in N2 atmosphere for 30 min.

The sheet resistance was measured with a four-point probe. Thephase formation of the films was investigated by X-ray diffraction(XRD) (Cu Ka radiation). The film composition and atomic depthprofile were characterized by Auger electron spectroscopy (AES)with an average etch rate of 17 nm/min. The surface morphologyof the Cu/Ru based barrier/Si samples was observed using scanningelectron microscopy (SEM).

3. Results and discussion

3.1. Ru–N diffusion barrier

The thermal stability of the Ru–N thin films with different Ar:N2

ratio on the Si substrate as diffusion barrier for Cu is investigated.Fig. 1 shows the sheet resistance variation as a function of anneal-ing temperatures for the Cu(100 nm)/Ru–N(10 nm)/Si structureswith different Ru–N barriers. For the RuN B sample (Ar:N2 =27:3) and RuN C sample (Ar:N2 = 20:4), the sheet resistance in-creases abruptly when the annealing temperature is higher than500 �C; while the RuN A (Ar:N2 = 30:3) sample with least N2 con-tent has the best thermal stability, with resistance increased only13% after annealing at 500 �C.

Fig. 2 gives the XRD patterns of the Cu/RuN A/Si samplesafter annealing at different temperatures. For the as-depositedsample, only the peaks of Cu(1 1 1) and Cu(2 0 0) can be observed,indicating an amorphous structure of the as-deposited Ru–N film.

Fig. 1. The sheet resistance variation of the Cu/Ru–N/Si samples with the differentannealing temperatures.

Fig. 2. The XRD patterns of the Cu/Ru–N A/Si samples annealed at differenttemperatures.

636 J. Li et al. / Microelectronic Engineering 88 (2011) 635–640

After annealing at 500 �C, no Cu3Si or Ru2Si3 peak can be found. Forthe Cu/Ru/Si sample, annealing at 400 �C already caused the failureof the whole system [7]. This demonstrates the good thermal sta-bility and diffusion barrier properties of the N incorporated Ru–Nlayer. Also we can not observe the diffraction peak from Ru afterannealing at 500 �C, which is normally observed for the Cu/Ru/Sistructure after annealing at 400–500 �C [9]. So the incorporationof N in the Ru changes the original nanocrystalline Ru to amor-phous structure. Damayanti et al. also observed that nitrogen dis-solved in Ru film during PVD process could lead to substantialinternal stress and therefore promote nanocrystalline structurewith shorter range ordering or even an amorphous structure for

Fig. 3. SEM images of the Cu/Ru–N A/Si sample

Ru [9]. The amorphous structure of Ru is beneficial to block Cu dif-fusion. The reason why the Ru–N film with less N2 has better ther-mal stability is under investigation.

The surface SEM images of the Cu/RuN/Si after annealing at dif-ferent temperatures are shown in Fig. 3. The SEM images show thatthe Cu surface remains smooth and uniform with no agglomerationafter annealing at 400 �C and 450 �C, however, there appear a lot ofpinholes on the Cu surface. This is because the dissolved nitrogen isstarting to diffuse from the Ru–N layer into Cu under 275 �C whichwas also observed by Kim et al. for the Ru–N layer deposited in theN2 atmosphere [9]. After annealing at 500 �C, severe Cu oxidationcan be observed.

s after annealing at different temperatures.

Fig. 4. The sheet resistance variation of the Cu/Ru–Ti(10 nm)/Si structures withannealing temperatures.

Fig. 5. XRD patterns of the (a) Cu/Ru2Ti1/Si samples and (b) C

Fig. 6. AES depth profile of the Cu/Ru3Ti1/Si sample for annealin

J. Li et al. / Microelectronic Engineering 88 (2011) 635–640 637

3.2. Ru–Ti diffusion barrier

Since pinholes will be formed in the Cu/RuN/Si system afterannealing, we then investigate another dopant, Ti in the Ru.Ru–Ti layer was formed by co-sputtering of Ru and Ti. Twokinds of volume ratio were tried, that is 2:1 (named Ru2Ti1),3:1 (named Ru3Ti1). However, the AES data show only slightTi (about 5%) was incorporated into the Ru film, which reasonneeds to be checked further. Fig. 4 gives the resistance varia-tion of the Cu(100 nm)/Ru–Ti(10 nm)/Si samples as a functionof annealing temperatures. Both samples with Ru2Ti 1 andRu3Ti1 films show no increase of sheet resistance after anneal-ing at 450 �C. But the sample with Ru2Ti1 has better thermalstability, with 30% sheet resistance increase after annealing at500 �C; while the resistance for one with Ru3Ti1 increases200%.

The XRD patterns in Fig. 5 show for the Cu/Ru2Ti1/Si samples,although annealing at 500 �C already results in the formation ofRu2Si3, but the copper peak intensity is still very strong. For the

u/Ru3Ti1/si samples annealed at different temperatures.

g after (a) as-deposited, (b) 400 �C, (c) 450 �C and (d) 500 �C.

Fig. 8. The sheet resistance variation of the Cu/Ru–Ti/Si and Cu/Ru–Ti–N/Si samplewith annealing temperatures.

638 J. Li et al. / Microelectronic Engineering 88 (2011) 635–640

Cu/Ru3Ti1/Si sample, after annealing at 500 �C, the intensity of theCu(111) peak decreases; after annealing at 550 �C, a very smallCu3Si peak appears. A small peak of Ru(0 0 2) can be observedwhich is due to the crystallization of the Ru film because there ishigher Ru content in the Ru3Ti1 film. This also causes the barrierfailure and the increase of the sheet resistance in Fig. 4. Our previ-ous work has shown that after annealing at 500 �C for 1 min. Ru2Si3

phase has already formed in the structure of Cu/Ru/Si with an in-crease of sheet resistance [10]. Adding Ti into Ru can successfullydelay Ru reaction with Si.

The AES depth profiles of the Cu/Ru3Ti1/Si samples beforeand after annealing are shown in Fig. 6. The Ti content is verylow in the Ru–Ti layer. In the as-deposited sample, the inter-faces between Cu/RuTi, RuTi/Si are both distinct. After annealingat 400 �C for 30 min, Cu has already diffused into the Ru–Tilayer; after annealing at 450 �C, more Cu diffuses into the Ru–Ti layer, also the oxygen content in the Ru–Ti layer increases.After annealing at 500 �C, Cu has diffused to the RuTi/Siinterface.

Fig. 7. SEM images of Cu surface of the Cu/Ru–Ti/Si sample after annealing at (a) 400 �C, (b) 450 �C, and (c) 500 �C, with the Ru:Ti ratio of 2:1; and (d) 400 �C, (e) 450 �C, and (f)500 �C, with the Ru:Ti ratio of 3:1.

J. Li et al. / Microelectronic Engineering 88 (2011) 635–640 639

The SEM results (Fig. 7) shows after annealing at 400 �C and450 �C, the Cu surfaces on the Ru3Ti1 are quite smooth, while se-vere oxidation can be observed on the Cu/Ru2Ti1/Si sample whichis maybe due to higher Ti content. No pinholes can been found inboth samples after annealing. The Ti alloying into Ru may blockthe Ru columnar grain boundaries and improve the barrier proper-ties. Chen et al. observed the improvement of barrier properties byadding Ta in the Ru layer [7]. Tagami et al. reported improved bar-rier properties by using the Ru/Ti bilayers [11]. Ti alloying into Ruwill make the Ru film amorphous and possibly impede the crystal-lization of the Ru, thus increase the thermal stability. The resultsalso show that the quantity of Ti is also important. While higherTi content may make the Ru more amorphous; the film with lessTi content can not impede the crystallization of Ru. After annealingat 500 �C, the density of the rectangular crystallites in the Cu/Ru3Ti1/Si system is higher than that on the Cu/Ru2Ti1/Si, whichis in accordance with the XRD results that the Cu peak intensityis low for the Cu/Ru3Ti1/Si sample annealed at 500 �C.

Fig. 9. The XRD patterns of the Cu/Ru–Ti–N/Si samples annealed a

Fig. 10. AES depth profile of the Cu/Ru3Ti1N/Si samples anne

3.3. Ru–Ti–N diffusion barrier

The sheet resistances of the Cu/Ru–Ti–N/Si samples with vol-ume ratio of Ru:Ti = 2 (named as Ru2Ti1N) and 3 (named as Ru3-Ti1N) after annealing at different temperatures are shown inFig. 8. From the results of Section 3.1, we choose Ar to N2 ratio of30:3. The Cu/Ru–Ti/Si samples were used as reference. The sheetresistances of the Cu/Ru–Ti–N/Si sample keep decreasing afterannealing at 400 �C and 450 �C. After annealing at 550 �C, the sheetresistance increased 19% for the system with Ru3Ti1N but de-creased by 3% for that with Ru2Ti1N, while the sheet resistanceof the Cu/Ru–Ti/Si structure increases significantly. This may becaused by the higher nitrogen absorbed by Ti during the PVDprocess, which impedes the crystallization of the film and the dif-fusion of Cu.

Fig. 9 shows the XRD patterns of the Cu/Ru–Ti–N/Si samplesafter annealing at different temperatures. No Ru or Ti peak couldbe observed, indicating the incorporation of N into Ru–Ti matrix

t different temperatures with different volume ratio of Ru:Ti.

aling at (a) as-dep, (b) 400 �C, (c) 450 �C and (d) 500 �C.

640 J. Li et al. / Microelectronic Engineering 88 (2011) 635–640

interferes the crystallization of the film. Similar to the XRD withRu2Ti1, after a annealing at 550 �C, the Ru2Si3 peak is also observedin the Cu/Ru2Ti1N/Si sample.

The AES depth profiles of the Cu/Ru–Ti–N/Si samples with vol-ume ratio of Ru:Ti = 3:1 before and after annealing are shown inFig. 10. The Ti content is very low in the Ru–Ti–N layer. In theas-deposited sample, the Cu/Ru3Ti1N/Si interfaces are distinct.After annealing at 400 �C for 30 min, it can be observed that thereis no diffusion of Cu into the Si layer in the Cu/Ru–Ti–N/Si structure.However, after annealing, Ru has diffused into the Si layer and theO content in the Ru–Ti–N layer is very high. The AES depth profileresults show that by incorporation with Ti and N, the Ru–Ti–N filmhas very good thermal stability. Pinholes can still be observed onthe Cu surface (not shown here) but the density is less than thaton the Cu with the Ru–N layer.

4. Conclusion

In summary, the barrier properties of the Ru–N and Ru–Ti andRu–Ti–N films were comparatively investigated by examiningproperties of the Cu/barrier/Si structures annealed at differenttemperatures. The Ru–N barrier has a good thermal stability butthere are a lot of pinholes on the Cu surface after annealing. TheRu–Ti film has effectively prevented the diffusion of Cu into Si afterannealing at 450 �C. The Ru–Ti–N barrier also exhibits good ther-mal stability. Comparing to the Ru–Ti alloy, both sheet resistanceand AES results show that the Cu film on the Ru–Ti–N layer re-mains stable after annealing at 550 �C, while Cu on the RuTi alloy

film has already diffused into Si and causes the sheet resistance in-crease abruptly. The results show that the Ru–Ti–N film can be ofgreat potential as a single diffusion barrier to Cu.

Acknowledgements

This work was supported by National Basic Research Program ofChina (No. 2006CB302703), State Key Research Program (No.2009ZX02308-005), and Science and Technology Committee ofShanghai Municipality (No. 08QH14002).

References

[1] L. Peters, Semicond. Int. 26 (2003) 50.[2] The International Technology Roadmap for Semiconductors. Available from:

<http://www.itrs.net/>.[3] O. Chyan, T.N. Arunagiri, T. Ponuswamy, J. Electrochem. Soc. 150 (2003) C347.[4] R. Chan, T.N. Arunagiri, Y. Zhang, O. Chyan, R.M. Wallace, M.J. Kim, T.Q. Hurd,

Electrochem. Solid-State Lett. 7 (2004) G154.[5] X.P. Qu, J.J. Tan, M. Zhou, T. Chen, Q. Xie, G.P. Ru, B.Z. Li, Appl. Phys. Lett. 88

(2006) 1.[6] S.-W. Kim, S.-H. Kwon, S.-J. Jeong, S.-W. Kang, J. Electrochem. Soc. 155 (2008)

H885.[7] C.-W. Chen, J.S. Chen, J.-S. Jeng, J. Electrochem. Soc. 155 (12) (2008) H1003.[8] S.-H. Kwon, O.-K. Kwon, J.-S. Min, S.-W. Kang, J. Electrochem. Soc. 153 (6)

(2006) G578–G581.[9] M. Damayanti, T. Sritharan, S.G. Mhaisalkar, Z.H. Gan, Appl. Phys. Lett. 88

(2006) 044101.[10] J.J. Tan, X.P. Qu, Q. Xie, Y. Zhao, G.P. Ru, B.Z. Li, Thin Solid Films 504 (2006) 231.[11] M. Tagami, N. Furutake, S. Saito, Y. Hayashi, In: Proceedings of the Eleventh

International Interconnect Technology Conference, IEEE, San Francisco, 2008,pp. 156.