Atomic layer deposition of amorphous Ni-Ta-N films

Atomic layer deposition of amorphous Ni-Ta-N films for Cu diffusion barrier Yong-Ping Wang, Zi-Jun Ding, Bao Zhu, Wen-Jun Liu, David Wei Zhang, and Shi-Jin Ding

Citation: Journal of Vacuum Science & Technology A 36, 031502 (2018); doi: 10.1116/1.5002727 View online: https://doi.org/10.1116/1.5002727 View Table of Contents: http://avs.scitation.org/toc/jva/36/3 Published by the American Vacuum Society

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Atomic layer deposition of amorphous Ni-Ta-N films for Cu diffusion barrier Yong-Ping Wang, Zi-Jun Ding, Bao Zhu, Wen-Jun Liu, David Wei Zhang, and Shi-Jin Dinga) State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai 200433, China

(Received 1 September 2017; accepted 20 February 2018; published 6 March 2018) Novel Ni-doped TaN (Ni-Ta-N) films are deposited by remote plasma-enhanced atomic layer deposition (ALD) with pentakis(dimethylamino)tantalum, nickelocene, and NH3 precursors for Cu diffusion barriers. Various Ni-Ta-N films with different compositions are achieved by changing the deposition cycles (n) of Ni sublayer while fixing the deposition cycles of TaN sublayer at 2. As n increases from 1 to 6, the root-mean-square roughness of the deposited film increases from 0.150 to 0.527 nm, and the resistivity decreases from 0.18 to 1.1 102 X cm. After annealing at 400 C for 30 min in the forming gas (N2/H2), these films still maintain an amorphous texture and demonstrate a negligible reduction of resistivity and a weak increase of density. Subsequently, the barrier effects of the Ni-Ta-N films with different compositions are compared against Cu diffusion after annealing. The results reveal that the Ni-Ta-N films with n 4 exhibit barrier effects comparable with the ALD TaN film even after annealing at 550 C. Further, a 3 nm ultrathin Ni-Ta-N film with n ¼ 4, corresponding to an addition of 22 at. % Ni to TaN, cannot only reduce the film resistivity by 78% but also effectively block Cu diffusion after annealing at 450 C for 30 min. Published by the AVS. https://doi.org/10.1116/1.5002727

I. INTRODUCTION One of the most important issues in fabricating high-speed ultralarge scale integrated circuits is the propagation signal delay (i.e., a product of resistance and capacitance) of copper interconnect lines.1–3 Furthermore, copper diffusion is also one of the critical issues for Cu interconnects. Therefore, to prevent Cu atoms from diffusing into the intermetal dielectrics, a robust barrier is indispensable. For this purpose, various refractory metal and metal nitride films have been examined as Cu diffusion barriers, such as Ru,4 Ta,5 RuMn,4 RuCo,6 TaNi,7 TaN,5,8–10 TiN,10,11 WNxCy,12,13 NbN,14 RuTaN,15 and RuTiN.16 However, most of them have a higher resistivity (50–2 108lX cm) than Cu (1.72 lX cm), hence leading to a significant increase in interconnect resistance with the feature size scaling down.17,18 Therefore, it is expected that an ultrathin barrier less than 5 nm can be used in Cu interconnects because this can reduce the volume of diffusion barrier (normally with a relatively higher resistivity) in the whole interconnect lines.18–22 Among various diffusion barriers, tantalum nitride (TaN) has been widely used in CMOS integrated circuits due to its good barrier effect, thermally stable texture, and excellent adhesion to dielectrics.23,24 In modern semiconductor processing, TaN thin films are usually deposited by physical vapor deposition (PVD) technique, but the PVD technique still faces some challenges, for instance, difficulty of forming conformal films on holes owing to the line-of-sight approach.22,25,26 Furthermore, chemical vapor deposition (CVD) also has been studied to grow diffusion barriers in the Cu interconnect technology, but it requires a high deposition temperature above 450 C to produce a high-quality film with few impurities and low resistivity.27–30 This makes the a)

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031502-1 J. Vac. Sci. Technol. A 36(3), May/Jun 2018

CVD barriers incompatible with the Cu back end of line process. In recent years, it has been reported that atomic layer deposition (ALD) can guarantee depositions of ultrathin films with a precisely determined film thickness and excellent step coverage by sequential self-limiting reactions between the gas phase and substrate or film surface.21,31,32 In particular, most ALD films can be obtained at relatively low temperatures (350 C).17,33 To date, ALD TaN films have been reported by using various Ta precursors such as halide precursors [TaF5,25 TaCl5,21,30 and TaBr5 (Ref. 30)] and amide based precursors {Ta[N(CH3)(C2H5)]5,34 Ta[N(CH3)2]5,9 and Ta[N(C2H5)2]3[NC(CH3)3]19,32}. However, the halide precursors tend to produce corrosive hydrogen halide byproducts and also require a relatively high deposition temperature of above 400 C.9 On the other hand, Ta is in a þ5 oxidation state in all volatile Ta sources, such as TaCl5, Ta[N(CH3)2]5 and Ta[N(CH3)(C2H5)]5, so the ALD Ta-N film is usually in the form of nonconductive Ta3N5 (2 108 lX cm) instead of conductive TaN (Ta oxidation state ¼ þ3).35 This leads to high resistivity of 103–2 108 lX cm for most of the ALD Ta-N films.15,32,35 To reduce the resistivity of the Ta-N film, it is considered as a good strategy that a low resistivity metal is incorporated into the conventional Ta-N barrier, such as Ta1-xAlxNy,8 RuTaN,15 and Ta(Al)N(C).30 Our previous research indicates that the plasma-enhanced (PE) ALD Ni film has resistivity as low as 71 lX cm.36 Therefore, it is desired to explore Ni-doped TaN (Ni-Ta-N) films for Cu diffusion barrier from the viewpoints of science and technology. In this article, the Ni-Ta-N films with different compositions are prepared by PE-ALD, and the corresponding characteristics are characterized comprehensively. In particular, the barrier effects of 3 nm ultrathin Ni-Ta-N films are evaluated accurately by means of leakage current measurements.

0734-2101/2018/36(3)/031502/9/$30.00

Published by the AVS. 031502-1

031502-2 Wang et al.: ALD of amorphous Ni-Ta-N films

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FIG. 1. (Color online) (a) Schematic diagram of one growth period of Ni-Ta-N film, including 2 cycles of TaN and n cycles of Ni; (b) cross-section view and (c) top view of the Mo(50 nm)/Cu(50 nm)/barrier(3 nm)/SiO2(20 nm)/Si structure for I-V measurements.

II. EXPERIMENT P-type Si (100) wafers were cleaned using the standard Radio Corporation of America process and used as the starting substrates. The Ni-Ta-N films were deposited by PE-ALD at 250 C. Pentakis(dimethylamino)tantalum (PDMAT) and NH3 plasma were used as the reagents for ALD TaN, and nickelocene (NiCp2) and NH3 plasma were used as the reagents for ALD Ni. PDMAT was stored in a container at 120 C and NiCp2 was store in another container at 80 C. PDMAT and NiCp2 were transferred into the deposition chamber with N2 carrier gas. The flow rate of N2 was kept at 50 sccm, and the working pressure of the deposition chamber was 1200 Pa during the film growth. The flow rate of NH3 was kept at 130 sccm, and NH3 plasma was generated by a remote plasma generator under a power of 3000 W, acting as the reducing agent. The distance between the plasma shower head and the substrate is 70–80 cm. During the film deposition process, the pulse times of PDMAT, NiCp2 and NH3 plasma were kept at 2, 2, and 10 s, respectively. As shown in Fig. 1(a), the Ni-Ta-N films with different compositions were deposited by changing the deposition cycles (n) of Ni sublayer while fixing the deposition cycles of TaN sublayer at 2. To investigate the barrier effect of the Ni-Ta-N films, Cu(50 nm)/barrier(5 nm)/Si stacks with different barrier compositions were prepared and annealed at different temperatures for 30 min in the forming gas (N2/4%-H2), then the corresponding microstructures and sheet resistances were characterized. To further evaluate the barrier effects of various ultrathin (3 nm) Ni-Ta-N films, metal-oxide-semiconductor (MOS) capacitors of Mo(50 nm)/Cu(50 nm)/barrier(3 nm)/ SiO2(20 nm)/Si were fabricated using a lift-off process, shown in Figs. 1(b) and 1(c), and were annealed at different temperatures for 30 min in the forming gas. Subsequently, current–voltage (I–V) measurements were conducted on the as-fabricated and postannealed MOS capacitors. The thickness and density of the film were deduced by xray reflection, and the microstructure of the film was determined by grazing incidence x-ray diffraction (XRD) on a diffractometer (Bruker D8 Discover) with Cu Ka radiation. The elemental composition and chemical bonds of the film were analyzed by x-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra DLD). The surface morphology of the film was observed with atomic force microscopy (AFM) (Bruker Icon). The sheet resistance of the film was measured J. Vac. Sci. Technol. A, Vol. 36, No. 3, May/Jun 2018

by four-point-probe, and the film resistivity was calculated based on the film thickness and sheet resistance. The cross section high resolution transmission electron microscopy (TEM) image, high angle annular dark field (HAADF) scanning TEM image and energy dispersive X-ray (EDX) composition map of the sample were measured by TEM (FEI Talos F200X). The I–V measurements were performed on a semiconductor device analyzer (B1500A, Agilent Technologies). III. RESULTS AND DISCUSSION A. Characterization of the as-deposited films

To ascertain the elemental composition of the deposited film with different n, Fig. 2 illustrates an evolution of the XPS survey spectrum of the deposited film as a function of n. In the case of n ¼ 0, corresponding to the TaN film, the photoelectron peaks of Ta, C, N, and O elements are observed. When n ¼ 2, additional signals resulting from Ni are observed, such as Ni 2p, Ni 3p, etc. Further, with raising the value of n, the intensities of the photoelectron peaks associated with Ni increase gradually, meanwhile, the peak intensities of Ta 4f, Ta 4d and N 1s become weaker and weaker. This is due to the changes of the relative contents of Ni and TaN in the deposited films. In addition, the weak photoelectron peaks of O and C do not show discernible

FIG. 2. (Color online) Evolution of the XPS survey spectrum as a function of n for the as-deposited films with 6 min in situ Ar ion bombardment, which aims to remove the surface contamination.


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TABLE I. Elemental percentages of the deposited film as a function of n. Cycles of each Ni sublayer n¼0 n¼2 n¼4 n¼6

C (at. %)

O (at. %)

N (at. %)

Ta (at. %)

Ni (at. %)

6 9 10 11

7 9 8 7

41 36 31 27

46 35 29 25

0 11 22 30

changes. The aforementioned results indicate that the deposited films contain small quantities of C and O impurities. Table I lists the elemental percentages of the films corresponding to different n values. As n increases from 0 to 6, the relative atomic percentage of Ni increases gradually from 0% to 30% in the deposited films, and the relative contents of Ta and N decrease by 21% and 14%, respectively. Furthermore, the percentage of C exhibits a gradual and small rise with increasing n. This indicates the ALD Ni film contains more C impurity atoms than the ALD TaN film, and the detected C atoms should originate from the metal organic precursors of PDMAT and NiCp2. Moreover, the relative percentage of O shows a stable value (around 8%) in all the films. It is thus indicated that the incorporated O atoms might originate from the deposition system. It is worthwhile to mention that the existence of O degenerates the resistivity of the barrier film, but a small amount of O incorporated into a barrier matrix can improve the thermal stability and strengthen the barrier performance.7,37 Figure 3 shows high-resolution Ta 4f XPS spectra of the deposited films corresponding to different n. It is found that each Ta 4f spectrum can be well separated into three doublet-peaks using the Gaussian–Lorentzian function. The peaks centered at 23.8 6 0.2 and 23 6 0.1 eV should be

FIG. 4. (Color online) High-resolution Ni 2p3/2 XPS spectra of the deposited Ni-Ta-N films with different n.

FIG. 5. (Color online) High-resolution C 1s XPS spectra of the deposited films with different n.

FIG. 3. (Color online) High-resolution Ta 4f XPS spectra of the deposited films with different n. JVST A - Vacuum, Surfaces, and Films

FIG. 6. Extracted RMS roughness of the Ni-Ta-N film as a function of n.


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FIG. 7. (Color online) Representative AFM images of the deposited films (20–25 nm) with different compositions: (a) TaN; (b) Ni; (c) n ¼ 2; and (d) n ¼ 4.

attributed to Ta–N (Refs. 38–41) and Ta–C (Refs. 39 and 42) bonds, respectively. The peak located at 26 6 0.2 eV should be associated with Ta–O bond.43,44 Based on the peak area, the relative percentages of different components are calculated, not showing any discernable trend of variation. This is because the deposition process of TaN is changeless for different Ni-Ta-N films, thus resulting in similar chemical compositions in each TaN sublayer. High-resolution Ni 2p3/2 XPS spectra are shown in Fig. 4 for different Ni-Ta-N films. It is seen that the Ni 2p3/2 envelope can be easily deconvolved into three components, which are located at 852.7 6 0.1, 853.5 6 0.1 and 854.4 6 0.1 eV, respectively. The strong peak at the lowest binding energy should come from metallic Ni atoms (Ni0).36 The peak at the highest binding energy is attributed to the Ni–N bond,36 and the peak at 853.5 6 0.1 eV is ascribed to the Ni–C bond.36 In addition, the relative percentage of each component also shows a negligible variation.

FIG. 8. Cross-section TEM image of the Cu/Ni-Ta-N/Si sample with an interfacial layer of SiON. J. Vac. Sci. Technol. A, Vol. 36, No. 3, May/Jun 2018

FIG. 9. (Color online) XRD patterns of the deposited films (20–25 nm) with different n.

FIG. 10. (Color online) Dependences of the resistivity and density of the asdeposited film on n.


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FIG. 11. (Color online) Resistivity, density, and thickness reduction of the postannealed films with different n.

Figure 5 demonstrates high-resolution C 1s XPS spectra of the deposited films corresponding to different n. In terms of n ¼ 0 (i.e., the TaN film), the C1s spectrum shows a strong peak at 282.7 6 0.1 eV, which should originate from tantalum carbide (Ta–C).39,42 As n increases from 2 to 6, the peak at 282.7 6 0.1 eV becomes weaker and weaker, and more and more asymmetric. This should be attributed to the gradually enhanced contribution from the C–Ni bonds, which is usually located at around 283.2 eV.36 Furthermore, a wide peak can be observed in the region of 284–287 eV for the Ni-Ta-N films with n 2, whose relative intensity increases gradually with the increment of n. This could be ascribed to superposition of three components associated with C–O (286.3 eV),42,43 C–C (284.8 eV),36 and C–N (285.7 eV)36 bonds. The above analyses also indicate that the components of C–Ni, C–C, and C–N mainly originate from the Ni sublayers. Figure 6 shows the root-mean-square (RMS) roughness of the Ni-Ta-N film as a function of n, which is extracted by the AFM measurement. It is found that the RMS value rises from 0.15 to 0.527 nm with increasing n from 1 to 6. This indicates that the surface roughness of the Ni-Ta-N film increases gradually with the increment of incorporated Ni.

FIG. 12. (Color online) XRD patterns of the postannealed films with different n. JVST A - Vacuum, Surfaces, and Films

FIG. 13. (Color online) Sheet resistances of the Cu(50 nm)/barrier(5 nm)/ interfacial layer/Si stacks with different barrier compositions as a function of annealing temperature.

Figure 7 exhibits the representative AFM images of the films with different compositions. It is seen that the TaN film has a very smooth surface with a RMS of 0.146 nm; however, the Ni film has a much higher RMS of 0.721 nm, revealing

FIG. 14. (Color online) XRD patterns of the Cu/barrier(5 nm)/interfacial layer/Si stacks with different barrier compositions after 30 min annealing at (a) 450 C and (b) 500 C, respectively.


small hillocks on its surface. As a result, it is easy to infer that the surface smoothness of the Ni-Ta-N film exhibits gradual degeneration with increasing Ni content, as demonstrated in Figs. 7(c) and 7(d). Further, the cross-section TEM image of the Ni-Ta-N film with n ¼ 6 is shown in Fig. 8. It is found that the barrier layer is amorphous, and displays an approximately uniform matrix. This indicates that the growth mode of Ni sublayers is not layer-by-layer but in the form of islandslike, which should be ascribed to few reactive sites (i.e., nucleation sites) on the growth surface. In addition, an interfacial layer (i.e., SiON) can be observed between NiTa-N and Si substrate, which could result from the reaction between the substrate surface and NH3 plasma. Figure 9 shows the XRD patterns of the deposited films with different n. As n increases from 0 to 6, all the films demonstrate an amorphous texture. Such amorphous microstructure free of grain boundaries can eliminate dominant diffusion channels in the barrier layer at low temperatures. Therefore, the resulting amorphous microstructure can effectively block interdiffusion between different materials while preserving good conductivity offered by constitutive metals.7 Figure 10 shows dependence of the density and resistivity of the deposited film on n. As n increases from 0 to 6, the film density decreases gradually from 11.43 to 10.14 g/cm3, and the film resistivity reduces from 0.18 to 1.1 102 X cm. These should be attributed to an increase in the relative content of Ni, because Ni has a lower density of 8.26 g/cm3 and a lower resistivity of 71 lX cm compared with TaN. As an example, regarding n ¼ 4, the deposited Ni-Ta-N film exhibits a decrease of 78% in resistivity compared with the TaN film. In a word, the introduction of Ni into the TaN matrix

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cannot only reduce its resistivity but also maintain an amorphous microstructure. B. Characteristics of the postannealed films

To investigate the thermal stabilities of the deposited films, all the films are annealed at 400 C for 30 min. Figure 11 presents the resistivity, density, and thickness reduction of the postannealed film in the case of different n. The resistivity of the postannealed film decreases from 1.7 101 to 1.0 102 X cm with increasing n from 0 to 6. This reveals a small reduction in resistivity for the postannealed films in comparison with the as-deposited films. Moreover, as n increases from 0 to 6, the film density reduces from 11.68 to 10.54 g/cm3. That is, the postannealing leads to an increase of 0.3 g/cm3 in density. This is in good agreement with a decrease of about 1.8 nm in the postannealed film thickness, as indicated in Fig. 11. The XRD patterns of the postannealed films are shown in Fig. 12. It is found that all of the postannealed films still demonstrate an amorphous texture, revealing a good microstructure stability for the Ni-Ta-N films. C. Barrier effect of the Ni-Ta-N film against Cu diffusion

To investigate Ni content-dependent barrier effect against copper diffusion for various films, Fig. 13 shows dependence of sheet resistance on annealing temperature for the Cu(50 nm)/barrier(5 nm)/interfacial layer/Si stacks with different barrier compositions. For all the barriers (n ¼ 0–6), the sheet resistance remains at a low value of 1.6 X/ⵧ till 450 C annealing. When the annealing temperature increases

FIG. 15. (Color online) Cross-section HAADF STEM image and elemental maps of the Cu (50 nm)/Ni-Ta-N barrier (5 nm, n ¼ 6)/interfacial layer/Si sample: (a) as-prepared and (b) annealed at 500 C for 30 min. J. Vac. Sci. Technol. A, Vol. 36, No. 3, May/Jun 2018


to 500 C, the sheet resistance exhibits a sharp rise for the barriers with n ¼ 5 and 6. This indicates that the Ni-Ta-N films with n ¼ 5 and 6 fail against Cu diffusion after annealing at 500 C for 30 min. Furthermore, a slight decrease in sheet resistance can be observed for the barriers with n 4 after annealing at 500 C. This may be attributed to the grain growth of the Cu layer and the reduction of crystal defects during annealing.45 Further, when the annealing temperature increases up to 550 C, the Ni-Ta-N barriers with n 4 also fail, but demonstrate comparable performance with the TaN film. To understand the failure of barrier in relation to its sheet resistance, XRD measurements are carried out on the annealed stacks of Cu(50 nm)/barrier(5 nm)/interfacial layer/ Si. As shown in Fig. 14(a), regarding the stacks annealed at 450 C for 30 min, only Cu (111) and Cu (200) signals can be observed at 2h ¼ 43.5 (Refs. 46–48) and 50.4 .46–48 However, after annealing at 500 C for 30 min, as shown in Fig. 14(b), a new diffraction peak appears at 2h ¼ 45.3 for the barriers with n ¼ 5 and 6, which corresponds to the formation of Cu3Si.46,47 This reveals that Cu atoms diffuse through the barrier into the Si substrate and react therein. Meanwhile, it is also indicated that the barriers with n 4 have a better barrier effect against Cu diffusion than those with n ¼ 5 and 6 after 500 C annealing. The results are also in accord with the changes of sheet resistance in Fig. 13. To understand the failure mechanism of the Ni-Ta-N barrier with n ¼ 6, Fig. 15 shows the cross-section HAADF STEM images and EDX elemental maps of the as-prepared Cu (50 nm)/Ni-Ta-N barrier (5 nm)/interfacial layer/Si sample in comparison with those of the annealed sample at 500 C. For the as-prepared sample, sharp interfaces of Cu/Ni-Ta-N/SiON/Si can be clearly observed, as shown in Fig. 15(a). The maps of Si, Ta, N, Ni, and Cu have distinguishable and regular shapes. It should be noted that the sample was put on a Cu holder during our STEM measurement; therefore, the Cu signal from the back Cu holder could affect the map of Cu. This is why weak Cu signals can be observed in the Si and Ni-Ta-N regions in the map of Cu. After the annealing, the interfaces of Si/SiON/Ni-Ta-N/Cu become blurry and indiscernible, as shown in Fig. 15(b). In particular, the distributions of Si, Ni, and Ta broaden significantly. These results indicate that the annealing leads to interdiffusion at the interfaces of Si/Ni-Ta-N/Cu. Based on the maps of Si and Ni, it seems that Si and Ni atoms diffuse remarkably into the barrier and Cu layer, respectively. Moreover, Ni atoms exhibit more serious diffusion than Ta atoms according to the changes of Ta and Ni map widths. This should be related to their different chemical bonds, which are dominated by Ta-N and Ni-Ni in the barrier. The former has stronger bonding than the latter; thus, Ni atoms are easier to diffuse at a high temperature. Once Ni atoms diffuse into the Cu layer, vacancy defects are generated in the barrier, hence providing diffusion paths for Cu atoms through the barrier. In a word, to achieve a good barrier effect, the content of Ni in the TaN matrix should be optimized, and the spatial configuration of the barrier should be engineered carefully. JVST A - Vacuum, Surfaces, and Films

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In order to evaluate the functions of 3 nm ultrathin diffusion barriers with different compositions, I-V measurements are performed on the MOS capacitors of Mo/Cu/barrier/ SiO2/Si. Figure 16 depicts the distributions of breakdown fields and the I-V characteristics of the MOS capacitors annealed at 450 and 500 C, respectively. After annealing at 450 C, the barriers with n 4 generate breakdown electric fields higher than 12 MV/cm, which are larger than those of the barriers with n ¼ 5 and 6, as shown in Fig. 16(a). Moreover, the I-V curves of the capacitors also indicate that the Ni-Ta-N films with n ¼5 and 6 start to generate a significant increase in the leakage current at about 10 V in comparison with the Ni-Ta-N films with n 4, as shown in the inset of Fig. 16(a). For example, the former results in a leakage current of 1.1 109 A at 13 V, and the latter exhibits a small leakage current of 12 MV/cm, and the others result in relatively lower breakdown electric fields ranging from 10.3 to 12 MV/ cm, as shown in Fig. 16(b). This indicates that the Ni-Ta-N

FIG. 16. (Color online) distributions of breakdown fields and I-V curves of the MOS capacitors of Mo/Cu/barrier (3 nm)/SiO2/Si with different barrier compositions: (a) annealed at 450 C for 30 min; (b) annealed at 500 C for 30 min.


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TABLE II. Comparison between other reported barriers and our work.

Barrier

Resistivity (lX cm)

RuTiN — WNxCy 792.3 Ru 50 Ru-Mn 250 Ru-(TaN) 300–7 104 TaN — Ru-TaN (Ru:Ta ¼ 24) — TaN 1550 TaAlNC (0.66–6.4) 104 Ta-Ni-N (n ¼ 4) 4.4 104 Ta-Ni-N (n ¼ 4) 4.4 104

Thickness of barrier Highest working temperature Failure temperature (nm) ( C) ( C)

Annealing time

References