Ta-N bi-layer

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 774–778

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

Fabrication and diffusion barrier properties of nanoscale Ta/Ta-N bi-layer Ji-cheng Zhou ∗ , You-zhen Li, Di-hui Huang School of Physics Science and Technology, Central South University, Changsha 410083, PR China

a r t i c l e

i n f o

a b s t r a c t

Article history:

One of the most important processes in Cu metallization for ultra large scale integrated cir-

Received 30 April 2007

cuits (ULSI) is to fabricate better diffusion barrier. In this paper, Ta/Ta-N films were fabricated

Received in revised form

by dc magnetron reactive sputtering (DCMS) in N2 /Ar ambient, then Cu/Ta/Ta-N/Si multi-

26 January 2008

structures were prepared in suite. The thin-film samples were rapid thermal annealed (RTA)

Accepted 23 February 2008

at variational temperatures in N2 ambient. Alpha-Step IQ Profiler, four-point probe (FPP) sheet resistance measurer, atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD) and tape test were used to characterize the microstructure

Keywords:

and diffusion properties of the thin-films. The results show that the nanoscale Ta/Ta-N thin-

Cu interconnection

films have smooth surface, and the thermal stability and barrier performance are good. After

Ta/Ta-N diffusion barrier

600 ◦ C/300 s RTA, Ta (40 nm)/Ta-N (60 nm) thin-films can effectively block against Cu diffu-

RTA

sion and keep good adhesion strength with Cu films. After higher temperature RTA process,

Failure mechanism

Cu atoms penetrated through the barrier and reacted with silicon, the barrier fail. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

With the character scale of ultra large scale integrated circuit (ULSI) keeping down, new generation metal interconnect systems are needed to replace traditional Al/SiO2 systems. Cu has much lower resistance (1.67 ␮ cm), better electron migration and stress migration stability than aluminum or aluminum alloy, So Cu interconnection technology has been widely studied during the last 20 years (Wang and Kang, 2002; Wang et al., 2004a,b). The main problem of Cu interconnection is the fast diffusion of Cu in Si, SiO2 and many kinds of dielectrics. Cu atoms can react with Si at 200 ◦ C to form Cu–Si compounds, thus decrease the reliability of integrated circuits and even cause to fail (Chen et al., 2001). So, a good diffusion barrier between Cu and Si is needed, the barrier should be effective to prevent Cu atoms diffusion and enhance the adhesion strength between Cu thin-films and substrate. For the reason of the barrier should increase the resistance of Cu intercon-

∗

Corresponding author. E-mail address: [email protected] (J.-c. Zhou). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.02.077

nection wire, the barrier should be as thin as possible and with lower resistance. Refractory metal and its compounds with N, C, Si doping were widely studied because they have high melting point, low resistance and good adhesion strength with Cu thin-films (Yin et al., 2001). Recently, some research groups focused the research on the Ta, Ta-N and ternary amorphous structures such as Ta–Si-N, Zr–Si-N (Hecker et al., 2002; Khin et al., 2001; Song et al., 2004; Yuan et al., 2003). The main problem to improve the barrier properties is to decrease the micro-cracks and grain boundaries. Metal barrier (such as Ta and Ti) has low resistance, good adhesion strength with Cu thin-films, but it will form many grain boundaries after high temperature annealing. During the deposition process, adding elements like “N, C or Si” into the metal barrier can decrease the grain boundaries and micro-cracks, while at the same time, it will cause the resistance increase and the adhesion strength with Cu thin-film decrease. Amorphous ternary barrier has better thermal stability, the failure temperature is

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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 774–778

Table 1 – Deposition parameters of the specimens Thin-films Ta Ta-N Ta/Ta-N Cu

Sputtering power (W) 100 100 100 100

N2 flow rate (sccm) 0 10 10 0

Film thickness (nm) 100 100 40/60 150

Deposition rate (nm/min) 30 55 30/55 40

Pressure (Pa) 0.9 0.9 0.9 0.9

mainly dominated by the crystallization temperature (Mukesh and Dinesh, 2005; Xie et al., 2006). But the amorphous ternary barrier resistance is relatively high. With the rapid development of Cu interconnection integrated circuits technology, to find good barrier material and to develop barrier fabrication technology become more and more important and urgent. In this paper, based on the advantages of Ta and Ta-N barrier, the Ta/Ta-N bi-layer was fabricated by DCMS. The microstructure and barrier performance were characterized, and the thin-film barrier failure mechanism was probed into.

2.

Experimental details

The p-type Si(1 1 1) wafers with nominal resistivity of 33–35  cm were used as the substrates. And they were RCA (the name from the Radio Corporation of American) cleaned before loaded into the vacuum chamber. Before the deposition process, the base pressure of the vacuum chamber was pumped less than 1 × 10−3 Pa. The substrates were roasted for 20 min at 200 ◦ C. And before deposition, the targets were sputtered by Argon ions to remove the target surface’s impurity and oxide. The substrates went round and round with the velocity of 5 rpm during the deposition process. The deposition conditions are as follows: target–substrate distance was 6 cm; Ta target purity was 99.99%, Cu target purity was 99.99%, both N2 and Argon gas purity were 99.99%; Ta-N thin-films were deposited by dc magnetron reactive sputtering at N2 /Ar ambient, Ta thin-films were deposited by dc magnetron sputtering, and the Cu thin-films were directly sputtered in suite to form Cu/Ta/Ta-N/Si structure; Cu/Ta/Si, Cu/Ta-N/Si and Cu/Si structures were also fabricated at the same experimental conditions to compare with the Ta/Ta-N bi-layer. The deposition processes parameters were summarized in Table 1. The thickness of the thin-films were measured by Alpha-step IQ profiler. The sheet resistance were measured by SDY-4D type fourpoint probe (the accuracy is 0.01 /). The specimens were rapid thermal annealed for 300 s in N2 atmosphere at different temperatures varied from 200 to 800 ◦ C. The surface morphology was observed by means of AFM and SEM. The surface root mean square (RMS) roughness was calculated by the AFM attachment software. The microstructure was analysed based on the D/MAX 2550 XRD patterns and energy dispersive spectrum (EDS).

3.

Results and discussion

In Table 1, the thickness of the thin-films were the mean value of five measured points, the deposition rate were determined by the thickness dividing the deposition time. Fig. 1 shows the effect of annealing temperature on the sheet resistance of

Fig. 1 – Sheet resistance as the function of annealing temperature.

the Cu/barrier/Si specimens. Since the Cu thin-films are much thicker than the barrier and have obviously lower resistivity than the barrier materials, so the sheet resistance is dominated by the Cu thin-films. As shown in Fig. 1, for as-deposited samples, the sheet resistance are very low. For the specimens after less than 600 ◦ C RTA, the sheet resistance just slightly decreased, this may be due to the reduction of crystal defects and grain growth in the Cu thin-films (Yuan et al., 2004), while for Cu/Si sample, the sheet resistance increased abruptly after 200 ◦ C RTA, which means that Cu had already reacted with silicon and formed Cu silicon compounds at this temperature. After RTA at 600 ◦ C, the sheet resistance of Cu/Ta/Si samples increased, while the other samples still keep stable. After RTA at 700 ◦ C, the sheet resistance of Cu/Ta-N/Si, Cu/Ta/Ta-N/Si multi-layer also increased, and the Cu/Ta/Si sample increased abruptly. When the RTA temperature was up to 800 ◦ C, the sheet resistance of all kinds of the samples increased abruptly, and the resistance of Cu/Ta-N/Si sample is much higher than that of Cu/Ta/Ta-N/Si sample. The abruptly increasing resistance means that the structure of Cu thin-films was destroyed, the composition has changed and Cu atoms might diffuse through the barrier and formed Cu3 Si (Chen et al., 2001; Yin et al., 2001). Obviously, the Ta/Ta-N barrier increased the diffusion react temperature to 700 ◦ C and from Fig. 1, we can conclude that the Ta/Ta-N bi-layer did a better job than single Ta or Ta-N barrier. Fig. 2 shows the surface morphology of as-deposited (Fig. 2a–c) and 400 ◦ C/300 s RTA (Fig. 2d–f) Ta, Ta-N and Ta/Ta-N specimens, the RMS roughness are also shown in the images. For as deposited specimens, the surface of Ta-N is smoother than that of Ta and Ta/Ta-N, and surface of Ta is the roughest.

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Fig. 2 – AFM images of as deposited and 400 ◦ C/300 s FA samples’ surface: Ta-N(a and d), Ta/Ta-N(b and e), and Ta(c and f).

This may be attributed to that the addition of “N” makes the Ta grain thin, and makes the Ta-N tends to be nano-crystalline or amorphous-like (Damayantia et al., 2006). So the surface of Ta-N is smoother than the others. And the direction-oriented growth of Ta caused the Ta surface rougher (Grace et al., 2005). For Ta/Ta-N samples, some of the additional N atoms in TaN layer diffused into Ta layer and thus caused the surface smooth. The barrier surface status must be considered seriously because it will directly influence the adhesion strength with Cu thin-films, thermal stability and barrier properties. After 400 ◦ C/300 s RTA, the surface particle size became larger, the surface all became rougher (Fig. 2d–f). The possibility of Cu diffusion increased. After 800 ◦ C/300 s RTA, the surface was all distorted, hillocks and voids can be seen by naked eyes, so the surface morphology were not observed by AFM.

Fig. 3 shows the surface morphology of as-deposited and thermal-treated Cu/Ta/Ta-N/Si specimens. For as-deposited samples, the surface is very fine and smooth (Fig. 3a); after 400 ◦ C/300 s RTA, the surface became rough for the grain growth (Fig. 3b); after 800 ◦ C/300 s RTA, the surface was destroyed, hillocks and holes were observed (Fig. 3c). In the diffusion and reaction procedure of Cu and silicon, Cu is more active. When Cu atoms diffuse through the barrier into silicon, Cu can react with silicon and form Cu–Si compounds (like Cu3 Si) at 200 ◦ C, thus remaining holes in the Cu film surface. At the same time, hillocks occurred in the surface because that silicon atom also diffused to the Cu surface and Cu–Si reaction caused the volume to increase (Kuo et al., 2003). A big hillock is shown in Fig. 3c and it was proved to be Cu3 Si by EDS analysis.

Fig. 3 – SEM images of Cu/Ta/Ta-N/Si surface (a) as deposited, (b) 400 ◦ C/300 s RTA and (c) 800 ◦ C/300 s RTA.

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Table 2 – Percentage of copper removal by Tape test for as deposited and rapid thermal treated samples Treated conditions

Percentage of copper removal Cu/Ta/Si

Cu/Ta-N/Si

8.2 6.5 18.2 42

12.7 10.2 26.1 48

As deposited 400 ◦ C/300 s RTA 600 ◦ C/300 s RTA 800 ◦ C/300 s RTA

Fig. 4 – XRD patterns of the Cu/Ta/Ta-N/Si multi-layer structure.

The XRD patterns of as-deposited and thermal-treated Cu/Ta/Ta-N/Si samples were shown in Fig. 4 (silicon(1 1 1) peaks were ignored). For as-deposited sample, only Cu diffraction peaks were observed, and with the RTA temperature increase, the intensity of Cu diffraction peaks increase. This indicated the Cu grain grown with the increase of annealing temperature. Fig. 4 also shows that for all the specimens, Cu(1 1 1) oriented diffraction peaks are much higher than the others. This means that Cu thin-films here all have a (1 1 1) oriented texture. The strongly (1 1 1) oriented texture can remarkably enhance the electro-migration stability of the Cu thin-films (Wang et al., 2004a,b). The structure of the Cu thinfilms is mainly responsible for the deposition techniques and the substrate. Obviously, in our experiments, all of the Cu thinfilms on the Ta/Ta-N bi-layer have (1 1 1) oriented texture. This indicated that Ta/Ta-N bi-layer is suitable to be the barrier for Cu interconnection. On the other side, Ta or Ta-N diffraction peaks are not observed in the as-deposited status thin-film samples, this may because that the Ta and Ta-N thin-films are very thin, but after RTA, the broad Ta peaks appeared, this means that Ta grain also grown after thermal annealing. When the RTA temperature was up to 700 ◦ C, Cu3 Si diffraction peak was observed, this indicated that Cu atoms have already diffused through the barrier and reacted with silicon. When the specimens were annealed at 800 ◦ C, seriously destroyed Cu films can be seen by naked eyes. And high intensity of Cu3 Si diffraction peak was observed by using XRD method. This means that the barrier has already failed, the color of Cu thin-film changed from golden into gray, defects just as black voids and hillocks appeared on the surface. Cu thin-films have poor adhesion strength with most dielectric materials. And in IC produce processes, the adhesion strength between Cu thin-films and barrier is a very important guide line to appraise the barrier properties. So the thin-film adhesion tape test were performed based on the standard “Permacel 99”, which was widely used and accepted as the method to measure the thin-film adhesion, and was introduced by Kim and Alford (2004). Table 2 shows the test results. Adhesion test

Cu/Ta/Ta-N/Si 8.8 7.1 18.1 39

is considered as failed, if more than 25% of the total area of the sample was removed. The removal area was measured by using an optical microscope. These results indicated that the Cu thin-films removal percentage for all as-deposited samples are below 15%, but after 400 ◦ C/300 s RTA, the results are below 10%. After 600 ◦ C/300 s RTA, the results of Ta and Ta/Ta-N samples are still below 25%, while Ta-N sample become greater than 40%. After 800 ◦ C/300 s RTA, for all the samples, the adhesion test failed, and some barrier peeled off with Cu thin-films. The fact confirmed that the adhesion of Ta with Cu thin-films was better than that of Ta-N’s. This maybe attributed to the rough surface of Ta thin-films offered better binding strength to Cu thin-films.

4.

Conclusions

1. The sheet resistance of Cu/Ta/Ta-N/Si is lower than Cu/TaN/Si system. 2. Ta/Ta-N bi-layer barrier undergoing 600 ◦ C/300 s RTA, with the thickness of 40 nm/60 nm, can keep stable and effective against Cu diffusion. After 800 ◦ C/300 s RTA, the barrier failed. 3. Ta/Ta-N bi-layer hold up the excellent advantages of Ta and Ta-N barrier, such as the lower resistance, better thermal stability and barrier properties. And at the same time, this multi-layer improved the adhesion with Cu thin-films. 4. The failure mechanism of Ta/Ta-N bi-layer is similar with Ta and Ta-N. After RTA processes up to 800 ◦ C/300 s, Cu atoms diffuse through the grain boundary of barrier and reacted with silicon to form Cu3 Si, voids and hillocks were found on the surface. 5. Ta/Ta-N bi-layer is benefit for the (1 1 1) oriented growth of Cu thin-films. And it is a good kind of barrier for Cu interconnection integrated circuits.

Acknowledgments The authors would like to thank the National Natural Science Foundation of China for the financial support of this work (60371046). Thanks a lot to Haibo Chen et al. for all kind of helps in film preparation and property test.

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