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TaN double layers for Cu metallization
Applied Surface Science 315 (2014) 353–359
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Diffusion barrier performance of novel Ti/TaN double layers for Cu metallization Y.M. Zhou ∗ , M.Z. He, Z. Xie School of Physics and Microelectronics Science, Hunan University, Hunan 410082, China
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Article history: Received 26 December 2013 Received in revised form 9 July 2014 Accepted 24 July 2014 Available online 1 August 2014 Keywords: Ti/TaN films Diffusion barrier Thermal stability Failure mechanism Copper metallization
a b s t r a c t Novel Ti/TaN double layers offering good stability as a barrier against Cu metallization have been made achievable by annealing in vacuum better than 1 × 10−3 Pa. Ti/TaN double layers were formed on SiO2 /Si substrates by DC magnetron sputtering and then the properties of Cu/Ti/TaN/SiO2 /Si film stacks were studied. It was found that the Ti/TaN double layers provide good diffusion barrier between Cu and SiO2 /Si up to 750 ◦ C for 30 min. The XRD, Auger and EDS results show that the Cu–Si compounds like Cu3 Si were formed by Cu diffusion through Ti/TaN barrier for the 800 ◦ C annealed samples. It seems that the improved diffusion barrier property of Cu/Ti/TaN/SiO2 /Si stack is due to the diffusion of nitrogen along the grain boundaries in Ti layer, which would decrease the defects in Ti film and block the diffusion path for Cu diffusion with increasing annealing temperature. The failure mechanism of Ti/TaN bi-layer is similar to the Cu/TaN/Si metallization system in which Cu atoms diffuse through the grain boundary of barrier and react with silicon to form Cu3 Si. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As device dimensions in integrated circuits (ICs) are scaled down, copper metallization has been received extensive attention as a very interesting potential candidate for aluminum because of its low resistivity, high electro-migration and stress-migration resistance, and high melting point [1–5]. However, the challenges of Cu integration are that Cu shows poor adhesion to oxide and most dielectric materials, rapidly diffuses into SiO2 and Si, and easily react with Si at a temperature as low as 200 ◦ C to form Cu–Si compounds [4,6–8]. Therefore, an appropriate barrier layer between Cu and SiO2 or Si is necessary in order to prevent the inter-diffusion between Cu and Si. Generally, a good barrier layer should have high thermal stability, low resistivity and fine adhesion with Cu and Si. Hence, many refractory metals and their nitrides, such as W and WN [9–11], Ti and TiN [7,11–14], Ta and TaN [3,5,6,15–17], have been chosen as potential diffusion barriers for Cu interconnects because of their excellent thermal stability and good electrical conductivity. Recently, the tantalum nitride (TaN) film has been widely studied in silicon devices as a diffusion barrier for Cu metallization due to its relative good barrier effect and stable structure [18–20]. A lot
∗ Corresponding author. Tel.: +86 731 88822892; fax: +86 731 88822858. E-mail addresses: [email protected] (Y.M. Zhou), [email protected] (M.Z. He), [email protected] (Z. Xie). http://dx.doi.org/10.1016/j.apsusc.2014.07.146 0169-4332/© 2014 Elsevier B.V. All rights reserved.
of investigations have conformed that the phase composition and microstructure of as-deposited Ta–N films depend strongly on the change in the N to Ta atomic ratio, which can be controlled by deposition process or annealing ambient [21–24]. With increasing the ratio value of N2 to the sputtering gas, the phase in sputtered tantalum film is usually transformed from metastable tetragonal -Ta to nanocrystalline bcc Ta(N), hexagonal Ta2 N and NaCl-structure TaN. The order of the effectiveness of these barriers according chemical inertness for copper metallization is TaN, Ta2 N, ␣-Ta and -Ta. Compared to Ta, although stoichiometric TaN shows excellent thermal stability with a high melting temperature up to 3090 ◦ C, unfortunately, TaN presents a high electrical resistivity and a poor adhesion with Cu thin film. With in-depth understanding of the properties of pure Ta and of pure TaN films, stacked layers such as Ta/TaN and TaN/Ta/TaN were recently designed as diffusion for Cu metallization. It was suggested that these stacked bi-layers prevent Cu to diffuse effectively up to a higher temperature compared with that of the corresponding monolayer. While the stacked Ta/TaN barriers have received in considerable interest in recent years because of their inherent properties, little is reported on microstructure and thermal stability of graded Ti/TaN diffusion barriers. We know that pure Ti film has gained much interest due to its potential applications in different areas of silicon device technology. It is the aim of this work to elucidate the properties for novel Ti/TaN bi-layer structure deposited on SiO2 , which have little report to the best of our knowledge.
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Fig. 1. the XRD patterns of: (a) TaN/SiO2 , (b) Ti/TaN/SiO2 and (c) Cu/Ti/TaN/SiO2 samples as-deposited and annealed at various temperatures.
2. Experimental details For the investigations of the stacked Cu/Ti/TaN/SiO2 diffusion barriers, thermally oxidized p-type Si (1 0 0) wafers, with a diameter of 100 mm and a resistivity of 6–9 cm, were utilized as substrates. Thermal oxidation was carried out at a temperature of 1000 ◦ C in an oxygen ambient to grow a 200 nm thick SiO2 film. Before being loaded into the sputtering vacuum chamber, these wafers were cut into rectangle with 5 mm × 10 mm and then standard Radio Corporation of America (RCA) cleaning was performed. Ti and Cu thin films were sputtered from a pure titanium and copper target under Ar, while TaN thin film was sputtered under Ar–N2 plasma. Titanium, copper and tantalum plates had a diameter of 50.8 mm, a thickness of 5 mm and a purity of 99.99%. The distance between the magnetron cathodes and the substrate holder was 80 mm. The base pressure of the sputtering chamber was kept at better than 4 × 10−4 Pa, while the working pressure was 0.7 Pa. During the TaN layer deposition, the N2 flow rate was kept 3.5 sccm and the remainder was supplied with pure argon gas to keep a constant total flow rate at 35 sccm. Both Argon and N2 gas purity were 99.9999%. The gas flows were adjusted by mass flow controllers. Before deposition, tantalum, titanium and copper targets were cleaned by pre-sputtering for 10 min one by one at an argon flow rate of 30 sccm, with a shutter above the sources, respectively. After deposition of the TaN layer with 50 nm thickness, a 30 nm thick titanium layer was subsequently sputtered onto the TaN thin film in pure Ar without breaking vacuum. In order to complete the metallization schemes, the Cu layer of about 200 nm was subsequently deposited onto the stacked Ti/TaN bi-layer without interrupting vacuum. The sputtering power of both TaN and Ti thin film was 100 W, while the sputtering power of Cu layer was 60W. During the deposition, the substrate temperature was kept at 200 ◦ C. The samples were then annealed at temperatures
ranging from 400 to 900 ◦ C in vacuum for 30 min, at a pressure better than 1 × 10−3 Pa. After annealed, the samples were cooled spontaneously to room temperature in vacuum. Cu/TaN/SiO2 /Si structure was fabricated at the same experimental conditions to compare with the Ti/TaN bi-layer. The phase and crystal structure of the samples was examined by X-ray diffraction (XRD) with a RIGAKUD/MAX2550VB + , operating at 40 kV and 30 mA with Cu K␣ radiation. The –2 scans range between 20◦ and 90◦ with a step size of 0.02◦ and a fixed dwelling time of 0.2 s. The morphology of the surface of stacked barriers was studied both with scanning electron microscopy (JEOL, JSM-6700F) equipped with energy dispersive spectroscopy (EDS) and atomic force microscopy (Russia, SolverP47-Pro) operating in non-contact mode. The chemical composition of the samples was detected by an energy dispersive spectrometer (OXFORD INCA). An Auger electron microscope was used for Auger Electron Spectroscopy (ULVAC, PHI700) to characterize composition, diffusion and depth distribution of atoms in the Cu/Ti/TaN/SiO2 samples before and after annealing. The sheet resistance of all samples before and after annealing was evaluated by SX-1934 digital type four-point probe instrument with 0.1 / accuracy. The leakage current measurements were performed by Keithley 4200 SCS for both TaN and Ti/TaN barriers. 3. Results and discussion Fig. 1a shows the XRD profiles of the as-deposited and 700–800 ◦ C-annealed TaN/SiO2 /Si samples. For the as-deposited and 700 ◦ C-annealed samples, no obvious TaN peaks can be observed, which demonstrates that the as-deposited and annealed TaN film at temperature as lower than 700 ◦ C are amorphous. The peaks belonging to TaN (1 1 1) and TaN (2 0 0) appear for the sample after annealed at 800 ◦ C, indicating the crystallization of the TaN film in the TaN/SiO2 /Si sample. Fig. 1b shows the XRD profiles
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Fig. 2. Sheet resistance of Cu/TaN/SiO2 /Si and Cu/Ti/TaN/SiO2 /Si as a function of annealing temperature.
of the Ti/TaN/SiO2 /Si samples before and after different temperatures annealing for 30 min. Since the as-deposited TaN thin film is amorphous, only Ti (1 1 0) diffraction peak can be observed, which indicates that the Ti thin-film here presents a (1 1 0) oriented texture. The diffraction patterns reveal that the structure has no obvious change after annealing at temperature up to 700 ◦ C, while peaks belonging to TaN and Ti (2 2 0) are found after 800 ◦ C. These observed indicated that the lattice discontinuity at the Ti/Ta–N interfaces may play an important role in reducing the diffusions. Fig. 1c shows the XRD spectra of as-deposited and thermal-treated Cu/Ti/TaN/SiO2 samples. For as-deposited sample, only one distinct Cu diffraction peak, with 2 position at 43.32◦ , is identified as Cu (1 1 1). After annealed at 600–700 ◦ C, the diffraction patterns reveal that the structure remained unchanged. However, the 750 ◦ C annealed sample shows the Cu3 Si(3 0 0) peak. The appearance of the Cu3 Si peaks, which due to the reaction in sample, demonstrates the failure of the barrier. The reaction in the Cu/Ti/TaN/SiO2 structure is induced by the diffusion of Cu into the Ti/TaN barrier layer and the simultaneous diffusion of Si onto the surface. The reaction of Cu with Si causes to the formation of Cu3 Si. When the annealing temperature was increased to 800 ◦ C, besides the major Cu and Cu3 Si (3 0 0) peaks, another two weak peaks of Cu3 Si (0 2 0) and (2 0 1) appear. In addition, the Ti or TaN diffraction peaks are not observed in the as-deposited or annealed Cu/Ti/TaN/SiO2 /Si stacked samples. Possible explanation is that Ti and TaN thin films are very thin. Another, the Cu grain growth can also be noticed. The intensity of the Cu (1 1 1) peak increased with increasing annealing temperature up to 700 ◦ C and remained level after annealing 800 ◦ C, as can be seen from the comparison of the Cu intensity peaks between the as-deposited and the annealed samples in Fig. 1c. The above results indicate that Cu thin films in our samples all have a (1 1 1) oriented texture. Since the Cu thin-films are much thicker than the barrier and have obviously lower resistivity than the barrier materials, the variation of Cu sheet resistance as a function of annealing temperature is commonly used to examine the capability of diffusion barrier against Cu diffusion. Fig. 2 shows the effect of annealing temperature on the sheet resistance of the Cu/Ti/TaN/SiO2 /Si and Cu/TaN/SiO2 /Si specimens. As shown in Fig. 2, the sheet resistances of as-deposited samples are very low. The sheet resistances of Cu deposited on the TaN and Ti/TaN layer are virtually decreased to 650 ◦ C and slightly increase after annealed 700 ◦ C. This may be due to the reduction of crystal defects and the Cu grain growth during annealing at these temperatures. After annealed at 750 ◦ C, the sheet resistance of Cu/TaN/SiO2 /Si sample increases abruptly,
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while the value of Cu/Ti/TaN/SiO2 /Si sample is still kept stable. Because the Cu layer has a lower resistivity than TaN monolayer or Ti/TaN bi-layer, the total electrical resistance is mainly dominated by the Cu layer in our samples. It is necessary to note that the sheet resistance of Cu/Ti/TaN/SiO2 /Si sample noticeably increased but still maintained a low value even after annealed at 850 ◦ C. The superior resistance stability of the Cu/Ti/TaN/SiO2 /Si sample upon annealing is attributed to the stable Cu morphology. It may be indicated that the obviously increase in the sheet resistance of the Cu/Ti/TaN/SiO2 /Si sample can be owed to the formation of Cu3 Si precipitates from XRD measurement (shown in Fig. 1) and the Cu agglomeration [6,23]. It is reported that diffusion is a primary factor that affects the sheet resistance after annealing [24]. For the metal barrier, some of metal atoms can diffuse into the Si or SiO2 substrates. Similarly, some atoms in the substrates can easily diffuse into the metal barrier to form resistive metal silicide with increasing annealing temperature. For the TaN layer, the nitrogen in the film mostly accommodate near the grain boundaries, which can reduce effectively intermixing. Therefore, the sheet resistance of Cu/TaN/SiO2 /Si sample has no obvious change even after being annealed at 700 ◦ C. The Cu/Ti/TaN/SiO2 /Si sample always provides a lower sheet resistance than the Cu/TaN/SiO2 /Si sample when the annealing temperature is higher than 700 ◦ C. These results indicate that the Ti/TaN bi-layer does a better job than single TaN barrier for the Cu metallization system from the views of resistivity, expected reliable electro-migration resistance, and thermal stability. SEM images are used to examine the morphological evolution of the sample surfaces after annealing. Fig. 3 compares the SEM results of Cu/TaN/SiO2 /Si stacks (Fig. 3a–c) and Cu/Ti/TaN/SiO2 /Si samples (Fig. 3d–f). For the Cu/TaN/SiO2 /Si sample annealed at 600 ◦ C, the surface is very fine and smooth, and a large number of “bright boundary” are observed, as shown in Fig. 3a. The possible explanation is that, as the recrystallization temperature range of pure Cu is 400–500 ◦ C, Cu grains grow at 600 ◦ C, resulting in the first agglomeration of Cu at the boundary. While after annealed at 700 ◦ C, the sample surface become relatively rough, some nanovoids and “bright spots” are observed in Fig. 3b. This may be caused by the formation of Cu grain clusters originating from thermal stress in the Cu thin films, which produces a synergistic effect to develop hillocks and holes in the samples. Another possible reason is that Cu atoms diffuse into the Si substrate along grain boundaries in the barrier layer, which promotes the appearance of hillocks and porosites. After annealing at 800 ◦ C, the sample surface appears very rough and splits owing to Cu agglomeration as shown in Fig. 3c, corresponding to an obvious increase in the sheet resistance as shown in Fig. 2. Simultaneously, the Cu/Ti/TaN/SiO2 /Si sample annealed at 700 ◦ C is continuous and there are no obvious grain boundaries, seen in Fig. 3d. After annealing at 800 ◦ C, many obvious boundaries and “bright spots” are also observed, and the surface also becomes relatively rough for the Cu grains grow in Fig. 3e. With increasing annealing temperature up to 850 ◦ C, the surface roughing is observed and the etch-pits appear which formed by Cu atoms diffuse into the Si surface, as shown in Fig. 3f. These phenomena indicate that the Cu atoms penetrate through the Ti/TaN bi-layer and react with Si substrate to form Cu3 Si. The results are content with the forgoing XRD analysis and the sheet resistance data. In addition, at the same annealing temperature, the surface of Cu/Ti/TaN/SiO2 /Si sample is less rough than that of Cu/TaN/SiO2 /Si sample. These results show that a Ti interlayer between TaN and Cu layer significantly improves the ability of TaN film as a diffusion barrier. To further explore any change in the surface chemistry due to annealing, chemical analysis of the surface for the annealed samples was carried out by energy dispersive spectroscopy (EDS). For comparison, Fig. 4a and b show the EDS analysis results for the top layers in the Cu/TaN/SiO2 /Si and Cu/Ti/TaN/SiO2 /Si barrier
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Fig. 3. SEM micrographs of surface of (a) Cu/TaN/SiO2 /Si annealed at 600 ◦ C, (b) Cu/TaN/SiO2 /Si annealed at 700 ◦ C, (c) Cu/TaN/SiO2 /Si annealed at 800 ◦ C, (d) Cu/Ti/TaN/SiO2 /Si annealed at 700 ◦ C, (e) Cu/Ti/TaN/SiO2 /Si annealed at 800 ◦ C, and (f) Cu/Ti/TaN/SiO2 /Si annealed at 850 ◦ C.
structures annealed at 750 ◦ C. Samples of the Cu/TaN/SiO2 /Si annealed up to 750 ◦ C show Si, O and Ta peaks with Cu peaks, indicating the interdiffusion of Cu and SiO2 or Si through TaN layers, which agrees with the sheet resistance analysis as show
in Fig. 2. Based on the EDS analyses in Fig. 4b, it is seen that the samples of the Cu/Ti/TaN/SiO2 /Si annealed at 750 ◦ C contains Si, Cu, O, Ti and Ta peaks, but the ratio of Si, O and Ta to Cu is much smaller than in Fig. 4a, which may indicate that a Ti/TaN bi-layer
Fig. 4. Energy dispersive spectrometer (EDS) spectra of (a) Cu/TaN/SiO2 /Si annealed at 750 ◦ C, (b) Cu/Ti/TaN/SiO2 /Si annealed at 750 ◦ C, and (c) Cu/Ti/TaN/SiO2 /Si annealed at 850 ◦ C.
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Fig. 5. AES depth profiles of the Cu/TaN/SiO2 /Si samples (a) as-deposited; (b) annealed at 700 ◦ C; (c) annealed at 750 ◦ C.
is superior to a single TaN layer to prevent diffusive intermixing of Cu and SiO2 /Si. In Fig. 4c, chemical analysis results of the etch-pits, observed in the SEM of the samples of Cu/Ti/TaN/SiO2 /Si annealed up to 850 ◦ C in Fig. 3f, also shows Si, Cu, O, Ti and Ta peaks.
Similarly, the ratio of Si, O, Ta and Ti to Cu is much larger than in Fig. 4b. These results show that the diffusion barrier performance of Ti/TaN bi-layer gets worse. The formation of the copper-silicide at Cu/Ti/Ta–N/SiO2 /Si stack proves again that the Ti/TaN barrier
Fig. 6. AES depth profiles of the Cu/Ti/TaN/SiO2 /Si samples (a) as-deposited; (b) annealed at 750 ◦ C; (c) annealed at 850 ◦ C.
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Fig. 7. The schematic diagrams of the leakage current measurement for: (a) TaN barrier sample, (b) Ti/TaN bi-layer barrier sample.
fails at 850 ◦ C. Based on these EDS results, it also can be confirmed that the Ti/TaN bi-layer serves as a better diffusion barrier against Cu attack on Si contacts than TaN single layer. In order to further characterize and compare the thermal stability of the TaN and Ti/TaN bi-layer barriers, the Cu/TaN/SiO2 /Si and Cu/Ti/TaN/SiO2 /Si contact samples for as-deposited and after annealing were carried out by the depth-profiling Auger electron spectroscopy (AES) measurement. It can be seen from Fig. 5a that there is a clear boundary between layers in the as-deposited Cu/TaN/SiO2 /Si stack and there is a uniform distribution in depth. There is almost no change of element composite annealed at 700 ◦ C, as shown in Fig. 5b, while remarkable profiles appear after annealing at 750 ◦ C. It can be seen from Fig. 5c that the depth profiles of the Cu/TaN/SiO2 /Si show not only the Cu diffusion across the TaN barrier but also the motion of Ta, N, and O towards the Cu surface. With the migration of many Cu atoms through the Ta–N barrier to the Ta–N/SiO2 interface, some Cu atoms diffuse into the SiO2 /Si
substrate. Meanwhile, the TaN barrier shifts to the top of the sample. However, the signal of the element Cu is stronger than that of the element Ta and N, which indicates that the amount of Cu is still larger than that of Ta–N at the top of the sample. This mutual diffusion implies that there are reactions occurring among Si, Ta, N, and Cu elements during annealing at 750 ◦ C. For comparison, AES depth profiles of the as-deposited and annealed Cu/Ti/TaN/SiO2 /Si samples are also shown in Fig. 6. For the as-deposited sample, the interfaces between layers are also abrupt as shown in Fig. 6a. Nearly no change are found in the profiles after annealing at 750 ◦ C, as shown in Fig. 6b. Cu and Si signals are not detected on the opposite sides, indicating no diffusion of Cu through Ti/TaN bi-layer to SiO2 /Si substrate. Comparing with Fig. 6a and b, for the Ti/TaN interface, the Ti intensity of the sample annealed at 750 ◦ C declines smoother than that of the as-deposited sample as the scan moves into TaN layer. Similarly, the Ta and N intensities of the latter also rise smoother than those of the former. This indicates
Fig. 8. I–V characteristics for TaN and Ti/TaN bi-layer barriers: (a) Cu/TaN/SiO2 /Si annealed at 600 and 700 ◦ C, (b) Cu/Ti/TaN/SiO2 /Si annealed at 600 and 700 ◦ C, (c) Cu/TaN/SiO2 /Si annealed at 800 ◦ C, and (d) Cu/Ti/TaN/SiO2 /Si annealed at 800 ◦ C.
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that the diffusion between Ti and TaN layer occurs after annealing at 750 ◦ C. Some of the additional N atoms in TaN layer diffuse into Ti layer and thus Ti would incorporate with N. AES depth profile shown in Fig. 6b demonstrates that a very limited diffusion of Cu through the Ti/TaN system, which is in good with the sheet resistances measurement and the EDS analysis. Only the Cu diffuses into the Ti layer and stops on the Ti/TaN interface. This phenomenon is attributed to the diffusion of nitrogen along the grain boundaries in Ti layer, which has reduced the defects in Ti barrier and blocked the diffusion path for Cu diffusion with increasing annealing temperature. Another, grain boundary mismatch between Ti and TaN would also increase the difficulty for Cu diffusion through Ti/TaN bi-layer structure. However, after annealing at 850 ◦ C, AES spectrum shown in Fig. 6c reveals a high degree of interdiffusion. The Cu atoms have penetrated through the Ti/TaN bi-layer perfectly, and the Si diffuses into the Ti and stops with the Ti/TaN bi-layer. These results observed by AES are rather consistent with those obtained from XRD results, four-probe measurements data, SEM and EDS analysis. To further determine the thermal stability, the leakage current measurements were done using Keithley 4200 SCS for both TaN and Ti/TaN barriers after annealing at 600, 700 and 800 ◦ C. To prepare the samples for leakage current measurements, an Au film dot was initially deposited on one border area of SiO2 /Si substrate to act as bottom electrode. The TaN, Ti and Cu layers were then deposited sequentially on the SiO2 /Si substrate, partially-covered the predeposited Au electrode. Fig. 7 shows the schematic diagram of the leakage current measurement. A voltage of −10 to 10 V (or −5 to 5 V) was applied to the TaN or Ti/TaN barrier, with the negative terminal connected to the copper electrode and the positive terminal applied to the pre-deposited Au electrode. The I–V curves were recorded to monitor the current passing through the TaN or Ti/TaN barrier (from pre-deposited Au electrode to the copper electrode). As shown in Fig. 8a and b, the electrons were obstructed by the barrier layer at 600 and 700 ◦ C. There is a large change in the characteristic of the samples after annealing at 800 ◦ C for both TaN single barrier and Ti/TaN bi-layer compared to the diode annealed at 600 and 700 ◦ C. When the temperature approached 800 ◦ C, the leakage current increased to 83.2 A at −5 V, reflecting the collapse of the TaN single barrier layer, seen in Fig. 8c. When we increased the temperature to 800 ◦ C, the leakage current 1.19 A at −5 V, suggesting that some of the copper atoms had diffused through the barrier in the Cu/Ti/TaN/SiO2 /Si diode, as seen in Fig. 8d. Therefore, the Ti/TaN bi-layer barrier exhibits better thermal stability than the TaN single layer barrier. These results observed by leakage current measurements are also consistent with those obtained from XRD results, four-probe measurements data, SEM and EDS analysis. From these observations, the Ti/TaN bi-layer structure shows superior diffusion barrier properties for copper, without Cu3 Si formation until annealing up to 800 ◦ C. The Ti/TaN bi-layer serves as a better diffusion barrier performance than the single Ta–N layer for silicon devices. 4. Conclusion In this work, sputtered TaN and Ti/TaN bi-layer have been investigated and compared as potential alternatives for the barrier layer
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of Cu metallization. The results show that the sheet resistance of Cu/Ti/TaN/SiO2 /Si is lower than that of Cu/TaN/SiO2 /Si system. The Ti/TaN bi-layer structure shows superior diffusion barrier properties for copper, without Cu3 Si formation until annealing up to 800 ◦ C. The failure mechanism of Ti/TaN bi-layer is similar with single TaN barrier. After annealing at 800 ◦ C, Cu atoms diffuse through the grain boundary of Ti/TaN bi-layer. Voids and hillocks are also found on the surface. The Ti/TaN bi-layer has already failed after annealing up to 850 ◦ C, while the single Ta–N barrier does not work after annealing at 750 ◦ C. The incorporation of nitrogen atoms of TaN layer into Ti film is shown to be very beneficial improving the thermal stability of the Ti/TaN bi-layer between Cu and SiO2 /Si substrate. The diffusion of nitrogen along the grain boundaries in Ti layer would reduce the defects in Ti film and block the diffusion path for Cu diffusion with increasing annealing temperature. Grain boundary mismatch between Ti and TaN would also increase the difficulty for Cu diffusion through Ti/TaN bi-layer structure. Acknowledgements This project is sponsored by the young teachers’ growth plan of Hunan University and the Fundamental Research Funds for the Central Universities of China under Grant No. 531107040232 and No. 53110704334. References [1] P. Majumder, C.G. Takoudis, Appl. Phys. Lett. 91 (2007) 1621081–1621083. [2] S. Rawal, D.P. Norton, K.C. Kim, T.J. Anderson, L.M. White, Appl. Phys. Lett. 89 (2006) 2319141–2319143. [3] H. Wang, A. Tiwari, A. Kvit, X. Zhang, J. Narayan, Appl. Phys. Lett. 80 (2002) 2323–2325. [4] S. Li, H.S. Park, M.H. Liang, T.H. Yip, O. Prabhakar, Thin Solid Films 462–463 (2004) 192–196. [5] T. Laurila, K. Zeng, J.K. Kivilahti, J. Molarius, I. Suni, J. Appl. Phys. 88 (2000) 3377–3384. [6] M.H. Tsai, S.C. Sun, C.E. Tsai, S.H. Chuang, H.T. Chiu, J. Appl. Phys. 79 (1996) 6932–6938. [7] J. Baumann, T. Werner, A. Ehrlich, M. Rennau, Ch. Kaufmann, T. Gessner, Microelectron. Eng. 37–38 (1997) 221–228. [8] Z.H. Cao, K. Hu, X.K. Meng, J. Appl. Phys. 106 (2009) 1135131–1135135. [9] Y.H. Song, J.H. Son, B.J. Kim, H.K. Yu, C.J. Yoo, J.L. Lee, Appl. Phys. Lett. 99 (2011) 2335021–2335023. [10] B.S. Suh, H.K. Cho, Y.J. Lee, W.J. Lee, C.O. Park, J. Appl. Phys. 89 (2001) 4128–4133. [11] H. Kizil, C. Steinbruchel, Thin Solid Films 449 (2004) 158–165. [12] H.C. Chen, B.H. Tseng, M.P. Houng, Y.H. Wang, Thin Solid Films 445 (2003) 112–117. [13] A.R. Phani, J.E. Krzanowski, Appl. Surf. Sci. 174 (2001) 132–137. [14] V. Lingwal, N.S. Panwar, J. Appl. Phys. 97 (2005) 1049021–1049028. [15] L. Chen, N. Magtoto, B. Ekstron, J. Kelber, Thin Solid Films 376 (2000) 115–123. [16] C. Zhao, Zs. Tokei, A. Haider, S. Demuynck, Microelectron. Eng. 84 (2007) 2669–2674. [17] Y.Y. Wu, A. Kohn, M. Eizenberg, J. Appl. Phys. 95 (2004) 6167–6174. [18] P. Violet, E. Blanquet, O.L. Bacq, Microelectron. Eng. 83 (2006) 2077–2081. [19] H. Kim, C. Lavoie, M. Copel, V. Narayanan, D.-G. Park, S.M. Rossnagel, J. Appl. Phys. 95 (2004) 5848–5855. [20] M. Hecker, D. Fischer, V. Hoffmann, H.-J. Engelmann, A. Voss, N. Mattern, C. Wenzel, C. Vogt, E. Zschech, Thin Solid Films 414 (2002) 184–191. [21] T. Riekkinen, J. Molarius, T. Laurila, A. Nurmela, I. Suni, J.K. Kiviahti, Microelectron. Eng. 64 (2002) 289–297. [22] H.C. Chung, C.P. Liu, Surf. Coat. Technol. 200 (2006) 3122–3126. [23] T. Laurila, K. Zeng, J.K. Kivilahti, J. Molarius, I. Suni, Thin Solid Films 373 (2000) 64–67. [24] Z.W. Yang, D.H. Zhang, C.Y. Li, C.M. Tan, K. Prasad, Thin Solid Films 462–463 (2004) 288–291.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Diffusion barrier performance of novel Ti/TaN double layers for Cu metallization Y.M. Zhou ∗ , M.Z. He, Z. Xie School of Physics and Microelectronics Science, Hunan University, Hunan 410082, China
a r t i c l e
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Article history: Received 26 December 2013 Received in revised form 9 July 2014 Accepted 24 July 2014 Available online 1 August 2014 Keywords: Ti/TaN films Diffusion barrier Thermal stability Failure mechanism Copper metallization
a b s t r a c t Novel Ti/TaN double layers offering good stability as a barrier against Cu metallization have been made achievable by annealing in vacuum better than 1 × 10−3 Pa. Ti/TaN double layers were formed on SiO2 /Si substrates by DC magnetron sputtering and then the properties of Cu/Ti/TaN/SiO2 /Si film stacks were studied. It was found that the Ti/TaN double layers provide good diffusion barrier between Cu and SiO2 /Si up to 750 ◦ C for 30 min. The XRD, Auger and EDS results show that the Cu–Si compounds like Cu3 Si were formed by Cu diffusion through Ti/TaN barrier for the 800 ◦ C annealed samples. It seems that the improved diffusion barrier property of Cu/Ti/TaN/SiO2 /Si stack is due to the diffusion of nitrogen along the grain boundaries in Ti layer, which would decrease the defects in Ti film and block the diffusion path for Cu diffusion with increasing annealing temperature. The failure mechanism of Ti/TaN bi-layer is similar to the Cu/TaN/Si metallization system in which Cu atoms diffuse through the grain boundary of barrier and react with silicon to form Cu3 Si. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As device dimensions in integrated circuits (ICs) are scaled down, copper metallization has been received extensive attention as a very interesting potential candidate for aluminum because of its low resistivity, high electro-migration and stress-migration resistance, and high melting point [1–5]. However, the challenges of Cu integration are that Cu shows poor adhesion to oxide and most dielectric materials, rapidly diffuses into SiO2 and Si, and easily react with Si at a temperature as low as 200 ◦ C to form Cu–Si compounds [4,6–8]. Therefore, an appropriate barrier layer between Cu and SiO2 or Si is necessary in order to prevent the inter-diffusion between Cu and Si. Generally, a good barrier layer should have high thermal stability, low resistivity and fine adhesion with Cu and Si. Hence, many refractory metals and their nitrides, such as W and WN [9–11], Ti and TiN [7,11–14], Ta and TaN [3,5,6,15–17], have been chosen as potential diffusion barriers for Cu interconnects because of their excellent thermal stability and good electrical conductivity. Recently, the tantalum nitride (TaN) film has been widely studied in silicon devices as a diffusion barrier for Cu metallization due to its relative good barrier effect and stable structure [18–20]. A lot
∗ Corresponding author. Tel.: +86 731 88822892; fax: +86 731 88822858. E-mail addresses: [email protected] (Y.M. Zhou), [email protected] (M.Z. He), [email protected] (Z. Xie). http://dx.doi.org/10.1016/j.apsusc.2014.07.146 0169-4332/© 2014 Elsevier B.V. All rights reserved.
of investigations have conformed that the phase composition and microstructure of as-deposited Ta–N films depend strongly on the change in the N to Ta atomic ratio, which can be controlled by deposition process or annealing ambient [21–24]. With increasing the ratio value of N2 to the sputtering gas, the phase in sputtered tantalum film is usually transformed from metastable tetragonal -Ta to nanocrystalline bcc Ta(N), hexagonal Ta2 N and NaCl-structure TaN. The order of the effectiveness of these barriers according chemical inertness for copper metallization is TaN, Ta2 N, ␣-Ta and -Ta. Compared to Ta, although stoichiometric TaN shows excellent thermal stability with a high melting temperature up to 3090 ◦ C, unfortunately, TaN presents a high electrical resistivity and a poor adhesion with Cu thin film. With in-depth understanding of the properties of pure Ta and of pure TaN films, stacked layers such as Ta/TaN and TaN/Ta/TaN were recently designed as diffusion for Cu metallization. It was suggested that these stacked bi-layers prevent Cu to diffuse effectively up to a higher temperature compared with that of the corresponding monolayer. While the stacked Ta/TaN barriers have received in considerable interest in recent years because of their inherent properties, little is reported on microstructure and thermal stability of graded Ti/TaN diffusion barriers. We know that pure Ti film has gained much interest due to its potential applications in different areas of silicon device technology. It is the aim of this work to elucidate the properties for novel Ti/TaN bi-layer structure deposited on SiO2 , which have little report to the best of our knowledge.
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Fig. 1. the XRD patterns of: (a) TaN/SiO2 , (b) Ti/TaN/SiO2 and (c) Cu/Ti/TaN/SiO2 samples as-deposited and annealed at various temperatures.
2. Experimental details For the investigations of the stacked Cu/Ti/TaN/SiO2 diffusion barriers, thermally oxidized p-type Si (1 0 0) wafers, with a diameter of 100 mm and a resistivity of 6–9 cm, were utilized as substrates. Thermal oxidation was carried out at a temperature of 1000 ◦ C in an oxygen ambient to grow a 200 nm thick SiO2 film. Before being loaded into the sputtering vacuum chamber, these wafers were cut into rectangle with 5 mm × 10 mm and then standard Radio Corporation of America (RCA) cleaning was performed. Ti and Cu thin films were sputtered from a pure titanium and copper target under Ar, while TaN thin film was sputtered under Ar–N2 plasma. Titanium, copper and tantalum plates had a diameter of 50.8 mm, a thickness of 5 mm and a purity of 99.99%. The distance between the magnetron cathodes and the substrate holder was 80 mm. The base pressure of the sputtering chamber was kept at better than 4 × 10−4 Pa, while the working pressure was 0.7 Pa. During the TaN layer deposition, the N2 flow rate was kept 3.5 sccm and the remainder was supplied with pure argon gas to keep a constant total flow rate at 35 sccm. Both Argon and N2 gas purity were 99.9999%. The gas flows were adjusted by mass flow controllers. Before deposition, tantalum, titanium and copper targets were cleaned by pre-sputtering for 10 min one by one at an argon flow rate of 30 sccm, with a shutter above the sources, respectively. After deposition of the TaN layer with 50 nm thickness, a 30 nm thick titanium layer was subsequently sputtered onto the TaN thin film in pure Ar without breaking vacuum. In order to complete the metallization schemes, the Cu layer of about 200 nm was subsequently deposited onto the stacked Ti/TaN bi-layer without interrupting vacuum. The sputtering power of both TaN and Ti thin film was 100 W, while the sputtering power of Cu layer was 60W. During the deposition, the substrate temperature was kept at 200 ◦ C. The samples were then annealed at temperatures
ranging from 400 to 900 ◦ C in vacuum for 30 min, at a pressure better than 1 × 10−3 Pa. After annealed, the samples were cooled spontaneously to room temperature in vacuum. Cu/TaN/SiO2 /Si structure was fabricated at the same experimental conditions to compare with the Ti/TaN bi-layer. The phase and crystal structure of the samples was examined by X-ray diffraction (XRD) with a RIGAKUD/MAX2550VB + , operating at 40 kV and 30 mA with Cu K␣ radiation. The –2 scans range between 20◦ and 90◦ with a step size of 0.02◦ and a fixed dwelling time of 0.2 s. The morphology of the surface of stacked barriers was studied both with scanning electron microscopy (JEOL, JSM-6700F) equipped with energy dispersive spectroscopy (EDS) and atomic force microscopy (Russia, SolverP47-Pro) operating in non-contact mode. The chemical composition of the samples was detected by an energy dispersive spectrometer (OXFORD INCA). An Auger electron microscope was used for Auger Electron Spectroscopy (ULVAC, PHI700) to characterize composition, diffusion and depth distribution of atoms in the Cu/Ti/TaN/SiO2 samples before and after annealing. The sheet resistance of all samples before and after annealing was evaluated by SX-1934 digital type four-point probe instrument with 0.1 / accuracy. The leakage current measurements were performed by Keithley 4200 SCS for both TaN and Ti/TaN barriers. 3. Results and discussion Fig. 1a shows the XRD profiles of the as-deposited and 700–800 ◦ C-annealed TaN/SiO2 /Si samples. For the as-deposited and 700 ◦ C-annealed samples, no obvious TaN peaks can be observed, which demonstrates that the as-deposited and annealed TaN film at temperature as lower than 700 ◦ C are amorphous. The peaks belonging to TaN (1 1 1) and TaN (2 0 0) appear for the sample after annealed at 800 ◦ C, indicating the crystallization of the TaN film in the TaN/SiO2 /Si sample. Fig. 1b shows the XRD profiles
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Fig. 2. Sheet resistance of Cu/TaN/SiO2 /Si and Cu/Ti/TaN/SiO2 /Si as a function of annealing temperature.
of the Ti/TaN/SiO2 /Si samples before and after different temperatures annealing for 30 min. Since the as-deposited TaN thin film is amorphous, only Ti (1 1 0) diffraction peak can be observed, which indicates that the Ti thin-film here presents a (1 1 0) oriented texture. The diffraction patterns reveal that the structure has no obvious change after annealing at temperature up to 700 ◦ C, while peaks belonging to TaN and Ti (2 2 0) are found after 800 ◦ C. These observed indicated that the lattice discontinuity at the Ti/Ta–N interfaces may play an important role in reducing the diffusions. Fig. 1c shows the XRD spectra of as-deposited and thermal-treated Cu/Ti/TaN/SiO2 samples. For as-deposited sample, only one distinct Cu diffraction peak, with 2 position at 43.32◦ , is identified as Cu (1 1 1). After annealed at 600–700 ◦ C, the diffraction patterns reveal that the structure remained unchanged. However, the 750 ◦ C annealed sample shows the Cu3 Si(3 0 0) peak. The appearance of the Cu3 Si peaks, which due to the reaction in sample, demonstrates the failure of the barrier. The reaction in the Cu/Ti/TaN/SiO2 structure is induced by the diffusion of Cu into the Ti/TaN barrier layer and the simultaneous diffusion of Si onto the surface. The reaction of Cu with Si causes to the formation of Cu3 Si. When the annealing temperature was increased to 800 ◦ C, besides the major Cu and Cu3 Si (3 0 0) peaks, another two weak peaks of Cu3 Si (0 2 0) and (2 0 1) appear. In addition, the Ti or TaN diffraction peaks are not observed in the as-deposited or annealed Cu/Ti/TaN/SiO2 /Si stacked samples. Possible explanation is that Ti and TaN thin films are very thin. Another, the Cu grain growth can also be noticed. The intensity of the Cu (1 1 1) peak increased with increasing annealing temperature up to 700 ◦ C and remained level after annealing 800 ◦ C, as can be seen from the comparison of the Cu intensity peaks between the as-deposited and the annealed samples in Fig. 1c. The above results indicate that Cu thin films in our samples all have a (1 1 1) oriented texture. Since the Cu thin-films are much thicker than the barrier and have obviously lower resistivity than the barrier materials, the variation of Cu sheet resistance as a function of annealing temperature is commonly used to examine the capability of diffusion barrier against Cu diffusion. Fig. 2 shows the effect of annealing temperature on the sheet resistance of the Cu/Ti/TaN/SiO2 /Si and Cu/TaN/SiO2 /Si specimens. As shown in Fig. 2, the sheet resistances of as-deposited samples are very low. The sheet resistances of Cu deposited on the TaN and Ti/TaN layer are virtually decreased to 650 ◦ C and slightly increase after annealed 700 ◦ C. This may be due to the reduction of crystal defects and the Cu grain growth during annealing at these temperatures. After annealed at 750 ◦ C, the sheet resistance of Cu/TaN/SiO2 /Si sample increases abruptly,
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while the value of Cu/Ti/TaN/SiO2 /Si sample is still kept stable. Because the Cu layer has a lower resistivity than TaN monolayer or Ti/TaN bi-layer, the total electrical resistance is mainly dominated by the Cu layer in our samples. It is necessary to note that the sheet resistance of Cu/Ti/TaN/SiO2 /Si sample noticeably increased but still maintained a low value even after annealed at 850 ◦ C. The superior resistance stability of the Cu/Ti/TaN/SiO2 /Si sample upon annealing is attributed to the stable Cu morphology. It may be indicated that the obviously increase in the sheet resistance of the Cu/Ti/TaN/SiO2 /Si sample can be owed to the formation of Cu3 Si precipitates from XRD measurement (shown in Fig. 1) and the Cu agglomeration [6,23]. It is reported that diffusion is a primary factor that affects the sheet resistance after annealing [24]. For the metal barrier, some of metal atoms can diffuse into the Si or SiO2 substrates. Similarly, some atoms in the substrates can easily diffuse into the metal barrier to form resistive metal silicide with increasing annealing temperature. For the TaN layer, the nitrogen in the film mostly accommodate near the grain boundaries, which can reduce effectively intermixing. Therefore, the sheet resistance of Cu/TaN/SiO2 /Si sample has no obvious change even after being annealed at 700 ◦ C. The Cu/Ti/TaN/SiO2 /Si sample always provides a lower sheet resistance than the Cu/TaN/SiO2 /Si sample when the annealing temperature is higher than 700 ◦ C. These results indicate that the Ti/TaN bi-layer does a better job than single TaN barrier for the Cu metallization system from the views of resistivity, expected reliable electro-migration resistance, and thermal stability. SEM images are used to examine the morphological evolution of the sample surfaces after annealing. Fig. 3 compares the SEM results of Cu/TaN/SiO2 /Si stacks (Fig. 3a–c) and Cu/Ti/TaN/SiO2 /Si samples (Fig. 3d–f). For the Cu/TaN/SiO2 /Si sample annealed at 600 ◦ C, the surface is very fine and smooth, and a large number of “bright boundary” are observed, as shown in Fig. 3a. The possible explanation is that, as the recrystallization temperature range of pure Cu is 400–500 ◦ C, Cu grains grow at 600 ◦ C, resulting in the first agglomeration of Cu at the boundary. While after annealed at 700 ◦ C, the sample surface become relatively rough, some nanovoids and “bright spots” are observed in Fig. 3b. This may be caused by the formation of Cu grain clusters originating from thermal stress in the Cu thin films, which produces a synergistic effect to develop hillocks and holes in the samples. Another possible reason is that Cu atoms diffuse into the Si substrate along grain boundaries in the barrier layer, which promotes the appearance of hillocks and porosites. After annealing at 800 ◦ C, the sample surface appears very rough and splits owing to Cu agglomeration as shown in Fig. 3c, corresponding to an obvious increase in the sheet resistance as shown in Fig. 2. Simultaneously, the Cu/Ti/TaN/SiO2 /Si sample annealed at 700 ◦ C is continuous and there are no obvious grain boundaries, seen in Fig. 3d. After annealing at 800 ◦ C, many obvious boundaries and “bright spots” are also observed, and the surface also becomes relatively rough for the Cu grains grow in Fig. 3e. With increasing annealing temperature up to 850 ◦ C, the surface roughing is observed and the etch-pits appear which formed by Cu atoms diffuse into the Si surface, as shown in Fig. 3f. These phenomena indicate that the Cu atoms penetrate through the Ti/TaN bi-layer and react with Si substrate to form Cu3 Si. The results are content with the forgoing XRD analysis and the sheet resistance data. In addition, at the same annealing temperature, the surface of Cu/Ti/TaN/SiO2 /Si sample is less rough than that of Cu/TaN/SiO2 /Si sample. These results show that a Ti interlayer between TaN and Cu layer significantly improves the ability of TaN film as a diffusion barrier. To further explore any change in the surface chemistry due to annealing, chemical analysis of the surface for the annealed samples was carried out by energy dispersive spectroscopy (EDS). For comparison, Fig. 4a and b show the EDS analysis results for the top layers in the Cu/TaN/SiO2 /Si and Cu/Ti/TaN/SiO2 /Si barrier
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Fig. 3. SEM micrographs of surface of (a) Cu/TaN/SiO2 /Si annealed at 600 ◦ C, (b) Cu/TaN/SiO2 /Si annealed at 700 ◦ C, (c) Cu/TaN/SiO2 /Si annealed at 800 ◦ C, (d) Cu/Ti/TaN/SiO2 /Si annealed at 700 ◦ C, (e) Cu/Ti/TaN/SiO2 /Si annealed at 800 ◦ C, and (f) Cu/Ti/TaN/SiO2 /Si annealed at 850 ◦ C.
structures annealed at 750 ◦ C. Samples of the Cu/TaN/SiO2 /Si annealed up to 750 ◦ C show Si, O and Ta peaks with Cu peaks, indicating the interdiffusion of Cu and SiO2 or Si through TaN layers, which agrees with the sheet resistance analysis as show
in Fig. 2. Based on the EDS analyses in Fig. 4b, it is seen that the samples of the Cu/Ti/TaN/SiO2 /Si annealed at 750 ◦ C contains Si, Cu, O, Ti and Ta peaks, but the ratio of Si, O and Ta to Cu is much smaller than in Fig. 4a, which may indicate that a Ti/TaN bi-layer
Fig. 4. Energy dispersive spectrometer (EDS) spectra of (a) Cu/TaN/SiO2 /Si annealed at 750 ◦ C, (b) Cu/Ti/TaN/SiO2 /Si annealed at 750 ◦ C, and (c) Cu/Ti/TaN/SiO2 /Si annealed at 850 ◦ C.
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Fig. 5. AES depth profiles of the Cu/TaN/SiO2 /Si samples (a) as-deposited; (b) annealed at 700 ◦ C; (c) annealed at 750 ◦ C.
is superior to a single TaN layer to prevent diffusive intermixing of Cu and SiO2 /Si. In Fig. 4c, chemical analysis results of the etch-pits, observed in the SEM of the samples of Cu/Ti/TaN/SiO2 /Si annealed up to 850 ◦ C in Fig. 3f, also shows Si, Cu, O, Ti and Ta peaks.
Similarly, the ratio of Si, O, Ta and Ti to Cu is much larger than in Fig. 4b. These results show that the diffusion barrier performance of Ti/TaN bi-layer gets worse. The formation of the copper-silicide at Cu/Ti/Ta–N/SiO2 /Si stack proves again that the Ti/TaN barrier
Fig. 6. AES depth profiles of the Cu/Ti/TaN/SiO2 /Si samples (a) as-deposited; (b) annealed at 750 ◦ C; (c) annealed at 850 ◦ C.
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Fig. 7. The schematic diagrams of the leakage current measurement for: (a) TaN barrier sample, (b) Ti/TaN bi-layer barrier sample.
fails at 850 ◦ C. Based on these EDS results, it also can be confirmed that the Ti/TaN bi-layer serves as a better diffusion barrier against Cu attack on Si contacts than TaN single layer. In order to further characterize and compare the thermal stability of the TaN and Ti/TaN bi-layer barriers, the Cu/TaN/SiO2 /Si and Cu/Ti/TaN/SiO2 /Si contact samples for as-deposited and after annealing were carried out by the depth-profiling Auger electron spectroscopy (AES) measurement. It can be seen from Fig. 5a that there is a clear boundary between layers in the as-deposited Cu/TaN/SiO2 /Si stack and there is a uniform distribution in depth. There is almost no change of element composite annealed at 700 ◦ C, as shown in Fig. 5b, while remarkable profiles appear after annealing at 750 ◦ C. It can be seen from Fig. 5c that the depth profiles of the Cu/TaN/SiO2 /Si show not only the Cu diffusion across the TaN barrier but also the motion of Ta, N, and O towards the Cu surface. With the migration of many Cu atoms through the Ta–N barrier to the Ta–N/SiO2 interface, some Cu atoms diffuse into the SiO2 /Si
substrate. Meanwhile, the TaN barrier shifts to the top of the sample. However, the signal of the element Cu is stronger than that of the element Ta and N, which indicates that the amount of Cu is still larger than that of Ta–N at the top of the sample. This mutual diffusion implies that there are reactions occurring among Si, Ta, N, and Cu elements during annealing at 750 ◦ C. For comparison, AES depth profiles of the as-deposited and annealed Cu/Ti/TaN/SiO2 /Si samples are also shown in Fig. 6. For the as-deposited sample, the interfaces between layers are also abrupt as shown in Fig. 6a. Nearly no change are found in the profiles after annealing at 750 ◦ C, as shown in Fig. 6b. Cu and Si signals are not detected on the opposite sides, indicating no diffusion of Cu through Ti/TaN bi-layer to SiO2 /Si substrate. Comparing with Fig. 6a and b, for the Ti/TaN interface, the Ti intensity of the sample annealed at 750 ◦ C declines smoother than that of the as-deposited sample as the scan moves into TaN layer. Similarly, the Ta and N intensities of the latter also rise smoother than those of the former. This indicates
Fig. 8. I–V characteristics for TaN and Ti/TaN bi-layer barriers: (a) Cu/TaN/SiO2 /Si annealed at 600 and 700 ◦ C, (b) Cu/Ti/TaN/SiO2 /Si annealed at 600 and 700 ◦ C, (c) Cu/TaN/SiO2 /Si annealed at 800 ◦ C, and (d) Cu/Ti/TaN/SiO2 /Si annealed at 800 ◦ C.
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that the diffusion between Ti and TaN layer occurs after annealing at 750 ◦ C. Some of the additional N atoms in TaN layer diffuse into Ti layer and thus Ti would incorporate with N. AES depth profile shown in Fig. 6b demonstrates that a very limited diffusion of Cu through the Ti/TaN system, which is in good with the sheet resistances measurement and the EDS analysis. Only the Cu diffuses into the Ti layer and stops on the Ti/TaN interface. This phenomenon is attributed to the diffusion of nitrogen along the grain boundaries in Ti layer, which has reduced the defects in Ti barrier and blocked the diffusion path for Cu diffusion with increasing annealing temperature. Another, grain boundary mismatch between Ti and TaN would also increase the difficulty for Cu diffusion through Ti/TaN bi-layer structure. However, after annealing at 850 ◦ C, AES spectrum shown in Fig. 6c reveals a high degree of interdiffusion. The Cu atoms have penetrated through the Ti/TaN bi-layer perfectly, and the Si diffuses into the Ti and stops with the Ti/TaN bi-layer. These results observed by AES are rather consistent with those obtained from XRD results, four-probe measurements data, SEM and EDS analysis. To further determine the thermal stability, the leakage current measurements were done using Keithley 4200 SCS for both TaN and Ti/TaN barriers after annealing at 600, 700 and 800 ◦ C. To prepare the samples for leakage current measurements, an Au film dot was initially deposited on one border area of SiO2 /Si substrate to act as bottom electrode. The TaN, Ti and Cu layers were then deposited sequentially on the SiO2 /Si substrate, partially-covered the predeposited Au electrode. Fig. 7 shows the schematic diagram of the leakage current measurement. A voltage of −10 to 10 V (or −5 to 5 V) was applied to the TaN or Ti/TaN barrier, with the negative terminal connected to the copper electrode and the positive terminal applied to the pre-deposited Au electrode. The I–V curves were recorded to monitor the current passing through the TaN or Ti/TaN barrier (from pre-deposited Au electrode to the copper electrode). As shown in Fig. 8a and b, the electrons were obstructed by the barrier layer at 600 and 700 ◦ C. There is a large change in the characteristic of the samples after annealing at 800 ◦ C for both TaN single barrier and Ti/TaN bi-layer compared to the diode annealed at 600 and 700 ◦ C. When the temperature approached 800 ◦ C, the leakage current increased to 83.2 A at −5 V, reflecting the collapse of the TaN single barrier layer, seen in Fig. 8c. When we increased the temperature to 800 ◦ C, the leakage current 1.19 A at −5 V, suggesting that some of the copper atoms had diffused through the barrier in the Cu/Ti/TaN/SiO2 /Si diode, as seen in Fig. 8d. Therefore, the Ti/TaN bi-layer barrier exhibits better thermal stability than the TaN single layer barrier. These results observed by leakage current measurements are also consistent with those obtained from XRD results, four-probe measurements data, SEM and EDS analysis. From these observations, the Ti/TaN bi-layer structure shows superior diffusion barrier properties for copper, without Cu3 Si formation until annealing up to 800 ◦ C. The Ti/TaN bi-layer serves as a better diffusion barrier performance than the single Ta–N layer for silicon devices. 4. Conclusion In this work, sputtered TaN and Ti/TaN bi-layer have been investigated and compared as potential alternatives for the barrier layer
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of Cu metallization. The results show that the sheet resistance of Cu/Ti/TaN/SiO2 /Si is lower than that of Cu/TaN/SiO2 /Si system. The Ti/TaN bi-layer structure shows superior diffusion barrier properties for copper, without Cu3 Si formation until annealing up to 800 ◦ C. The failure mechanism of Ti/TaN bi-layer is similar with single TaN barrier. After annealing at 800 ◦ C, Cu atoms diffuse through the grain boundary of Ti/TaN bi-layer. Voids and hillocks are also found on the surface. The Ti/TaN bi-layer has already failed after annealing up to 850 ◦ C, while the single Ta–N barrier does not work after annealing at 750 ◦ C. The incorporation of nitrogen atoms of TaN layer into Ti film is shown to be very beneficial improving the thermal stability of the Ti/TaN bi-layer between Cu and SiO2 /Si substrate. The diffusion of nitrogen along the grain boundaries in Ti layer would reduce the defects in Ti film and block the diffusion path for Cu diffusion with increasing annealing temperature. Grain boundary mismatch between Ti and TaN would also increase the difficulty for Cu diffusion through Ti/TaN bi-layer structure. Acknowledgements This project is sponsored by the young teachers’ growth plan of Hunan University and the Fundamental Research Funds for the Central Universities of China under Grant No. 531107040232 and No. 53110704334. References [1] P. Majumder, C.G. Takoudis, Appl. Phys. Lett. 91 (2007) 1621081–1621083. [2] S. Rawal, D.P. Norton, K.C. Kim, T.J. Anderson, L.M. White, Appl. Phys. Lett. 89 (2006) 2319141–2319143. [3] H. Wang, A. Tiwari, A. Kvit, X. Zhang, J. Narayan, Appl. Phys. Lett. 80 (2002) 2323–2325. [4] S. Li, H.S. Park, M.H. Liang, T.H. Yip, O. Prabhakar, Thin Solid Films 462–463 (2004) 192–196. [5] T. Laurila, K. Zeng, J.K. Kivilahti, J. Molarius, I. Suni, J. Appl. Phys. 88 (2000) 3377–3384. [6] M.H. Tsai, S.C. Sun, C.E. Tsai, S.H. Chuang, H.T. Chiu, J. Appl. Phys. 79 (1996) 6932–6938. [7] J. Baumann, T. Werner, A. Ehrlich, M. Rennau, Ch. Kaufmann, T. Gessner, Microelectron. Eng. 37–38 (1997) 221–228. [8] Z.H. Cao, K. Hu, X.K. Meng, J. Appl. Phys. 106 (2009) 1135131–1135135. [9] Y.H. Song, J.H. Son, B.J. Kim, H.K. Yu, C.J. Yoo, J.L. Lee, Appl. Phys. Lett. 99 (2011) 2335021–2335023. [10] B.S. Suh, H.K. Cho, Y.J. Lee, W.J. Lee, C.O. Park, J. Appl. Phys. 89 (2001) 4128–4133. [11] H. Kizil, C. Steinbruchel, Thin Solid Films 449 (2004) 158–165. [12] H.C. Chen, B.H. Tseng, M.P. Houng, Y.H. Wang, Thin Solid Films 445 (2003) 112–117. [13] A.R. Phani, J.E. Krzanowski, Appl. Surf. Sci. 174 (2001) 132–137. [14] V. Lingwal, N.S. Panwar, J. Appl. Phys. 97 (2005) 1049021–1049028. [15] L. Chen, N. Magtoto, B. Ekstron, J. Kelber, Thin Solid Films 376 (2000) 115–123. [16] C. Zhao, Zs. Tokei, A. Haider, S. Demuynck, Microelectron. Eng. 84 (2007) 2669–2674. [17] Y.Y. Wu, A. Kohn, M. Eizenberg, J. Appl. Phys. 95 (2004) 6167–6174. [18] P. Violet, E. Blanquet, O.L. Bacq, Microelectron. Eng. 83 (2006) 2077–2081. [19] H. Kim, C. Lavoie, M. Copel, V. Narayanan, D.-G. Park, S.M. Rossnagel, J. Appl. Phys. 95 (2004) 5848–5855. [20] M. Hecker, D. Fischer, V. Hoffmann, H.-J. Engelmann, A. Voss, N. Mattern, C. Wenzel, C. Vogt, E. Zschech, Thin Solid Films 414 (2002) 184–191. [21] T. Riekkinen, J. Molarius, T. Laurila, A. Nurmela, I. Suni, J.K. Kiviahti, Microelectron. Eng. 64 (2002) 289–297. [22] H.C. Chung, C.P. Liu, Surf. Coat. Technol. 200 (2006) 3122–3126. [23] T. Laurila, K. Zeng, J.K. Kivilahti, J. Molarius, I. Suni, Thin Solid Films 373 (2000) 64–67. [24] Z.W. Yang, D.H. Zhang, C.Y. Li, C.M. Tan, K. Prasad, Thin Solid Films 462–463 (2004) 288–291.