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Atomic layer deposited self-forming Ru-Mn diffusion barrier for seedless Cu interconnects
Journal of Alloys and Compounds 686 (2016) 1025e1031
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Atomic layer deposited self-forming Ru-Mn diffusion barrier for seedless Cu interconnects Hyun-Jung Lee a, Tae Eun Hong b, Soo-Hyun Kim a, * a b
School of Materials Science and Engineering, Yeungnam University, Gyeongsan-si, 712-749, South Korea Busan Center, Korea Basic Science Institute, 1275 Jisadong, Gangseogu, Busan, 618-230, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2016 Received in revised form 20 June 2016 Accepted 27 June 2016 Available online 29 June 2016
Ru-based alloy thin films were prepared by atomic layer deposition (ALD) at a deposition temperature of 225 C for use as a diffusion barrier for seedless Cu interconnects. Ru-Mn alloy thin films were grown by a repetition of super-cycles consisting of multiple Ru and Mn ALD sub-cycles. Ru and Mn ALD was performed using a sequential supply of a Ru precursor [1-Isopropyl-4-methylbenzene (cyclohexa-1, 3-diene) Ru (0)] and NH3 plasma, and a Mn precursor [bis(1,4-di-isopropyl-1,3-diazabutadienyl)Mn(II)] and H2 plasma, respectively. Secondary ion mass spectrometry (SIMS) indicated that Mn had been incorporated successfully into the Ru film by the periodic addition of Mn ALD sub-cycles during the ALD-Ru process. Plan-view transmission electron microscopy (TEM) showed that the grain size was reduced obviously from ~15 nm (pristine Ru) to ~8 nm (Ru-Mn alloy) and continuous columnar grain growth of the ALD-Ru film was prevented by the addition of the periodic Mn ALD sub-cycles. After annealing at 500 C, the formation of a MnSiOx diffusion barrier was observed between the Ru-Mn film and underlying SiO2 surface by both cross-sectional view TEM and energy dispersive spectroscopy analysis and confirmed by SIMS depth profile. X-ray diffraction showed that the ultrathin ALD Ru-Mn thin film (~5 nm) prevented Cu diffusion up to an annealing temperature of 600 C, whereas the ALD-Ru thin film with the same thickness failed to prevent Cu diffusion only after annealing at 500 C. Finally, the capability of the direct plating of Cu was demonstrated on the annealed ALD Ru-Mn film. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ruthenium Manganese Ru-Mn alloy Atomic layer deposition Self-forming barrier Direct plating of Cu
1. Introduction As a part of a manufacturing process of integrated circuits, Cu prepared by electroplating (EP) has been implemented widely in interconnect materials of logic devices owing to its very low resistivity and inexpensive, simple, and high-volume manufacturing. On the other hand, because semiconductor devices have continued scaling down into the nanometer range, there have been critical issues on the drastic increase in the Cu line resistance [1,2]. Therefore, the volume of the EP-Cu filled into the trenches should be increased to prevent an increase in interconnect resistance, which has forced a decrease in the volume in a stack of three layers, which are deposited by physical vapor deposition (PVD), such as the TaN diffusion barrier, Ta adhesion promoter, and Cu seed layer. As a potential solution, by replacing the stack of three layers into a single layer, called a Cu direct plateable diffusion barrier, EP-Cu can
* Corresponding author. E-mail address: [email protected] (S.-H. Kim). http://dx.doi.org/10.1016/j.jallcom.2016.06.270 0925-8388/© 2016 Elsevier B.V. All rights reserved.
be readily filled and the increase in Cu line resistance can be minimized. In the pursuit of a suitable and reliable Cu direct plateable diffusion barrier, many materials and processes have been reported. Among them, CueMn films, which are typically deposited by PVD, have been quite promising as an alternate material for a Cu direct plateable diffusion barrier [3]. The concept of implanting the CueMn films is that, upon annealing, Mn segregates toward a dielectric, reacts with Si and oxygen in the underlying substrate, and forms a very conformal and ultrathin MnSiOx layer, as a selfforming barrier, at the interface between the dielectric material and CueMn films. Indeed, the formation of a MnSiOx layer, only 3 nm in thickness, was confirmed after the CueMn film (Mn content: 7.9 at.%) was annealed at 450 C for 30 min [4]. On the other hand, despite the successful demonstration, there might be some problems for CueMn films to be used as a Cu direct plateable diffusion barrier. The first problem is that Cu is in direct contact with a dielectric material and can diffuse into the dielectric material even faster than the formation of a MnSiOx layer. The second is a limitation of Cu plating over an entire wafer. A resistance of CueMn
H.-J. Lee et al. / Journal of Alloys and Compounds 686 (2016) 1025e1031
2. Experiments 2.1. Deposition
single unit cycle was comprised of a precursor injection pulse, a purge pulse, a reactant injection pulse, and another purge pulse. IMBCHDRu (0) [1-Isopropyl-4-methylbenzene) (cyclohexa-1, 3diene) Ru (0)] and Mn(II)[iPr(DAD)]2 [bis(1,4-di-isopropyl-1,3-diazabutadienyl)Mn(II)] were used as the Ru and Mn precursors, respectively. The Ru and Mn precursors are in a yellow liquid state at room temperature and were vaporized in a bubbler at 100 and 80 C, respectively, and carried to the chamber by Ar gas. NH3 and H2 plasmas were used as the reactants for the Ru and Mn precursors, respectively. Radio frequency (RF) powers of 150 W for NH3 plasma and 100 W for H2 plasma were applied to the shower-head to ignite the corresponding plasma. A basic ALD condition of this investigation was set as follows: Ru precursor pulse of 10 s, precursor purge of 10 s, NH3 reactant pulse of 15 s, and reactant purge of 10 s for Ru ALD; and Mn precursor pulse of 15 s, precursor purge of 10 s, H2 reactant pulse of 10 s, and reactant purge of 10 s for Mn ALD. By combining these two processes, ALD Ru-Mn films could be deposited, as illustrated in Fig. 1(a). Here, the number of Ru subcycles was fixed to 35 and the number of overall super-cycles, consisting of Ru and Mn sub-cycles, was 7. These super-cycles made the number of total cycles of ALD-Ru 280 because the process was terminated by an additional 35 cycles of ALD-Ru. The final ALD-Ru step was performed to prevent possible oxidation of the Mn film by air-exposure after deposition. In each super-cycle, the number of Mn ALD sub-cycles was varied from 1 to 5 cycles immediately after the Ru sub-cycles to prepare the ALD Ru-Mn films with various compositions. 2.2. Analysis of film properties The compositions of the pristine ALD-Ru and ALD Ru-Mn films were characterized by secondary ion mass spectrometry (SIMS, CAMECA IMS-6f in Korea Basic Science Institute). The microstructure of both ALD-Ru and Ru-Mn films were analyzed by plan-view transmission electron microscopy (TEM, Tecnai F20 equipped with
(a) Ru source pulse
Mn source pulse
Mn sub-cycles
Purge
Purge
Ru sub-cycles
Time
One cycles 100 80
(b)
C O Mn Ru
Ru
60 40 20 0
Ru-Mn films were deposited by repeating the super-cycles consisting of Ru and Mn ALD sub-cycles at a deposition temperature of 225 C and a working pressure of approximately 0.5 Torr using a shower-head type ALD reactor (Lucida-M100, NCD Technology, Korea). Each sub-cycle consisted of several unit cycles. A
H2 plasma Purge
NH3 plasma Purge
films is too high so the films might be damaged and etched by terminal effects during the EP of Cu [5]. The third is the lack of a suitable atomic layer deposition (ALD) Cu process that provides a perfect conformality for ever-shrinking semiconductor devices. A noble metal Ru was suggested as a replacement for both the Ta/TaN diffusion barrier and Cu seed layer for seedless Cu interconnects and Ru has advantages in a view of a Cu direct plateable diffusion barrier. For example, a successful EP-Cu on Ru has often been demonstrated [6] and many ALD processes of preparing Ru thin films have been demonstrated using a variety of Ru metallorganic precursors with excellent conformality on very narrow sized trenches and contact structures with a high aspect ratio (AR) [7e11]. On the other hand, Ru is not a suitable diffusion barrier against Cu itself due mainly to its poor microstructure with polycrystalline columnar grains [12,13]. To modify the microstructure of columnar Ru to an amorphous or a nanocrystalline structure, some compound materials, such TiN, TaN, WCN, SiNx, and AlOx [14e18], were incorporated into the Ru matrix during the ALD process, which resulted in an improvement of the diffusion barrier performance. The capability of an ALD-Ru process with a possible modification of the microstructure can shed light on the development of a Ru-Mn, Ru-based alloy, such as CueMn, as a potential Cu direct plateable diffusion barrier. According to the binary phase diagram of Ru-Mn, Ru has near-zero solubility of Mn and there are no intermetallic phases between Ru and Mn. Therefore, as in the concept of the self-forming barrier, upon annealing, Mn might diffuse toward an underlying dielectric material through Ru grain boundaries and the diffused Mn would form a highly uniform MnSiOx layer, only few nanometers in thickness. At the same time, almost pure Ru with a low resistivity would be formed on top the self-forming MnSiOx barrier. Consequently, the formation of a successful EP-Cu would be possible, avoiding the terminal effects, due to the sufficiently low resistivity of the annealed Ru-Mn films. In these regards, the Ru-Mn films can be considered a strong candidate for the Cu direct plateable diffusion barriers, as a replacement of the conventional Ta/TaN or CueMn barrier. In fact, previous reports showed that Ru-Mn films deposited by PVD offered an improved diffusion barrier performance against Cu compared to the pure Ru diffusion barrier as well as excellent Cu adhesion, but the uniformity of the PVD film on the trench structure was poor and too thick (~50 nm) if the scaling down of devices is considered [19e21]. This paper reports a new ALD process of Ru-Mn alloy thin films and their properties as a Cu direct plateable diffusion barrier. ALD Ru-Mn films were prepared using the concept of a super-cycle [17] consisting of alternate Ru and Mn ALD sub-cycles; the material properties of those films were studied using a range of characterization tools. Based on transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS), the diffusion of Mn into the underlying dielectric layer and the subsequent selfformation of a MnSiOx layer was confirmed after annealing. EP-Cu on the annealed ALD Ru-Mn barriers and the improved diffusion barrier performance of those against Cu, compared to pure ALD-Ru counterpart, were demonstrated.
Concentration (at.%)
1026
Mn 20
40
60
80
Sputter Depth (nm) Fig. 1. (a) Schematic diagram of a single ALD super-cycle consisting of Ru and Mn ALD sub-cycles, and (b) SIMS depth profile of the ALD Ru-Mn film deposited by repeating the 35 Ru-sub-cycles and 3 Mn sub-cycles.
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Fig. 2. (a) Plan-view TEM bright-field (BF) and (b) dark-field (DF) images of the ALD-Ru film. (c) plan-view TEM BF and (d) DF images of the ALD Ru-Mn film.
200 kV accelerating voltage and field emission gun) and selectedarea electron diffraction (SAED) analysis. The resistivities of the ALD Ru-Mn films were determined by measuring the sheet resistances using a four-point probe and the thicknesses by crosssectional view scanning electron microscopy (XSEM, Hitachi S4800 equipped with a 20 kV accelerating voltage and a field emission electron gun) and combining the two. Post-annealing of the Ru-Mn films was performed for 30 min at 400 and 500 C in H2 ambient to confirm the self-formation of MnSiOx barriers by crosssectional view TEM (XTEM). Before annealing, a furnace chamber was evacuated to approximately 5 106, and N2 (40 sccm) and H2 (100 sccm) gases were flowed into the chamber. Grazing-incidence angle (incident angle, q ¼ 3 ) X-ray diffraction (GID/GIXD, PANalytical X’-pert MRD with Cu-Ka radiation at 1.5 kW) was performed to elucidate microstructural evolution of the Ru-Mn films after annealing. Finally, the feasibility of Cu direct plating on the annealed ALD Ru-Mn film (10 nm in thickness) was tested by observing a plated Cu by XSEM. 3. Results and discussion The properties of the ALD-Ru films deposited by IBMCHD Ru and NH3 plasma are summarized in this paragraph because they are reported elsewhere [22]. The results showed that highly pure Ru films, without the incorporation of N, C, and O impurities based on the SIMS depth profiles, were obtained in this process. The resistivity of the pristine ALD-Ru film without the insertion of Mn sub-cycles was approximately 50 mU-cm. The properties of the pure ALD-Mn films deposited by Mn(II)[iPr(DAD)]2 and H2 plasma were also characterized. The SIMS depth profiles indicated that the ALD-
Mn films contained negligible impurities, such as C and O, and the resistivity of those was approximately 400 mU-cm, indicating that a metallic Mn film had been deposited. Indeed, SAED confirmed the formation of a cubic-phase Mn in those films. Details on the ALD characteristics and materials properties of the ALD-Mn films will be reported elsewhere. Based on those preliminary investigations, Ru-based alloy films, Ru-Mn, were deposited by repeating the super-cycles, where each super-cycle consisted of Ru ALD and Mn ALD sub-cycles, as illustrated in Fig. 1(a). The Ru-Mn films were deposited on a thermally grown SiO2 surface and analyzed by SIMS depth profiling (Fig. 1(b)). Here, the number of Ru and Mn sub-cycles in each super-cycle was 35 and 3, respectively, and the number of super-cycles was 7 making an overall number of Ru ALD cycles of 280. The SIMS depth profiles showed that the Ru and Mn (~4.2 at.%) contents through the entire film were uniform, even though the Ru and Mn ALD sub-cycles had been repeated alternately. This homogeneous composition is probably because the number of Ru and Mn sub-cycles was too small to form continuous, individual Ru and Mn films during the sub-cycles. In fact, when the number of Mn cycles was increased to 7, periodic fluctuations of the composition were observed (data not shown). The SIMS depth profiles also showed that very small amounts of carbon (~1 at.%) and oxygen (~2.3 at.%) were incorporated into the film during deposition. If the previous results on the Ru ALD process with negligible impurity levels are considered [23], these impurities might come from the Mn ALD process. The resistivity of the ALD Ru-Mn films was approximately 250 mU-cm, which is relatively higher than that of the pure Ru ALD-film (50 mU-cm) but lower than that of the pristine Mn films (400 mU-cm).
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Resistivity (μΩ-cm)
250
(a)
200 150 100 50 0
As-dep.
400
500
o
Ru (103)
Ru (110)
Ru (102)
Ru (101)
Ru (002)
Intensity (arb.units)
(b)
Ru (100)
Annealing temperature ( C)
500 C annealing
400 C annealing
As dep.
30
40
50
60
70
80
2Theta (degree) Fig. 3. (a) Resistivity of the ALD Ru-Mn films (Mn content: 4.2 at.%) annealed at various temperatures and (b) corresponding XRD results.
The relevant Ru-Mn films were analyzed by TEM to confirm the phases and characterize microstructures in detail. For comparison, TEM was also performed on the ALD-Ru film. The plan-view TEM bright-field (BF) image of the ALD-Ru film (Fig. 2(a)) showed polycrystalline grains and the corresponding SED pattern [inset of Fig. 2(a)] indicated that the films formed hexagonal-close-packed (HCP) Ru with randomly-oriented polycrystalline grains. The size of the Ru grains was measured by taking a dark-field (DF) TEM image with the objective aperture placed into the first bright ring of the SAED pattern. Fig. 2(b) shows that the grain size was approximately 15 nm Fig. 2(c) shows a plan-view TEM BF image of the ALD Ru-Mn film deposited with periodic Ru ALD and Mn ALD sub-cycles of 35 and 3, respectively. First, the grain size appeared to be much smaller than that of the pure ALD-Ru counterpart. The corresponding SED pattern, which was taken by placing the selected area aperture with the same size [inset of Fig. 2(c)] exhibited less spotty ring-type patterns than those of the pure ALD-Ru film. This result
would support the decrease in grain size by adding the periodic Mn sub-cycles during ALD-Ru growth. As revealed in the plan-view TEM DF image (Fig. 2(d)), the mean grain size of the ALD Ru-Mn film decreased to ~8 nm. Second, the SED pattern still coincided with that of HCP-Ru, suggesting that the grains observed in the BF and DF TEM images were crystalline Ru. This microstructural evolution was observed more clearly in the plan-view TEM BF image of the film deposited with a higher Mn content (data not shown). Fig. 3(a) shows the resistivity of the ALD-grown Ru-Mn films as a function of the post-annealing temperature. The samples were annealed in a hydrogen environment for 30 min. A drastic decrease in the resistivity of the ALD Ru-Mn thin film was observed after annealing at 400 C. When the annealing temperature increased further to 500 C, its resistivity decreased further to approximately 20 mU-cm and this value was even lower than that of the pure ALDRu deposited without the addition of the periodic Mn ALD subcycles (50 mU-cm). Fig. 3(b) shows the corresponding XRD results with various annealing temperatures. In the case of the asdeposited ALD Ru-Mn film, a broad hump, ranging from 2q of 38 and 44 , was detected, which consisted of three separate peaks that could be identified as those from HCP-Ru (100) [2q ¼ 38.38 , d ¼ 2.34 Å], (002) [2q ¼ 42.15 , d ¼ 2.14 Å], and (101) [2q ¼ 44.00 , d ¼ 2.05 Å]. On the other hand, once the sample was annealed at 400 C, the broad hump disappeared and three distinct main peaks from HCP-Ru appeared more clearly. The XRD results on the sample annealed at 500 C showed that the peak intensities from HCP-Ru significantly and the full widths at half maximum became narrower. These aspects indicate the improvement of the crystallinity and the increase in grains size. A new XRD peak from Ru also developed at 2q ¼ 58.32 . These results suggest that the drastic decrease in resistivity was due mainly to the improved crystallinity of Ru upon annealing. Fig. 4 shows the XTEM images of the ALD Ru-Mn films (a) before (as-deposited) and (b) after annealing at 500 C. The grains were quite small and showed continuous columnar grain growth (Fig. 4(a)), which is typically observed in ALD-Ru films, was prevented by the interruption of ALD-Mn inserted periodically during the ALD-Ru process. Porous regions between the small-sized grains were observed easily, indicating that the grains were loosely packed and the film density might be low. On the other hand, after annealing, well-developed columnar grains with a size of ~25 nm were observed; this columnar microstructure was clearly different from that of the as-grown film in Fig. 4(a). In addition, after postannealing, grain growth was also confirmed from the XTEM image of the corresponding film (Fig. 4(b)), supporting the drastic decrease in film resistivity in Fig. 3(a). Interestingly, at the interface of the annealed film and the underlying SiO2, an ultrathin (~2 nm in thickness) layer was clearly observed. Fig. 5 showed the SIMS depth profile for the annealed ALD RuMn film. SIMS depth profiles clearly showed that after annealing at 500 C, the most of Mn was moved to the interface between SiO2 and annealed ALD Ru-Mn and it was piled up there while the SIMS depth profiles of as-deposited ALD Ru-Mn film showed Mn (~4.2 at.%) contents through the entire film were seems to be uniform (Fig. 1(b)). Thus, SIMS depth profile clearly supported that the Mn in the as-deposited ALD Ru-Mn film diffused into the underlying SiO2 and Mn silicate was finally formed at the interface between the annealed ALD Ru-Mn film and SiO2 while quite pure Ru film was formed on it. It was also confirmed by energy dispersive spectroscopy (EDS) analysis of TEM (not shown here) that Mn in the as-deposited ALD Ru-Mn film diffused into the underlying SiO2 after annealed at 400 C and Mn silicate was finally self-formed at the interface between the annealed ALD Ru-Mn film and SiO2. XTEM showed that the thickness of as-deposited ALD Ru-Mn sample (~55 nm) had significantly decreased to ~30 nm after
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Fig. 4. Cross-sectional view TEM images of (a) as-deposited ALD Ru-Mn film and (b) annealed film at 500 C.
Concentration (at. %)
100
C O Si Mn Ru
Ru 80
O
60 40
Si
20 0
Mn 20
40
60
80
Sputter Depth (nm) Fig. 5. SIMS depth profile of ALD Ru-Mn film, which was prepared by repeating 35 Rusub-cycles and 3 Mn sub-cycles after annealing at 500 C for 30 min.
pure ALD-Ru film (~19 nm). This suggests that surface scattering to increase the resistivity is much higher for both the annealed ALD Ru-Mn and a pure ALD-Ru film. Consequently, surface scattering could not explain the higher resistivity of the as-deposited ALD RuMn film. Second, Mn impurities in the ALD Ru-Mn films can increase their resistivity and many studies of metal or metal alloy thin films showed an increase in resistivity with the incorporation of impurities [2527]. On the other hand, after annealing at 500 C, the Mn in the as-deposited ALD Ru-Mn film diffused into the underlying SiO2 and Mn silicate was finally formed at the interface between the annealed ALD Ru-Mn film and SiO2, and a pure Ru film was formed on top of it. The decrease in the resistivity of ALD RuMn after annealing was due partly to the elimination of impurities in the film. Third, TEM showed the grain size of pure ALD-Ru
Sheet Resistance (mΩ/seq.)
annealing at 500 C. The thickness reduction after annealing was attributed to the densification, impurity elimination with the selfformation of Mn silicate due to the diffusion of Mn toward SiO2, and grain growth, which was characterized by XTEM. The above investigations gave clues to understand the change in the film properties after annealing the ALD Ru-Mn film. The film resistivity of the as-deposited ALD Ru-Mn films (250 mU-cm) was higher than that of the pure ALD-Ru film deposited without the Mn ALD cycles (50 mU-cm). Note, however, that the resistivity of the ALD-RuMn film decreased drastically to 65 mU-cm after annealing at 400 C and decreased further to ~20 mU-cm after annealing at 500 C, as shown by Fig. 3(a). Several factors can be considered to explain the high resistivity of the as-deposited ALD Ru-Mn films compared to that of the annealed ALD Ru-Mn film and a pure ALDRu film. Generally, impurities incorporation, surface scattering (the “size effect”), and grain boundary scattering are responsible for increasing the resistivity of a metal film [23,24]. First, in this study, the thickness of the as-deposited ALD Ru-Mn film (~55 nm) was even higher than those of the annealed ALD Ru-Mn (~25 nm) and
300.0k
Cu/ALD-Ru/Si Cu/ALD-RuMn/Si
250.0k 200.0k 150.0k 100.0k 50.0k 4k 3k 2k 1k 0
400
450
500
550
600
650
Annealing Temperature (oC) Fig. 6. Sheet resistance of the Cu (50 nm)/ALD-Ru (5 nm)/Si and Cu (50 nm)/ALD RuMn (~5 nm)/Si structures after annealed.
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was approximately 15 nm, whereas the average grain size of the asdeposited ALD Ru-Mn film decreased to ~8 nm. TEM also revealed grain growth after annealing the ALD Ru-Mn. Therefore, the formation of much larger-sized Ru grains for both the pure ALD-Ru film and annealed ALD Ru-Mn film could be another reason for the reduced resistivity. Once the material properties of the ALD-grown Ru-Mn films were characterized, this study focused on evaluating the diffusion barrier performance of the ALD Ru-Mn films against Cu. For the evaluation, an ultrathin (5 nm) ALD-Ru or Ru-Mn film was grown on a Si wafer and Cu film (50 nm in thickness) was deposited on top of the diffusion barrier. Fig. 6 shows the sheet resistances of the Cu (50 nm)/ALD-Ru (5 nm)/Si wafer and Cu (50 nm)/ALD Ru-Mn
(~5 nm)/Si wafer samples as a function of the post-annealing temperature. The sheet resistance in this kind of stack is determined mostly by the conditions and quality of the Cu layer because it has the lowest resistance among the layers and carries almost all the sensor current, whereas the sheet resistance contributions from the Ru and Ru-Mn films can be neglected because of their high resistivity (Ru: 50 mU cm and Ru-Mn: 250 mU cm) and narrow thickness compared to those of Cu. As shown in Fig. 6, the sheet resistance of both samples decreased slightly after annealing at 450 C and 500 C, possibly due to the increased grain size and defect annihilation of the Cu film. In the case of the Cu (50 nm)/ Ru(5 nm)/Si sample, the sheet resistance began to increase once annealed at 550 C and increased drastically by several orders of magnitude after annealing at 600 C, which means the failure of the ALD-Ru diffusion barrier at that temperature. In contrast, in the case of the Cu (50 nm)/Ru Mn(~5 nm)/Si sample, there was no change in the sheet resistance until the samples were annealed at 600 C. As the temperature was increased to 650 C, the resistivity of the structure increased abruptly. These results strongly suggest that the ALD-grown Ru-Mn films can prevent Cu diffusion more effectively than the Ru counterpart. XRD was performed on the annealed samples to determine the possible reasons for the increase in sheet resistance of the Ru and Ru-Mn samples. Fig. 7(a) and (b) show the corresponding XRD
Fig. 7. XRD results of (a) Cu (50 nm)/ALD-Ru (5 nm)/Si and (b) Cu (50 nm)/ALD Ru-Mn (~5 nm)/Si structures as a function of the annealing temperature.
Fig. 8. (a) Plan-view and (b) cross-section SEM image of the electroplated Cu on the annealed ALD Ru-Mn film.
H.-J. Lee et al. / Journal of Alloys and Compounds 686 (2016) 1025e1031
patterns of the Cu/Ru/Si and Cu/Ru-Mn/Si samples, respectively, annealed at various temperatures. In the case of the sample with the ALD-Ru diffusion barrier (Fig. 7(a)), the Cu15Si4 (2q ¼ 34.74) peak was first detected after annealing at 500 C, indicating the failure of the diffusion barrier due to the diffusion of Cu through it. On the other hand, the intensity was weak and strong Cu peaks were still observed. When the annealing temperature was increased to 550 C, new peaks related to Cu3Si (2q ¼ 27.96, 44.57, 44.99, and 65.23) were observed with significantly high intensities. This result was well matched with the results of the sheet resistance showing an initial increase. Finally, various types of Cu silicides were detected with a concomitant decrease in intensity of the Cu peak when the annealing temperature was increased further to 600 C, corresponding to the abrupt increase in sheet resistance in Fig. 6(a). In contrast, the ALD Ru-Mn diffusion barrier samples showed no peaks related to Cu silicides until annealed at 550 C. Once the Ru-Mn samples were annealed at 600 C, small peaks related to Cu3Si were observed, but the peak intensity from Cu was still strong, suggesting that the diffusion barrier failure was only minimal. Therefore, the diffusion barrier performance of the ALDgrown Ru-Mn film appears to be superior to the ALD-Ru counterpart. This superior performance can be attributed to several aspects, such as the self-formation of a Mn silicate diffusion barrier, stuffing of the grain boundaries of Ru by the added Mn, and the prevention of columnar grain growth of the ALD-Ru film by inserting the periodic Mn ALD sub-cycles. Finally, the capability of the direct plating of Cu on top of the ALD Ru-Mn film was examined. The ALD Ru-Mn film was annealed at 400 C before direct plating and the resistivity of the annealed Ru-Mn film was as low as ~36 mU-cm, which would guarantee a successful EP-Cu. Fig. 8 shows a plan-view and cross-sectional view SEM image of a Cu film electrodeposited on the annealed Ru-Mn film. As expected from its low resistivity and the existence of Ru on the surface, a very high area density of extremely small Cu nuclei was observed, indicating the rapid nucleation of EP-Cu on the annealed ALD Ru-Mn films. The EP-Cu films were grown continuously on the annealed ALD-grown Ru-Mn film. Such capability of the direct Cu plating, in conjunction with the excellent step coverage and ultra-small volume enabled by the ALD process, should be highlighted as a potential replacement of the diffusion barrier and seed layer materials. 3. Summary and conclusions Ru-based alloy films, Ru-Mn, were prepared by repeating the super-cycles consisting of Ru and Mn ALD sub-cycles at 225 C for the diffusion barrier materials of seedless Cu interconnects. Here, Ru and Mn ALD was performed using the sequential supply of IMBCHDRu(0) precursor/NH3 plasma and Mn(II)[iPr(DAD)]2 precursor/H2 plasma, respectively. Mn could be incorporated into the ALD-Ru film by the periodic addition of Mn ALD sub-cycles during the ALD-Ru process. Plan-view TEM analysis showed that the grains can be reduced significantly from ~15 nm (pristine Ru) to ~8 nm (Ru-Mn) and continuous columnar grain growth of the ALD-Ru film could be prevented by the addition of a periodic Mn ALD sub-cycle. Both the XTEM and SIMS analysis further showed that the MnSiOx diffusion barrier layer was self-formed at the interface of the RuMn film and the underlying SiO2 surface after annealing the ALD-
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Ru-Mn at 500 C. Both the superior diffusion barrier performance of the ALD Ru-Mn film against Cu compared to the ALD-Ru counterpart and the successful EP-Cu on the annealed ALD Ru-Mn barrier were also demonstrated. Finally, these results suggest that the ALD-grown Ru-Mn films are viable candidates for a Cu direct plateable diffusion barrier for ever-shrinking semiconductor devices.
Acknowledgements This study (2015R1A2A2A04004945) was supported by Midcareer Researcher Program through NRF grant funded by the MEST and also partially supported by a KBSI Grant (T33524) to T.E. Hong.
References [1] S.M. Rossnagel, T.S. Kuan, J. Vac. Sci. Technol. B 22 (2004) 240. € gl, G. Schindler, G. Steinlesberger, M. Engelhardt, Phys. Rev. B 66 [2] W. Steinho (2002) 075414. [3] M. Haneda, J. Iijima, J. Koike, Appl. Phys. Lett. 90 (2007) 252107. [4] J. Koike, M. Wada, Appl. Phys. Lett. 87 (2005) 041911. [5] K.M. Takahashi, J. Electrochem. Soc. 147 (2000) 1414e1417. [6] C.-C. Yang, T. Spooner, S. Ponoth, K. Chanda, A. Simon, C. Lavoie, M. Lane, C.K. Hu, E. Liniger, L. Gignac, T. Shaw, S. Cohen, F. McFeely, D. Edelstein, Interconnect Technology Conf, IEEE, 2006, pp. 187e190. €n, M. Ritala, M. Leskela €, Chem. Vap. Depos. 9 (2003) 45. [7] T. Aaltonen, P. Ala [8] N. Leick, R.O.F. Verkuijlen, L. Lamagna, E. Langereis, S. Rushworth, F. Roozeboom, M.C.M. van de Sanden, W.M.M. Kessels, J. Vac. Sci. Technol. A 29 (2011) 021016. [9] S.-J. Park, W.-H. Kim, H.-B.-R. Lee, W.J. Maeng, H. Kim, Microelectron. Eng. 85 (2008) 39. [10] W.H. Kim, S.-J. Park, J.-Y. Son, H. Kim, Nanotechnology 19 (2008) 045302. [11] Seungmin Yeo, Ji-Yoon Park, Seung-Joon Lee, Do-Joong Lee, Jong Hyun Seo, Soo-Hyun Kim, Microelectron. Eng. 137 (2015) 16e22. [12] T.N. Arunagiri, Y. Zhang, O. Chyan, M. El-Bouanani, M.J. Kim, K.H. Chen, C.T. Wu, L.C. Chen, Appl. Phys. Lett. 86 (2005) 083104. [13] R. Chan, T.N. Arunagiri, Y. Zhang, O. Chyan, R.M. Wallace, M.J. Kim, T.Q. Hurd, Electrochem. Solid-state Lett. 7 (8) (2004) G154eG157. [14] S.-W. Kim, S.-H. Kwon, S.-J. Jeong, S.-W. Kang, J. Electrochem. Soc. 155 (2008) H885. [15] S. Kumar, D. Greenslit, T. Chakraborty, E.T. Eisenbraun, J. Vac. Sci. Technol. A 27 (2009) 572. [16] D. Greenslit, S. Kumar, T. Chakraborty, E.T. Eisenbraun, ECS Trans. 13 (8) (2008) 63. [17] T.-K. Eom, S.-H. Kim, K.-S. Park, S. Kim, H. Kim, Electrochem. Solid-State Lett. 14 (1) (2011) D10. [18] T. Cheon, S.-H. Chio, S.-H. Kim, D.-H. Kang, Electrochem. Solid-State Lett. 14 (5) (2011) D57eD61. [19] H. Wojcik, C. Krien, U. Merkel, J.W. Bartha, M. Knaut, M. Geidel, B. Adolphi, V. Neumann, C. Wenzel, M. Bendlin, K. Richter, D. Makarov, Microelectron. Eng. 112 (2013) 103e109. [20] H. Wojcik, R. Kaltofen, U. Merkel, C. Krien, S. Strehle, J. Gluch, M. Knaut, C. Wenzel, A. Preusse, J.W. Bartha, M. Geidel, B. Adolphi, V. Neumann, R. Liske, F. Munnik, Microelectron. Eng. 92 (2012) 71e75. [21] Henry Wojcik, et al., Investigations on Ru-Mn Films as Plateable Cu Diffusion barriers.” Interconnect Technology Conference and 2011 Materials for Advanced Metallization (IITC/MAM), 2011 IEEE International, IEEE, 2011, pp. 1e3. [22] W. Sari, T.-K. Eom, S.-H. Kim, H. Kim, J. Electrochem. Soc. 158 (1) (2011) D42eD47. [23] S.M. Rossnagel, J. Vac, Sci. Technol. B20 (2002) 2328. [24] T.S. Kuan, C.K. Inoki, G.S. Oehrlein, K. Ross, Y.-P. Zhao, G.-C. Wang, S.M. Rossnagel, C. Cabral Jr., Mater. Res. Soc. Symp. Proc. 612 (2000) D7, 1.1. [25] S.P. Murarka, in: Metallization: Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, MA, 1993, p. 50 (Chapter. 4). [26] I.A. Weerasekera, S.I. Shah, D.V. Baxter, K.M. Unruh, Appl. Phys. Lett. 64 (1994) 3231. [27] J. Ligot, S. Benayoun, J.J. Hantzpergue, J. Vac. Sci. Technol. A19 (2001) 798.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Atomic layer deposited self-forming Ru-Mn diffusion barrier for seedless Cu interconnects Hyun-Jung Lee a, Tae Eun Hong b, Soo-Hyun Kim a, * a b
School of Materials Science and Engineering, Yeungnam University, Gyeongsan-si, 712-749, South Korea Busan Center, Korea Basic Science Institute, 1275 Jisadong, Gangseogu, Busan, 618-230, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2016 Received in revised form 20 June 2016 Accepted 27 June 2016 Available online 29 June 2016
Ru-based alloy thin films were prepared by atomic layer deposition (ALD) at a deposition temperature of 225 C for use as a diffusion barrier for seedless Cu interconnects. Ru-Mn alloy thin films were grown by a repetition of super-cycles consisting of multiple Ru and Mn ALD sub-cycles. Ru and Mn ALD was performed using a sequential supply of a Ru precursor [1-Isopropyl-4-methylbenzene (cyclohexa-1, 3-diene) Ru (0)] and NH3 plasma, and a Mn precursor [bis(1,4-di-isopropyl-1,3-diazabutadienyl)Mn(II)] and H2 plasma, respectively. Secondary ion mass spectrometry (SIMS) indicated that Mn had been incorporated successfully into the Ru film by the periodic addition of Mn ALD sub-cycles during the ALD-Ru process. Plan-view transmission electron microscopy (TEM) showed that the grain size was reduced obviously from ~15 nm (pristine Ru) to ~8 nm (Ru-Mn alloy) and continuous columnar grain growth of the ALD-Ru film was prevented by the addition of the periodic Mn ALD sub-cycles. After annealing at 500 C, the formation of a MnSiOx diffusion barrier was observed between the Ru-Mn film and underlying SiO2 surface by both cross-sectional view TEM and energy dispersive spectroscopy analysis and confirmed by SIMS depth profile. X-ray diffraction showed that the ultrathin ALD Ru-Mn thin film (~5 nm) prevented Cu diffusion up to an annealing temperature of 600 C, whereas the ALD-Ru thin film with the same thickness failed to prevent Cu diffusion only after annealing at 500 C. Finally, the capability of the direct plating of Cu was demonstrated on the annealed ALD Ru-Mn film. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ruthenium Manganese Ru-Mn alloy Atomic layer deposition Self-forming barrier Direct plating of Cu
1. Introduction As a part of a manufacturing process of integrated circuits, Cu prepared by electroplating (EP) has been implemented widely in interconnect materials of logic devices owing to its very low resistivity and inexpensive, simple, and high-volume manufacturing. On the other hand, because semiconductor devices have continued scaling down into the nanometer range, there have been critical issues on the drastic increase in the Cu line resistance [1,2]. Therefore, the volume of the EP-Cu filled into the trenches should be increased to prevent an increase in interconnect resistance, which has forced a decrease in the volume in a stack of three layers, which are deposited by physical vapor deposition (PVD), such as the TaN diffusion barrier, Ta adhesion promoter, and Cu seed layer. As a potential solution, by replacing the stack of three layers into a single layer, called a Cu direct plateable diffusion barrier, EP-Cu can
* Corresponding author. E-mail address: [email protected] (S.-H. Kim). http://dx.doi.org/10.1016/j.jallcom.2016.06.270 0925-8388/© 2016 Elsevier B.V. All rights reserved.
be readily filled and the increase in Cu line resistance can be minimized. In the pursuit of a suitable and reliable Cu direct plateable diffusion barrier, many materials and processes have been reported. Among them, CueMn films, which are typically deposited by PVD, have been quite promising as an alternate material for a Cu direct plateable diffusion barrier [3]. The concept of implanting the CueMn films is that, upon annealing, Mn segregates toward a dielectric, reacts with Si and oxygen in the underlying substrate, and forms a very conformal and ultrathin MnSiOx layer, as a selfforming barrier, at the interface between the dielectric material and CueMn films. Indeed, the formation of a MnSiOx layer, only 3 nm in thickness, was confirmed after the CueMn film (Mn content: 7.9 at.%) was annealed at 450 C for 30 min [4]. On the other hand, despite the successful demonstration, there might be some problems for CueMn films to be used as a Cu direct plateable diffusion barrier. The first problem is that Cu is in direct contact with a dielectric material and can diffuse into the dielectric material even faster than the formation of a MnSiOx layer. The second is a limitation of Cu plating over an entire wafer. A resistance of CueMn
H.-J. Lee et al. / Journal of Alloys and Compounds 686 (2016) 1025e1031
2. Experiments 2.1. Deposition
single unit cycle was comprised of a precursor injection pulse, a purge pulse, a reactant injection pulse, and another purge pulse. IMBCHDRu (0) [1-Isopropyl-4-methylbenzene) (cyclohexa-1, 3diene) Ru (0)] and Mn(II)[iPr(DAD)]2 [bis(1,4-di-isopropyl-1,3-diazabutadienyl)Mn(II)] were used as the Ru and Mn precursors, respectively. The Ru and Mn precursors are in a yellow liquid state at room temperature and were vaporized in a bubbler at 100 and 80 C, respectively, and carried to the chamber by Ar gas. NH3 and H2 plasmas were used as the reactants for the Ru and Mn precursors, respectively. Radio frequency (RF) powers of 150 W for NH3 plasma and 100 W for H2 plasma were applied to the shower-head to ignite the corresponding plasma. A basic ALD condition of this investigation was set as follows: Ru precursor pulse of 10 s, precursor purge of 10 s, NH3 reactant pulse of 15 s, and reactant purge of 10 s for Ru ALD; and Mn precursor pulse of 15 s, precursor purge of 10 s, H2 reactant pulse of 10 s, and reactant purge of 10 s for Mn ALD. By combining these two processes, ALD Ru-Mn films could be deposited, as illustrated in Fig. 1(a). Here, the number of Ru subcycles was fixed to 35 and the number of overall super-cycles, consisting of Ru and Mn sub-cycles, was 7. These super-cycles made the number of total cycles of ALD-Ru 280 because the process was terminated by an additional 35 cycles of ALD-Ru. The final ALD-Ru step was performed to prevent possible oxidation of the Mn film by air-exposure after deposition. In each super-cycle, the number of Mn ALD sub-cycles was varied from 1 to 5 cycles immediately after the Ru sub-cycles to prepare the ALD Ru-Mn films with various compositions. 2.2. Analysis of film properties The compositions of the pristine ALD-Ru and ALD Ru-Mn films were characterized by secondary ion mass spectrometry (SIMS, CAMECA IMS-6f in Korea Basic Science Institute). The microstructure of both ALD-Ru and Ru-Mn films were analyzed by plan-view transmission electron microscopy (TEM, Tecnai F20 equipped with
(a) Ru source pulse
Mn source pulse
Mn sub-cycles
Purge
Purge
Ru sub-cycles
Time
One cycles 100 80
(b)
C O Mn Ru
Ru
60 40 20 0
Ru-Mn films were deposited by repeating the super-cycles consisting of Ru and Mn ALD sub-cycles at a deposition temperature of 225 C and a working pressure of approximately 0.5 Torr using a shower-head type ALD reactor (Lucida-M100, NCD Technology, Korea). Each sub-cycle consisted of several unit cycles. A
H2 plasma Purge
NH3 plasma Purge
films is too high so the films might be damaged and etched by terminal effects during the EP of Cu [5]. The third is the lack of a suitable atomic layer deposition (ALD) Cu process that provides a perfect conformality for ever-shrinking semiconductor devices. A noble metal Ru was suggested as a replacement for both the Ta/TaN diffusion barrier and Cu seed layer for seedless Cu interconnects and Ru has advantages in a view of a Cu direct plateable diffusion barrier. For example, a successful EP-Cu on Ru has often been demonstrated [6] and many ALD processes of preparing Ru thin films have been demonstrated using a variety of Ru metallorganic precursors with excellent conformality on very narrow sized trenches and contact structures with a high aspect ratio (AR) [7e11]. On the other hand, Ru is not a suitable diffusion barrier against Cu itself due mainly to its poor microstructure with polycrystalline columnar grains [12,13]. To modify the microstructure of columnar Ru to an amorphous or a nanocrystalline structure, some compound materials, such TiN, TaN, WCN, SiNx, and AlOx [14e18], were incorporated into the Ru matrix during the ALD process, which resulted in an improvement of the diffusion barrier performance. The capability of an ALD-Ru process with a possible modification of the microstructure can shed light on the development of a Ru-Mn, Ru-based alloy, such as CueMn, as a potential Cu direct plateable diffusion barrier. According to the binary phase diagram of Ru-Mn, Ru has near-zero solubility of Mn and there are no intermetallic phases between Ru and Mn. Therefore, as in the concept of the self-forming barrier, upon annealing, Mn might diffuse toward an underlying dielectric material through Ru grain boundaries and the diffused Mn would form a highly uniform MnSiOx layer, only few nanometers in thickness. At the same time, almost pure Ru with a low resistivity would be formed on top the self-forming MnSiOx barrier. Consequently, the formation of a successful EP-Cu would be possible, avoiding the terminal effects, due to the sufficiently low resistivity of the annealed Ru-Mn films. In these regards, the Ru-Mn films can be considered a strong candidate for the Cu direct plateable diffusion barriers, as a replacement of the conventional Ta/TaN or CueMn barrier. In fact, previous reports showed that Ru-Mn films deposited by PVD offered an improved diffusion barrier performance against Cu compared to the pure Ru diffusion barrier as well as excellent Cu adhesion, but the uniformity of the PVD film on the trench structure was poor and too thick (~50 nm) if the scaling down of devices is considered [19e21]. This paper reports a new ALD process of Ru-Mn alloy thin films and their properties as a Cu direct plateable diffusion barrier. ALD Ru-Mn films were prepared using the concept of a super-cycle [17] consisting of alternate Ru and Mn ALD sub-cycles; the material properties of those films were studied using a range of characterization tools. Based on transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS), the diffusion of Mn into the underlying dielectric layer and the subsequent selfformation of a MnSiOx layer was confirmed after annealing. EP-Cu on the annealed ALD Ru-Mn barriers and the improved diffusion barrier performance of those against Cu, compared to pure ALD-Ru counterpart, were demonstrated.
Concentration (at.%)
1026
Mn 20
40
60
80
Sputter Depth (nm) Fig. 1. (a) Schematic diagram of a single ALD super-cycle consisting of Ru and Mn ALD sub-cycles, and (b) SIMS depth profile of the ALD Ru-Mn film deposited by repeating the 35 Ru-sub-cycles and 3 Mn sub-cycles.
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Fig. 2. (a) Plan-view TEM bright-field (BF) and (b) dark-field (DF) images of the ALD-Ru film. (c) plan-view TEM BF and (d) DF images of the ALD Ru-Mn film.
200 kV accelerating voltage and field emission gun) and selectedarea electron diffraction (SAED) analysis. The resistivities of the ALD Ru-Mn films were determined by measuring the sheet resistances using a four-point probe and the thicknesses by crosssectional view scanning electron microscopy (XSEM, Hitachi S4800 equipped with a 20 kV accelerating voltage and a field emission electron gun) and combining the two. Post-annealing of the Ru-Mn films was performed for 30 min at 400 and 500 C in H2 ambient to confirm the self-formation of MnSiOx barriers by crosssectional view TEM (XTEM). Before annealing, a furnace chamber was evacuated to approximately 5 106, and N2 (40 sccm) and H2 (100 sccm) gases were flowed into the chamber. Grazing-incidence angle (incident angle, q ¼ 3 ) X-ray diffraction (GID/GIXD, PANalytical X’-pert MRD with Cu-Ka radiation at 1.5 kW) was performed to elucidate microstructural evolution of the Ru-Mn films after annealing. Finally, the feasibility of Cu direct plating on the annealed ALD Ru-Mn film (10 nm in thickness) was tested by observing a plated Cu by XSEM. 3. Results and discussion The properties of the ALD-Ru films deposited by IBMCHD Ru and NH3 plasma are summarized in this paragraph because they are reported elsewhere [22]. The results showed that highly pure Ru films, without the incorporation of N, C, and O impurities based on the SIMS depth profiles, were obtained in this process. The resistivity of the pristine ALD-Ru film without the insertion of Mn sub-cycles was approximately 50 mU-cm. The properties of the pure ALD-Mn films deposited by Mn(II)[iPr(DAD)]2 and H2 plasma were also characterized. The SIMS depth profiles indicated that the ALD-
Mn films contained negligible impurities, such as C and O, and the resistivity of those was approximately 400 mU-cm, indicating that a metallic Mn film had been deposited. Indeed, SAED confirmed the formation of a cubic-phase Mn in those films. Details on the ALD characteristics and materials properties of the ALD-Mn films will be reported elsewhere. Based on those preliminary investigations, Ru-based alloy films, Ru-Mn, were deposited by repeating the super-cycles, where each super-cycle consisted of Ru ALD and Mn ALD sub-cycles, as illustrated in Fig. 1(a). The Ru-Mn films were deposited on a thermally grown SiO2 surface and analyzed by SIMS depth profiling (Fig. 1(b)). Here, the number of Ru and Mn sub-cycles in each super-cycle was 35 and 3, respectively, and the number of super-cycles was 7 making an overall number of Ru ALD cycles of 280. The SIMS depth profiles showed that the Ru and Mn (~4.2 at.%) contents through the entire film were uniform, even though the Ru and Mn ALD sub-cycles had been repeated alternately. This homogeneous composition is probably because the number of Ru and Mn sub-cycles was too small to form continuous, individual Ru and Mn films during the sub-cycles. In fact, when the number of Mn cycles was increased to 7, periodic fluctuations of the composition were observed (data not shown). The SIMS depth profiles also showed that very small amounts of carbon (~1 at.%) and oxygen (~2.3 at.%) were incorporated into the film during deposition. If the previous results on the Ru ALD process with negligible impurity levels are considered [23], these impurities might come from the Mn ALD process. The resistivity of the ALD Ru-Mn films was approximately 250 mU-cm, which is relatively higher than that of the pure Ru ALD-film (50 mU-cm) but lower than that of the pristine Mn films (400 mU-cm).
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Resistivity (μΩ-cm)
250
(a)
200 150 100 50 0
As-dep.
400
500
o
Ru (103)
Ru (110)
Ru (102)
Ru (101)
Ru (002)
Intensity (arb.units)
(b)
Ru (100)
Annealing temperature ( C)
500 C annealing
400 C annealing
As dep.
30
40
50
60
70
80
2Theta (degree) Fig. 3. (a) Resistivity of the ALD Ru-Mn films (Mn content: 4.2 at.%) annealed at various temperatures and (b) corresponding XRD results.
The relevant Ru-Mn films were analyzed by TEM to confirm the phases and characterize microstructures in detail. For comparison, TEM was also performed on the ALD-Ru film. The plan-view TEM bright-field (BF) image of the ALD-Ru film (Fig. 2(a)) showed polycrystalline grains and the corresponding SED pattern [inset of Fig. 2(a)] indicated that the films formed hexagonal-close-packed (HCP) Ru with randomly-oriented polycrystalline grains. The size of the Ru grains was measured by taking a dark-field (DF) TEM image with the objective aperture placed into the first bright ring of the SAED pattern. Fig. 2(b) shows that the grain size was approximately 15 nm Fig. 2(c) shows a plan-view TEM BF image of the ALD Ru-Mn film deposited with periodic Ru ALD and Mn ALD sub-cycles of 35 and 3, respectively. First, the grain size appeared to be much smaller than that of the pure ALD-Ru counterpart. The corresponding SED pattern, which was taken by placing the selected area aperture with the same size [inset of Fig. 2(c)] exhibited less spotty ring-type patterns than those of the pure ALD-Ru film. This result
would support the decrease in grain size by adding the periodic Mn sub-cycles during ALD-Ru growth. As revealed in the plan-view TEM DF image (Fig. 2(d)), the mean grain size of the ALD Ru-Mn film decreased to ~8 nm. Second, the SED pattern still coincided with that of HCP-Ru, suggesting that the grains observed in the BF and DF TEM images were crystalline Ru. This microstructural evolution was observed more clearly in the plan-view TEM BF image of the film deposited with a higher Mn content (data not shown). Fig. 3(a) shows the resistivity of the ALD-grown Ru-Mn films as a function of the post-annealing temperature. The samples were annealed in a hydrogen environment for 30 min. A drastic decrease in the resistivity of the ALD Ru-Mn thin film was observed after annealing at 400 C. When the annealing temperature increased further to 500 C, its resistivity decreased further to approximately 20 mU-cm and this value was even lower than that of the pure ALDRu deposited without the addition of the periodic Mn ALD subcycles (50 mU-cm). Fig. 3(b) shows the corresponding XRD results with various annealing temperatures. In the case of the asdeposited ALD Ru-Mn film, a broad hump, ranging from 2q of 38 and 44 , was detected, which consisted of three separate peaks that could be identified as those from HCP-Ru (100) [2q ¼ 38.38 , d ¼ 2.34 Å], (002) [2q ¼ 42.15 , d ¼ 2.14 Å], and (101) [2q ¼ 44.00 , d ¼ 2.05 Å]. On the other hand, once the sample was annealed at 400 C, the broad hump disappeared and three distinct main peaks from HCP-Ru appeared more clearly. The XRD results on the sample annealed at 500 C showed that the peak intensities from HCP-Ru significantly and the full widths at half maximum became narrower. These aspects indicate the improvement of the crystallinity and the increase in grains size. A new XRD peak from Ru also developed at 2q ¼ 58.32 . These results suggest that the drastic decrease in resistivity was due mainly to the improved crystallinity of Ru upon annealing. Fig. 4 shows the XTEM images of the ALD Ru-Mn films (a) before (as-deposited) and (b) after annealing at 500 C. The grains were quite small and showed continuous columnar grain growth (Fig. 4(a)), which is typically observed in ALD-Ru films, was prevented by the interruption of ALD-Mn inserted periodically during the ALD-Ru process. Porous regions between the small-sized grains were observed easily, indicating that the grains were loosely packed and the film density might be low. On the other hand, after annealing, well-developed columnar grains with a size of ~25 nm were observed; this columnar microstructure was clearly different from that of the as-grown film in Fig. 4(a). In addition, after postannealing, grain growth was also confirmed from the XTEM image of the corresponding film (Fig. 4(b)), supporting the drastic decrease in film resistivity in Fig. 3(a). Interestingly, at the interface of the annealed film and the underlying SiO2, an ultrathin (~2 nm in thickness) layer was clearly observed. Fig. 5 showed the SIMS depth profile for the annealed ALD RuMn film. SIMS depth profiles clearly showed that after annealing at 500 C, the most of Mn was moved to the interface between SiO2 and annealed ALD Ru-Mn and it was piled up there while the SIMS depth profiles of as-deposited ALD Ru-Mn film showed Mn (~4.2 at.%) contents through the entire film were seems to be uniform (Fig. 1(b)). Thus, SIMS depth profile clearly supported that the Mn in the as-deposited ALD Ru-Mn film diffused into the underlying SiO2 and Mn silicate was finally formed at the interface between the annealed ALD Ru-Mn film and SiO2 while quite pure Ru film was formed on it. It was also confirmed by energy dispersive spectroscopy (EDS) analysis of TEM (not shown here) that Mn in the as-deposited ALD Ru-Mn film diffused into the underlying SiO2 after annealed at 400 C and Mn silicate was finally self-formed at the interface between the annealed ALD Ru-Mn film and SiO2. XTEM showed that the thickness of as-deposited ALD Ru-Mn sample (~55 nm) had significantly decreased to ~30 nm after
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Fig. 4. Cross-sectional view TEM images of (a) as-deposited ALD Ru-Mn film and (b) annealed film at 500 C.
Concentration (at. %)
100
C O Si Mn Ru
Ru 80
O
60 40
Si
20 0
Mn 20
40
60
80
Sputter Depth (nm) Fig. 5. SIMS depth profile of ALD Ru-Mn film, which was prepared by repeating 35 Rusub-cycles and 3 Mn sub-cycles after annealing at 500 C for 30 min.
pure ALD-Ru film (~19 nm). This suggests that surface scattering to increase the resistivity is much higher for both the annealed ALD Ru-Mn and a pure ALD-Ru film. Consequently, surface scattering could not explain the higher resistivity of the as-deposited ALD RuMn film. Second, Mn impurities in the ALD Ru-Mn films can increase their resistivity and many studies of metal or metal alloy thin films showed an increase in resistivity with the incorporation of impurities [2527]. On the other hand, after annealing at 500 C, the Mn in the as-deposited ALD Ru-Mn film diffused into the underlying SiO2 and Mn silicate was finally formed at the interface between the annealed ALD Ru-Mn film and SiO2, and a pure Ru film was formed on top of it. The decrease in the resistivity of ALD RuMn after annealing was due partly to the elimination of impurities in the film. Third, TEM showed the grain size of pure ALD-Ru
Sheet Resistance (mΩ/seq.)
annealing at 500 C. The thickness reduction after annealing was attributed to the densification, impurity elimination with the selfformation of Mn silicate due to the diffusion of Mn toward SiO2, and grain growth, which was characterized by XTEM. The above investigations gave clues to understand the change in the film properties after annealing the ALD Ru-Mn film. The film resistivity of the as-deposited ALD Ru-Mn films (250 mU-cm) was higher than that of the pure ALD-Ru film deposited without the Mn ALD cycles (50 mU-cm). Note, however, that the resistivity of the ALD-RuMn film decreased drastically to 65 mU-cm after annealing at 400 C and decreased further to ~20 mU-cm after annealing at 500 C, as shown by Fig. 3(a). Several factors can be considered to explain the high resistivity of the as-deposited ALD Ru-Mn films compared to that of the annealed ALD Ru-Mn film and a pure ALDRu film. Generally, impurities incorporation, surface scattering (the “size effect”), and grain boundary scattering are responsible for increasing the resistivity of a metal film [23,24]. First, in this study, the thickness of the as-deposited ALD Ru-Mn film (~55 nm) was even higher than those of the annealed ALD Ru-Mn (~25 nm) and
300.0k
Cu/ALD-Ru/Si Cu/ALD-RuMn/Si
250.0k 200.0k 150.0k 100.0k 50.0k 4k 3k 2k 1k 0
400
450
500
550
600
650
Annealing Temperature (oC) Fig. 6. Sheet resistance of the Cu (50 nm)/ALD-Ru (5 nm)/Si and Cu (50 nm)/ALD RuMn (~5 nm)/Si structures after annealed.
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was approximately 15 nm, whereas the average grain size of the asdeposited ALD Ru-Mn film decreased to ~8 nm. TEM also revealed grain growth after annealing the ALD Ru-Mn. Therefore, the formation of much larger-sized Ru grains for both the pure ALD-Ru film and annealed ALD Ru-Mn film could be another reason for the reduced resistivity. Once the material properties of the ALD-grown Ru-Mn films were characterized, this study focused on evaluating the diffusion barrier performance of the ALD Ru-Mn films against Cu. For the evaluation, an ultrathin (5 nm) ALD-Ru or Ru-Mn film was grown on a Si wafer and Cu film (50 nm in thickness) was deposited on top of the diffusion barrier. Fig. 6 shows the sheet resistances of the Cu (50 nm)/ALD-Ru (5 nm)/Si wafer and Cu (50 nm)/ALD Ru-Mn
(~5 nm)/Si wafer samples as a function of the post-annealing temperature. The sheet resistance in this kind of stack is determined mostly by the conditions and quality of the Cu layer because it has the lowest resistance among the layers and carries almost all the sensor current, whereas the sheet resistance contributions from the Ru and Ru-Mn films can be neglected because of their high resistivity (Ru: 50 mU cm and Ru-Mn: 250 mU cm) and narrow thickness compared to those of Cu. As shown in Fig. 6, the sheet resistance of both samples decreased slightly after annealing at 450 C and 500 C, possibly due to the increased grain size and defect annihilation of the Cu film. In the case of the Cu (50 nm)/ Ru(5 nm)/Si sample, the sheet resistance began to increase once annealed at 550 C and increased drastically by several orders of magnitude after annealing at 600 C, which means the failure of the ALD-Ru diffusion barrier at that temperature. In contrast, in the case of the Cu (50 nm)/Ru Mn(~5 nm)/Si sample, there was no change in the sheet resistance until the samples were annealed at 600 C. As the temperature was increased to 650 C, the resistivity of the structure increased abruptly. These results strongly suggest that the ALD-grown Ru-Mn films can prevent Cu diffusion more effectively than the Ru counterpart. XRD was performed on the annealed samples to determine the possible reasons for the increase in sheet resistance of the Ru and Ru-Mn samples. Fig. 7(a) and (b) show the corresponding XRD
Fig. 7. XRD results of (a) Cu (50 nm)/ALD-Ru (5 nm)/Si and (b) Cu (50 nm)/ALD Ru-Mn (~5 nm)/Si structures as a function of the annealing temperature.
Fig. 8. (a) Plan-view and (b) cross-section SEM image of the electroplated Cu on the annealed ALD Ru-Mn film.
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patterns of the Cu/Ru/Si and Cu/Ru-Mn/Si samples, respectively, annealed at various temperatures. In the case of the sample with the ALD-Ru diffusion barrier (Fig. 7(a)), the Cu15Si4 (2q ¼ 34.74) peak was first detected after annealing at 500 C, indicating the failure of the diffusion barrier due to the diffusion of Cu through it. On the other hand, the intensity was weak and strong Cu peaks were still observed. When the annealing temperature was increased to 550 C, new peaks related to Cu3Si (2q ¼ 27.96, 44.57, 44.99, and 65.23) were observed with significantly high intensities. This result was well matched with the results of the sheet resistance showing an initial increase. Finally, various types of Cu silicides were detected with a concomitant decrease in intensity of the Cu peak when the annealing temperature was increased further to 600 C, corresponding to the abrupt increase in sheet resistance in Fig. 6(a). In contrast, the ALD Ru-Mn diffusion barrier samples showed no peaks related to Cu silicides until annealed at 550 C. Once the Ru-Mn samples were annealed at 600 C, small peaks related to Cu3Si were observed, but the peak intensity from Cu was still strong, suggesting that the diffusion barrier failure was only minimal. Therefore, the diffusion barrier performance of the ALDgrown Ru-Mn film appears to be superior to the ALD-Ru counterpart. This superior performance can be attributed to several aspects, such as the self-formation of a Mn silicate diffusion barrier, stuffing of the grain boundaries of Ru by the added Mn, and the prevention of columnar grain growth of the ALD-Ru film by inserting the periodic Mn ALD sub-cycles. Finally, the capability of the direct plating of Cu on top of the ALD Ru-Mn film was examined. The ALD Ru-Mn film was annealed at 400 C before direct plating and the resistivity of the annealed Ru-Mn film was as low as ~36 mU-cm, which would guarantee a successful EP-Cu. Fig. 8 shows a plan-view and cross-sectional view SEM image of a Cu film electrodeposited on the annealed Ru-Mn film. As expected from its low resistivity and the existence of Ru on the surface, a very high area density of extremely small Cu nuclei was observed, indicating the rapid nucleation of EP-Cu on the annealed ALD Ru-Mn films. The EP-Cu films were grown continuously on the annealed ALD-grown Ru-Mn film. Such capability of the direct Cu plating, in conjunction with the excellent step coverage and ultra-small volume enabled by the ALD process, should be highlighted as a potential replacement of the diffusion barrier and seed layer materials. 3. Summary and conclusions Ru-based alloy films, Ru-Mn, were prepared by repeating the super-cycles consisting of Ru and Mn ALD sub-cycles at 225 C for the diffusion barrier materials of seedless Cu interconnects. Here, Ru and Mn ALD was performed using the sequential supply of IMBCHDRu(0) precursor/NH3 plasma and Mn(II)[iPr(DAD)]2 precursor/H2 plasma, respectively. Mn could be incorporated into the ALD-Ru film by the periodic addition of Mn ALD sub-cycles during the ALD-Ru process. Plan-view TEM analysis showed that the grains can be reduced significantly from ~15 nm (pristine Ru) to ~8 nm (Ru-Mn) and continuous columnar grain growth of the ALD-Ru film could be prevented by the addition of a periodic Mn ALD sub-cycle. Both the XTEM and SIMS analysis further showed that the MnSiOx diffusion barrier layer was self-formed at the interface of the RuMn film and the underlying SiO2 surface after annealing the ALD-
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Ru-Mn at 500 C. Both the superior diffusion barrier performance of the ALD Ru-Mn film against Cu compared to the ALD-Ru counterpart and the successful EP-Cu on the annealed ALD Ru-Mn barrier were also demonstrated. Finally, these results suggest that the ALD-grown Ru-Mn films are viable candidates for a Cu direct plateable diffusion barrier for ever-shrinking semiconductor devices.
Acknowledgements This study (2015R1A2A2A04004945) was supported by Midcareer Researcher Program through NRF grant funded by the MEST and also partially supported by a KBSI Grant (T33524) to T.E. Hong.
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