Metallorganic Chemical Vapor Deposition of Ru Films Using Cyclopentadienyl-Propylcyclopentadienylruthenium II and Oxygen

Transcription

1 /2002/149 6 /C317/7/$7.00 The Electrochemical Society, Inc. Metallorganic Chemical Vapor Deposition of Ru Films Using Cyclopentadienyl-Propylcyclopentadienylruthenium II and Oxygen Sang Yeol Kang, Ha Jin Lim, Cheol Seong Hwang,*,z and Hyeong Joon Kim* School of Materials Science and Engineering and Interuniversity Semiconductor Research Center, Seoul National University, Kwanak-ku, Seoul , Korea C317 Ru thin films were prepared by metallorganic chemical vapor deposition using a cyclopentadienyl-propylcyclopentadienylruthenium II precursor and O 2. The nucleation and deposition behavior, thermal stability of the film surface, reaction at the Ru/TiN interface, and the stresses of the Ru thin films were investigated. The films consisted of single phase metallic Ru under all deposition conditions and showed an electrical resistivity as low as 12 cm. The Ru thin films had a negligible oxygen content. Therefore, the interface between Ru and TiN was not changed after annealing at 600 C under a N 2 atmosphere. However, the poor nucleation property especially on a SiO 2 surface resulted in a rough surface morphology. It was found that the stress of the Ru thin film decreased as the deposition temperature and growth rate increased. The deposition behavior and physical properties of the films were investigated for use as an electrode in a metal-insulator-metal capacitor in future generation dynamic random access memory devices The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted August 23, 2001; revised manuscript received December 21, Available electronically April 12, Metal-insulator-metal MIM capacitors using Ta 2 O 5 and (Ba,Sr)TiO 3 BST dielectric films have been studied for use as storage capacitors to shrink the storage capacitor area in dynamic random access memories DRAMs having a minimum feature size 0.12 m. Due to the extremely small size of the capacitor, a threedimensional structure, such as hole-type, is required for Gbit-scale DRAMs to obtain sufficient storage capacitance even though dielectrics with a high permittivity are used. Therefore, a chemical vapor deposition CVD technique providing excellent conformality is required in order to fabricate the top and bottom electrodes as well as the dielectric films. 1 The Ru thin film is one of the promising materials for capacitor electrodes due to its excellent characteristics, such as low resistivity and good dry etching property. In addition, it forms conducting RuO 2 even when it oxidizes preventing the formation of a low dielectric interfacial layer at the interface with a high dielectric layer. Previous studies of the metallorganic CVD MOCVD of Ru metal films have utilized several precursors such as Ru(C 5 H 5 ) 2 (RuCp 2 ) 2-4 where Cp is cyclopentadienyl, Ru 3 (CO) 12, Ru(C 11 H 19 O 2 ) 3 Ru(dpm) 3 2,5 where dpm is dipivaloylmethanate, Ru(octanedionate) 3 Ru(OD) 3, 6,7 and Ru(C 5 H 4 C 2 H 5 ) 2 Ru(EtCp) 2 1,8,9 where Et is ethyl. Ru(EtCp) 2 appears to be the precursor of choice for Ru CVD these days due to its high vapor pressure and good thermal stability. Furthermore, it exists as a liquid at room temperature, making supply of the precursor stable either by liquid delivery or a standard bubbling technique. CVD Ru thin films grown using Ru(EtCp) 2 showed good properties such as low electrical resistivity, high deposition rates, and low impurity concentrations. However, Ru(EtCp) 2 is too expensive to be used as a precursor for Ru CVD in a mass production stage. Therefore, a new precursor that has a comparable performance is required for Ru CVD. In this study, cyclopentadienyl-propylcyclopentadienylruthenium II RuCp i-prcp, developed by Tanaka Kikinzoku Co., Japan, was used to deposit the Ru films. This new precursor is potentially cheaper than Ru(EtCp) 2 although it possesses comparable thermal properties. The growth behavior, structural, and electrical properties of the Ru films are reported in comparison with the results using the Ru(EtCp) 2 precursor. In particular, variations in the stresses of Ru thin films with the deposition conditions were investigated. This is * Electrochemical Society Active Member. z cheolsh@plaza.snu.ac.kr because the stress should be a critical factor for determining the thermal stability of the electrode films when the film thickness is as low as 10 nm owing to the integration purposes of DRAMs. Experimental The MOCVD apparatus consisted of a vertical warm wall reactor and a resistive substrate heater that could handle Si wafers of up to a 6 in. diam. A more detailed geometry of the deposition system was reported earlier. 10 The experimental conditions are summarized in Table I. RuCp i-prcp was vaporized at 85 C via a standard sublimation technique, where a vapor pressure of approximately 0.2 Torr was obtained. Figure 1 shows the molecular structure of RuCp i- PrCp. Ar was used as both a carrier gas and diluent gas. O 2 gas was introduced into the reactor in order to enhance metallorganic precursor decomposition and to reduce carbon incorporation into the films. Ru thin films were deposited on Si 100 and TiN 500 Å, sputtered /TiO Å, sputtered /SiO Å /Si substrates at temperatures ranging from 300 to 400 C. Some of the deposited films were annealed in an atmosphere-controlled furnace under N 2 or H 2 /Ar ambient. Film phase analysis and the resistivity measurements were performed by X-ray diffraction XRD and a four-point probe, respectively. The film thickness and surface morphology were observed by scanning electron microscopy SEM and atomic force microscopy AFM. The Ru/TiN interface was investigated by both Auger electron spectroscopy AES and transmission electron microscopy TEM. Stresses in the thin films were characterized from the measured curvature of the samples films and the Si substrate using a laser scanning technique in a commercial apparatus Tencor FLX and Stoney s equation. 11 The average thickness of the Ru thin films used in the stress measurements was 1000 Å. Results and Discussion Figure 2a and b shows the variations in XRD patterns of the thin films deposited on Si with the substrate temperatures when the O 2 flow rate was 50 standard cubic centimeters per minute sccm, and with the O 2 flow rate when the substrate temperature was 325 C, respectively. Below 300 C, no film was deposited under the given deposition conditions. Here, the range of deposition conditions were determined from the experimental conditions for Ru CVD using the Ru(EtCp) 2 which produced the phase-pure Ru films. Figure 2 shows that all the deposited films are Ru metal layers without RuO 2 found in the experimental results from the Ru(EtCp) 2 precursor. 8 It was estimated that the detection limit for XRD of the oxide phase, such

2 C318 Journal of The Electrochemical Society, C317-C Table I. Experimental conditions for Ru MOCVD. Precursor RuCp i-prcp Source temperature 85 C Carrier gas flow rate 100 sccm Deposition temperature C Reaction gas flow rate O sccm Total flow rate 1000 sccm Deposition pressure 3 Torr Figure 2. The XRD patterns of Ru thin films deposited on Si a at various substrate temperature with the addition of 50 sccm of oxygen and b at 325 C with various oxygen flow rates. as RuO 2, was approximately 1-2%. It has been observed that the RuO 2 phase was obtained from deposition under the conditions of a higher deposition temperature or a higher oxygen concentration. 8 Therefore, it was expected that deposition at temperatures higher than 400 C and an O 2 flow rate higher than 200 sccm would result in a slight incorporation of the oxide phase in the grown films. The variations in the crystallographic orientation of the grown films are also similar to the films grown using the Ru(EtCp) 2 precursor. Therefore, the thermal decomposition property of the RuCp i-prcp precursor and oxidation behavior of the grown Ru film using this precursor are similar to those when using Ru(EtCp) 2 precursor. Figure 3a and b shows the variations in resistivity of the Ru films as functions of the deposition temperature and oxygen flow rate, respectively. In Fig. 3a, the O 2 flow rate was 50 sccm. The resistivities of the Ru thin films were from approximately 12 to 13 cm, almost irrespective of temperature. In Fig. 3b, the growth temperature was 325 C. The resistivity of the Ru thin film deposited with the addition of 25 sccm was slightly larger than those of films deposited under other conditions. This might be due to the incomplete removal of carbon in the film when the oxygen flow rate was small, which corresponds to the case of the experiments using Ru(EtCp) 2. 8 This also corresponds to thermodynamic considerations that the removal of carbon, which is included in the precursor, occurs through the reaction with added oxygen to form primarily to CO 2 and CO. 8 In fact, this was the reason why metallic Ru films were obtained under the oxidizing atmosphere; almost all the oxygen was consumed to oxidize the carbon due to the larger oxidation potential of C compared to that of Ru. Therefore, it can be reasonably assumed that the phase-pure Ru film growth window in terms of the oxygen flow rate at a given temperature is dependent on the precursor delivery technique. For example, the liquid delivery technique, where an excessive amount of solvent C source is introduced to the chamber, may require a much larger O 2 flow rate to obtain metallic Ru films. The required amount of oxygen to completely oxidize and remove all the carbon in the system was larger than that expected from the thermodynamic calculation 8 due to the relatively small adsorption of oxygen onto the growing surface. However, it is apparent that the addition of a sufficient amount of Figure 1. The structure of RuCp i-prcp. Figure 3. The electrical resistivites of Ru thin films deposited a at various substrate temperatures with the addition of 50 sccm of oxygen, and b at 325 C with various oxygen flow rates.

3 C319 Figure 5. Cross-sectional TEM images of the Ru films deposited a at 375 C with 50 sccm of oxygen flow rate on TiN/TiO 2 /SiO 2 /Si substrate and b annealed under H 2 atmosphere at 400 C and then N 2 atmosphere at 600 C. Figure 4. The AES depth profiles of the Ru films deposited at 375 C with 50 sccm of oxygen flow rate on TiN/TiO 2 /SiO 2 /Si substrate and annealed under various conditions; a as-deposited, b postannealed under N 2 atmosphere at 600 C, c under H 2 atmosphere at 400 C, and d under H 2 atmosphere at 400 C and N 2 atmosphere at 600 C. oxygen, not enough to induce oxidation of the deposited Ru film, is essential to obtain high quality Ru films. This also corresponds to the experimental results when using Ru(EtCp) 2. Another potential contaminant in the Ru film is oxygen. Oxygen largely degrades the electrical conductivity compared to carbon. 8 Furthermore, oxygen in the Ru layer can oxidize the surface of the underlying barrier layer, such as TiN or TiAlN, when it is used as a bottom electrode. This results in a serious increase in the contact resistance of the integrated capacitors, even under the conditions where no further oxygen is introduced during dielectric deposition and postannealing. To investigate this potential problem, Ru films were deposited on the barrier TiN layer and the interfacial reaction between the Ru and TiN layers was investigated by both AES and TEM. It was a concern that oxygen in the Ru film could diffuse to the Ru/TiN interface during dielectric deposition and postannealing, and form a TiO x thin layer at the interface even when the amount of the oxygen is not very large. This concern is due to the much larger thermodynamic stability of TiO 2 compared to that of RuO 2 G f of TiO 2 and RuO 2 at 900 K are and kj/mol, respectively. 12 The TiO x layer can increase the contact resistance of the Ru/TiN interface. Figure 4 shows the AES depth profile of a typical Ru film, deposited at 375 C with 50 sccm of oxygen flow rate, on TiN/TiO 2 /SiO 2 /Si substrate and annealed under various conditions. It can be clearly observed that the Ru film contains a negligible amount of oxygen ( 1%) even in the as-deposited state. The relatively high oxygen concentration on the TiN surface is due to the exposure of the TiN film to air prior to Ru film growth. 15 to 20% of the oxygen concentration on the TiN surface after exposure to air is a well-known phenomenon. 13 It can be clearly seen that all annealing conditions do not induce change in the concentration distribution

4 C320 Journal of The Electrochemical Society, C317-C Figure 6. The Arrehnius plot of the growth rate of Ru film deposited at various substrate temperatures with 100 sccm of oxygen addition. of the constituent elements, except a slight decrease in the C concentration after the high temperature annealing. This implies that the initial oxygen concentration of the grown Ru film was small enough not to cause any interfacial oxidation or intermixing problem with the TiN barrier layer. Figure 5a and b shows cross-sectional TEM images of the asdeposited and postannealed films under H 2 atmosphere at 400 C followed by a N 2 atmosphere at 600 C, respectively. The slightly light contrast effect on the TiN surface is due to the initially high oxygen concentration. It can be confirmed that postannealing does not induce any structural change at the interface. Figure 6 shows the Arrehnius plot of the Ru film growth ratewith the deposition temperature when the O 2 flow rate was 100 sccm. The diffusion-limited-growth region was determined to be approximately 370 C because the growth rate at 375 C is almost the same as that at 400 C. Therefore, the growth rates at 325 and 350 C belong to the surface-chemical-reaction controlled region and the apparent activation energy for the surface-chemical-reaction was estimated from the slope of the straight line connecting the two points. The growth behavior changes from the surface chemical reaction controlled region to diffusion limited region at around 365 C, and the activation energy is approximately 0.8 ev, which is comparable to that from the experiment using Ru(EtCp) 2. Figure 7 shows the variation of the Ru film growth rate with O 2 flow rate at a deposition temperature of 350 C. The growth rate Figure 7. The variation of the growth rates of Ru films deposited at 350 C with varying O 2 flow rate. increases with increasing O 2 flow rate, and then saturates to approximately 250 Å/min. This behavior corresponds to the oxygen assisted decomposition behavior of the precursor. The growth rate has a very strong relationship with the stress behavior of the film as discussed later. The rough surface morphology of CVD Ru films has been known as one of the potential problems for its use as the bottom electrode. 14 Therefore, variations in surface morphology and roughness with the various deposition and postannealing conditions and types of substrate were investigated by SEM and AFM. Figure 8 shows the surface morphologies of 750 Å thick Ru thin films deposited at 325 C with various oxygen flow rates. The surface morphologies are almost independent of the oxygen flow rates and the root-mean-squared RMS roughness of the films are approximately 40 Å. However, the surface morphology was dependent on the deposition temperature. Figure 9 shows the surface morphologies of 1000 Å thick Ru films deposited at 325, 350, 375, and 400 C, respectively. The grain size increases with increasing growth temperature, as might be expected. The surface roughness of the Ru film depends on the types of substrate on which the films were grown. The surface dependent roughness might originate from the different nucleation properties of the Ru film on different surfaces. Figure 10a, b, c, and d shows the SEM micrographs of the films grown only for 1 min under a deposition temperature of 350 C and an O 2 flow rate 100 sccm on Si, SiO 2, TiN, and Ta 2 O 5 surfaces, respectively. The SiO 2, TiN, and Ta 2 O 5 films were prepared by thermal oxidation, reactive sputtering, Figure 8. The surface morphologies of Ru thin films deposited at 350 C with various O 2 flow rates in sccm ; a 25, b 50, and c 100.

5 C321 Figure 9. The surface morphologies of Ru thin films deposited at a 325 C, b 350 C, c 375 C, and d 400 C with 100 sccm of oxygen addition. and low pressure CVD, respectively. The Si surface actually had a very thin SiO 2 native oxide due to the air exposure of the substrate before the film deposition. The RMS roughnesses of the four samples measured by AFM were 82.7, 99.8, 43.7, and 34.8 C, respectively. It can be understood that the Ru film grown on the SiO 2 surface has a small nucleation probability, due probably to the high SiO 2 /Ru interface energy. The low nucleation probability, thereby large surface roughness, of the Ru film on the SiO 2 surface may induce a leakage current problem in the MIM capacitor structure of the integrated DRAM device. A certain nucleation layer or buffer layer, such as TiN, may be required to improve the nucleation property of the Ru film on a SiO 2 layer. However, this will make the integration process more complicated with the extremely small feature size ( 0.1 m). The poor nucleation of the Ru film on SiO 2 results in a Volmer-Weber type growth behavior, which greatly influences the stress behavior of the film. This sort of poor nucleation behavior of Ru is also observed on the Ta 2 O 5 film surface even though it is less severe compared to the Si or SiO 2 case. Figure 11 shows the AFM surface morphologies of the Ru film grown on the Ta 2 O 5 film surface. When the film is very thin, as in Fig. 11a, the connectivity between each nucleus is poor. Therefore, coalescence during the postannealing results in the island-like grains on the surface and the rougher surface Fig. 11c. However, as the film thickness increases, the surface is fully covered with the Ru film resulting in a more stable surface against postannealing, as in the case of Fig. 11d. This implies that there is a certain critical thickness under which the electrode is unstable to postannealing as the top electrode of the Ta 2 O 5 MIM capacitor. This critical value appeared to be approximately 600 Å in this study. The stress affects the thermal stabilities of the Ru films as the bottom and top electrodes. As the design rule of memory devices decreases, the residual stress in the films became increasingly crucial for the fabrication of memory devices and their reliability. Although the precise influence of stress on device production and reliability is still not clearly understood, residual stresses in the thin films on silicon substrates are generally considered to be of major concern in integrated circuit IC technology. This is especially the case as the feature size becomes smaller and smaller. High stress values in the thin films can cause a film to migrate, crack and even peel off. 15 Therefore, it is important to investigate the stress generated in Ru thin films for their use as the top as well as the bottom electrode of the DRAM capacitor. The stress of the thin films is generally classified into either thermal or intrinsic stresses. In general, for high melting point materials such as Ru and Pt, intrinsic tensile stress is mainly observed 16 and this type of stress is associated with island impingement and coalescence. 17 Figure 12a shows the variations in stresses in Ru thin films deposited on Si as a function of various substrate temperatures when the oxygen flow rate was 100 sccm. As the deposition temperature increases, the tensile stress decreases. Therefore, the generated stress in the film originates mainly from the intrinsic component and not from the thermal component of the stress. Figure 12b shows the stresses in the Ru thin films deposited at 350 C with various oxygen flow rates. The stress decreases with oxygen flow rates up to 200 sccm and then slightly increases. From the comparison between the variations in stress of the films Fig. 12a and b and the variations in the film growth rate Fig. 6 and 7 according to the various deposition conditions, it can be understood that the higher the growth rate, the smaller the stress. It is generally accepted that the tensile stress observed during the island coalescence stage of Volmer-Weber film growth is associated with the formation of grain boundaries. When two growing islands first touch, some portion of the adjacent surfaces of the islands zip together to form a grain boundary. Zipping, which is driven by the reduction in the overall surface energy, is accomplished through elastic deformation of the islands, thereby imposing a cost of increased strain energy. In contrast, the tensile stresses generated by grain boundary formation can be relieved by diffusion of adatoms on the free surface of a film into the grain boundary. 15 Therefore, the intrinsic stress generated in the film can be viewed as a by-product from competition between the zipping of adjacent Figure 10. The SEM micrographs of the Ru films grown on a Si, b SiO 2, c TiN, and d Ta 2 O 5 for 1 min under the conditions of 350 C deposition temperature and 100 sccm of O 2 flow rate.

6 C322 Journal of The Electrochemical Society, C317-C Figure 12. Stresses in Ru thin films deposited on Si a at various substrate temperature with 100 sccm of oxygen addition and b at 350 C with various oxygen addition. Most of the grain boundaries were formed by zipping when the film growth rate was low, whereas the zipping is suppressed by the fast accumulation of constituent atoms in the grain boundary area when the growth rate is high. This is a diffusive stress relaxation mechanism. In addition, for the case of Ru film deposition at high temperature, the surface diffusion of adatoms is more active compared to film deposition at low temperature resulting in a more effective release of thin film stress. Therefore, the tensile stress decreases very fast as the deposition temperature increases Fig. 12a. The high stress at 325 C is a serious concern since the typical Ru film growth temperature should be approximately 300 C considering the conformal deposition property at this low temperature. Further research will certainly be required to reduce the stress at low deposition temperatures. Figure 11. The AFM surface morphologies of the Ru film grown on the Ta 2 O 5 film surface for a, c 1 min and b, d 3 min at 350 C, respectively. And in c and d, the films were annealed at 600 C under N 2 atmosphere for 30 min. nuclei or grains, which increases the stress, and surface diffusion of adatoms into the grain boundary area, which reduces the stress when the film grows via the Volmer-Weber mechanism. The Volmer- Weber type growth behavior of the Ru films was confirmed by AFM. Conclusion Ru thin films were deposited using a RuCp i-prcp precursor and O 2 on Si wafers. The films consisted of a metallic Ru single phase under all deposition conditions and exhibited good electrical resistivities as low as 12 cm. Almost all the deposition behavior and physical characteristics of the Ru films are quite similar to the Ru films produced using the Ru(EtCp) 2 precursor, which is the most widely used precursor for Ru CVD. The deposited films had a negligible amount of residual oxygen which meant oxidation of the underlying barrier layer, TiN, after the postannealing at 600 C was negligible. However, the films showed a poor nucleation behavior especially on SiO 2 surface due to the nonwetting behavior of Ru on SiO 2 resulting in a rough surface morphology. The poor nucleation property also occurred on Ta 2 O 5 surface resulting in a increased roughness after postannealing at 600 C when the film thickness is very small. No increase in roughness after annealing was observed when the film thickness was larger than a critical thickness of approximately 600 Å. This nucleation behavior of the Ru film also affected the stress property of the grown film. The stress greatly increased with the decreasing growth rate almost irrespective of the deposition conditions. This behavior was explained in terms of intrinsic stress generation by zipping of the nuclei when the film growth rate was low. The decrease in stress with increasing growth rate and temperature was explained by a diffusive stress relaxation mechanism. Acknowledgments The authors acknowledge the financial support of Samsung electronics, and the Korean government through A Collaborative Project for Excellence in Basic System IC Technology System IC 2010 and National Research Laboratory NRL program. The authors also acknowledge Tanaka Kikinzoku Co. Japan for their supply of RuCp i-prcp. Seoul National University assisted in meeting the publication costs of this article.

7 C323 References 1. Y. Matsui, M. Hiratani, T. Nabatame, Y. Shimamoto, and S. Kimura, Electrochem. Solid-State Lett., 4, C M. L. Green, M. E. Gross, L. E. Papa, K. J. Schnoes, and D. Brasen, J. Electrochem. Soc., 132, J. Si and S. B. Desu, J. Mater. Res., 8, S.-E. Park, H.-M. Kim, K.-B. Kim, and S.-H. Min, J. Electrochem. Soc., 147, J. M. Lee, J. C. Shin, C. S. Hwang, H. J. Kim, and C.-G. Suk, J. Vac. Sci. Technol. A, 16, J.-H. Lee, J.-Y. Kim, S.-W. Rhee, D. Y. Yang, D.-H. Kim, C.-H. Yang, Y.-K. Han, and C.-J. Hwang, J. Vac. Sci. Technol. A, 18, J.-H. Lee, J.-Y. Kim, and S.-W. Rhee, Electrochem. Solid-State Lett., 2, S. Y. Kang, K. H. Choi, S. K. Lee, C. S. Hwang, and H. J. Kim, J. Electrochem. Soc., 147, T. Nabatame, M. Hiratani, M. Kadoshima, Y. Shimamoto, Y. Matsui, Y. Ohji, I. Asano, T. Fujiwara, and T. Suzuki, Jpn. J. Appl. Phys., Part 2, 39, L J. M. Lee, S.-K. Hong, C. S. Hwang, and H. J. Kim, J. Korean Phys. Soc., 33, S W.D.Nix,Metall. Trans. A, 20, I. Barin, Thermochemical Data of Pure Substances, VCH Publishers, NewYork M. Eizenberg, Mater. Res. Soc. Symp. Proc., 427, T. Aoyama, M. Kiyotoshi, S. Yamazaki, and K. Eguchi, Jpn. J. Appl. Phys., Part 1, 38, G. J. Leusink, T. G. M. Oosterlaken, G. C. A. M. Janssen, and S. Radelaar, J. Appl. Phys., 74, J. A. Thornton and D. W. Hoffman, Thin Solid Films, 171, J. A. Floro, S. J. Hearne, J. A. Hunter, P. Kotula, E. Chason, S. C. Seel, and C. V. Thompson, J. Appl. Phys., 89,