Structure and thermal stability of graded Ta–TaN diffusion barriers between Cu and SiO2

Thin Solid Films 437 (2003) 248–256

Structure and thermal stability of graded Ta–TaN diffusion barriers between Cu and SiO2 a, ¨ R. Hubner *, M. Heckera, N. Matterna, V. Hoffmanna, K. Wetziga, Ch. Wengerb, H.-J. Engelmannc, Ch. Wenzelb, E. Zschechc, J.W. Barthab a

b

Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstrasse 20, Dresden 01069, Germany Dresden University of Technology, Semiconductor and Microsystems Technology Laboratory, Dresden 01062, Germany c AMD Saxony LLC and Co. KG Dresden, Materials Analysis Department, Dresden 01330, Germany Received 1 November 2002; received in revised form 18 March 2003; accepted 14 April 2003

Abstract Sputter deposited Ta and TaN single layers of 10 nm thickness as well as graded TaNyTa and TayTaNyTa layer stacks that act as diffusion barriers for Cu metallization were investigated after annealing at temperatures between Tan s300 and 700 8C. By means of glancing angle X-ray diffraction, glow discharge optical emission spectroscopy and transmission electron microscopy, results of microstructure and phase characterization were correlated with diffusion phenomena. For the pure Ta barrier, Ta diffusion through the Cu cap layer to the sample surface is observed at Tans500 8C, and the transformation of initially grown metastable b-Ta into the equilibrium a-Ta phase occurs at Tans600 8C. In contrast, a fcc TaN layer remains stable at least up to Tans700 8C. In the case of the graded layer stacks, first signs of N diffusion out of the TaN film into the adjacent Ta layers are observed after annealing at Tans300 8C, and formation of hexagonal Ta2N starts at Tan s500 8C. Whereas in the course of thermal treatments for the threefold graded TayTaNyTa barrier all TaN reacts with Ta to form Ta2 N, some fcc TaN remains in the twofold graded TaNyTa barrier. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Cu metallization; Diffusion barriers; Annealing; X-ray diffraction

1. Introduction To prevent the diffusion of Cu into SiO2 and Si substrates of microelectronic devices, effective diffusion barriers are needed. Additionally, the barrier material has to be thermally and electrically conducting as well as resistant to mechanical and thermal stresses. Furthermore, it should show a strong adhesion to the adjacent materials w1x. The refractory metal Ta (Tmeltings3020 8C w2x) does not react with Cu w2,3x. The solubility of Ta in Cu and vice versa is very low in the solid state w2x, and silicide formation occurs at relatively high temperature only w4x. Thus, Ta-based layers are suited as diffusion barriers in Cu metallization w5–7x. In thin Ta films, three modifications can be observed: the thermodynamically stable body centered cubic (bcc) a*Corresponding author. Tel.: q49-351-4659-685; fax: q49-3514659-452. ¨ E-mail address: [email protected] (R. Hubner).

¯ (229) w8,9x, resistivity: ra-Taf25 Ta (space group: Im3m mVcm w10x), the metastable tetragonal b-Ta (space ¯ 1m (113) w11x, resistivity: rb-Taf200 mVcm group: P42 w10x) and sometimes a face centered cubic (fcc) phase w10x. Depending on the deposition parameters, a deposited Ta layer consists of either one or a mixture of the above-mentioned three phases w10x. At room temperature, metastable b-Ta usually grows on Si or SiO2 w4,5,12–15x. During thermal treatments, the metastable b-Ta phase can be transformed into the equilibrium aTa phase. Parameters determining this phase transformation are annealing temperature and time w4,16,17x, annealing ambience, substrate material w4x and film thickness w18x. Ta has a strong adhesion to Cu w19x, but shows only a moderate adhesion to SiO2 w20x. Due to the polycrystalline structure of Ta, grain boundary diffusion of Cu atoms becomes relevant at elevated temperatures w21x. One way to improve the thermal stability of Ta barrier layers is the addition of

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N during the deposition process. A lot of investigations were carried out to determine phase composition and microstructure of as-deposited Ta-N films in dependence on the N content w22–25x. According to Stavrev et al. w24x, a small addition of N2 to the sputtering gas results in the transition from metastable tetragonal b-Ta to nanocrystalline bcc Ta(N). Further increase of the N2 flow leads to the formation of nanocrystalline fcc TaN. Stoichiometric TaN has a high melting temperature (Tmeltings3090 8C w26x), and therefore, it shows enhanced thermal stability. Compared to Ta, its adhesion to SiO2 is increased w20,27x. Unfortunately, TaN has a high electrical resistivity. With the knowledge of the properties of pure Ta and of pure TaN films, further optimization of the barrier performance by combining both layers is possible. Edelstein et al. w27x investigated the graded TayTaN layer stack between Cu and SiO2. Using a specified range of PVD conditions, they were able to deposit hexagonal close-packed (hcp) TaN with a small fraction of the fcc TaN modification. Compared to the latter one, hcp TaN shows a lower stress and a lower electrical resistivity for similar adhesion to SiO2. The Ta layer consists of the low resistivity bcc a-Ta, which is formed spontaneously on TaN and which adheres well to the Cu top layer. Although there is a growing interest in graded Ta– TaN barrier layer stacks w27–29x, less is known about microstructure and thermal stability of such systems. It is the aim of this publication to elucidate these properties for TaNyTa and TayTaNyTa barriers deposited onto SiO2 and to compare their features with those of pure Ta and TaN films. The characterization of the asdeposited samples was followed by thermal treatments carried out at different annealing temperatures between Tans300 and 700 8C. Structural and compositional changes in the layer stacks were monitored with glancing angle X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) investigations. Glow discharge optical emission spectroscopy (GDOES) was used for depth profile analysis. 2. Experimental details For the investigations of the graded Ta–TaN diffusion barriers, thermally oxidized (100) Si wafers with a diameter of 100 mm were used. Thermal oxidation was carried out at a temperature of 1000 8C for 120 min in O2 ambience to grow a 140 nm thick SiO2 film. A standard RCA clean was performed prior to loading the wafers into the load–lock. After soft-etching at 200 W in Ar plasma without interrupting the vacuum, the Tabased films were radio frequency (rf) magnetron sputtered at 1 kW forward power from a Ta target (99.95%). The base pressure in the PVD chamber was 3=10y5 Pa and the target-to-substrate distance 125 mm. During

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Ta and TaN layer deposition, the N2 flow was 0 and 3.5 sccm, respectively, and the Ar flow was kept at 5 sccm. This results in process pressures between 0.15 and 0.24 Pa. The nominal thickness of each Ta and TaN layer was 10 nm, so that the total thicknesses of the TaNyTa and TayTaNyTa layers were 20 and 30 nm, respectively. To complete the metallization schemes, a 50 nm thick Cu film was direct current (dc) magnetron sputtered on top of the Ta-based layers without interrupting the vacuum. After film deposition, the wafers were cut into pieces of approximately 20=30 mm2. For all samples, thermal treatments were performed under vacuum conditions (pf10y4 Pa) within a temperature range between Tans300 and 700 8C for an annealing time of tans1 h. For each thermal treatment a different sample was used. Glancing angle XRD measurements with an incidence angle of vs28 were done at room temperature in parallel beam geometry employing a Philips X’Pert ˚ and diffractometer with Cu-Ka radiation (ls1.5418 A) thin film equipment. The registered diffraction angle range was 2us20–958 with a step size D2us0.058 and a measuring time of 40 s per step. Cross-sectional TEM investigations were carried out with a FEI 200 kV CM200FEG microscope. The element depth distributions by GD-OES were determined using a modified LECO-GD750 tool. Since the SiO2 ySi substrates are non-conducting, rf sputtering was applied. The corresponding home-made rf equipment is characterized by the fast stability in milliseconds due to the use of a free running rf generator w30x. To analyze the N content of the TaN film, Rutherford backscattering spectrometry (RBS) measurements were carried out, and the NyTa ratio was determined to 1.1. 3. Experimental results 3.1. Characterization of the pure Ta barrier In the as-deposited state, the barrier layer predominantly consists of metastable tetragonal b-Ta, which can be derived from the broad Bragg reflections (JCPDSICDD 25-1280) marked in Fig. 1. Additional peaks appearing at 2us43.4, 50.6, 74.2 and 90.18 in all diffraction patterns belong to the 111, 200, 220 and 311 Bragg reflections of polycrystalline Cu. Due to the used diffraction geometry with an incidence angle of vs28, a strong Cu 220 reflection points to the preference of a Cu N111M texture. This result was verified by an XRD measurement in Bragg–Brentano geometry (not shown here). During thermal treatments up to Tans500 8C, no changes of the b-Ta reflections occur, whereas peak shifts are visible after the 600 8Cy1 h anneal. These peak shifts may be caused by stress relaxation phenomena. According to Clevenger et al. w16x, who investigated 100 nm thick initially compressively stressed (ssy1.6

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Fig. 1. Glancing angle XRD diagrams of the CuyTaySiO2 ySi samples heat treated at different temperatures Tan for a duration of tans1 h. (Only the positions of the observed Bragg peaks of the different phases are marked).

GPa) uncovered b-Ta films on SiO2, a stress relaxation of approximately 10% occurring in the temperature range 600 8C-Tan-750 8C can be attributed to plastic deformation of the Ta film. Annealing at Tans700 8C, results in the transformation of the metastable b-Ta into the thermodynamically stable a-Ta (Fig. 1). This phase transformation is concluded from the arising Bragg peaks at 2us38.6, 56.0 and 69.78, which are, however, shifted to higher diffraction angles relative to their ideal positions (JCPDS-ICDD 04-0788). Clevenger et al. w16x assumed that the main relaxation of the initial compressive stress occurs during the b-™a-Ta phase transformation. Because of the observed peak shift, it seems that the grown a-Ta grains are tensile stressed. The commencing phase transition is also observed in the diffraction diagram after annealing the sample at Tans 600 8C for tans2 h (not shown here). Heat treatments at Tans600 8C up to tans16 h result in both an increase of the a-Ta intensity and a shift of the a-Ta peaks to smaller diffraction angles. Whereas the continued b-™ a-Ta transformation is the reason for the intensity increase, the peak shift may be caused by a relaxation of a possible tensile stress or the incorporation of additional atoms into the a-Ta lattice. The diffraction diagrams of the samples taken for annealing times tan) 16 h show a continuous decrease in the a-Ta intensity and some additional peaks, which cannot be uniquely identified. The element depth profiles of the as-deposited state clearly confirm the nominal sample setup. Fig. 2 indicates that there is no Ta in the Cu layer and the SiO2 substrate. In accordance with the glancing angle XRD measurements, no changes occur during heat treatment up to Tans400 8C. However, at Tans500 8C, Ta atoms diffuse through the Cu layer to the sample surface (Fig.

Fig. 2. GD-OES depth profiles of the Ta distribution of the CuyTaySiO2ySi samples heat treated at different temperatures Tan for a duration of tans1 h.

2, curve (3)). Such Ta diffusion was also observed by Jang et al. w31x. In the depth profile taken after annealing at Tans700 8C (Fig. 2, curve (5)), Ta is not only present at the sample surface, but also in the lower part of the Cu layer. Because of the shape of the Ta profile, it is more likely that the latter signal is caused by a continued Ta diffusion to the sample surface than by an a-Ta crystallite growth into the Cu layer. Additional TEM investigations support this interpretation. Fig. 3 indicates that the a-Ta grains grow onto the SiO2 substrate and within the original barrier region. Using energy disper-

Fig. 3. Cross-sectional TEM image of the CuyTaySiO2 ySi layer stack after heat treatment at Tans600 8C for a duration of tans16 h.

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Up to Tans700 8C, the GD-OES depth profiles remain unchanged. In particular, no Ta signal is observed at the sample surface. This means that in contrast to the pure Ta barrier, no Ta atoms diffused from the TaN film through the Cu cap layer. Oxygen atoms from the residual gas in the annealing ambience, however, penetrate into the Cu and accumulate at the CuyTaN interface already during heat treatment at Tans600 8C. 3.3. Characterization of the graded TaNyTa barrier

3.2. Characterization of the pure TaN barrier

The diffraction diagram of the as-deposited sample can be interpreted as a superposition of the patterns from the above described b-Ta and fcc TaN films (Fig. 5). Due to absorption in the upper TaN layer, the peak intensities of the underlying b-Ta film are significantly reduced. Anneals of the layer stack at Tans300 and 400 8C lead to shifts of the TaN and Ta peak positions, which indicate lattice parameter changes. Annealing the sample at Tans500 8C results in the commencing formation of hexagonal Ta2N, being enhanced after thermal treatment at Tans600 8C (Fig. 5). It should be mentioned that some fcc TaN still remains in the barrier layer at Tans700 8C. Fig. 6 and Fig. 7 represent the GD-OES depth profiles of Ta and N, respectively. In the as-deposited state, a large Ta signal occurs in the Ta layer and a smaller one in the TaN film, whereas an obvious N signal can be found only in the upper barrier region. Already for heat treatments at Tans300 and 400 8C, the depth profiles show signs of increasing N content in the region of the Ta layer (Fig. 7, curves (2, 3)). This result can be explained by the diffusion of N atoms from the TaN layer into the Ta film, leading to the observed lattice parameter alterations. Annealing at higher temperatures (Tans500 8C) results in an enhanced N diffusion (Fig. 7, curve (4)), but the N distribution in the whole barrier

In the CuyTaNySiO2 ySi system, the barrier layer mainly consists of nanocrystalline fcc TaN (Fig. 4). However, compared with the ideal peak positions for fcc TaN (JCPDS-ICDD 49–1283), the measured ones are slightly shifted. One reason for this shift may be the large compressive stress in the thin barrier layer w24x. Another possible explanation is the incorporation of excess N atoms into the TaN lattice. The high background within the angular range 308-2u-338 indicates that an additional fraction of the barrier is amorphous. As well as for the pure Ta barrier, the Cu cap layer is N111M textured. According to c-scans at the Cu 111 reflection (not shown here), the N111M texture component is larger in the case of TaN. Annealing up to Tans 700 8Cytans1 h leads to a reduction of the amorphous part of the barrier and to a minor decrease of the half widths of the fcc TaN peaks, which is correlated to crystallite growth.

Fig. 5. Glancing angle XRD diagrams of the CuyTaNyTaySiO2 ySi samples heat treated at different temperatures Tan for a duration of tans1 h.

Fig. 4. Glancing angle XRD diagrams of the CuyTaNySiO2 ySi sample in the as-deposited state and after heat treatment at Tans700 8C for a duration of tans1 h.

sive X-ray spectrometry (EDXS) and electron energy loss spectrometry (EELS) line scans, the approximately 5 nm thick surface layer in Fig. 3 was shown to consist of TaOx, which seems to result from the reaction of tantalum atoms diffused out of the barrier layer to the sample surface with oxygen from the residual gas of the annealing ambience. Finally, it should be mentioned that after heat treatments at Tans600 8CytanG16 h and Tans 700 8Cytans1 h, changes in the course of the Si depth profile at the TaySiO2 interface (not shown here) are observed. Furthermore, the TEM image in Fig. 3 shows a 2–3 nm thick interfacial layer on the SiO2 substrate. Together with the XRD results, both observations give reason to the assumption that at higher temperatures a reaction between Ta and interfacial SiO2 becomes possible.

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Fig. 6. GD-OES depth profiles of the Ta distribution of the CuyTaNyTaySiO2 ySi samples heat treated at different temperatures Tan for a duration of tans1 h.

Fig. 7. GD-OES depth profiles of the N distribution of the CuyTaNyTaySiO2 ySi samples heat treated at different temperatures Tan for a duration of tans1 h.

layer remains inhomogeneous, even after heat treatment at Tans700 8C (Fig. 7, curve (5)). It seems that not all fcc TaN reacts with Ta to form Ta2N, so that some residual nanocrystalline fcc TaN remains in its original layer. Ta diffusion to the sample surface, which was observed for the system with single Ta barrier at Tans 500 8C (Fig. 2, curve (3)), starts in the case of the layer stack with the twofold graded TaNyTa barrier at Tans650 8C. It is shown in Fig. 6 for the Ta profile, taken after annealing at Tans700 8C. The comparatively inhibited Ta diffusion for the layer stack with the TaNy Ta bilayer may be due to the nanocrystalline TaN, which has to be passed by the Ta atoms before they can diffuse through the Cu layer.

of the TaNyTa bilayer, these alterations seem to be caused by a commencing N diffusion out of the TaN layer into both adjacent Ta films. At Tans500 8C, Bragg reflections belonging to hexagonal Ta2N appear (Fig. 8). Anneals at higher temperatures lead to further Ta2N formation, which is completed after heat treatment at Tans700 8C. The GD-OES depth profiles of the as-deposited state show clearly the sample setup. Especially, the gradation of the whole barrier into three parts can be derived from the Ta and N depth profiles shown in Fig. 9 and Fig. 10, respectively. As in the case of the TaNyTa bilayer, the N diffusion out of the TaN film into the adjacent Ta layers starts during thermal treatment at Tans300 8C and is enhanced for elevated temperatures. At Tans600 8C, the Ta and N distributions across the whole barrier region are nearly equal, and they do not change during

3.4. Characterization of the graded TayTaNyTa barrier For the as-deposited state, the diffraction diagram of the threefold graded TayTaNyTa layer stack contains— as well as for the TaNyTa bilayer—Bragg reflections of the fcc TaN and only signs of weak b-Ta peaks (Fig. 8). However, additional peaks indicate the appearance of N110M oriented bcc a-Ta. In agreement with the observation of a preferred growth of a-Ta deposited onto TaN w27x and our results obtained for the TaNyTa bilayer, we thus conclude that the a-Ta is located in the upper Ta film, whereas the lower one consists of b-Ta and the layer in between of fcc TaN. As for the three systems described above, the crystallites within the Cu layer are preferentially oriented. The N111M texture component is even stronger than in the case of the layer stack with the pure TaN barrier. Heat treatments of the sample at Tans300 and 400 8C lead to changes in the peak positions. Especially, the a-Ta reflections are shifted to smaller diffraction angles (Fig. 8). Thus, the corresponding lattice parameter increases. As in the case

Fig. 8. Glancing angle XRD diagrams of the CuyTayTaNyTaySiO2 ySi samples heat treated at different temperatures Tan for a duration of tans1 h.

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Fig. 9. GD-OES depth profiles of the Ta distribution of the CuyTayTaNyTaySiO2 ySi samples heat treated at different temperatures Tan for a duration of tans1 h.

Fig. 10. GD-OES depth profiles of the N distribution of the CuyTayTaNyTaySiO2 ySi samples heat treated at different temperatures Tan for a duration of tans1 h.

further heat treatment. This observation correlates well with the results of the diffraction experiments that the formation of Ta2N is nearly completed at Tans600 8C, since only minor remnants of the as-deposited phases are visible. Finally, it should be noted that in the case of the TayTaNyTa trilayer, Ta starts to diffuse through the Cu to the sample surface at Tans500 8C (Fig. 9 curve (4)). The TEM image in Fig. 11 shows the setup of the TayTaNyTa layer stack in the as-deposited state. Since the interfaces between the different films are sharp, the threefold gradation of the barrier can be seen very well. Whereas the upper and lower Ta layers are made up of crystallites in the dimension of the layer thickness, the TaN grains in the middle barrier region have a size of approximately 3 nm. These observations confirm both, the GD-OES and the glancing angle XRD results. An EELS line scan reveals oxygen in the amorphous region between Cu and the upper Ta film. However, the corresponding GD-OES depth profile does not show an enhanced O signal at this position. The reason for the formation of the amorphous region is not completely understood. We assume that it is formed by surface oxidation after TEM sample preparation. This assumption is supported by the investigations of Wang et al. w32x, who observed ‘amorphized’ regions between Cu and Ta only for TEM specimens, which had been exposed to oxygen. Investigations of the layer stacks directly after TEM sample preparation showed sharp interfaces between Cu and Ta without any amorphous layer. Compared to Fig. 11, the TEM image of the sample heat treated at Tans600 8C (Fig. 12) shows no longer a threefold graded layer stack, but rather a single layer indicating a homogeneous distribution of the elements within the barrier. According to the glancing angle XRD results, this 30 nm thick film consists mainly

of Ta2N. It should be mentioned that the interface to the SiO2 remains sharp, which indicates that no reaction occurred between the barrier and the substrate. 4. Discussion Depending on the barrier layer composition, diffusion of Ta atoms through the Cu to the sample surface can occur during thermal treatment. Whereas it starts at Tans500 8C in the case of the pure Ta barrier, no indications of such Ta diffusion out of the TaN layer are observed up to Tans700 8C. Therefore, we conclude

Fig. 11. Cross-sectional TEM image CuyTayTaNyTaySiO2 ySi layer stack.

of

the

as-deposited

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Fig. 12. Cross-sectional TEM image of the CuyTayTaNyTaySiO2 ySi layer stack after heat treatment at Tan s600 8C for a duration of tans 1 h.

that the N content in the Ta-based barrier can suppress this diffusion process. The local arrangement of Ta and TaN films within a graded barrier has an influence on the onset of the Ta diffusion, too. For the TaNyTa bilayer, a detectable Ta surface signal is observed after a heat treatment at Tans650 8C for the first time. This may be caused by the fact that the Ta atoms primarily have to diffuse through the nanocrystalline TaN before they can pass the Cu layer. Using a TayTaNyTa trilayer, Ta diffusion is already observed at Tans500 8C. Compared to the results obtained for the TaNyTa bilayer and the pure Ta barrier, it can be concluded that these Ta atoms are originated from the upper a-Ta layer and that the Ta diffusion out of a a-Ta as well as a b-Ta film to the sample surface starts at almost the same temperature (Tans500 8C). It should be noted that such Ta diffusion leads to a decrease of the barrier layer thickness, which may have an influence on its thermal stability. The accumulation of the diffused Ta atoms in the Cu metallization layer can also change its electrical properties. For these reasons, the Ta diffusion is a disadvantageous process, which should be suppressed. Combining Ta and TaN films to graded barriers can lead to another diffusion phenomenon. Already during heat treatment at Tans300 8C, N atoms start to diffuse out of the TaN layer into the adjacent Ta films. According to the RBS measurement, the TaN layer has nearly a stoichiometric composition with a small excess of nitrogen. Maybe these additional N atoms, which can be incorporated into the TaN lattice as well as into the

adjacent grain boundaries, are the starting point for the N diffusion, which is observed for both, the TaNyTa bilayer and the TayTaNyTa trilayer. As indicated in the diffraction diagrams, annealing at Tans500 8C results in the commencing formation of Ta2N, which seems to be the product of the reaction between Ta and TaN. Caused by the Ta diffusion to the sample surface, in the case of the TaNyTa bilayer, not all fcc TaN can react with Ta to form Ta2N. A small fraction of fcc TaN remains in the barrier (Fig. 5). Using a TayTaNyTa trilayer, the amount of Ta in the lower and upper barrier film is sufficient to transform all fcc TaN into Ta2N. After heat treatment at Tans700 8C, there is a good correspondence between the presence of Ta2N as the only observed phase in the diffraction diagram apart from Cu (Fig. 8) and the nearly homogenous Ta and N distribution across the barrier in the GD-OES depth profiles (Fig. 9 and Fig. 10). Both, the N diffusion and the subsequent Ta2N formation can alter the original barrier properties, like electrical resistivity or adhesion characteristics. In particular, the thermal stability is affected due to the changed microstructure. To test the barrier properties more sensitively, additional annealing experiments were performed for Ta-based barrier layers deposited directly onto Si. Using the appearance of Cu3Si peaks in the diffraction diagrams as an indicator for a Cu diffusion into the Si substrate, a 10 nm thick TaN barrier between Cu and Si failed during annealing at Tans800 8C w33x. However, for a 30 nm thick graded TayTaNyTa trilayer between Cu and Si, both, Cu and Ta silicides were already formed at Tans700 8C (Fig. 13). This means that a graded barrier formed by a combination of Ta layers with the thermally very stable TaN film shows a reduced thermal stability. Using SiO2 instead of Si as substrate material for the Cuybarrier layer stack, no significant Cu diffusion

Fig. 13. Glancing angle XRD diagrams of the CuyTayTaNyTaySi samples heat treated at different temperatures Tan for a duration of tans1 h.

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through the barrier layer was observed. Thus, the barriers show a different behavior during thermal treatment. Whereas for a Ta layer between Cu and Si significant diffusion of Cu atoms into the substrate with subsequent Cu3Si and TaSi2 formation is already observed at Tans 550 8C w34x, for the same Ta film deposited between Cu and SiO2 the Ta is transformed from tetragonal bTa into bcc a-Ta prior to any reaction with the SiO2. Annealing the TayTaNyTa barrier between Cu and Si at Tans650 and 700 8C results in a commencing formation of TaSi2 and Cu3Si, respectively (Fig. 13). At these temperatures, hexagonal Ta2N is the only phase that is observed between Cu and SiO2 (Fig. 8). However, it should be noted that further investigations, e.g. using atomic absorption spectrometry, are necessary to prove a possible trace diffusion of Cu into the SiO2 substrate. 5. Conclusions The annealing behavior of ultrathin sputter deposited single layered Ta and TaN as well as that of graded TaNyTa and TayTaNyTa diffusion barriers between Cu and SiO2 was investigated by means of X-ray diffraction, GD-OES depth profile analysis and transmission electron microscopy. Whereas in the as-deposited state the Ta layer adjacent to the dielectric mainly consists of metastable tetragonal b-Ta, bcc a-Ta with an increased electrical conductivity grows onto nanocrystalline fcc TaN. Depending on the N content in the single layer barrier and on the arrangement of these single layers within a graded barrier, different behavior during thermal treatment of the samples was observed. In the case of the pure Ta barrier, Ta diffusion to the sample surface occurs during annealing at Tans500 8C leading to a reduction of the original barrier layer thickness. At Tans 600 8C, the remaining b-Ta transforms into the thermodynamically stable cubic a-Ta modification. The TaN layer is stable up to Tans700 8C. Using a TaNyTa bilayer or a TayTaNyTa trilayer, N diffusion out of the TaN layer into the adjacent Ta films appears already during thermal treatment at Tans300 8C, and formation of hexagonal Ta2N is detected at Tans500 8C. Whereas the Ta content within the TaNyTa layer stack seems to be too small to transform all the fcc TaN into Ta2N, in the case of the threefold graded barrier, all TaN reacts with Ta to form a nearly homogenous Ta2N film, which remains stable up to Tans700 8C. For a TayTaNyTa trilayer deposited directly onto Si, Ta2N starts to form also at Tans500 8C. At Tans650 8C, however, TaSi2 peaks are observed in the diffraction pattern, and significant Cu diffusion into the substrate occurs at Tans700 8C. Furthermore, the barrier layers deposited between Cu and SiO2 are significantly more stable than those prepared between Cu and Si, since in the latter case a reaction between Cu traces, barrier atoms and the substrate is possible at lower temperatures.

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