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Thermal stability of RuZr alloy thin films as the diffusion barrier in Cu metallization
Journal of Alloys and Compounds 588 (2014) 461–464
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Thermal stability of RuZr alloy thin films as the diffusion barrier in Cu metallization Y. Meng a, Z.X. Song a, D. Qian a, W.J. Dai a, J.F. Wang c, F. Ma a,⇑, Y.H. Li a,⇑, K.W. Xu a,b a
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China Department of Physics and Opt-Electronic Engineering, Xi’an University of Arts and Science, Xi’an 710065, Shaanxi, China c Linyi University, Linyi 276000, Shandong, China b
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 1 September 2013 Accepted 14 November 2013 Available online 22 November 2013 Keywords: RuZr alloy Amorphous film Thermal stability Cu interconnect
a b s t r a c t Thin films of RuZr/Cu stacking were deposited on Si substrates by magnetron sputtering technology. The as-deposited RuZr thin films were amorphous, while Cu thin films were polycrystalline with (1 1 1) preferred orientation. The films were then annealed at given temperatures to evaluate the thermal stability. It was demonstrated that the amorphous state could be maintained up to 450 °C and, the inter-diffusion between Cu and Si atoms was effectively suppressed. However, the atom diffusion became significant at higher temperatures and resulted in the formation of high-resistance Cu3Si phase. So RuZr amorphous alloy thin films can be readily used for Cu metallization, but the working temperature should be not higher than 450 °C. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of very-large-scale integrated (VLSI) circuits, the application of Cu metallization becomes more and more popular due to its low resistivity, high electro-migration and excellent conductivity [1–3]. However, the inter-diffusion between Cu and Si atoms is significant and they will react to produce high-resistance phase of Cu3Si, even at low temperature [4,5]. This usually leads to the degradation and even failure of the electronic devices [5]. Therefore, it is indispensable to add a thin barrier layer between Cu interconnects and Si substrates to suppress the interdiffusion between Cu and Si atoms, to improve the stability and reliability of the electronic devices. The diffusion barrier materials should usually have low resistivity, high thermal stability, good adhesion with Cu and the dielectric layer. Most importantly, it should effectively block the unwanted atomic diffusion. It has been extensively reported that refractory metals such as Ta, Ti, Zr and Ru can be used as a perfect barrier in Cu metallization, in particular Ta/TaN barrier system has been put into device applications [6]. According to the requirements of the International Technology Roadmap for Semiconductors (ITRS) [7], the barrier layers should become thinner and thinner [8]. Thus, more research works are needed to exploit new barrier layers to meet the requirement during device minimization.
⇑ Corresponding authors. E-mail addresses: [email protected] (F. Ma), [email protected] (Y.H. Li). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.085
Ruthenium (Ru), as a transition noble metal, has a low bulk electrical resistivity of 7.1 lX cm [6–9] and can adhere well with Cu film. The solubility of Ru in Cu is negligible [10]. However, Ru itself has poor barrier performance because of its intrinsic polycrystalline structure in which the grain boundaries provide fast diffusion paths [11]. Specifically, the 5 nm Ru layer fail to block the diffusion of Cu atoms only at 300 °C [12]. Recently, many research works have been done to improve the barrier performance of Ru against Cu diffusion [12–17]. Since the grain boundaries are the main path, it is mandatory to eliminate them in order to suppress the atomic diffusion. Singe-crystal and amorphous thin films are suitable for that purpose because of the absence of grain boundaries, but the former is high cost, so amorphous thin films are preferred. The introduction of a small amount of a second element in Ru thin films will disturb the crystal structure, suppress the crystallization, and thus improve the diffusion barrier performance. Ru–Zr system has been receiving more and more concerns in this field [18–22]. The solid solubility of Zr in Ru is only about 2% [18], and the compositions of Zr (at.%) required for the formation of two intermetallic compounds, Ru2Zr and RuZr, are 33% and 48% [19]. If the Zr composition is in the intermediate range, no intermetallic compound will be formed and, the crystallization of Ru–Zr thin films will be suppressed, which is expected for highperformance diffusion barriers. In this paper, Ru–Zr alloy thin films were fabricated and annealed at different temperatures to explore the thermal stability.
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Cu(111) Cu3 Si Cu(200)
Intensity/a.u.
500°C 450°C 400°C 350°C As-deposited
35
40
45
50
55
60
65
2θ/deg. Fig. 1. XRD patterns of the Cu/RuZr dual layered thin films, as-deposited and annealed at various temperatures.
water. RuZr thin films 40 nm in thickness were firstly deposited on Si substrates by magnetron co-sputtering in argon atmosphere at room temperature using Ru target (U75 mm 5 mm, 99.999% purity) and Zr target (U75 mm 5 mm, 99.999% purity) with a power ratio of Ru:Zr targets is 1:3 for the best thermal stability and barrier performance. The Ar gas flow rate was set to 20 sccm. Cu thin films were then deposited on the RuZr layer using the copper target (U75 mm 5 mm, 99.999% purity) without breaking vacuum. The Cu/RuZr/Si samples were annealed at a given different temperatures from 350 °C to 500 °C for half an hour in N2/H2 mixture ambient (N2:H2 = 9:1). Phase composition and crystalline structure of the thin films were determined by the X-ray diffraction (XRD) measurement using Cu Ka radiation, and the in-plane grain sizes in Cu thin films were evaluated according to Scherrer’s equation. The sheet resistance was measured by the four-point probe (FPP) technology. The high-resolution transmission electron microscopy (HRTEM, JEM-2100F) with EDS was employed to characterize the cross-section morphology as well as the evolution of the microstructures of the film system. The TEM sample was prepared in the following steps: the thin films on Si substrates were cut into square pieces 2 mm 2 mm in size, then glued together face to face, followed by thermal annealing in an incubator at 120 °C for 2 h. After that, the samples were mechanically grinded from the cross-section direction using multi-granularity sandpapers until the cross-section thickness was reduced down to several micrometers. Finally, ion polishing was done to further reduce the thickness in the center region down to about 50 nm so that the electron beam can penetrate.
Table 1 The grain size (unit in nm) of Cu thin films.
Cu(1 1 1) Cu(2 0 0)
As-deposited
350 °C
400 °C
450 °C
19.67 17.41
54.66 54.19
52.71 58.34
52.69 56.16
2. Experiment The p-type (1 0 0) Si wafer was used as the substrates. Prior to deposition, the substrates were first cleaned by ultrasonic technology with alcohol and acetone for 10 min, respectively, to remove organic contaminants. Then the wafers were dipped in 10% HF for 5 min to clear the oxides and finally rinsed in de-ionized
3. Results and discussion The atomic concentration of Zr in the Ru–Zr thin films is determined by EDS data. It is about 20%, just in the intermediate range stated above. Hence, no intermetallic compounds can be formed and the Ru–Zr thin films should be amorphous. Fig. 1 presents the XRD patterns of Cu/RuZr dual-layer stacked thin films as deposited as well as those annealed at the temperatures from 350 °C to 500 °C. It is obvious that the Cu film layer is polycrystalline even for the as-deposited samples, while there is no diffraction peak of RuZr alloy, indicating the amorphous nature even upon
Fig. 2. Cross-sectional TEM images of Cu/RuZr thin films annealed at 450 °C for 30 min: (a) bright-field image, (b) high-resolution image of the local region in Cu thin film, (c) high-resolution image of the local region in RuZr barrier layer with the diffraction pattern in the inset and (d) EDS line scanning across the film interface.
Y. Meng et al. / Journal of Alloys and Compounds 588 (2014) 461–464
annealing. The two diffraction peaks at 43.3° and 50.3° correspond to the (1 1 1) and (2 0 0) planes of face-centered-cubic copper [23,24]. These two peaks become sharper after thermal annealing, indicating the increased grain size, as evaluated according to Scherrer’s equation and listed in Table 1. The radius of Zr (0.16 nm) is bigger than Cu (0.1278 nm) by 25%. If Zr atoms are diffused into the Cu lattice, the lattice constant should be increased considerably, and the corresponding XRD peaks will shift to small-angle values. However, the XRD patterns in this work indicate a slight shift towards large-angle direction, for an example, the 2h of Cu (1 1 1) plane is increased slightly from 43.3° of the as-deposited thin film to 43.4° of the one annealed at 400 °C for half an hour. Hence, Zr atoms do not dissolved into the lattice of Cu, but, if any, should be randomly distributed in the grain boundaries of Cu thin films. It is worth noting that the high-resistance phase of Cu3Si appears when the Cu/RuZr dual-layer stacked thin films were annealed at 500 °C, as indicated in the XRD pattern. At the same time, the intensity of Cu (1 1 1) and Cu (2 0 0) peaks is reduced considerably. It implies that the reaction occurred between the Cu cap and the underlying Si substrate. In another word, the RuZr alloy diffusion barrier is invalid at 500 °C. As an example, Fig. 2 displays the cross-sectional TEM micrograph of the Cu/RuZr dual layered thin films annealed at 450 °C. As shown in the bright-field image in Fig. 2(a), the grains in Cu thin films exhibit columnar morphology and the RuZr diffusion barrier layer keeps distinct interfaces with the Cu capping layer and Si substrate upon annealing at 450 °C for half an hour. The glue between Cu and the barrier comes from the preparation process of TEM sample. Fig. 2(b) shows the high-resolution image of the local region in the Cu film as marked by the black square. Fig. 2(c) shows the high-resolution image of the local region in the RuZr layer as
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marked by the black square, the corresponding nano-beam diffraction pattern is displayed in the inset of the figure. Both of the disordered atomic configuration in high-resolution image and the halo ring of the diffraction pattern corroborate the amorphous nature of ZrRu thin films even after thermal annealing at 450 °C for half an hour. This is consistent with the XRD result. As mentioned above, there is no effective diffusion path in the amorphous films, which should suppress the atom diffusion significantly. Fig. 2(d) presents the line scanning of the element distribution cross the film interface. The Cu and Si components change sharply at the film interface. As expected, the inter-diffusion is effectively suppressed. Fig. 3 shows the cross-sectional TEM images of the Cu/RuZr dual-layered thin films annealed at 500 °C for half an hour. As displayed in the bright-field image in Fig. 3(a), the layer interface disappears as a result of significant diffusion of Cu and Si atoms. It is further confirmed by the scanning image of Si, Ru and Cu elements, as shown in Fig. 3(b–d). The element components are obtained through the point spectrum of the local region in the bright field. The ratio of Cu and Si atoms is nearly 3:1. Fig. 4 presents the electron diffraction pattern of the sample upon annealing at 500 °C. It demonstrates the formation of closed-packed hexagonal Cu3Si phase, as illustrated in the XRD pattern in Fig. 1. The weak spots in the pattern are caused by the long-period anti-phase domains [25]. Fig. 5 shows the sheet resistance of the Cu/RuZr dual-layered thin films after annealing at different temperatures. Below 450 °C, the sheet resistance decreases slightly compared with that of the as-deposited samples owing to the elimination of the defects and the grain growth in Cu films [7]. However, the resistance begins to increase upon annealing at 450 °C. In particular, the sheet resistance increased sharply for the samples annealed at 500 °C.
Fig. 3. Cross-sectional TEM images of Cu/RuZr thin films annealed at 500 °C for 30 min: (a) bright-field image and the EDS area scanning of (b) Si (c) Ru (d) Cu.
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nealed at 350, 400, 450 and 500 °C respectively, and XRD, HRTEM and FPP characterizations were done to evaluate the thermal stability as well as the diffusion barrier performance of amorphous RuZr layer. The results indicate that the diffraction peaks of Cu thin films become sharper with the increasing annealing temperature as a result of grain growth. The distinct interface is maintained up to a temperature of 450 °C, indicating the effective suppression of the inter-diffusion between Cu and Si atoms. However, the new peak of Cu3Si phase appears at 500 °C owing to the inter-diffusion between Cu and Si atoms. This leads to the sharp increase in the sheet resistance of Cu interconnects. The improvement on the thermal stability and blocking performance can be ascribed to the suppressed crystallization owing to the introduction of Zr. All in all, amorphous RuZr layer can be a candidate as the diffusion barrier for Cu metallization, but the working temperature should be not higher than 450 °C. Acknowledgements
Fig. 4. Electron diffraction pattern of the sample annealed at 500 °C for 30 min.
14000
RuZr
sheet resistance/mΩ
12000
References
10000 8000 6000 4000 2000 0 0
100
This work was supported by Natural Basic Research Program of China (973 program) (Grant No. 2010CB31002), the National Natural Science Foundation of China (Grant Nos. 51071119, 51101081 and 51271139), and New Century Excellent Talents in University (NCET-10-0679).
200
300
400
500
Temperature/°C Fig. 5. Sheet resistance of as-deposited and annealed thin films.
This is mainly due to the formation of the high-resistance phase of Cu3Si [26]. From another aspect, it demonstrated the significant inter-diffusion between Cu and Si layers as well as the failure of the RuZr alloy diffusion barrier. The failure temperature is raised considerably from 300 °C of the single-element Ru thin films [12] to higher than 450 °C of the Ru–Zr alloy ones. It can be ascribed to suppression of crystallization due to of the introduction of Zr. Similar results have also been observed in Cr and Mo doped film systems [4,7]. It suggests us that the introduction of alloy element might improve the diffusion barrier performance of Cu metallization to some degree. 4. Conclusions In the work of this paper, the amorphous RuZr thin films and polycrystalline Cu thin films were successively deposited on Si substrates by magnetron sputtering. Then the samples were an-
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Thermal stability of RuZr alloy thin films as the diffusion barrier in Cu metallization Y. Meng a, Z.X. Song a, D. Qian a, W.J. Dai a, J.F. Wang c, F. Ma a,⇑, Y.H. Li a,⇑, K.W. Xu a,b a
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China Department of Physics and Opt-Electronic Engineering, Xi’an University of Arts and Science, Xi’an 710065, Shaanxi, China c Linyi University, Linyi 276000, Shandong, China b
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 1 September 2013 Accepted 14 November 2013 Available online 22 November 2013 Keywords: RuZr alloy Amorphous film Thermal stability Cu interconnect
a b s t r a c t Thin films of RuZr/Cu stacking were deposited on Si substrates by magnetron sputtering technology. The as-deposited RuZr thin films were amorphous, while Cu thin films were polycrystalline with (1 1 1) preferred orientation. The films were then annealed at given temperatures to evaluate the thermal stability. It was demonstrated that the amorphous state could be maintained up to 450 °C and, the inter-diffusion between Cu and Si atoms was effectively suppressed. However, the atom diffusion became significant at higher temperatures and resulted in the formation of high-resistance Cu3Si phase. So RuZr amorphous alloy thin films can be readily used for Cu metallization, but the working temperature should be not higher than 450 °C. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of very-large-scale integrated (VLSI) circuits, the application of Cu metallization becomes more and more popular due to its low resistivity, high electro-migration and excellent conductivity [1–3]. However, the inter-diffusion between Cu and Si atoms is significant and they will react to produce high-resistance phase of Cu3Si, even at low temperature [4,5]. This usually leads to the degradation and even failure of the electronic devices [5]. Therefore, it is indispensable to add a thin barrier layer between Cu interconnects and Si substrates to suppress the interdiffusion between Cu and Si atoms, to improve the stability and reliability of the electronic devices. The diffusion barrier materials should usually have low resistivity, high thermal stability, good adhesion with Cu and the dielectric layer. Most importantly, it should effectively block the unwanted atomic diffusion. It has been extensively reported that refractory metals such as Ta, Ti, Zr and Ru can be used as a perfect barrier in Cu metallization, in particular Ta/TaN barrier system has been put into device applications [6]. According to the requirements of the International Technology Roadmap for Semiconductors (ITRS) [7], the barrier layers should become thinner and thinner [8]. Thus, more research works are needed to exploit new barrier layers to meet the requirement during device minimization.
⇑ Corresponding authors. E-mail addresses: [email protected] (F. Ma), [email protected] (Y.H. Li). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.085
Ruthenium (Ru), as a transition noble metal, has a low bulk electrical resistivity of 7.1 lX cm [6–9] and can adhere well with Cu film. The solubility of Ru in Cu is negligible [10]. However, Ru itself has poor barrier performance because of its intrinsic polycrystalline structure in which the grain boundaries provide fast diffusion paths [11]. Specifically, the 5 nm Ru layer fail to block the diffusion of Cu atoms only at 300 °C [12]. Recently, many research works have been done to improve the barrier performance of Ru against Cu diffusion [12–17]. Since the grain boundaries are the main path, it is mandatory to eliminate them in order to suppress the atomic diffusion. Singe-crystal and amorphous thin films are suitable for that purpose because of the absence of grain boundaries, but the former is high cost, so amorphous thin films are preferred. The introduction of a small amount of a second element in Ru thin films will disturb the crystal structure, suppress the crystallization, and thus improve the diffusion barrier performance. Ru–Zr system has been receiving more and more concerns in this field [18–22]. The solid solubility of Zr in Ru is only about 2% [18], and the compositions of Zr (at.%) required for the formation of two intermetallic compounds, Ru2Zr and RuZr, are 33% and 48% [19]. If the Zr composition is in the intermediate range, no intermetallic compound will be formed and, the crystallization of Ru–Zr thin films will be suppressed, which is expected for highperformance diffusion barriers. In this paper, Ru–Zr alloy thin films were fabricated and annealed at different temperatures to explore the thermal stability.
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Cu(111) Cu3 Si Cu(200)
Intensity/a.u.
500°C 450°C 400°C 350°C As-deposited
35
40
45
50
55
60
65
2θ/deg. Fig. 1. XRD patterns of the Cu/RuZr dual layered thin films, as-deposited and annealed at various temperatures.
water. RuZr thin films 40 nm in thickness were firstly deposited on Si substrates by magnetron co-sputtering in argon atmosphere at room temperature using Ru target (U75 mm 5 mm, 99.999% purity) and Zr target (U75 mm 5 mm, 99.999% purity) with a power ratio of Ru:Zr targets is 1:3 for the best thermal stability and barrier performance. The Ar gas flow rate was set to 20 sccm. Cu thin films were then deposited on the RuZr layer using the copper target (U75 mm 5 mm, 99.999% purity) without breaking vacuum. The Cu/RuZr/Si samples were annealed at a given different temperatures from 350 °C to 500 °C for half an hour in N2/H2 mixture ambient (N2:H2 = 9:1). Phase composition and crystalline structure of the thin films were determined by the X-ray diffraction (XRD) measurement using Cu Ka radiation, and the in-plane grain sizes in Cu thin films were evaluated according to Scherrer’s equation. The sheet resistance was measured by the four-point probe (FPP) technology. The high-resolution transmission electron microscopy (HRTEM, JEM-2100F) with EDS was employed to characterize the cross-section morphology as well as the evolution of the microstructures of the film system. The TEM sample was prepared in the following steps: the thin films on Si substrates were cut into square pieces 2 mm 2 mm in size, then glued together face to face, followed by thermal annealing in an incubator at 120 °C for 2 h. After that, the samples were mechanically grinded from the cross-section direction using multi-granularity sandpapers until the cross-section thickness was reduced down to several micrometers. Finally, ion polishing was done to further reduce the thickness in the center region down to about 50 nm so that the electron beam can penetrate.
Table 1 The grain size (unit in nm) of Cu thin films.
Cu(1 1 1) Cu(2 0 0)
As-deposited
350 °C
400 °C
450 °C
19.67 17.41
54.66 54.19
52.71 58.34
52.69 56.16
2. Experiment The p-type (1 0 0) Si wafer was used as the substrates. Prior to deposition, the substrates were first cleaned by ultrasonic technology with alcohol and acetone for 10 min, respectively, to remove organic contaminants. Then the wafers were dipped in 10% HF for 5 min to clear the oxides and finally rinsed in de-ionized
3. Results and discussion The atomic concentration of Zr in the Ru–Zr thin films is determined by EDS data. It is about 20%, just in the intermediate range stated above. Hence, no intermetallic compounds can be formed and the Ru–Zr thin films should be amorphous. Fig. 1 presents the XRD patterns of Cu/RuZr dual-layer stacked thin films as deposited as well as those annealed at the temperatures from 350 °C to 500 °C. It is obvious that the Cu film layer is polycrystalline even for the as-deposited samples, while there is no diffraction peak of RuZr alloy, indicating the amorphous nature even upon
Fig. 2. Cross-sectional TEM images of Cu/RuZr thin films annealed at 450 °C for 30 min: (a) bright-field image, (b) high-resolution image of the local region in Cu thin film, (c) high-resolution image of the local region in RuZr barrier layer with the diffraction pattern in the inset and (d) EDS line scanning across the film interface.
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annealing. The two diffraction peaks at 43.3° and 50.3° correspond to the (1 1 1) and (2 0 0) planes of face-centered-cubic copper [23,24]. These two peaks become sharper after thermal annealing, indicating the increased grain size, as evaluated according to Scherrer’s equation and listed in Table 1. The radius of Zr (0.16 nm) is bigger than Cu (0.1278 nm) by 25%. If Zr atoms are diffused into the Cu lattice, the lattice constant should be increased considerably, and the corresponding XRD peaks will shift to small-angle values. However, the XRD patterns in this work indicate a slight shift towards large-angle direction, for an example, the 2h of Cu (1 1 1) plane is increased slightly from 43.3° of the as-deposited thin film to 43.4° of the one annealed at 400 °C for half an hour. Hence, Zr atoms do not dissolved into the lattice of Cu, but, if any, should be randomly distributed in the grain boundaries of Cu thin films. It is worth noting that the high-resistance phase of Cu3Si appears when the Cu/RuZr dual-layer stacked thin films were annealed at 500 °C, as indicated in the XRD pattern. At the same time, the intensity of Cu (1 1 1) and Cu (2 0 0) peaks is reduced considerably. It implies that the reaction occurred between the Cu cap and the underlying Si substrate. In another word, the RuZr alloy diffusion barrier is invalid at 500 °C. As an example, Fig. 2 displays the cross-sectional TEM micrograph of the Cu/RuZr dual layered thin films annealed at 450 °C. As shown in the bright-field image in Fig. 2(a), the grains in Cu thin films exhibit columnar morphology and the RuZr diffusion barrier layer keeps distinct interfaces with the Cu capping layer and Si substrate upon annealing at 450 °C for half an hour. The glue between Cu and the barrier comes from the preparation process of TEM sample. Fig. 2(b) shows the high-resolution image of the local region in the Cu film as marked by the black square. Fig. 2(c) shows the high-resolution image of the local region in the RuZr layer as
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marked by the black square, the corresponding nano-beam diffraction pattern is displayed in the inset of the figure. Both of the disordered atomic configuration in high-resolution image and the halo ring of the diffraction pattern corroborate the amorphous nature of ZrRu thin films even after thermal annealing at 450 °C for half an hour. This is consistent with the XRD result. As mentioned above, there is no effective diffusion path in the amorphous films, which should suppress the atom diffusion significantly. Fig. 2(d) presents the line scanning of the element distribution cross the film interface. The Cu and Si components change sharply at the film interface. As expected, the inter-diffusion is effectively suppressed. Fig. 3 shows the cross-sectional TEM images of the Cu/RuZr dual-layered thin films annealed at 500 °C for half an hour. As displayed in the bright-field image in Fig. 3(a), the layer interface disappears as a result of significant diffusion of Cu and Si atoms. It is further confirmed by the scanning image of Si, Ru and Cu elements, as shown in Fig. 3(b–d). The element components are obtained through the point spectrum of the local region in the bright field. The ratio of Cu and Si atoms is nearly 3:1. Fig. 4 presents the electron diffraction pattern of the sample upon annealing at 500 °C. It demonstrates the formation of closed-packed hexagonal Cu3Si phase, as illustrated in the XRD pattern in Fig. 1. The weak spots in the pattern are caused by the long-period anti-phase domains [25]. Fig. 5 shows the sheet resistance of the Cu/RuZr dual-layered thin films after annealing at different temperatures. Below 450 °C, the sheet resistance decreases slightly compared with that of the as-deposited samples owing to the elimination of the defects and the grain growth in Cu films [7]. However, the resistance begins to increase upon annealing at 450 °C. In particular, the sheet resistance increased sharply for the samples annealed at 500 °C.
Fig. 3. Cross-sectional TEM images of Cu/RuZr thin films annealed at 500 °C for 30 min: (a) bright-field image and the EDS area scanning of (b) Si (c) Ru (d) Cu.
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nealed at 350, 400, 450 and 500 °C respectively, and XRD, HRTEM and FPP characterizations were done to evaluate the thermal stability as well as the diffusion barrier performance of amorphous RuZr layer. The results indicate that the diffraction peaks of Cu thin films become sharper with the increasing annealing temperature as a result of grain growth. The distinct interface is maintained up to a temperature of 450 °C, indicating the effective suppression of the inter-diffusion between Cu and Si atoms. However, the new peak of Cu3Si phase appears at 500 °C owing to the inter-diffusion between Cu and Si atoms. This leads to the sharp increase in the sheet resistance of Cu interconnects. The improvement on the thermal stability and blocking performance can be ascribed to the suppressed crystallization owing to the introduction of Zr. All in all, amorphous RuZr layer can be a candidate as the diffusion barrier for Cu metallization, but the working temperature should be not higher than 450 °C. Acknowledgements
Fig. 4. Electron diffraction pattern of the sample annealed at 500 °C for 30 min.
14000
RuZr
sheet resistance/mΩ
12000
References
10000 8000 6000 4000 2000 0 0
100
This work was supported by Natural Basic Research Program of China (973 program) (Grant No. 2010CB31002), the National Natural Science Foundation of China (Grant Nos. 51071119, 51101081 and 51271139), and New Century Excellent Talents in University (NCET-10-0679).
200
300
400
500
Temperature/°C Fig. 5. Sheet resistance of as-deposited and annealed thin films.
This is mainly due to the formation of the high-resistance phase of Cu3Si [26]. From another aspect, it demonstrated the significant inter-diffusion between Cu and Si layers as well as the failure of the RuZr alloy diffusion barrier. The failure temperature is raised considerably from 300 °C of the single-element Ru thin films [12] to higher than 450 °C of the Ru–Zr alloy ones. It can be ascribed to suppression of crystallization due to of the introduction of Zr. Similar results have also been observed in Cr and Mo doped film systems [4,7]. It suggests us that the introduction of alloy element might improve the diffusion barrier performance of Cu metallization to some degree. 4. Conclusions In the work of this paper, the amorphous RuZr thin films and polycrystalline Cu thin films were successively deposited on Si substrates by magnetron sputtering. Then the samples were an-
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