Characteristics of Hafnium-Aluminum-Oxide Thin Films Deposited by Using Atomic Layer Deposition with Various Aluminum Compositions

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1 Journal of the Korean Physical Society, Vol. 47, No. 3, September 2005, pp Characteristics of Hafnium-Aluminum-Oxide Thin Films Deposited by Using Atomic Layer Deposition with Various Aluminum Compositions Jaehyoung Koo, Janghee Lee, Seokhoon Kim, Young Do Kim and Hyeongtag Jeon Division of Materials Science and Engineering, Hanyang University, Seoul Deok-Soo Kim Department of Industrial Engineering, Hanyang University, Seoul Yangdo Kim School of Materials Science and Engineering, Pusan National University, Busan (Received 17 June 2005) Thin films of hafnium-aluminum-oxide [(HfO 2) x(al 2O 3) 1 x] were investigated as a potential replacement for SiO 2 gate dielectrics. Al 2O 3, HfO 2, and hafnium-aluminum-oxide films were successfully deposited using the atomic layer deposition (ALD) method at 300 C. The Al 2O 3 films showed amorphous structures while the HfO 2 films showed a randomly oriented polycrystalline structure with Hf-silicate and/or SiO x interfacial layers. Hafnium-aluminum-oxide films showed a much stronger resistance to oxygen diffusion than pure HfO 2 films during annealing, and the crystallization temperature increased with further Al addition to the hafnium-aluminum-oxide film. The leakage current densities of the Al 2O 3, HfO 2, and hafnium-aluminum-oxide films were measured at a gate bias voltage of V G V F B = 2 V, yielding , , and A/cm 2 (with a Al 2O 3 : HfO 2 ratio of 1 : 1), respectively. The corresponding calculated equivalent oxide thickness (EOT) values were approximately 2.2, 1.8, and 1.6 nm, respectively. The hafnium-aluminum-oxide film had a higher dielectric constant and a lower EOT than the Al 2O 3 film and exhibited a lower leakage current and higher breakdown voltage than the HfO 2 films. The dielectric constant, the leakage current, the flat band voltage, and the crystallinity of the hafnium-aluminum-oxide film exhibited a strong dependence on the Al-Hf composition ratio of the films. PACS numbers: f, Fy Keywords: ALD, Hafnium-aluminum-oxide, HfO 2, Al 2 O 3, Interfacial layer I. INTRODUCTION Silicon dioxide (SiO 2 ) is the most commonly used gate dielectric in metal-oxide-semiconductor (MOS) devices due to its chemical and thermal stability on Si substrates [1]. The SiO 2 thickness has been decreased significantly in the development of high-speed advanced semiconductor devices [2,3]. However, the gate oxide thickness cannot be reduced below 2 nm because of reliability problems associated with conventional SiO 2 and high leakage currents caused by direct tunneling across the gate dielectric film [4, 5]. Based on these restrictions, highdielectric-constant materials are required as alternative hjeon@hanyang.ac.kr; Tel: ; Fax: gate insulators in advanced MOS devices [6, 7]. Compared to SiO 2 films, high-dielectric-constant films can provide larger physical thicknesses and significant reductions in leakage currents for the same equivalent oxide thickness (EOT) [8,9]. Among the high-dielectric-constant materials, HfO 2, Al 2 O 3, ZrO 2, and Ta 2 O 5 have been extensively studied to solve the excessively high leakage-current concern for future advanced high-performance devices [10,11]. Al 2 O 3 and HfO 2 are considered to be the most attractive materials among the high dielectric materials. Also, Al 2 O 3 has been studied in ultra-large-scale integrated devices because it remains amorphous even after annealing at temperatures as high as 1000 C [12]. In addition, Al 2 O 3 exhibits a large band gap ( 8.8 ev), a high field strength, a excellent thermal stability, and large band offsets [13].

2 -502- Journal of the Korean Physical Society, Vol. 47, No. 3, September 2005 However, Al 2 O 3 has some drawbacks, such as its high oxide trap charge density and relatively low dielectric constant (9 12), which limit the application of this material in actual devices [14]. Another candidate is HfO 2 because of its high dielectric constant (25 30), high density ( 9.68 g/cm 3 ), and large band gap ( 5.68 ev) [15,16]. However, HfO 2 crystallizes at temperatures less than 500 C [17]. Most high-κ dielectric films are susceptible to crystallization during either deposition or subsequent high temperature processing. Crystallization of thin films generates grain boundaries in thin dielectric films and these cause an increase in leakage current. Grain boundaries in crystalline gate dielectrics provide fast diffusion paths for oxygen or dopants into the dielectric, causing uncontrolled interlayer growth at the dielectric/si interface, threshold voltage instability, and defect generation [18]. This interlayer formation limits further scaling of gate dielectric films. Thus, if lower leakage currents are to be achieved, it is important for a film to maintain an amorphous structure after thermal processing. Recently, several research groups have reported that the incorporation of nitrogen or aluminum into high-κ materials improves the thermal stability of high-κ dielectrics [19,20]. We noted that the two gate dielectrics, Al 2 O 3 and HfO 2, have complementary characteristics to each other and it is possible to achieve a high-κ gate oxide with low leakage current and with large band gap characteristics by using a composite of these two oxides [21, 22]. In this study, we deposited (HfO 2 ) x (Al 2 O 3 ) 1 x (hafnium-aluminum-oxide) films deposited by using the atomic layer deposition (ALD) method and investigated the physical, chemical, and electrical characteristics of the hafnium-aluminum-oxide films, comparing them to those of Al 2 O 3 and HfO 2 films. In addition, we evaluated the effects of compositional variation on the electrical characteristics of hafnium-aluminum oxide. II. EXPERIMENT Al 2 O 3, HfO 2, and hafnium-aluminum-oxide films were deposited using the ALD method on 4-inch, p-type silicon (100) substrates with a 5 10 Ω cm resistivity. The Si substrates were cleaned by dipping them first 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 : H 2 O = 1 : 100) for 2 min to remove organics and native oxides, respectively. The Al 2 O 3, HfO 2, and hafnium-aluminum-oxide thin films were deposited at 300 C using trimethylaluminum (TMA), Al(CH 3 ) 3, and hafnium tetrachloride, HfCl 4, as the Al and the Hf precursors, respectively.the basic first cycle consisted of applying either an Al or a Hf precursor and using H 2 O as a reactant gas. A N 2 purge gas was introduced to separate the precursor and the reactant gases. The sequential processing times of the precursor [Al(CH 3 ) 3 or HfCl 4 ], the N 2 purge, the H 2 O reactants, and another N 2 purge were 5, 8, 5, and 8 s, respectively. Three different sets of hafnium-aluminumoxide films were prepared with various Al and Hf compositions. The composition of the hafnium-aluminum-oxide films was controlled by using the ratio of Al 2 O 3 to HfO 2 deposition cycles. The number of cycles (and hence the film thickness) was fixed at 60 for samples bound for TEM, XPS, C-V, and I-V analyses and at 600 for samples bound for RBS and XRD analyses. The first data set had a 3 : 1 Al 2 O 3 : HfO 2 deposition ratio, repeated 15 times for the 60-cycle test samples and 150 times for the 600-cycle test samples. The second data set had a 1 : 1 Al 2 O 3 : HfO 2 deposition ratio, repeated 30 times for the 60-cycle test samples and 300 times for the 600- cycle test samples. Lastly, the third data set had a 1 : 3 Al 2 O 3 : HfO 2 ratio, repeated 15 times for the 60-cycle test samples and 150 times for the 600-cycle test samples. All samples were rapidly thermal annealed at 800 C for 10 s in a nitrogen atmosphere after deposition. After annealing, a Pt electrode with a thickness of approximately 1000 Å was deposited using an electron-beam evaporator. The forming gas annealing was carried out in a H 2 and N 2 ambient atmosphere at 450 C for 30 min. The physical and the chemical characteristics of the Al 2 O 3, HfO 2, and hafnium-aluminum-oxide films were analyzed by using cross-sectional transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The electrical properties, including the EOT, the hysteresis, the leakage current, and the capacitance, were analyzed by using currentvoltage (I-V) and capacitance-voltage (C-V) measurements. III. RESULTS AND DISCUSSION The microstructure and the chemical bonding structures of Al 2 O 3 and HfO 2 films were investigated by using cross-sectional TEM and XPS as shown in Figure 1. The XPS spectra were measured at a take-off angle of 90 using an Al Kα source with ev. Figures 1(a) and (b) show the cross-sectional TEM images of the Al 2 O 3 and the HfO 2 films deposited at 300 C, respectively. As Figure 1 (a) shows, the as-deposited Al 2 O 3 film had an amorphous structure with a thickness of approximately 50 Å. The XPS spectrum of the Si 2p peak taken from the Al 2 O 3 film, shown in the inset of Figure 1 (a), exhibited a binding energy of around 99.0 ev and no other peaks. This is an indication that no interfacial layers were formed in the as-deposited Al 2 O 3 film. However, the cross-sectional TEM image of the HfO 2 film, shown in Figure 1 (b), indicated a randomly oriented polycrystalline structure with an interfacial layer. In the inset of Figure 1 (b), the XPS spectrum of Si 2p peaks taken from the HfO 2 film showed binding energies of around 99.0 and ev, indicating that the interfacial layer shown in

3 Characteristics of Hafnium-Aluminum-Oxide Thin Films Deposited by Jaehyoung Koo et al Fig. 1. Cross-sectional TEM micrographs of the (a) Al 2O 3 and the (b) HfO 2 films deposited at a temperature of 300 C. The insets show the XPS spectra of the Si 2p peak in Al 2O 3 and HfO 2. Fig. 2. High-frequency (a) C-V and (b) I-V curves of Al 2O 3 and HfO 2 films after rapid thermal annealing. the XTEM image of the HfO 2 film was in a Hf-silicate and/or SiO x structure. The total thickness was approximately 65 Å. The interfacial layer was approximately 20-Å thick and exhibited an amorphous structure. Figures 2 (a) and (b) show the high-frequency C-V and I-V curves of the as-deposited Al 2 O 3 and HfO 2 films, respectively. The gate voltage was swept from accumulation to inversion and back, between 3.0 and 3.0 V. The corresponding EOT values of Al 2 O 3 and HfO 2, calculated from the maximum accumulation capacitances were approximately 2.2 nm and 1.8 nm, respectively. A quantum-mechanical correction was applied in analyzing these EOT values. The dielectric constants of Al 2 O 3 and HfO 2, calculated from the EOT values and the physical thicknesses measured from the cross-sectional TEM, were approximately 9.0 and 13.7, respectively. We postulated that the dielectric constant of the HfO 2 thin film was lower than its bulk value (25 30) due to the formation of a low-dielectric-constant interfacial layer. The leakage current densities were measured at a gate bias voltage of V G V F B = 2 and were approximately and A/cm 2 for Al 2 O 3 and HfO 2 respectively. The breakdown voltages of the Al 2 O 3 and the HfO 2 films were approximately 9 V and 6 V, respectively, which are approximately 18 and 9 MV/cm. The Al 2 O 3 film showed significantly improved I-V characteristics over the HfO 2 film. The low leakage current of the Al 2 O 3 film was attributed to its amorphous structure and larger band gap energy. Figure 3 shows the XRD spectra of (a) a HfO 2 film and (b) a hafnium-aluminum-oxide film with an Al 2 O 3 : HfO 2 ratio of 1 : 3 both before and after RTA treatment at 800 C for 10 s. The XRD spectra were scanned from 20 to 80 by using the 2/ method and an applied voltage and current of 40 kv and 80 ma, respectively. The thicknesses of the HfO 2 and the hafnium-aluminum-oxide films were measured to be approximately 610 Å and 650 Å, respectively, by using an ellipsometer. The HfO 2 XRD spectrum in Figure 3 (a) shows a polycrystalline structure with mixed monoclinic and tetragonal phase peaks. This indicates that the films are composed of mostly monoclinic HfO 2 with a small amount of tetragonal HfO 2. The as-deposited HfO 2 film was already crystallized before RTA, showing three crystalline peaks related to monoclinic (111), (002) and tetragonal (002) around the Si (200) peak. For the HfO 2 film annealed at 800 C, both the number and the intensity of the crystalline HfO 2 peaks increased. This indicates that the HfO 2 film crystallized during deposition and further crystallized during RTA treatment at 800 C. For both

4 -504- Journal of the Korean Physical Society, Vol. 47, No. 3, September 2005 Table 1. Chemical composition in hafnium-aluminum-oxide films measured by RBS. Hf content (at.%) Al content (at.%) O content (at.%) Al 2O 3 3/HfO Al 2O 3 1/HfO Al 2O 3 1/HfO Fig. 3. XRD data of the (a) HfO 2 film with 600 cycles and of the (b) hafnium-aluminum-oxide film with a Al 2O 3 : HfO 2 of 1 : 3 both before and after RTA treatment at 800 C for 10 s. the as-deposited and the annealed hafnium-aluminumoxide films in Figure 3 (b), no crystalline peaks were observed near Si (200) or Si (400). After the annealing of the as-deposited samples, the hafnium-aluminumoxide films maintained a mostly amorphous structure. Nonetheless, a broad, weak peak corresponding to a HfO 2 monoclinic (111) was observed. The broad width and the low intensity of this peak suggest that a very small amount of crystalline structure was locally embedded in the amorphous structure. This indicated that the hafnium-aluminum-oxide films maintained a mostly amorphous structure until annealing at high temperatures (800 C). Thus, these XRD results indicated that crystallization in the hafnium-aluminum-oxide structure was suppressed by adding Al 2 O 3 to the mix. Compared to the HfO 2 film, the hafnium-aluminum-oxide films showed a significant improvement in the thermodynamic stability of their amorphous phase. The chemical composition of hafnium-aluminum-oxide films was analyzed by using RBS with a He + beam at 2 MeV. Table 1 summarizes the chemical compositions of the hafnium-aluminum-oxide films. From the RBS analysis, the Hf, Al, and O concentrations were, respectively, 22, 9 and 69 at% in the film with a HfO 2 : Al 2 O 3 ratio of 3 : 1, 16, 22, and 62 at% in the film with a HfO 2 : Al 2 O 3 ratio of 1 : 1, and 9, 32, and 59 at% in the film with a HfO 2 : Al 2 O 3 ratio of 1 : 3. The results showed that the concentrations of Al and Hf in the films could be controlled by the number of process cycles used to form the Al 2 O 3 and the HfO 2 films. The O content increased as the Hf at% increased. This relationship in the concentration of hafnium-aluminum-oxide films was considered to be affected by the chemical stoichiometry of the Al 2 O 3 and the HfO 2 structures. The C and the Cl contents in every hafnium-aluminum-oxide film were detected as less than 1 at%. The low impurity content of the ALD films is probably due to the complete separation of each precursor and the reactant gas supply caused by using a N 2 purging. Another possibility is the relatively high reactivity of the H 2 O reactant vapor. The microstructure and the interface morphologies of the hafnium-aluminum-oxide films were investigated using cross-sectional TEM, as shown in Figure 4. All of the hafnium-aluminum-oxide films were analyzed by using cross-sectional TEM both before and after the RTA treatment at 800 C. Figures 4 (a), (b), and (c) are TEM micrographs of the as-deposited hafniumaluminum-oxide films with HfO 2 : Al 2 O 3 ratios of 1 : 3, 1 : 1, and 3 : 1, respectively. All of the as-deposited hafnium-aluminum-oxide films exhibited amorphous structures. The total thickness of the asdeposited hafnium-aluminum-oxide films increased from 50 Å to 57 Å as the Hf content was increased. Also, the thickness of the interfacial layer in the as-deposited hafnium-aluminum-oxide films thinned from 6 Å to 3 Å as the Al content increased. The interfacial layer thickness of as-deposited hafnium-aluminum-oxide in Figure 4 was less than that of the the as-deposited HfO 2 film in Figure 1(b). As Figure 4(c) shows, the hafniumaluminum oxide film with a HfO 2 : Al 2 O 3 ratio of 3 : 1 had the largest total thickness of approximately 57 Å and an interfacial layer of approximately 6 Å among the as-deposited samples. It is postulated that the total thickness of hafnium-aluminum-oxide films is affected by the thickness of the interfacial layer and the formation

5 Characteristics of Hafnium-Aluminum-Oxide Thin Films Deposited by Jaehyoung Koo et al Table 2. Summary of the electrical characteristics of hafnium-aluminum-oxide films Al2 O3 3/HfO2 1 Al2 O3 1/HfO2 1 Al2 O3 1/HfO2 3 EOT (nm) Dielectric constant Fig. 4. TEM micrographs of the hafnium-aluminum-oxide with (a) HfO2 : Al2 O3 of 1 : 3, (b) HfO2 : Al2 O3 of 1 : 1, and (c) HfO2 : Al2 O3 of 3 : 1 films in the as-deposited state and with (d) HfO2 : Al2 O3 of 1 : 3, (e) HfO2 : Al2 O3 of 1 : 1, and (f) HfO2 : Al2 O3 of 3 : 1 after RTA treatment at 800 C for 10 s. of an interfacial layer in a hafnium-aluminum-oxide film is suppressed by adding Al2 O3. Figures 4 (d), (e), and (f) show the TEM micrographs of hafnium-aluminumoxide films with HfO2 : Al2 O3 ratios of 1 : 3, 1 : 1, and 3 : 1, respectively, after RTA treatment at 800 C for 10 sec. The total thickness of both the hafniumaluminum-oxide films and the interfacial layer after annealing increased as the Hf content was increased. The growth of the interfacial layer is likely due to residual oxygen which is present in the N2 ambient atmosphere during the RTA process. The residual oxygen diffuses Flatband voltage (V) Leakage current density (A/cm2 ) 8.1E 8 6.5E 7 1.8E 5 through the HfO2 or hafnium-aluminum-oxide film and reacts with the Si substrate to form the interfacial layer. The hafnium-aluminum-oxide film with a HfO2 : Al2 O3 ratio of 3 : 1 (shown in Figure 4 (f)) was the thickest film, approximately 65 A, and had the thickest interfacial layer, approximately 20 A. The growth of the interfacial layer in the hafnium-aluminum-oxide film due to the annealing process was suppressed by adding Al2 O3. With the addition of Al2 O3, the corresponding film remained amorphous even after annealing at temperatures as high as 800 C. The Al-rich film, among hafnium-aluminumoxide films, is the most attractive because it exhibits the best thermodynamic stability and Si-interface properties. Figure 5 shows (a) high frequency C-V and (b) IV curves of the three different hafnium-aluminum-oxide films. The variation in hafnium-aluminum-oxide film s electrical properties, such as the EOT, the hysteresis and the flat band voltage, with changes in the relative Al and Hf composition was investigated by using C-V measurements, as shown in Figure 5(a). Table 2 summarizes the electrical characteristics of hafnium-aluminum-oxide films. The changes in the capacitance and the flat band voltage depended upon the chemical composition ratio of Hf to Al. The corresponding EOT values measured from maximum accumulation capacitances in the C-V curve of the hafnium-aluminum-oxide films were 1.4, 1.6, and 1.7 nm for films with HfO2 : Al2 O3 ratios of 3 : 1, 1 : 1, and 1 : 3, respectively. The dielectric constants calculated from the EOT values measured in the C-V curves of Figure 5(a) and the physical thicknesses measured in the TEM micrographs in Figures 4(a), (b), and (c) were 18.1, 14.6, and 13.1, respectively. As the Hf content was increased from 9 to 22 at%, the dielectric constant increased from 13.1 to This indicates that a low fraction of Al2 O3 in hafnium-aluminum-oxide films has the advantage of a smaller dielectric constant reduction because the dielectric constant of Al2 O3 is much smaller than that of HfO2. The flat band voltages of hafniumaluminum-oxide films were 0.7, 0.9, and 1.0 V when the HfO2 : Al2 O3 ratios were 3 : 1, 1 : 1, and 1 : 3, respectively. From these results, it can be seen that the flat band voltages of all hafnium-aluminum-oxide films shifted slightly in a positive direction due to the incorporation of negatively charged Al2 O3 films. This weak dependence of the flat band voltage on the composition of hafnium-aluminum-oxide film is fairly advantageous because of the tighter threshold voltage control of MOSFETs.

6 -506- Journal of the Korean Physical Society, Vol. 47, No. 3, September C with HfCl 4 and TMA as the Hf and the Al precursors and with H 2 O as a reaction gas. The Al 2 O 3 film showed an amorphous structure without an Al 2 O 3 - SiO 2 interfacial layer while the HfO 2 film showed a randomly oriented polycrystalline structure with an Al 2 O 3 - SiO 2 interfacial layer. The Al 2 O 3 film showed lower leakage currents than the HfO 2 film while the HfO 2 film showed a higher dielectric constant than the Al 2 O 3 film. The growth of the interfacial layer was suppressed in the hafnium-aluminum-oxide films by adding Al 2 O 3 to the HfO 2 film. Because of the addition of Al 2 O 3, the hafnium-aluminum-oxide films remained in an amorphous structure even after annealing at temperatures as high as 800 C, except in the film with a HfO 2 : Al 2 O 3 ratio of 3 : 1. The leakage currents at a gate bias voltage of V G V F B = 2 for the Al 2 O 3, the HfO 2, and the 1 : 1 HfO 2 : Al 2 O 3 hafnium-aluminum-oxide films were approximately , , and A/cm 2 with calculated equivalent oxide thicknesses of about 2.2, 1.8, and 1.6 nm, respectively. The hafniumaluminum-oxide films showed higher dielectric constants than the Al 2 O 3 films, and significantly improved leakage currents and breakdown voltages than the HfO 2 films. The dielectric constant, the leakage current, the flat band voltage, and the crystallinity of hafnium-aluminum-oxide films could be controlled by varying the relative composition of Al and Hf. Fig. 5. High-frequency (a) C-V and (b) I-V curves of the hafnium-aluminum-oxide films after annealing. As Figure 5(b) shows, the measured leakage-current densities of the hafnium-aluminum-oxide films at a gate bias voltage of V G V F B = 2 were , , and A/cm 2 for HfO 2 : Al 2 O 3 ratios of 3 : 1, 1 : 1, and 1 : 3, respectively. The breakdown voltages of each of the three hafnium-aluminum-oxide films were 5.0 V, 6.0 V, and 6.5 V, respectively. Generally, the amorphous phase created by adding Al 2 O 3 improved the leakage-current and the breakdown-voltage properties. However, the relatively high leakage current value of the 3 : 1 HfO 2 : Al 2 O 3 hafnium-aluminum-oxide film was due to the formation of locally crystallized structures caused by the low Al content. The 1 : 3 HfO 2 : Al 2 O 3 hafnium-aluminum-oxide films showed the lowest leakage current and the highest breakdown voltage, despite also having the smallest physical thickness. The I-V characteristics of all hafnium-aluminum-oxide films showed lower leakage current densities than the HfO 2 films in Figure 2(b) because the thermal stability was improved and because the band offset was increased by the addition of Al. IV. CONCLUSIONS The HfO 2, Al 2 O 3, and hafnium-aluminum-oxide films were successfully deposited using the ALD method at ACKNOWLEDGMENTS 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] A. Khandelwal, H. Niimi, G. Lucovsky and H. Henry Lamb, J. Vac. Sci. Technol. 20, 1989 (2002). [2] P. A. Packan, Science 285, 2079 (1999). [3] M. Schulz, Nature 399, 729 (1999). [4] H. Fukuda, M. Yasuda and T. Iwabuchi, Appl. Phys. Lett. 61, 693 (1992). [5] M. Copel, M. A. Gribelyuk and E. P. Gusev, Appl. Phys. Lett. 76, 436 (2000). [6] A. I. Kingon, J. P. Maria and S. K. Streiffer, Nature 406, 1032 (2000). [7] C. Chaneliere, J. L. Autran, R. A. B. Devine and B. Balland, Mater. Sci. Eng. R 22, 269 (1998). [8] G. D. Wilk, R. M. Wallace and J. M. Anthony, J. Appl. Phys. 87, 484 (2000). [9] Y. Kim, J. Koo, J. Han, S. Choi, H. Jeon and C. Park, J. Appl. Phys. 92, 5443 (2002). [10] S. Gopalan, K. Onishi, R. Nieh, C. S. Kang, R. Choi, H. J. Cho, S. Krishna and J. C. Lee, Appl. Phys. Lett. 80, 4416 (2002).

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