Characteristics of Cobalt Films Deposited by Using a Remote Plasma ALD Method with a CpCo(CO) 2 Precursor

Transcription

1 Journal of the Korean Physical Society, Vol. 50, No. 4, April 2007, pp Characteristics of Cobalt Films Deposited by Using a Remote Plasma ALD Method with a CpCo(CO) 2 Precursor Keunwoo Lee, Keunjun Kim and Hyeongtag Jeon Division of Materials Science and Engineering, Hanyang University, Seoul Youngjin Lee, Jeongtae Kim and Seungjin Yeom R&D Division, Hynix Semiconductor Inc., Icheon (Received 24 December 2006, in final form 22 January 2007) Co films deposited by using a remote plasma atomic layer deposition (RPALD) method with cyclopentadienyl cobalt dicarbonyl (CpCo(CO) 2) as a precursor were investigated. The process parameters, such as the deposition temperature, the plasma power, the process pressure, and the plasma gas, were varied, and the resulting Co films were characterized. The growth rate of the Co film was about 1.1 Å/cycle for a process window between 125 and 175 C. The impurity content of the Co films was minimized using a H 2 plasma in the process pressure range between 0.1 and 2 Torr at a plasma power of 300 W. The carbon and the oxygen contents of the Co films were about 7 at.% and below 1 at.%, the detection limit, respectively. The Co films showed a very uniform surface with a root-mean-square (RMS) roughness of 1.51 Å, as determined by using an atomic force microscopy (AFM) analysis. The Co films deposited on contact holes, about 0.12 µm wide and 1.8 µm deep, showed excellent conformal coverage. The compositions of the Co films on the tops and the sidewalls of the contact holes were examined with Auger electron spectroscopy (AES), and the results showed nearly identical compositions. PACS numbers: Je, Rx, Ls Keywords: RPALD, Co silicide, Step coverage I. INTRODUCTION As complementary metal oxide semiconductor (CMOS) devices are being scaled down to the nanometer scale, silicide formation has become essential for the improvement of device and circuit performance. Increasingly, more severe requirements have been placed on the silicide properties and performances to improve device properties such as resistance and leakage current [1]. Among the metal silicides used in devices such as interconnects, and ohmic contacts and used for low resistivity metallization, Co silicide has been extensively investigated due to its good thermal and chemical stability, low resistivity and small lattice mismatch with the Si substrate [2,3]. In a typical silicide process, Co silicide is produced mainly by using the physical vapor deposition (PVD) [4, 5] and the chemical vapor deposition (CVD) techniques [6, 7]. Co silicide produced by using the PVD method provides a good quality thin film, but unfortunately has poor step coverage. The CVD method is a good ap- hjeon@hanyang.ac.kr; Fax: proach to solve the step coverage problem, but it generally leads to a film that contains a large amount of impurities [7]. To solve these problems associated with the PVD and CVD methods, the atomic layer deposition (ALD) method was introduced as a deposition technique [8]. In general, it is well known that the ALD method has many advantages, such as ultra-thin film growth, conformal deposition, and thickness control. In these respects, the ALD method is a very attractive candidate for forming uniform and continuous thin films. The RPALD technique to deposit Co thin films was introduced previously [9, 10]. In addition, suitable precursors were examined. Typically, a suitable precursor for use in ALD should have the following properties: high volatility, low decomposition temperature, and resistance to premature decomposition; preferably, it should be a liquid. Co films have been studied previously with various precursors such as cobalt tricarbonyl nitrosyl (Co(CO) 3 NO) [11, 12], dicobalt octacarbonyl (Co 2 (CO) 8 ) [13 15], cobaltocene (CoCp 2 ) [16,17], and cyclopentadienylcobalt dicarbonyl ((C 5 H 5 )Co(CO) 2 )[(C 5 H 5 ) = Cp] [16]. Co(CO) 3 NO has zero oxidation state among these metal organic precur

2 Journal of the Korean Physical Society, Vol. 50, No. 4, April 2007 sors, so Co(CO) 3 NO yields a pure Co film without reduction reactants. However, the deposition temperature of Co(CO) 3 NO is relatively high, making it very difficult to handle. Furthermore, Co 2 (CO) 8 and CoCp 2 are solids at room temperature and are very unstable during the process [16]. Finally, CpCo(CO) 2 is a liquid with a high vapor pressure at room temperature, and the decomposition temperature is higher than those of the other Co metalorganic precursors. Therefore, the risk of premature decomposition of the CpCo(CO) 2 precursor before reaching the Si substrate is smaller than those of the others [17]. Based on these criteria, we chose CpCo(CO) 2 as the Co metalorganic precursor. In this study, we focused on Co films deposited by using the RPALD method with a CpCo(CO) 2 precursor and various reactant plasmas. Additionally, we studied the effect of the process parameters during deposition on the characteristics of the Co films deposited by using the RPALD method. Finally, these Co films were deposited under optimized process conditions on contact holes to confirm the step coverage. II. EXPERIMENTS Co films were deposited on p-type Si (100) substrates with a resistivity of 3 9 Ωcm by using the RPALD method with a CpCo(CO) 2 precursor. A downstreamtype RPALD reactor with a MHz radio-frequency (rf) power source was used in this study. The Si substrates were cleaned by dipping them in a piranha solution (H 2 SO 4 : H 2 O 2 = 4 : 1) for 10 min and then in a dilute HF solution (HF : D.I. water = 1 : 50) for 1 min to remove organic and native oxides, and then immediately loaded into the ALD reactor. The process parameters, such as deposition temperature, plasma power, process pressure, and plasma gases, were varied to optimize the process conditions. The CpCo(CO) 2 as the Co precursor was delivered from an external reservoir at 25 C and introduced into the reactor by using an Ar carrier gas with a flow rate of 5 sccm. The reactant gas flow rate was maintained between 20 and 190 sccm, and the corresponding process pressure was 0.1 to 2 Torr. The plasma power was varied between 50 and 300 W. One deposition cycle consisted of the injection of the CpCo(CO) 2 precursor, a purge with argon gas, an exposure to the reactant plasma, and a purge with argon gas again. The flow rate and time of the Ar purge gas were fixed at 100 sccm and 10 sec, respectively. To confirm the step coverage of the thin films, a Co film was deposited on contact holes of about 0.12 µm in width and 1.8 µm in depth with an aspect ratio of 15 : 1. The chemical composition and the impurity content were examined by using Auger electron spectroscopy (AES) and Rutherford backscattering spectroscopy (RBS). The thickness and the microstructure of the Co films were measured with a cross-sectional trans- Fig. 1. (a) Growth rate of Co films as a function of the deposition temperature. (b) XTEM micrograph of the Co film deposited by using the RPALD method. mission electron microscope (XTEM) and a field emission scanning electron microscope (FESEM). III. RESULTS AND DISCUSSION Figure 1 shows the growth rate of the Co films deposited by using the RPALD method with a CpCo(CO) 2 precursor at various temperatures. The growth rate of the Co film was about 1.1 Å per cycle in the ALD process window with a saturated temperature range between 125 and 175 C. The decomposition temperature of CpCo(CO) 2 is reported to be around 140 C [6]. In the inset of Figure 1(a), the Co film thickness was plotted against the number of deposition cycles at 150 C. The film thickness showed a linear dependence on the number of cycles. The linearity of the growth behavior as a function of the cycle number is a feature of the ALD process. However, the growth rate of the Co films increased rapidly as the deposition temperature was increased above the ALD process window due to the contribution of a CVD-like growth behavior caused by the

3 Characteristics of Cobalt Films Deposited by Using a Remote Keunwoo Lee et al Table 1. AES analysis of the impurity contents of the Co films for various reactant plasma gases. Reactant Gas Contents (at.%) Carbon Oxygen Nitrogen H < 1.0 < 1.0 H 2+Ar 7.8 < 1.0 < 1.0 NH < N < Fig. 2. AES analysis of the impurity contents in the Co films as functions of the plasma power. self-decomposition of the precursors. Also, the increased growth rate below 125 C in the Co film deposition is due to the effect of precursor condensation at low temperatures. If condensation occurs during an ALD cycle, undesirable or uncontrollable reactions may occur, resulting in the formation of porous and impure films [18]. The thickness of the Co film was found to be about 110 Å by using a XTEM analysis, as shown in Figure 1(b). This Co film showed a very uniform surface and a smooth interface. Particularly, the surface roughness of the Co films measured by using an AFM analysis showed a RMS value of 1.51 Å for a thickness of 110 Å (data not shown). To investigate the effect of the plasma power on the impurity content, Co films were deposited at various plasma powers in the range of W at 150 C with a H 2 plasma. Figure 2 shows the influence of the H 2 plasma power on the carbon and the oxygen contents in the Co films deposited using a CpCo(CO) 2 precursor. The carbon content of the Co films decreased gradually with increasing H 2 plasma power. At this temperature, the Co film deposited at a plasma power of 300 W showed the lowest carbon content of 7 at.%. Although the H 2 plasma power was high, the carbon content slightly decreased in the Co film due to incomplete decomposition ofn the CpCo(CO) 2 precursor at the deposition temperature. However, Co films deposited with the CpCo(CO) 2 precursor exhibited an oxygen impurity content below the detection limit (<1 at.%) of the AES analysis, regardless of the power of the various reactant plasmas. A decrease in the oxygen impurity content is likely to be associated with the promotion of the reactivity of hydrogen radicals along with an increase in the H 2 plasma power. To verify the effect of the process pressure on the decomposition of the CpCo(CO) 2 precursor, we varied the hydrogen flow rate between 20 to 190 sccm, which corresponded to hydrogen partial pressures in the range between 0.1 and 2 Torr. This H 2 pressure range was limited to maintain the proper mean free path so that a plasma could be generated. Then, the plasma power was maintained at 300 W. The variations in the oxygen and the carbon contents were remarkably small throughout the range. The oxygen and carbon contents were about 7 at.% and below 1 at.%, irrespective of the hydrogen partial pressure. These results indicate that the amount of hydrogen in the range between 0.1 and 2 Torr dose not play a significant role in the deposition of Co films from a CpCo(CO) 2 precursor. Finally, the variation in the impurity content when using this precursor was closely related with H 2 plasma power. We observed that the variation in the impurity content depended on the plasma gases, such as H 2, a H 2 +Ar mixture, NH 3, and N 2 gas, under a constant pressure of 0.1 Torr at 300 W, as shown in Table 1. The Co film deposited with the H 2 plasma had the lowest impurity content. The carbon and the oxygen were detected at around 7 at.% and below 1 at.%, the detection limit, respectively, for a H 2 plasma at a flow rate of 20 sccm. The mixed plasma with H 2, 20 sccm, and Ar, 40 sccm, caused a slight increase in the oxygen concentration. In particular, the nitrogen contents of the Co films deposited with the NH 3 and the N 2 plasmas were observed to be about 12.5 at.% and 18.2 at.% because of the high nitrogen incorporation in the plasma process. However, the carbon content of the Co film deposited using the NH 3 plasma was lower than that the of Co film deposited using the N 2 plasma due to highly effective physical and chemical reactions induced by the enhanced reactivity of ammonia radicals. Therefore, the Co film deposited with the N 2 plasma showed the highest impurity concentration. The physical reaction induced by the N 2 plasma was not sufficient to decrease the impurity content. And, the Co film made using the H 2 plasma showed the lowest impurity content of the films made by using various plasma gases. The reason Co with a H 2 plasma exhibits a low impurity content is that the Cp-Co bond is weaker than the bonds in the Cp ring itself; therefore, the Cp- Co bond in the CpCo(CO) 2 precursor was easily broken under a hydrogen-reducing atmosphere [17]. The Co films deposited using CpCo(CO) 2 showed an oxygen impurity content below the detection limit (< 1 at.%) of the AES analysis, regardless of the plasma gases, including H 2, the H 2 +Ar mixture, NH 3, and N 2 gas. This suggests that a Cp ligand in CpCo(CO) 2 reacts efficiently with the radicals formed by the H 2 plasma and is then

4 Journal of the Korean Physical Society, Vol. 50, No. 4, April 2007 Fig. 4. (a) FESEM and XTEM micrograghs of the (b) top, (c) sidewall, and (d) bottom of the Co film deposited on a contact hole, about 0.12 µm wide and 1.8 µm deep with a 15 : 1 aspect ratio. Fig. 3. (a) RBS analysis and (b) AES depth profile of Co films deposited by using the RPALD method. With a H 2 plasma at 300 W and 0.1 Torr. released as byproducts, such as hydrocarbons (C x H y ). The chemical reaction leading to the deposition of the Co film is represented generically by the following equation: C 5 H 5 Co(CO) 2 (g) + 1/2nH 2 (g) Co(s) + 2CO(g) + C 5 H 5 + n(g) [6,17]. However, the carbon incorporation in the Co film is attributed to the decomposition of CpCo(CO) 2 by thermal disproportion, leading to an unstable Co-CO ligand. The Co films were deposited under optimized process conditions: a H 2 reactant plasma, a process range between 0.1 and 2 Torr, and a plasma power of 300 W. Figure 3(a) shows that the carbon and the oxygen contents of the Co films were about 7 at.% and 1 at.%, as confirmed by RBS analysis. The oxygen on the surfaces of the Co films is likely to be contamination due to atmospheric exposure. The low carbon content in the Co films deposited with the CpCo(CO) 2 precursor is attributed to the chemical reaction between the H 2 plasma and the precursor. The Cp ligand in CpCo(CO) 2 reacts with H 2 to form stable and volatile hydrocarbons, which are easily removed. Although the Co film deposited by using the RPALD with CpCo(CO) 2 contains about 7 at.% carbon as an impurity, this is much lower than the approximate 50 at.% carbon impurity in the Co films deposited by using the MOCVD with the same precursor under a H 2 atmosphere [19]. This result indicated that the reactive hydrogen radical is more effective at breaking the Cp-Co bond than molecular hydrogen gas. Figure 3(b) shows AES data fitted by the absolute content value of the RBS analysis after the depth profiling. The step coverage and the impurity content of the films on contact holes are very important issues in the deposition in ultra large scale integration (ULSI) technology. Poor step coverage and inhomogeneous impurity content in the films generally results in poor adhesion and high contact resistance [20]. However, the ALD technique is known to be excellent at preventing these problems. We previously investigated similar chemical compositional variations in the step coverage of a TiN film deposited as a diffusion barrier by using the RPALD method. The chemical composition of the TiN film was very uniform on the top and the sidewalls of the contact hole [21]. In this study, the step coverage and the impurity content of the Co films deposited on contact holes by using the RPALD method were carefully investigated. Figures 4(a) (d) shows the FESEM and the XTEM micrographs of Co films deposited under optimized process conditions on contact holes, about 0.12 µm wide and 1.8 µm deep with an aspect ratio of 15:1. The deposition thicknesses of the Co films deposited on the top, the sidewalls, and the bottom of the contact holes were about 200 Å. The Co film showed excellent conformal deposition on the

5 Characteristics of Cobalt Films Deposited by Using a Remote Keunwoo Lee et al uniform composition and low impurity content. IV. CONCLUSION In summary, we have investigated Co films deposited by using the RPALD method with a CpCo(CO) 2 precursor. The growth rate of the Co film was about 1.1 Å/cycle in an ALD process window between 125 and 175 C. The impurity content of the Co films was minimized when the process was performed with a H 2 plasma within the process pressure range between 0.1 and 2 Torr at a plasma power of 300 W. Moreover, the impurity content decreased due to the Cp-Co bond in CpCo(CO) 2 precursor being easily broken under a H 2 plasma with a reducing atmosphere. The carbon and the oxygen contents of the Co films were about 7 at.% and below 1 at.%, the detection limit, respectively. As the plasma power is increases, the reactivity of the hydrogen radicals to the precursor seems to be promoted, resulting in a decrease in impurities. The Co films deposited on contact holes about 0.12 µm-wide and 1.8 µm-deep by using the RPALD method showed excellent conformal deposition. Also, the oxygen and the carbon contents of the Co films on the top and the sidewalls of the contact holes were found to be almost identical. ACKNOWLEDGMENTS Fig. 5. AES depth profiles of the Co films deposited on the (a) top and (b) sidewall of contact holes by using the RPALD method. contact holes. We also investigated the impurity content of the Co film deposited on the top and the sidewalls of the contact holes. Figure 5 shows the AES depth profiles of the Co film on the (a) top and the (b) sidewalls of the contact hole. To obtain these analyses for the contact hole, we used a field emission gun AES system with a minimum beam size of 10 nm at 10 kv. These contact hole samples were loaded in the analysis chamber and tilted 30 to focus the beam at the contact hole. We achieved the smallest beam size with the lowest beam current and highest acceleration voltage possible. As a result, the carbon and the oxygen contents at the top and the sidewalls of the contact holes with Co films deposited by using the RPALD method exhibited almost uniform compositional variations. Particularly, the oxygen and the carbon contents of Co films on the top and the sidewalls of the contact holes were examined and found to be almost identical. This indicated that the radicals reacted uniformly at the top and the sidewalls of the contact holes [22]. These results showed that RPALD is a promising method to obtain conformal thin films with a This work was supported by the National Program for Tera-level Nanodevices of the Ministry of Science and Technology as one of the 21 st Century Frontier Programs. REFERENCES [1] S. P. Murarka, Intermetallics 3, 173 (1995). [2] S. P. Murarka, Silicides for VLSI Applications (Academic Press, London, 1983), p. 30. [3] K. Maex, Mater. Sci. Eng. R 11, 53 (1993). [4] J. Heo and H. Jeon, Thin Solid Films 379, 265 (2000). [5] J. S. Byun, D. H. Kim, W. S. Kim and H. J. Kim, J. Appl. Phys. 78, 1725 (1995). [6] M. F. Chioncel and P. W. Haycick, Chem. Vapor Depos. 11, 235 (2005). [7] M. E. Gross, K. S. Kranz, D. Brasen and H. Luftman, J. Vac. Sci. Technol. B 6, 1548 (1988). [8] B. S. Lim, A. Rahtu and R. G. Gordon, Nature 406, 1032 (2000). [9] T. Suntola, Handbook of Thin Film Process Technology, 1st Ed., (Institute of Physics, London, 1995). [10] J. Koo, S. Kim, S. Jeon, Y. Kim, Y. Won and H. Jeon, J. Korean Phys. Soc. 48, 131 (2006). [11] A. R. Londergan, G. Nuesca, C. Goldberg, G. Peterson, A. E. Kaloyerors, B. Arkles and J. J. Sullivan, J. Electrochem. Soc. 148, 21 (2001).

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