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Atomic layer deposition of HfO2 thin films using H2O2 as oxidant
Applied Surface Science 301 (2014) 451–455
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Atomic layer deposition of HfO2 thin films using H2 O2 as oxidant Min-Jung Choi a,b , Hyung-Ho Park b , Doo Seok Jeong a , Jeong Hwan Kim c , Jin-Sang Kim a , Seong Keun Kim a,∗ a b c
Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Department of Nano Manufacturing Technology, Korea Institute of Machinery & Materials, Daejeon, 305-343, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 December 2013 Received in revised form 17 February 2014 Accepted 17 February 2014 Available online 24 February 2014 Keywords: Atomic layer deposition HfO2 H2 O2
a b s t r a c t HfO2 films were deposited by atomic layer deposition (ALD) using Hf[(C2 H5 )(CH3 )N]4 and H2 O2 at a temperature range of 175–325 ◦ C. The growth per cycle of the HfO2 films decreased with increasing temperature up to 280 ◦ C and then abruptly increased above 325 ◦ C as a result of the thermal decomposition of the precursor. Although the HfO2 films grown with H2 O2 exhibited slightly higher carbon contents than those grown with H2 O, the leakage properties of the HfO2 films grown with H2 O2 were superior to those of the HfO2 films grown with H2 O. This is because the HfO2 films grown with H2 O2 were fully oxidized as a result of the strong oxidation potential of H2 O2 . The use of the ALD process with H2 O2 also revealed the conformal growth of HfO2 films on a SiO2 hole structure with an aspect ratio of ∼15. This demonstrates that using the ALD process with H2 O2 shows great promise for growing robust HfO2 films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction HfO2 has recently received much attention as a dielectric material because of its high dielectric constant and the excellent leakage property produced by its wide band gap (∼5.8 eV). In particular, HfO2 films have been extensively studied as a high-k gate dielectric in metal-oxide-semiconductor field–effect transistors (MOSFET) because HfO2 is thermodynamically stable in contact with Si and reveals a low trap density at an HfO2 /Si interface [1–3]. The thermal stability of HfO2 enables it to efficiently suppress the formation of a low-k SiOx interfacial layer, even at a high temperature, and its low interfacial trap density allows high field–effect mobility of an electronic carrier in a MOSFET. In addition, ferroelectricity has recently been reported for HfO2 [4–6]. The well-known HfO2 crystalline structures such as its monoclinic, tetragonal, and cubic phases are centrosymmetric phases, which cannot induce ferroelectricity. However, it has been reported that under certain conditions, HfO2 is transformed into a non-centrosymmetric orthorhombic phase (Pbc21 ), resulting in ferroelectricity [4–6]. The discovery of ferroelectricity in HfO2 makes it possible to widen the applications of HfO2 films in the electronics industry. In the scaling-down of devices, the physical thickness of HfO2 thin films should be reduced to less than 10 nm. Among the
∗ Corresponding author. Tel.: +82 2 958 5462. E-mail addresses: [email protected], [email protected] (S.K. Kim). http://dx.doi.org/10.1016/j.apsusc.2014.02.098 0169-4332/© 2014 Elsevier B.V. All rights reserved.
various growth techniques available for the growth of such a thin HfO2 film, atomic layer deposition (ALD) is one of the most promising methods. ALD, which is based on self-limiting growth, allows excellent thickness control at the atomic scale, uniform film growth over large areas, and excellent conformality on threedimensional structures. Therefore, the ALD process of HfO2 thin films has been extensively investigated [7,8]. The oxygen vacancies in HfO2 thin films are one of the critical factors in the deterioration of the leakage properties. The oxygen vacancies in HfO2 give rise to defect levels in the band gap. Therefore, such oxygen vacancies could be the origin of a conduction pathway in HfO2 films [9,10]. In ALD, the use of an oxygen source with a strong oxidation potential is one of the most efficient ways to reduce the oxygen vacancies in HfO2 . Although H2 O, which is a common oxygen source in ALD, is a favorable oxidizing agent for ligand exchange in the ALD reaction, an oxide film grown using H2 O is likely to be less insulating because of its relatively low oxidation potential [11]. O3 , which has a strong oxidation potential, is a very effective oxygen source that can improve the leakage property of the oxide dielectric in ALD [12]. O3 at a high concentration has been widely used in ALD for the growth of insulating oxide films [11–14]. However, it has been reported that using a very high concentration of O3 in ALD for HfO2 has resulted in excess oxygen in the films, inducing the formation of an interfacial layer on the Si, along with the degradation of the electrical properties [15]. H2 O2 is a reactive, safe, and versatile oxygen source for ALD [16]. The oxidation potential (1.8 eV) of H2 O2 is comparable to that
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(2.1 eV) of O3 . Thus, H2 O2 is also expected to efficiently reduce the oxygen vacancies in HfO2 thin films. In addition, an H2 O2 -based ALD system does not require a complex and expensive system for the generation of O3 , in contrast to O3 -based ALD. Therefore, the ALD reaction and characteristics of HfO2 thin films deposited using H2 O2 as an oxygen source were investigated in this study. The electrical and chemical properties of the HfO2 films were compared to those of HfO2 films grown using H2 O. 2. Experiments HfO2 thin films were grown by ALD using a travelling-wave type reactor (CN-1 Co., Atomic Classic). The films were grown on (0 0 1) Si and sputtered Pt(100 nm)/Ti(50 nm)/SiO2 (300 nm)/Si substrates at a temperature range of 175–325 ◦ C. Ar was used as a purging and carrier gas. Tetrakis-ethylmethylaminohafnium (Hf[(C2 H5 )(CH3 )N]4 , TEMAHf) was used as a Hf source. The Hf precursor, which was held in a bubbler at 50 ◦ C, was delivered into the reactor using an Ar gas flow rate of 400 sccm, and 50 wt% H2 O2 was used as an oxygen source. The H2 O2 solution was held in a Pyrex cylinder at room temperature in order to minimize the selfdecomposition of H2 O2 . For comparison, de-ionized water cooled at 5 ◦ C was also used as an oxygen source. H2 O vapor was injected for 2 s. Between the injections of the Hf precursor and oxygen source, the reactor was purged using an Ar gas flow rate of 900 sccm. The film thickness was evaluated using ellipsometry and the X-ray reflectivity (XRR). The densities of the HfO2 films were compared from a critical angle in the XRR spectra. The impurity concentration in the films was examined using auger electron spectroscopy (AES). An X-ray photoelectron spectroscopy (XPS) technique was utilized for the analysis of the chemical binding states of the films. The crystalline structure was examined using grazing incident X-ray diffraction (GIXRD). An atomic force microscope was employed to examine the surface of the HfO2 films. The step coverage of the HfO2 film grown on a SiO2 hole structure with an aspect ratio of ∼15 was observed using scanning electron microscopy (SEM). Metal–insulator–metal (MIM) capacitors were fabricated to evaluate the electrical properties of the HfO2 films. Pt top electrodes were deposited by DC sputtering using a shadow mask. The MIM capacitors were annealed at 400 ◦ C for 30 min under an O2 atmosphere after the deposition of the Pt top electrodes. The capacitance of each capacitor was measured using a Agilent 4294A impedance analyzer at 10 kHz. The current density–applied field (J–E) curves of the capacitors were obtained using a Keithley 4200 semiconductor characterization system. 3. Results and discussion First, the self-saturating behavior of the ALD process using TEMAH and H2 O2 for HfO2 films was examined. Fig. 1(a) shows the change in the physical thickness of the HfO2 films grown at 260 ◦ C for 120 cycles as a function of the TEMAH feeding time. The purging times for the TEMAH and H2 O2 were fixed at 30 s and 20 s, respectively, to avoid the intermixing of these precursors. The film thickness was saturated to approximately 10 nm above a TEMAH feeding time of 1 s. We also examined the self-saturating behavior of HfO2 ALD with respect to H2 O2 . As shown in Fig. 1(b), the physical thickness of the HfO2 films is almost constant over the whole range of H2 O2 feeding times, indicating the self-saturating behavior even at a short H2 O2 feeding time of 0.1 s. Therefore, the results in Fig. 1(a) and (b) confirm the genuine ALD reaction of TEMAH and H2 O2 . Based on the optimized growth sequence, which showed the self-saturating behavior, the growth per cycle (GPC) of the HfO2 films was examined using a growth sequence of TEMAH feeding
(2 s), TEMAH purging (5 s), H2 O2 feeding (2 s), and H2 O2 purging (20 s). Although the film thickness did not vary with the H2 O2 feeding time, as shown in Fig. 1(b), the amount of the injected oxygen source could influence the leakage properties of the films because of the change in the concentration of the oxygen vacancies in the films. Therefore, most of the films in this study were grown using an H2 O2 feeding time of 2 s. Fig. 1(c) shows the variation in the film thickness as a function of the number of cycles. Fig. 1(c) also includes the variation in the thickness of the HfO2 films grown using H2 O. An excellent linearity was observed in the plots of the thickness vs. the number of cycles for both HfO2 films. The GPC (0.095 nm/cycle) of the HfO2 grown with H2 O2 , which can be calculated from the slope in Fig. 1(c), is slightly lower than the value (0.10 nm/cycle) for the HfO2 grown with H2 O. The temperature dependence of the GPC values of the HfO2 films grown with TEMAH and H2 O2 was examined. Fig. 2 shows the variation in the GPC values of the HfO2 films as a function of the growth temperature. The GPC values of the HfO2 films grown with H2 O2 decreased with the growth temperature from 0.11 to 0.09 nm/cycle in a growth temperature range of 175–280 ◦ C. The HfO2 films grown with H2 O also showed a similar behavior, although the GPC values of the HfO2 films grown with H2 O2 were slightly lower than those of the HfO2 films grown with H2 O over the whole temperature range. However, the GPC value abruptly increased to ∼0.12 nm/cycle at a growth temperature of 325 ◦ C. The HfO2 films grown with H2 O also exhibited an abrupt increase in the GPC value at 325 ◦ C. This increase is attributed to the thermal decomposition of TEMAH. Therefore, it was confirmed that the ALD window for growing HfO2 films with TEMAH and H2 O2 is in the range of 175–300 ◦ C. The temperature dependences of the film density and surface roughness of the HfO2 films were also examined. Fig. 2(b) shows the XRR spectra of the HfO2 films grown with H2 O2 at 175, 260, and 325 ◦ C. The density of the films is directly related to a critical angle, where the intensity of the reflected X-ray beam is at the half maximum in the XRR spectra. The inset figure, which is a magnification near a critical angle, clearly shows the variation in the density of the HfO2 films grown with H2 O2 . The film density increased with the growth temperature within the ALD window. However, the density of the film grown at 325 ◦ C, which is outside the ALD window, was much lower than that of the film grown even at 175 ◦ C. This suggests that the thermal decomposition of TEMAH severely deteriorated the film properties. The HfO2 films grown with H2 O also showed a similar temperature dependence, and the density of the films was almost identical to that of the films grown with H2 O2 (data not shown). The amplitude of the oscillation in the XRR spectra of the HfO2 films suddenly decreases at 325 ◦ C in Fig. 2(b), indicating surface and interface roughening. This result was verified using an AFM analysis (data not shown). Impurities such as carbon and nitrogen could remain in the ALD-grown films as a result of an incomplete ALD reaction or the insufficient reactivity of the precursors, resulting in the deterioration of the electrical properties of the grown films. Hence, the impurity contents in the HfO2 films were examined using AES. Fig. 3(a)–(c) shows the AES depth profiles of HfO2 films grown with H2 O2 at 175, 260, and 325 ◦ C, respectively. The carbon contents in all the films are approximately 2–3 at%. No substantial difference in the carbon content with respect to the growth temperature was observed. On the other hand, the HfO2 films grown with H2 O exhibited relatively low carbon impurity levels, as shown in Fig. 3(e)–(g). In particular, the carbon contents were negligible in the HfO2 films grown at 175 and 260 ◦ C. The HfO2 film grown at 325 ◦ C, which was outside the ALD window, showed slightly higher carbon concentrations, which can be attributed to the thermal decomposition of TEMAH. The nitrogen impurity levels of all the films were below the AES detection limit.
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Fig. 1. Variations in physical thickness of HfO2 films grown at 260 ◦ C for 120 cycles as a function of (a) TEMAH and (b) H2 O2 feeding times. (c) Variation in the physical thickness of HfO2 films grown at 260 ◦ C as a function of the number of cycles.
Fig. 2. (a) Variation in GPC of HfO2 films as a function of growth temperature. (b) XRR spectra of HfO2 films grown at 175, 260, and 325 ◦ C with H2 O2 . Inset shows the magnified spectra near a critical angle.
Higher impurity contents in HfO2 films can lead to the degradation of the electrical properties. Therefore, the electrical properties of the MIM capacitors composed of HfO2 films and Pt electrodes were investigated. Although the HfO2 films in this study were grown at a wide temperature range of 175–325 ◦ C, we only examined the electrical properties of the HfO2 films grown at 260 ◦ C, which had a higher density and less carbon contamination. Fig. 4(a) shows the variation in the equivalent oxide thickness (tox ) of the
HfO2 films grown at 260 ◦ C using H2 O2 and H2 O as oxidants. The variable tox indicates the physical film thickness multiplied by 3.9/dielectric constant (εr ). The tox value of the films linearly increased with the film thickness, indicating that the εr value of the film did not depend on the film thickness. The bulk εr values, which could be calculated from the inverse slope of the linear fit graphs, were 14 and 15 for HfO2 films grown with H2 O2 and H2 O, respectively. These εr values coincide with the value of monoclinic
Fig. 3. AES depth profiles of HfO2 films on Si grown at (a) 175, (b) 260, and (c) 325 ◦ C using H2 O2 and at (d) 175, (e) 260, and (f) 325 ◦ C using H2 O.
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Fig. 4. (a) Variation in equivalent oxide thickness (tox ) of HfO2 films grown at 260 ◦ C with H2 O2 and H2 O as oxidants. (b) J–E curves of 7-nm-thick HfO2 films grown with H2 O2 and H2 O.
Fig. 5. Hf 4f XPS spectra of approximately 20-nm-thick HfO2 films grown with (a) H2 O2 and (b) H2 O.
HfO2 [17]. The crystalline structure of the HfO2 films grown with H2 O2 and H2 O was also confirmed to be monoclinic based on the GIXRD analysis (data not shown). Fig. 4(b) shows the J–E curves of 7-nm-thick HfO2 films grown with H2 O2 and H2 O. The leakage current measurement of the capacitors was carried out under the application of a positive bias to the top electrode. The leakage properties of the HfO2 films grown with H2 O2 are superior to the properties of the HfO2 films grown with H2 O over the entire electric field range, although the film grown with H2 O2 contained larger carbon contents compared to the films grown with H2 O. The oxygen vacancies in films significantly influence the leakage properties of oxide dielectric films. The difference in the oxidation potentials of H2 O2 and H2 O in this study could lead to a change in the oxygen vacancy concentration and binding status of the HfO2 films, eventually affecting the leakage properties. Therefore, an XPS analysis was performed to elucidate the chemical properties of both HfO2 films. Here, the O 1s XPS spectra of the HfO2 films were not considered because the O 1s spectra were distorted by of the large number of carbonyl or hydroxyl groups on the surface. Fig. 5(a) and (b) shows the Hf 4f XPS spectra of approximately 20-nm-thick HfO2 films grown with H2 O2 and H2 O, respectively. The influence of the substrate and interfacial layer on the XPS spectra can be excluded because of the relatively large thickness. The Hf 4f XPS spectra of the HfO2 films grown with H2 O2 clearly show a spin-orbit doublet corresponding to Hf 4f5/2 –4f7/2 peaks, indicating that the Hf ions in the HfO2 films have almost a single binding state corresponding to Hf4+ in HfO2 . On the other hand, the Hf 4f XPS spectra of the HfO2 films grown with H2 O shown in Fig. 5(b) are broader than the spectra of the HfO2 films grown with H2 O2 , suggesting mixed chemical binding states for the HfO2 films. Considering the negligible impurities in the films, as shown in Fig. 3, the chemical shift in Fig. 5(b) is not caused by Hf–C and Hf–N bondings but by an oxygen deficient phase in the HfO2 films due to the weak oxidation potential of H2 O.
Fig. 6. Cross-sectional SEM image of HfO2 film grown at 260 ◦ C on SiO2 contact hole structure with aspect ratio of approximately 15.
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Therefore, it was verified that the worse leakage properties of the HfO2 films grown with H2 O could be attributed to the oxygen deficiency of the films. Although we attempted to vary the growth temperature to alleviate the oxygen deficiency, the chemical binding status of the films was not influenced by the growth temperature. The conformality of the HfO2 films grown with TEMAH and H2 O2 was also examined. Fig. 6 shows a cross-sectional SEM image of an HfO2 film grown at 260 ◦ C on a SiO2 contact hole structure with an aspect ratio of approximately 15. An excellent thickness uniformity (>90%) for the HfO2 film was achieved in the hole structures. This demonstrates that the H2 O2 -based ALD process is favorable for growing conformal HfO2 films. 4. Conclusion We developed an ALD process for growing HfO2 films using TEMAH and 50 wt% H2 O2 . The self-limiting behavior of the ALD growth of the HfO2 films was confirmed at an intermediate temperature range of 175–300 ◦ C. Interestingly, the HfO2 films grown with H2 O2 as an oxygen source showed superior insulating properties compared to the HfO2 films grown with H2 O, which is the most common oxygen source used in ALD, even though the HfO2 ALD process using H2 O2 exhibited a comparable GPC, density, and impurity content. The difference in the oxidation potentials of H2 O2 and H2 O led to a change in the chemical binding states of the growing films, eventually affecting the leakage properties. The HfO2 ALD process using H2 O2 even enabled conformal growth on a contact hole structure with an aspect ratio of ∼15. The ALD process using H2 O2 , which is much cheaper than an ALD process using O3 for the growth of dense and robust films, suggests the possible further utility of HfO2 films.
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Acknowledgments This work was supported by the Future Semiconductor Device Technology Development Program (10047231) funded by MOTIE (Ministry of Trade, Industry & Energy) and KSRC (Korea Semiconductor Research Consortium) and by the Korea Institute of Science and Technology (KIST through 2E24001). References [1] K.J. Hubbard, D.G. Schlom, J. Mater. Res. 11 (1996) 2757. [2] J. Robertson, Eur. Phys. J. Appl. Phys. 28 (2004) 265. [3] M. Cho, J. Park, H.B. Park, C.S. Hwang, J. Jeong, K.S. Hyun, Appl. Phys. Lett. 81 (2002) 334. [4] T.S. Böscke, J. Müller, D. Bräuhaus, U. Schröder, U. Böttger, Appl. Phys. Lett. 99 (2011) 102903. [5] J. Müller, T.S. Böscke, U. Schröder, S. Mueller, D. Bräuhaus, U. Böttger, L. Frey, T. Mikolajick, Nano Lett. 12 (2012) 4318. [6] M.H. Park, H.J. Kim, Y.J. Kim, W. Lee, T. Moon, C.S. Hwang, Appl. Phys. Lett. 102 (2013) 242905. [7] J. Aarik, A. Aidla, H. Mändar, T. Uustare, K. Kukli, M. Schuisky, Appl. Surf. Sci. 173 (2001) 15. [8] J. Aarik, A. Aidla, A. Kikas, T. Käämbre, R. Rammula, P. Ritslaid, T. Uustare, V. Sammelselg, Appl. Surf. Sci. 230 (2004) 292. [9] K. Xiong, J. Robertson, M.C. Gibson, S.J. Clark, Appl. Phys. Lett. 87 (2005) 183505. [10] K. Xiong, J. Robertson, S.J. Clark, J. Appl. Phys. 99 (2006) 044105. [11] M. Cho, D.S. Jeong, J. Park, H.B. Park, S.W. Lee, T.J. Park, C.S. Hwang, G.H. Jang, J. Jeong, Appl. Phys. Lett. 85 (2004) 5953. [12] S.K. Kim, G.-J. Choi, S.Y. Lee, M. Seo, S.W. Lee, J.H. Han, H.-S. Ahn, S. Han, C.S. Hwang, Adv. Mater. 20 (2008) 1429. [13] J.H. Kim, T.J. Park, S.K. Kim, D.-Y. Cho, H.-S. Jung, S.Y. Lee, C.S. Hwang, Appl. Surf. Sci. 292 (2014) 852. [14] S.K. Kim, S. Han, W. Jeon, J.H. Yoon, J.H. Han, W. Lee, C.S. Hwang, ACS Appl. Mater. Interfaces 4 (2012) 4726. [15] J. Park, M. Cho, S.K. Kim, T.J. Park, S.W. Lee, S.H. Hong, C.S. Hwang, Appl. Phys. Lett. 86 (2005) 112907. [16] B.B. Burton, S.W. Kang, S.W. Rhee, S.M. George, J. Phys. Chem. C 113 (2009) 8249. [17] M. Seo, S.K. Kim, J.H. Han, C.S. Hwang, Chem. Mater. 22 (2010) 4419.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Atomic layer deposition of HfO2 thin films using H2 O2 as oxidant Min-Jung Choi a,b , Hyung-Ho Park b , Doo Seok Jeong a , Jeong Hwan Kim c , Jin-Sang Kim a , Seong Keun Kim a,∗ a b c
Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Department of Nano Manufacturing Technology, Korea Institute of Machinery & Materials, Daejeon, 305-343, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 December 2013 Received in revised form 17 February 2014 Accepted 17 February 2014 Available online 24 February 2014 Keywords: Atomic layer deposition HfO2 H2 O2
a b s t r a c t HfO2 films were deposited by atomic layer deposition (ALD) using Hf[(C2 H5 )(CH3 )N]4 and H2 O2 at a temperature range of 175–325 ◦ C. The growth per cycle of the HfO2 films decreased with increasing temperature up to 280 ◦ C and then abruptly increased above 325 ◦ C as a result of the thermal decomposition of the precursor. Although the HfO2 films grown with H2 O2 exhibited slightly higher carbon contents than those grown with H2 O, the leakage properties of the HfO2 films grown with H2 O2 were superior to those of the HfO2 films grown with H2 O. This is because the HfO2 films grown with H2 O2 were fully oxidized as a result of the strong oxidation potential of H2 O2 . The use of the ALD process with H2 O2 also revealed the conformal growth of HfO2 films on a SiO2 hole structure with an aspect ratio of ∼15. This demonstrates that using the ALD process with H2 O2 shows great promise for growing robust HfO2 films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction HfO2 has recently received much attention as a dielectric material because of its high dielectric constant and the excellent leakage property produced by its wide band gap (∼5.8 eV). In particular, HfO2 films have been extensively studied as a high-k gate dielectric in metal-oxide-semiconductor field–effect transistors (MOSFET) because HfO2 is thermodynamically stable in contact with Si and reveals a low trap density at an HfO2 /Si interface [1–3]. The thermal stability of HfO2 enables it to efficiently suppress the formation of a low-k SiOx interfacial layer, even at a high temperature, and its low interfacial trap density allows high field–effect mobility of an electronic carrier in a MOSFET. In addition, ferroelectricity has recently been reported for HfO2 [4–6]. The well-known HfO2 crystalline structures such as its monoclinic, tetragonal, and cubic phases are centrosymmetric phases, which cannot induce ferroelectricity. However, it has been reported that under certain conditions, HfO2 is transformed into a non-centrosymmetric orthorhombic phase (Pbc21 ), resulting in ferroelectricity [4–6]. The discovery of ferroelectricity in HfO2 makes it possible to widen the applications of HfO2 films in the electronics industry. In the scaling-down of devices, the physical thickness of HfO2 thin films should be reduced to less than 10 nm. Among the
∗ Corresponding author. Tel.: +82 2 958 5462. E-mail addresses: [email protected], [email protected] (S.K. Kim). http://dx.doi.org/10.1016/j.apsusc.2014.02.098 0169-4332/© 2014 Elsevier B.V. All rights reserved.
various growth techniques available for the growth of such a thin HfO2 film, atomic layer deposition (ALD) is one of the most promising methods. ALD, which is based on self-limiting growth, allows excellent thickness control at the atomic scale, uniform film growth over large areas, and excellent conformality on threedimensional structures. Therefore, the ALD process of HfO2 thin films has been extensively investigated [7,8]. The oxygen vacancies in HfO2 thin films are one of the critical factors in the deterioration of the leakage properties. The oxygen vacancies in HfO2 give rise to defect levels in the band gap. Therefore, such oxygen vacancies could be the origin of a conduction pathway in HfO2 films [9,10]. In ALD, the use of an oxygen source with a strong oxidation potential is one of the most efficient ways to reduce the oxygen vacancies in HfO2 . Although H2 O, which is a common oxygen source in ALD, is a favorable oxidizing agent for ligand exchange in the ALD reaction, an oxide film grown using H2 O is likely to be less insulating because of its relatively low oxidation potential [11]. O3 , which has a strong oxidation potential, is a very effective oxygen source that can improve the leakage property of the oxide dielectric in ALD [12]. O3 at a high concentration has been widely used in ALD for the growth of insulating oxide films [11–14]. However, it has been reported that using a very high concentration of O3 in ALD for HfO2 has resulted in excess oxygen in the films, inducing the formation of an interfacial layer on the Si, along with the degradation of the electrical properties [15]. H2 O2 is a reactive, safe, and versatile oxygen source for ALD [16]. The oxidation potential (1.8 eV) of H2 O2 is comparable to that
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(2.1 eV) of O3 . Thus, H2 O2 is also expected to efficiently reduce the oxygen vacancies in HfO2 thin films. In addition, an H2 O2 -based ALD system does not require a complex and expensive system for the generation of O3 , in contrast to O3 -based ALD. Therefore, the ALD reaction and characteristics of HfO2 thin films deposited using H2 O2 as an oxygen source were investigated in this study. The electrical and chemical properties of the HfO2 films were compared to those of HfO2 films grown using H2 O. 2. Experiments HfO2 thin films were grown by ALD using a travelling-wave type reactor (CN-1 Co., Atomic Classic). The films were grown on (0 0 1) Si and sputtered Pt(100 nm)/Ti(50 nm)/SiO2 (300 nm)/Si substrates at a temperature range of 175–325 ◦ C. Ar was used as a purging and carrier gas. Tetrakis-ethylmethylaminohafnium (Hf[(C2 H5 )(CH3 )N]4 , TEMAHf) was used as a Hf source. The Hf precursor, which was held in a bubbler at 50 ◦ C, was delivered into the reactor using an Ar gas flow rate of 400 sccm, and 50 wt% H2 O2 was used as an oxygen source. The H2 O2 solution was held in a Pyrex cylinder at room temperature in order to minimize the selfdecomposition of H2 O2 . For comparison, de-ionized water cooled at 5 ◦ C was also used as an oxygen source. H2 O vapor was injected for 2 s. Between the injections of the Hf precursor and oxygen source, the reactor was purged using an Ar gas flow rate of 900 sccm. The film thickness was evaluated using ellipsometry and the X-ray reflectivity (XRR). The densities of the HfO2 films were compared from a critical angle in the XRR spectra. The impurity concentration in the films was examined using auger electron spectroscopy (AES). An X-ray photoelectron spectroscopy (XPS) technique was utilized for the analysis of the chemical binding states of the films. The crystalline structure was examined using grazing incident X-ray diffraction (GIXRD). An atomic force microscope was employed to examine the surface of the HfO2 films. The step coverage of the HfO2 film grown on a SiO2 hole structure with an aspect ratio of ∼15 was observed using scanning electron microscopy (SEM). Metal–insulator–metal (MIM) capacitors were fabricated to evaluate the electrical properties of the HfO2 films. Pt top electrodes were deposited by DC sputtering using a shadow mask. The MIM capacitors were annealed at 400 ◦ C for 30 min under an O2 atmosphere after the deposition of the Pt top electrodes. The capacitance of each capacitor was measured using a Agilent 4294A impedance analyzer at 10 kHz. The current density–applied field (J–E) curves of the capacitors were obtained using a Keithley 4200 semiconductor characterization system. 3. Results and discussion First, the self-saturating behavior of the ALD process using TEMAH and H2 O2 for HfO2 films was examined. Fig. 1(a) shows the change in the physical thickness of the HfO2 films grown at 260 ◦ C for 120 cycles as a function of the TEMAH feeding time. The purging times for the TEMAH and H2 O2 were fixed at 30 s and 20 s, respectively, to avoid the intermixing of these precursors. The film thickness was saturated to approximately 10 nm above a TEMAH feeding time of 1 s. We also examined the self-saturating behavior of HfO2 ALD with respect to H2 O2 . As shown in Fig. 1(b), the physical thickness of the HfO2 films is almost constant over the whole range of H2 O2 feeding times, indicating the self-saturating behavior even at a short H2 O2 feeding time of 0.1 s. Therefore, the results in Fig. 1(a) and (b) confirm the genuine ALD reaction of TEMAH and H2 O2 . Based on the optimized growth sequence, which showed the self-saturating behavior, the growth per cycle (GPC) of the HfO2 films was examined using a growth sequence of TEMAH feeding
(2 s), TEMAH purging (5 s), H2 O2 feeding (2 s), and H2 O2 purging (20 s). Although the film thickness did not vary with the H2 O2 feeding time, as shown in Fig. 1(b), the amount of the injected oxygen source could influence the leakage properties of the films because of the change in the concentration of the oxygen vacancies in the films. Therefore, most of the films in this study were grown using an H2 O2 feeding time of 2 s. Fig. 1(c) shows the variation in the film thickness as a function of the number of cycles. Fig. 1(c) also includes the variation in the thickness of the HfO2 films grown using H2 O. An excellent linearity was observed in the plots of the thickness vs. the number of cycles for both HfO2 films. The GPC (0.095 nm/cycle) of the HfO2 grown with H2 O2 , which can be calculated from the slope in Fig. 1(c), is slightly lower than the value (0.10 nm/cycle) for the HfO2 grown with H2 O. The temperature dependence of the GPC values of the HfO2 films grown with TEMAH and H2 O2 was examined. Fig. 2 shows the variation in the GPC values of the HfO2 films as a function of the growth temperature. The GPC values of the HfO2 films grown with H2 O2 decreased with the growth temperature from 0.11 to 0.09 nm/cycle in a growth temperature range of 175–280 ◦ C. The HfO2 films grown with H2 O also showed a similar behavior, although the GPC values of the HfO2 films grown with H2 O2 were slightly lower than those of the HfO2 films grown with H2 O over the whole temperature range. However, the GPC value abruptly increased to ∼0.12 nm/cycle at a growth temperature of 325 ◦ C. The HfO2 films grown with H2 O also exhibited an abrupt increase in the GPC value at 325 ◦ C. This increase is attributed to the thermal decomposition of TEMAH. Therefore, it was confirmed that the ALD window for growing HfO2 films with TEMAH and H2 O2 is in the range of 175–300 ◦ C. The temperature dependences of the film density and surface roughness of the HfO2 films were also examined. Fig. 2(b) shows the XRR spectra of the HfO2 films grown with H2 O2 at 175, 260, and 325 ◦ C. The density of the films is directly related to a critical angle, where the intensity of the reflected X-ray beam is at the half maximum in the XRR spectra. The inset figure, which is a magnification near a critical angle, clearly shows the variation in the density of the HfO2 films grown with H2 O2 . The film density increased with the growth temperature within the ALD window. However, the density of the film grown at 325 ◦ C, which is outside the ALD window, was much lower than that of the film grown even at 175 ◦ C. This suggests that the thermal decomposition of TEMAH severely deteriorated the film properties. The HfO2 films grown with H2 O also showed a similar temperature dependence, and the density of the films was almost identical to that of the films grown with H2 O2 (data not shown). The amplitude of the oscillation in the XRR spectra of the HfO2 films suddenly decreases at 325 ◦ C in Fig. 2(b), indicating surface and interface roughening. This result was verified using an AFM analysis (data not shown). Impurities such as carbon and nitrogen could remain in the ALD-grown films as a result of an incomplete ALD reaction or the insufficient reactivity of the precursors, resulting in the deterioration of the electrical properties of the grown films. Hence, the impurity contents in the HfO2 films were examined using AES. Fig. 3(a)–(c) shows the AES depth profiles of HfO2 films grown with H2 O2 at 175, 260, and 325 ◦ C, respectively. The carbon contents in all the films are approximately 2–3 at%. No substantial difference in the carbon content with respect to the growth temperature was observed. On the other hand, the HfO2 films grown with H2 O exhibited relatively low carbon impurity levels, as shown in Fig. 3(e)–(g). In particular, the carbon contents were negligible in the HfO2 films grown at 175 and 260 ◦ C. The HfO2 film grown at 325 ◦ C, which was outside the ALD window, showed slightly higher carbon concentrations, which can be attributed to the thermal decomposition of TEMAH. The nitrogen impurity levels of all the films were below the AES detection limit.
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Fig. 1. Variations in physical thickness of HfO2 films grown at 260 ◦ C for 120 cycles as a function of (a) TEMAH and (b) H2 O2 feeding times. (c) Variation in the physical thickness of HfO2 films grown at 260 ◦ C as a function of the number of cycles.
Fig. 2. (a) Variation in GPC of HfO2 films as a function of growth temperature. (b) XRR spectra of HfO2 films grown at 175, 260, and 325 ◦ C with H2 O2 . Inset shows the magnified spectra near a critical angle.
Higher impurity contents in HfO2 films can lead to the degradation of the electrical properties. Therefore, the electrical properties of the MIM capacitors composed of HfO2 films and Pt electrodes were investigated. Although the HfO2 films in this study were grown at a wide temperature range of 175–325 ◦ C, we only examined the electrical properties of the HfO2 films grown at 260 ◦ C, which had a higher density and less carbon contamination. Fig. 4(a) shows the variation in the equivalent oxide thickness (tox ) of the
HfO2 films grown at 260 ◦ C using H2 O2 and H2 O as oxidants. The variable tox indicates the physical film thickness multiplied by 3.9/dielectric constant (εr ). The tox value of the films linearly increased with the film thickness, indicating that the εr value of the film did not depend on the film thickness. The bulk εr values, which could be calculated from the inverse slope of the linear fit graphs, were 14 and 15 for HfO2 films grown with H2 O2 and H2 O, respectively. These εr values coincide with the value of monoclinic
Fig. 3. AES depth profiles of HfO2 films on Si grown at (a) 175, (b) 260, and (c) 325 ◦ C using H2 O2 and at (d) 175, (e) 260, and (f) 325 ◦ C using H2 O.
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Fig. 4. (a) Variation in equivalent oxide thickness (tox ) of HfO2 films grown at 260 ◦ C with H2 O2 and H2 O as oxidants. (b) J–E curves of 7-nm-thick HfO2 films grown with H2 O2 and H2 O.
Fig. 5. Hf 4f XPS spectra of approximately 20-nm-thick HfO2 films grown with (a) H2 O2 and (b) H2 O.
HfO2 [17]. The crystalline structure of the HfO2 films grown with H2 O2 and H2 O was also confirmed to be monoclinic based on the GIXRD analysis (data not shown). Fig. 4(b) shows the J–E curves of 7-nm-thick HfO2 films grown with H2 O2 and H2 O. The leakage current measurement of the capacitors was carried out under the application of a positive bias to the top electrode. The leakage properties of the HfO2 films grown with H2 O2 are superior to the properties of the HfO2 films grown with H2 O over the entire electric field range, although the film grown with H2 O2 contained larger carbon contents compared to the films grown with H2 O. The oxygen vacancies in films significantly influence the leakage properties of oxide dielectric films. The difference in the oxidation potentials of H2 O2 and H2 O in this study could lead to a change in the oxygen vacancy concentration and binding status of the HfO2 films, eventually affecting the leakage properties. Therefore, an XPS analysis was performed to elucidate the chemical properties of both HfO2 films. Here, the O 1s XPS spectra of the HfO2 films were not considered because the O 1s spectra were distorted by of the large number of carbonyl or hydroxyl groups on the surface. Fig. 5(a) and (b) shows the Hf 4f XPS spectra of approximately 20-nm-thick HfO2 films grown with H2 O2 and H2 O, respectively. The influence of the substrate and interfacial layer on the XPS spectra can be excluded because of the relatively large thickness. The Hf 4f XPS spectra of the HfO2 films grown with H2 O2 clearly show a spin-orbit doublet corresponding to Hf 4f5/2 –4f7/2 peaks, indicating that the Hf ions in the HfO2 films have almost a single binding state corresponding to Hf4+ in HfO2 . On the other hand, the Hf 4f XPS spectra of the HfO2 films grown with H2 O shown in Fig. 5(b) are broader than the spectra of the HfO2 films grown with H2 O2 , suggesting mixed chemical binding states for the HfO2 films. Considering the negligible impurities in the films, as shown in Fig. 3, the chemical shift in Fig. 5(b) is not caused by Hf–C and Hf–N bondings but by an oxygen deficient phase in the HfO2 films due to the weak oxidation potential of H2 O.
Fig. 6. Cross-sectional SEM image of HfO2 film grown at 260 ◦ C on SiO2 contact hole structure with aspect ratio of approximately 15.
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Therefore, it was verified that the worse leakage properties of the HfO2 films grown with H2 O could be attributed to the oxygen deficiency of the films. Although we attempted to vary the growth temperature to alleviate the oxygen deficiency, the chemical binding status of the films was not influenced by the growth temperature. The conformality of the HfO2 films grown with TEMAH and H2 O2 was also examined. Fig. 6 shows a cross-sectional SEM image of an HfO2 film grown at 260 ◦ C on a SiO2 contact hole structure with an aspect ratio of approximately 15. An excellent thickness uniformity (>90%) for the HfO2 film was achieved in the hole structures. This demonstrates that the H2 O2 -based ALD process is favorable for growing conformal HfO2 films. 4. Conclusion We developed an ALD process for growing HfO2 films using TEMAH and 50 wt% H2 O2 . The self-limiting behavior of the ALD growth of the HfO2 films was confirmed at an intermediate temperature range of 175–300 ◦ C. Interestingly, the HfO2 films grown with H2 O2 as an oxygen source showed superior insulating properties compared to the HfO2 films grown with H2 O, which is the most common oxygen source used in ALD, even though the HfO2 ALD process using H2 O2 exhibited a comparable GPC, density, and impurity content. The difference in the oxidation potentials of H2 O2 and H2 O led to a change in the chemical binding states of the growing films, eventually affecting the leakage properties. The HfO2 ALD process using H2 O2 even enabled conformal growth on a contact hole structure with an aspect ratio of ∼15. The ALD process using H2 O2 , which is much cheaper than an ALD process using O3 for the growth of dense and robust films, suggests the possible further utility of HfO2 films.
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