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Effect of hydrogen atmosphere in Cu thin film growth by chemical vapor deposition using Cu(dmamb)2
Microelectronic Engineering 89 (2012) 109–115
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Effect of hydrogen atmosphere in Cu thin film growth by chemical vapor deposition using Cu(dmamb)2 Jong Mun Choi a, Dohan Lee a, Ji Hun Park a, Chang Gyoun Kim b, Taek-Mo Chung b, Baek-Mann Kim c, Dongjin Byun a,d,⇑ a
Material Science and Engineering, Korea University, Seoul 136-713, South Korea Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejeon 305-600, South Korea R&D Division, Hynix Semiconductor Inc., Icheon 467-701, South Korea d Nano Semiconductor Engineering, Korea University, Seoul 136-713, South Korea b c
a r t i c l e
i n f o
Article history: Available online 22 September 2011 Keywords: Interconnect Cu seed layer CVD Thin film growth Cu(II) precursor
a b s t r a c t A high quality Cu seed layer was successfully prepared by chemical vapor deposition (CVD) from the newly synthesized Cu(dmamb)2 precursor. The growth behavior of Cu thin film was systematically investigated, with particular focus on the effect of growth temperature and atmosphere on the Cu seed layer properties. The grain size of the Cu thin film was enhanced by hydrogen atmosphere and the residual impurity in the Cu thin film was effectively reduced. The resistivity of the Cu thin film depended on the grain size and impurity concentration. A Cu seed layer was successfully obtained with resistivity of 37 l X cm and thickness of 27 nm. These results demonstrated the possibility of fabricating a high quality Cu seed layer by CVD using the Cu(dmamb)2 precursor. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction Copper is suitable as an interconnect metal due to its low resistivity, high thermal conductivity and high electromigration resistance compared with aluminum [1]. Interconnects for the next generation of ultra large scale integration require a low resistance–capacitance delay, which necessitates the combination of low-k intermetal dielectric (IMD) and low resistivity metal like copper. To insert copper into the metal line/via interconnect, the dual damascene process is required due to the difficulty in etching copper [2]. Among the various methods for depositing copper, the metal organic chemical vapor deposition (MOCVD) processes have several advantages such as the ability to achieve good conformality and nanoscale integration. In addition, the low-k IMDs are a composite of polymeric and porous materials that are needed for the deposition of copper film at low temperature. Cu thin film as the seed layer for interconnect application has required film properties such as continuous surface morphology and high purity. However as a precursor for Cu CVD, conventional Cu(I) precursors such as (hfac)Cu(VTMS) suffer the drawback of poor thermal and longterm stability [3]. In contrast, the Cu(II) precursor with good stability is not suitable for low-k IMDs application because it is solid at room temperature and has a high deposition temperature. Therefore, to overcome these difficulties on Cu CVD, a newly ⇑ Corresponding author at: Material Science and Engineering, Korea University, Seoul 136-713, South Korea. Tel.: +82 17 223 3280; fax: +82 2 928 3584. E-mail address: [email protected] (D. Byun).
synthesized bis(dimethylamino-2-methyl-2-butoxy)Cu(II), Cu(dmamb)2 precursor was adopted as a metal organic precursor. A Cu thin film prepared by the Cu(II) precursor exhibited superior growth behavior in hydrogen atmosphere. It has been reported that the reducing atmosphere was provided by hydrogen [4,5]. In this paper, a high quality Cu seed layer was fabricated by the newly synthesized Cu(dmamb)2 and the effect of the hydrogen atmosphere on Cu thin film growth behavior was systematically investigated. 2. Experiment A Ta thin film was deposited by RF sputtering on a SiO2/Si substrate as a barrier layer. The main parameters were the growth temperature and atmosphere. The deposition was carried out in a cold wall, vertical-type quartz tube, MOCVD reactor. Cu(dmamb)2 was adopted as the metal organic precursor and its molecular structure is shown in Fig. 1. Cu(dmamb)2 offers the advantages of good thermal stability (activation energy of 113 kJ/mol), liquid phase at room temperature (all of the conventional Cu(II) was solid), high vapor pressure (0.9 Torr at 75 °C) and low temperature deposition (possible to deposit on polymer material). Moreover, Cu(dmamb)2 does not include any F in the ligand. Previous studies have shown that the hydrogen significantly improved the purity of Cu thin film. Nevertheless, it is difficult to completely remove any F remaining at the interface between the Cu thin film and their under-layer [6,7]. Therefore, it is an important advantage for the fabrication of high purity Cu thin film.
0167-9317/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.08.009
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Fig. 1. Molecular structure of Cu(dmamb)2.
The Cu(dmamb)2 metal organic precursor in the bubbler was vaporized and introduced with the carrier gas, which was either hydrogen or argon at 35 SCCM or 50 SCCM, respectively, though a cylindrical shower head type distributor. The mixed gas flowed vertically down toward the substrate surface. The precursor bubbler and gas line temperature were maintained at 70 °C to prevent condensation. The process conditions for the Cu thin film deposition were a pressure of 0.5 Torr and a substrate temperature of 210–370 °C. The thermochemical property of Cu(dmamb)2 was measured by thermogravimetric differential thermal analysis (TG-DTA). Resistivity measurements were performed by a fourpoint probe. The microstructure of the Cu thin film was observed from field-emission scanning electron microscopic (FESEM, Hitachi S4800) micrographs. The structural properties of the Cu thin film were determined by high resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30 S-twin) and X-ray diffractometer (XRD, Rigaku D) with Cu Ka radiation operating at 40 kV, 200 mA. The chemical composition and impurity concentration of the thin films grown under different conditions were measured by auger electron spectroscopy (AES, Physical Electronics PHI680).
Cu(dmamb)2 was measured as 199.7 °C and its dissociation temperature was similar to that of the Cu(I) type precursor with a range of 150–200 °C. The DTA result indicated that the maximum reaction rate of Cu(dmamb)2 approached to about 230 °C. This revealed that Cu thin film deposition at low temperature was possible. On the other hand, the endothermic peak revealed in the DTA measurement met with the dissociation temperature at 199.7 °C from the TG measurement. This result was a specific property of the Cu(II) precursor and suggested that other than Cu(I), the ligand dissociation did not undergo the disproportionation reaction. The Cu(I) precursor has been reported to have two endothermic peak by neutral ligand dissociation and dissociation of consequent Cu(hfac)2 [8]. Fig. 3 shows an FESEM image of the Cu thin film grown on the Ta thin film in argon atmosphere. It had relative high process temperature using argon atmosphere compare to hydrogen atmosphere. The nucleation density increased, but no grain growth appeared with increasing growth temperature. In addition, the film structure only exhibited growth in argon atmosphere at a temperature over 340 °C. Fig. 4 shows an FESEM image of the Cu thin film deposited in hydrogen atmosphere. The Cu thin film deposition was possible at approaching the source dissociation temperature at 230 °C in the hydrogen atmosphere, with a resulting large grain size and continuous surface morphology. With increasing growth temperature, the grain size slightly decreased (Fig. 4c and d). This changing growth behavior was attributed to the Volmer–Weber growth mechanism of the Cu thin film that was grown by CVD [9]. Volmer–Weber growth is an island-type growth behavior that produces dramatically formed clusters and numerous voids at high growth temperature. The result shows that the Cu thin film growth behavior occurred as a seed layer in specific areas under suitable conditions. The effect of the growth atmosphere on the growth behavior can be explained by considering the difference of the decomposition reaction of the precursor in each atmosphere. The surface reaction mechanism of Cu(dmamb)2 in argon atmosphere can be presented as follows:
Cu½OCR1 R2 ðCH2 ÞNR3 R4 2 ðlÞ 3. Results and discussion To observe the thermo chemical property of Cu(dmamb)2, TG-DTA was carried out. Fig. 2 shows the TG-DTA results for Cu(dmamb)2. The Cu(II) precursor is recognized for its high dissociation temperature compared to that of the Cu(I) precursor. However, according to the TG data, the dissociation temperature of
! CuðsÞ þ HOCR1 R2 ðCH2 ÞNR3 R4 ðgÞ þ R1 R2 COðgÞ þ R3 R4 NCðHÞ ¼ CðHÞNR3 R4 ðgÞ where, R1 = C2H5, R2, 3, 4 = CH2 In inert atmosphere, this reaction has been reported to comprise the removal of the ligand hydrogen by the thermal energy and the reduction of oxygen with Cu by the electron thereby generated as a b-hydrogen elimination reaction. This is the so called self reduction reaction. Cu(dmamb)2 has been used to produce solid copper and by-products such as amino alcohols, ketone and endiamine due to the self reduction reaction in argon atmosphere. These by-products could easily be removed from the substrate as volatile hydrocarbon. On the other hand, ligand decomposition in hydrogen atmosphere was proposed through the following comparatively simple process:
Cu½OCR1 R2 ðCH2 ÞNR3 R4 2 ðlÞ þ H2 ðgÞ ! CuðsÞ þ 2HOCR1 R2 ðCH2 ÞNR3 R4 ðgÞ
Fig. 2. TG-DTA graph of Cu(dmamb)2.
From the reaction formula, the precursor was decomposed through the process in which hydrogen atoms acted as reducing agents and then reduced oxygen directly. This was a much more effective reaction pathway than that by self-reduction in the former inert atmosphere and this was the reason for the difference of thin film growth temperature and growth behavior in each atmosphere.
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Fig. 3. FESEM image of Cu thin film grown in argon atmosphere at (a) 270 °C, (b) 300 °C, (c) 340 °C, and (d) 370 °C.
Fig. 4. FESEM image of Cu thin film grown in hydrogen atmosphere at (a) 210 °C, (b) 220 °C, (c) 230 °C, and (d) 250 °C.
In addition, another reason for having a higher growth temperature than that of hydrogen atmosphere in argon atmosphere is that the argon atmosphere has high activation energy for the surface reaction. In the CVD process, the thickness of the thermal boundary layer occurred with main reaction of the precursor from the substrate is proportional to the square root of the thermal diffusivity [10]. The thermal diffusivity of argon and hydrogen is 525 cm2 s1 and 2939 cm2 s1 at the pressure of 0.3 Torr and room temperature, respectively [11]. The thermal diffusivity of hydrogen is six times higher than argon. Therefore, the thickness of the thermal boundary layer at the reaction surface was smaller in argon atmosphere than in hydrogen. Although the temperature was
sufficient for source decomposition, it apparently did not allow active growth of the Cu thin film in the argon atmosphere. Furthermore, Rha et al. [12] has reported that hydrogen inhibited the generation of CuO in the grain boundary and had an effect on enhancing grain growth. Therefore deposition of Cu thin film in hydrogen atmosphere would be desirable for Cu seed layer application. To evaluate the properties as a seed layer, several properties of the grown Cu thin film are listed in Table 1. The thin film grown in argon atmosphere had a small grain size of approximately 30 nm and able to cover the whole Ta surface. However, the resistivity of the Cu thin film grown in argon atmosphere was too high (104 l O cm) to meet the property
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Table 1 Cu thin film properties from the Ar and H2 atmosphere. Growth ambient
Ar atmosphere
Growth temperature (°C) Number of grain (in 200 200 nm) Grain size (nm) Cu film thickness (nm) Resistivity ( l X cm)
270 47ea 15 Non layer structure –
H2 atmosphere 300 38ea 30 Non layer structure –
required as a seed layer. On the contrary, the Cu thin film grown in hydrogen atmosphere had a grain size of approximately 60 nm with comparatively continuous morphology. The resistivity value as 37 l O cm improved remarkably. This difference in resistivity was attributed to the mean free path of electrons in the Cu thin film. The electron mean free path of copper was 39.3 nm [13]. Moreover, the extrinsic resistivity of the Cu thin film was determined by impurities, defects and electron scattering wrought in the grain boundary and thin film surface. Therefore, when the grain size and thin film thickness were over 39.3 nm, the electron scattering in the surface, interface and grain boundary sharply
340 49ea 30 35 93
370 50ea 30 32 104
210 62ea 10 Non layer structure –
220 34ea 45 Non layer structure –
230 9ea 80 34 54
250 15ea 60 27 37
decreased. The grain size of the Cu thin film grown in hydrogen atmosphere was around 60 nm, compared to 30 nm for that grown in argon atmosphere, which could not reach the electron mean free path of copper. Accordingly, the Cu thin film grown in argon atmosphere experienced electron scattering within the short distance of the mean free path of copper, thereby significantly increasing the resistivity. Similarly, the grain size of Cu thin film grown in hydrogen was beyond the mean free path of copper, electron scattering was largely reduced and the resistivity also decreased. To observe the structural properties of Cu thin film, TEM analysis was carried out. Fig. 5 shows the TEM cross-sectional image of
Fig. 5. TEM cross-sectional image of the Cu thin film and EDS spectra obtained from bright and dark area of the Cu thin film. (a) TEM cross-sectional image of the Cu thin film. (b) EDS spectra obtained from bright area. (c) EDS spectra obtained from dark area.
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Fig. 6. AES depth profile of Cu thin film grown in each atmosphere (after 3 nm sputtering): (a) argon atmosphere and (b) hydrogen atmosphere.
the Cu thin film grown in hydrogen atmosphere and EDS spectra obtained from the bright and dark area in that image. A large amount of Cu component was detected in EDS result with dark area. On the contrary, the bright area relatively a small amount of Cu component was detected. It is supposed that the contrast ratio of Cu thin film depended on the Cu composition. Therefore, these results are indicated that the Cu thin film was comprised of continuous single phase. To observe the effect on purity of the thin film based on growth atmosphere, AES analysis was conducted. Fig. 6 is the AES depth profile of the Cu thin film grown in argon atmosphere and hydrogen atmosphere. The concentration of carbon and oxygen impurities significantly decreased in hydrogen atmosphere, compared to
argon atmosphere. The resistivity of the Cu thin film depended on the impurity concentration with each atmosphere condition. These effects by hydrogen have already been reported in several previous studies. Awaya and Arita [14] improved the thin film purity and resistivity by using hydrogen in an experiment with the Cu(I) precursor, and Choi et al. [15] removed the carbon and oxygen impurities of the Cu thin film using hydrogen plasma. Systematic investigation using SIMS analysis suggested that the residual ligand and impurity were combined with hydrogen to produce the volatile species. In order to clarify structural, compositional properties of Cu/ Ta/SiO2 multilayer structure, HRTEM analysis was carried out. Fig. 7 shows the HRTEM image of Cu/Ta/SiO2 interface grown in
Fig. 7. HRTEM image of Cu/Ta/SiO2 stack layer (Cu grown in argon atmosphere).
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Fig. 8. XRD patterns of the Cu/Ta thin film grown in each atmosphere.
argon atmosphere. A large amount of oxygen was detected in the EDS result at the Cu/Ta interface. Actually, AES result was revealed that the interlayer contains a significant amount of oxygen. Moreover, fast Fourier transform (FFT) diffractogram from Cu/Ta interlayer region shows a diffused ring (Fig. 7A region). Therefore, indicating that it is amorphous. The results clearly demonstrate the interlayer of amorphous TaOx. Whereas, the HRTEM image with FFT diffractogram in Fig. 7(B) region shows that Ta is a beta-phase (body-center-tetragonal) in the form of microcrystallinity. On the other hand, some oxygen was detected in the AES result with hydrogen atmosphere. This oxygen was originated from the native oxide of the Ta surface [16]. It has been reported that the Ta oxide was generated also in in situ fabricating Cu/Ta thin film interface due to the exposure to atmosphere. These effects are confirmed that oxygen in atmosphere move to Cu/Ta interface through the grain boundary of Cu thin film. Therefore, AES results in case of hydrogen are supposed that the Ta oxide layer was not affected by deposition process. Fig. 8 shows the XRD pattern of Cu/Ta thin film grown in each atmosphere. It was observed that the Cu(1 1 1) peaks tend to increase with increasing deposition temperature to 230 °C with hydrogen atmosphere but decrease with hydrogen at above 250 °C. Whereas, the thin film grown in argon atmosphere exhibited strong Cu(2 0 0) peaks in all the temperature ranges. The reliability of copper interconnects is expected to be dependent on the microstructure. It is well known that copper film having (1 1 1) preferred crystal orientation results in excellent electromigration resistance [17]. From above result, the Cu thin film growth with hydrogen atmosphere is helpful to get the highly (1 1 1) oriented Cu thin film leading to a higher resistance to electromigration. The reason for the formation of Cu film with enhanced (1 1 1) preferred orientation in hydrogen atmosphere is that the Cu thin film grown in hydrogen atmosphere exhibited higher purity than argon atmosphere as shown in the AES results. These effects by hydrogen with the relationship between the film purity and structural property are consistent with several previous studies [15,18–20].
4. Conclusion A high quality Cu seed layer was successfully prepared by CVD method from the newly synthesized Cu(dmamb)2 precursor. Hydrogen atmosphere enhanced the grain size and purity of the Cu thin film. The hydrogen acted as a reducing agent to enhance the decomposition reaction of Cu(dmamb)2 and effectively removed the residual impurity in the Cu thin film. Island growth behavior was initiated by copper agglomeration due to the main parameters including temperature. Therefore, Cu thin film growth occurred as a seed layer in specific areas under suitable conditions. AES and HRTEM results for Cu/Ta interface clearly demonstrate the interlayer of amorphous TaOx grown in argon atmosphere. XRD results showed that the intensity of the Cu(1 1 1) direction is higher for the film grown in hydrogen atmosphere. It was demonstrated that the Cu thin film growth with hydrogen atmosphere is helpful to get the highly (1 1 1) oriented Cu thin film leading to a higher resistance to electromigration. In summary, The Cu thin film deposition was possible at low temperature (230 °C) in the hydrogen atmosphere, with a resulting continuous surface morphology, low impurity concentration and low resistivity (37 l X cm at thickness of 27 nm). These results demonstrated the possibility of fabricating a high quality Cu seed layer by CVD using the Cu (dmamb)2 precursor. References [1] ITRS, ITRS Executive summary, 2007 ed., (2007) pp. 4. [2] T.N. Theis, IBM J. Res. Dev. 44 (3) (2000) 379. [3] T. Kodas, M. Hampden-Smith, The Chemistry of Metal CVD, VCH, New York, 1994. pp. 175. [4] R.L. Van Hemert, L.B. Spendlove, R.E. Sievers, J. Electrochem. Soc. 112 (11) (1965) 1123. [5] K. Takenaka, K. Koga, M. Shiratani, Y. Watanabe, T. Shingen, Thin Solid Films 506 (2006) 197. [6] H.J. Jin, M. Shiratani, Y. Nakatake, T. Fukuzawa, T. Kinishita, Y. Watanabe, M. Toyohuku, Jpn. J. Appl. Phys. 38 (1999) 4492. [7] H.J. Jin, M. Shiratani, T. Kawasaki, T. Fukuzawa, T. Kinoshita, Y. Watanabe, J. Vac. Sci. Technol. A 17 (3) (1999) 726. [8] S.W. Rhee, S.W. Kang, S.H. Han, Electrochem. Solid-State Lett. 3 (3) (2000) 135. [9] S.K. Kwak, K.S. Chung, I. Park, H. Lim, Curr. Appl. Phys. 2 (2002) 205.
J.M. Choi et al. / Microelectronic Engineering 89 (2012) 109–115 [10] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, John & Wiley & Sons, Inc., New York, 1960. pp. 368. [11] K.K. Choi, S.W. Rhee, J. Electrochem. Soc. 148 (7) (2001) C473. [12] S.K. Rha, W.J. Lee, S.Y. Lee, D.W. Kim, C.O. Park, J. Mater. Sci. Mater. Electron. 8 (1997) 217. [13] J.A. Davis, R. Venkatesan, A. Kaloyeros, M. Beylansky, S.J. Souri, K. Banerjee, K.C. Saraswat, A. Rahman, R. Reif, J.D. Meindl, Proc. IEEE 89 (3) (2001). [14] N. Awaya, Y. Arita, Jpn. J. Appl. Phys. 30 (1991) 1813.
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[15] K.K. Choi, J.H. Yun, S.W. Rhee, Thin Solid Films 429 (2003) 255. [16] K.M. Yin, L. Chang, F.R. Chen, J.J. Kai, C.C. Chiang, G. Chung, P. Ding, B. Chin, H. Zhang, F. Chen, Thin Solid Films 388 (2001) 27. [17] M.J. Attardo, R. Rosenberg, J. Appl. Phys. 41 (1970) 2381. [18] C.L. Lin, P.S. Chen, C.L. Chang, M.C. Chen, J. Vac. Sci. Technol. B 20 (5) (2002) 1947. [19] J.K. Lee, B.J. Jeon, S.W. Lee, Mater. Sci. Forum 510 (2006) 666. [20] C.L. Lin, P.S. Chen, M.C. Chen, J. Electrochem. Soc. 149 (5) (2002) C237.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Effect of hydrogen atmosphere in Cu thin film growth by chemical vapor deposition using Cu(dmamb)2 Jong Mun Choi a, Dohan Lee a, Ji Hun Park a, Chang Gyoun Kim b, Taek-Mo Chung b, Baek-Mann Kim c, Dongjin Byun a,d,⇑ a
Material Science and Engineering, Korea University, Seoul 136-713, South Korea Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejeon 305-600, South Korea R&D Division, Hynix Semiconductor Inc., Icheon 467-701, South Korea d Nano Semiconductor Engineering, Korea University, Seoul 136-713, South Korea b c
a r t i c l e
i n f o
Article history: Available online 22 September 2011 Keywords: Interconnect Cu seed layer CVD Thin film growth Cu(II) precursor
a b s t r a c t A high quality Cu seed layer was successfully prepared by chemical vapor deposition (CVD) from the newly synthesized Cu(dmamb)2 precursor. The growth behavior of Cu thin film was systematically investigated, with particular focus on the effect of growth temperature and atmosphere on the Cu seed layer properties. The grain size of the Cu thin film was enhanced by hydrogen atmosphere and the residual impurity in the Cu thin film was effectively reduced. The resistivity of the Cu thin film depended on the grain size and impurity concentration. A Cu seed layer was successfully obtained with resistivity of 37 l X cm and thickness of 27 nm. These results demonstrated the possibility of fabricating a high quality Cu seed layer by CVD using the Cu(dmamb)2 precursor. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction Copper is suitable as an interconnect metal due to its low resistivity, high thermal conductivity and high electromigration resistance compared with aluminum [1]. Interconnects for the next generation of ultra large scale integration require a low resistance–capacitance delay, which necessitates the combination of low-k intermetal dielectric (IMD) and low resistivity metal like copper. To insert copper into the metal line/via interconnect, the dual damascene process is required due to the difficulty in etching copper [2]. Among the various methods for depositing copper, the metal organic chemical vapor deposition (MOCVD) processes have several advantages such as the ability to achieve good conformality and nanoscale integration. In addition, the low-k IMDs are a composite of polymeric and porous materials that are needed for the deposition of copper film at low temperature. Cu thin film as the seed layer for interconnect application has required film properties such as continuous surface morphology and high purity. However as a precursor for Cu CVD, conventional Cu(I) precursors such as (hfac)Cu(VTMS) suffer the drawback of poor thermal and longterm stability [3]. In contrast, the Cu(II) precursor with good stability is not suitable for low-k IMDs application because it is solid at room temperature and has a high deposition temperature. Therefore, to overcome these difficulties on Cu CVD, a newly ⇑ Corresponding author at: Material Science and Engineering, Korea University, Seoul 136-713, South Korea. Tel.: +82 17 223 3280; fax: +82 2 928 3584. E-mail address: [email protected] (D. Byun).
synthesized bis(dimethylamino-2-methyl-2-butoxy)Cu(II), Cu(dmamb)2 precursor was adopted as a metal organic precursor. A Cu thin film prepared by the Cu(II) precursor exhibited superior growth behavior in hydrogen atmosphere. It has been reported that the reducing atmosphere was provided by hydrogen [4,5]. In this paper, a high quality Cu seed layer was fabricated by the newly synthesized Cu(dmamb)2 and the effect of the hydrogen atmosphere on Cu thin film growth behavior was systematically investigated. 2. Experiment A Ta thin film was deposited by RF sputtering on a SiO2/Si substrate as a barrier layer. The main parameters were the growth temperature and atmosphere. The deposition was carried out in a cold wall, vertical-type quartz tube, MOCVD reactor. Cu(dmamb)2 was adopted as the metal organic precursor and its molecular structure is shown in Fig. 1. Cu(dmamb)2 offers the advantages of good thermal stability (activation energy of 113 kJ/mol), liquid phase at room temperature (all of the conventional Cu(II) was solid), high vapor pressure (0.9 Torr at 75 °C) and low temperature deposition (possible to deposit on polymer material). Moreover, Cu(dmamb)2 does not include any F in the ligand. Previous studies have shown that the hydrogen significantly improved the purity of Cu thin film. Nevertheless, it is difficult to completely remove any F remaining at the interface between the Cu thin film and their under-layer [6,7]. Therefore, it is an important advantage for the fabrication of high purity Cu thin film.
0167-9317/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.08.009
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Fig. 1. Molecular structure of Cu(dmamb)2.
The Cu(dmamb)2 metal organic precursor in the bubbler was vaporized and introduced with the carrier gas, which was either hydrogen or argon at 35 SCCM or 50 SCCM, respectively, though a cylindrical shower head type distributor. The mixed gas flowed vertically down toward the substrate surface. The precursor bubbler and gas line temperature were maintained at 70 °C to prevent condensation. The process conditions for the Cu thin film deposition were a pressure of 0.5 Torr and a substrate temperature of 210–370 °C. The thermochemical property of Cu(dmamb)2 was measured by thermogravimetric differential thermal analysis (TG-DTA). Resistivity measurements were performed by a fourpoint probe. The microstructure of the Cu thin film was observed from field-emission scanning electron microscopic (FESEM, Hitachi S4800) micrographs. The structural properties of the Cu thin film were determined by high resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30 S-twin) and X-ray diffractometer (XRD, Rigaku D) with Cu Ka radiation operating at 40 kV, 200 mA. The chemical composition and impurity concentration of the thin films grown under different conditions were measured by auger electron spectroscopy (AES, Physical Electronics PHI680).
Cu(dmamb)2 was measured as 199.7 °C and its dissociation temperature was similar to that of the Cu(I) type precursor with a range of 150–200 °C. The DTA result indicated that the maximum reaction rate of Cu(dmamb)2 approached to about 230 °C. This revealed that Cu thin film deposition at low temperature was possible. On the other hand, the endothermic peak revealed in the DTA measurement met with the dissociation temperature at 199.7 °C from the TG measurement. This result was a specific property of the Cu(II) precursor and suggested that other than Cu(I), the ligand dissociation did not undergo the disproportionation reaction. The Cu(I) precursor has been reported to have two endothermic peak by neutral ligand dissociation and dissociation of consequent Cu(hfac)2 [8]. Fig. 3 shows an FESEM image of the Cu thin film grown on the Ta thin film in argon atmosphere. It had relative high process temperature using argon atmosphere compare to hydrogen atmosphere. The nucleation density increased, but no grain growth appeared with increasing growth temperature. In addition, the film structure only exhibited growth in argon atmosphere at a temperature over 340 °C. Fig. 4 shows an FESEM image of the Cu thin film deposited in hydrogen atmosphere. The Cu thin film deposition was possible at approaching the source dissociation temperature at 230 °C in the hydrogen atmosphere, with a resulting large grain size and continuous surface morphology. With increasing growth temperature, the grain size slightly decreased (Fig. 4c and d). This changing growth behavior was attributed to the Volmer–Weber growth mechanism of the Cu thin film that was grown by CVD [9]. Volmer–Weber growth is an island-type growth behavior that produces dramatically formed clusters and numerous voids at high growth temperature. The result shows that the Cu thin film growth behavior occurred as a seed layer in specific areas under suitable conditions. The effect of the growth atmosphere on the growth behavior can be explained by considering the difference of the decomposition reaction of the precursor in each atmosphere. The surface reaction mechanism of Cu(dmamb)2 in argon atmosphere can be presented as follows:
Cu½OCR1 R2 ðCH2 ÞNR3 R4 2 ðlÞ 3. Results and discussion To observe the thermo chemical property of Cu(dmamb)2, TG-DTA was carried out. Fig. 2 shows the TG-DTA results for Cu(dmamb)2. The Cu(II) precursor is recognized for its high dissociation temperature compared to that of the Cu(I) precursor. However, according to the TG data, the dissociation temperature of
! CuðsÞ þ HOCR1 R2 ðCH2 ÞNR3 R4 ðgÞ þ R1 R2 COðgÞ þ R3 R4 NCðHÞ ¼ CðHÞNR3 R4 ðgÞ where, R1 = C2H5, R2, 3, 4 = CH2 In inert atmosphere, this reaction has been reported to comprise the removal of the ligand hydrogen by the thermal energy and the reduction of oxygen with Cu by the electron thereby generated as a b-hydrogen elimination reaction. This is the so called self reduction reaction. Cu(dmamb)2 has been used to produce solid copper and by-products such as amino alcohols, ketone and endiamine due to the self reduction reaction in argon atmosphere. These by-products could easily be removed from the substrate as volatile hydrocarbon. On the other hand, ligand decomposition in hydrogen atmosphere was proposed through the following comparatively simple process:
Cu½OCR1 R2 ðCH2 ÞNR3 R4 2 ðlÞ þ H2 ðgÞ ! CuðsÞ þ 2HOCR1 R2 ðCH2 ÞNR3 R4 ðgÞ
Fig. 2. TG-DTA graph of Cu(dmamb)2.
From the reaction formula, the precursor was decomposed through the process in which hydrogen atoms acted as reducing agents and then reduced oxygen directly. This was a much more effective reaction pathway than that by self-reduction in the former inert atmosphere and this was the reason for the difference of thin film growth temperature and growth behavior in each atmosphere.
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Fig. 3. FESEM image of Cu thin film grown in argon atmosphere at (a) 270 °C, (b) 300 °C, (c) 340 °C, and (d) 370 °C.
Fig. 4. FESEM image of Cu thin film grown in hydrogen atmosphere at (a) 210 °C, (b) 220 °C, (c) 230 °C, and (d) 250 °C.
In addition, another reason for having a higher growth temperature than that of hydrogen atmosphere in argon atmosphere is that the argon atmosphere has high activation energy for the surface reaction. In the CVD process, the thickness of the thermal boundary layer occurred with main reaction of the precursor from the substrate is proportional to the square root of the thermal diffusivity [10]. The thermal diffusivity of argon and hydrogen is 525 cm2 s1 and 2939 cm2 s1 at the pressure of 0.3 Torr and room temperature, respectively [11]. The thermal diffusivity of hydrogen is six times higher than argon. Therefore, the thickness of the thermal boundary layer at the reaction surface was smaller in argon atmosphere than in hydrogen. Although the temperature was
sufficient for source decomposition, it apparently did not allow active growth of the Cu thin film in the argon atmosphere. Furthermore, Rha et al. [12] has reported that hydrogen inhibited the generation of CuO in the grain boundary and had an effect on enhancing grain growth. Therefore deposition of Cu thin film in hydrogen atmosphere would be desirable for Cu seed layer application. To evaluate the properties as a seed layer, several properties of the grown Cu thin film are listed in Table 1. The thin film grown in argon atmosphere had a small grain size of approximately 30 nm and able to cover the whole Ta surface. However, the resistivity of the Cu thin film grown in argon atmosphere was too high (104 l O cm) to meet the property
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Table 1 Cu thin film properties from the Ar and H2 atmosphere. Growth ambient
Ar atmosphere
Growth temperature (°C) Number of grain (in 200 200 nm) Grain size (nm) Cu film thickness (nm) Resistivity ( l X cm)
270 47ea 15 Non layer structure –
H2 atmosphere 300 38ea 30 Non layer structure –
required as a seed layer. On the contrary, the Cu thin film grown in hydrogen atmosphere had a grain size of approximately 60 nm with comparatively continuous morphology. The resistivity value as 37 l O cm improved remarkably. This difference in resistivity was attributed to the mean free path of electrons in the Cu thin film. The electron mean free path of copper was 39.3 nm [13]. Moreover, the extrinsic resistivity of the Cu thin film was determined by impurities, defects and electron scattering wrought in the grain boundary and thin film surface. Therefore, when the grain size and thin film thickness were over 39.3 nm, the electron scattering in the surface, interface and grain boundary sharply
340 49ea 30 35 93
370 50ea 30 32 104
210 62ea 10 Non layer structure –
220 34ea 45 Non layer structure –
230 9ea 80 34 54
250 15ea 60 27 37
decreased. The grain size of the Cu thin film grown in hydrogen atmosphere was around 60 nm, compared to 30 nm for that grown in argon atmosphere, which could not reach the electron mean free path of copper. Accordingly, the Cu thin film grown in argon atmosphere experienced electron scattering within the short distance of the mean free path of copper, thereby significantly increasing the resistivity. Similarly, the grain size of Cu thin film grown in hydrogen was beyond the mean free path of copper, electron scattering was largely reduced and the resistivity also decreased. To observe the structural properties of Cu thin film, TEM analysis was carried out. Fig. 5 shows the TEM cross-sectional image of
Fig. 5. TEM cross-sectional image of the Cu thin film and EDS spectra obtained from bright and dark area of the Cu thin film. (a) TEM cross-sectional image of the Cu thin film. (b) EDS spectra obtained from bright area. (c) EDS spectra obtained from dark area.
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Fig. 6. AES depth profile of Cu thin film grown in each atmosphere (after 3 nm sputtering): (a) argon atmosphere and (b) hydrogen atmosphere.
the Cu thin film grown in hydrogen atmosphere and EDS spectra obtained from the bright and dark area in that image. A large amount of Cu component was detected in EDS result with dark area. On the contrary, the bright area relatively a small amount of Cu component was detected. It is supposed that the contrast ratio of Cu thin film depended on the Cu composition. Therefore, these results are indicated that the Cu thin film was comprised of continuous single phase. To observe the effect on purity of the thin film based on growth atmosphere, AES analysis was conducted. Fig. 6 is the AES depth profile of the Cu thin film grown in argon atmosphere and hydrogen atmosphere. The concentration of carbon and oxygen impurities significantly decreased in hydrogen atmosphere, compared to
argon atmosphere. The resistivity of the Cu thin film depended on the impurity concentration with each atmosphere condition. These effects by hydrogen have already been reported in several previous studies. Awaya and Arita [14] improved the thin film purity and resistivity by using hydrogen in an experiment with the Cu(I) precursor, and Choi et al. [15] removed the carbon and oxygen impurities of the Cu thin film using hydrogen plasma. Systematic investigation using SIMS analysis suggested that the residual ligand and impurity were combined with hydrogen to produce the volatile species. In order to clarify structural, compositional properties of Cu/ Ta/SiO2 multilayer structure, HRTEM analysis was carried out. Fig. 7 shows the HRTEM image of Cu/Ta/SiO2 interface grown in
Fig. 7. HRTEM image of Cu/Ta/SiO2 stack layer (Cu grown in argon atmosphere).
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Fig. 8. XRD patterns of the Cu/Ta thin film grown in each atmosphere.
argon atmosphere. A large amount of oxygen was detected in the EDS result at the Cu/Ta interface. Actually, AES result was revealed that the interlayer contains a significant amount of oxygen. Moreover, fast Fourier transform (FFT) diffractogram from Cu/Ta interlayer region shows a diffused ring (Fig. 7A region). Therefore, indicating that it is amorphous. The results clearly demonstrate the interlayer of amorphous TaOx. Whereas, the HRTEM image with FFT diffractogram in Fig. 7(B) region shows that Ta is a beta-phase (body-center-tetragonal) in the form of microcrystallinity. On the other hand, some oxygen was detected in the AES result with hydrogen atmosphere. This oxygen was originated from the native oxide of the Ta surface [16]. It has been reported that the Ta oxide was generated also in in situ fabricating Cu/Ta thin film interface due to the exposure to atmosphere. These effects are confirmed that oxygen in atmosphere move to Cu/Ta interface through the grain boundary of Cu thin film. Therefore, AES results in case of hydrogen are supposed that the Ta oxide layer was not affected by deposition process. Fig. 8 shows the XRD pattern of Cu/Ta thin film grown in each atmosphere. It was observed that the Cu(1 1 1) peaks tend to increase with increasing deposition temperature to 230 °C with hydrogen atmosphere but decrease with hydrogen at above 250 °C. Whereas, the thin film grown in argon atmosphere exhibited strong Cu(2 0 0) peaks in all the temperature ranges. The reliability of copper interconnects is expected to be dependent on the microstructure. It is well known that copper film having (1 1 1) preferred crystal orientation results in excellent electromigration resistance [17]. From above result, the Cu thin film growth with hydrogen atmosphere is helpful to get the highly (1 1 1) oriented Cu thin film leading to a higher resistance to electromigration. The reason for the formation of Cu film with enhanced (1 1 1) preferred orientation in hydrogen atmosphere is that the Cu thin film grown in hydrogen atmosphere exhibited higher purity than argon atmosphere as shown in the AES results. These effects by hydrogen with the relationship between the film purity and structural property are consistent with several previous studies [15,18–20].
4. Conclusion A high quality Cu seed layer was successfully prepared by CVD method from the newly synthesized Cu(dmamb)2 precursor. Hydrogen atmosphere enhanced the grain size and purity of the Cu thin film. The hydrogen acted as a reducing agent to enhance the decomposition reaction of Cu(dmamb)2 and effectively removed the residual impurity in the Cu thin film. Island growth behavior was initiated by copper agglomeration due to the main parameters including temperature. Therefore, Cu thin film growth occurred as a seed layer in specific areas under suitable conditions. AES and HRTEM results for Cu/Ta interface clearly demonstrate the interlayer of amorphous TaOx grown in argon atmosphere. XRD results showed that the intensity of the Cu(1 1 1) direction is higher for the film grown in hydrogen atmosphere. It was demonstrated that the Cu thin film growth with hydrogen atmosphere is helpful to get the highly (1 1 1) oriented Cu thin film leading to a higher resistance to electromigration. In summary, The Cu thin film deposition was possible at low temperature (230 °C) in the hydrogen atmosphere, with a resulting continuous surface morphology, low impurity concentration and low resistivity (37 l X cm at thickness of 27 nm). These results demonstrated the possibility of fabricating a high quality Cu seed layer by CVD using the Cu (dmamb)2 precursor. References [1] ITRS, ITRS Executive summary, 2007 ed., (2007) pp. 4. [2] T.N. Theis, IBM J. Res. Dev. 44 (3) (2000) 379. [3] T. Kodas, M. Hampden-Smith, The Chemistry of Metal CVD, VCH, New York, 1994. pp. 175. [4] R.L. Van Hemert, L.B. Spendlove, R.E. Sievers, J. Electrochem. Soc. 112 (11) (1965) 1123. [5] K. Takenaka, K. Koga, M. Shiratani, Y. Watanabe, T. Shingen, Thin Solid Films 506 (2006) 197. [6] H.J. Jin, M. Shiratani, Y. Nakatake, T. Fukuzawa, T. Kinishita, Y. Watanabe, M. Toyohuku, Jpn. J. Appl. Phys. 38 (1999) 4492. [7] H.J. Jin, M. Shiratani, T. Kawasaki, T. Fukuzawa, T. Kinoshita, Y. Watanabe, J. Vac. Sci. Technol. A 17 (3) (1999) 726. [8] S.W. Rhee, S.W. Kang, S.H. Han, Electrochem. Solid-State Lett. 3 (3) (2000) 135. [9] S.K. Kwak, K.S. Chung, I. Park, H. Lim, Curr. Appl. Phys. 2 (2002) 205.
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