Microelectronics Reliability

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1 Microelectronics Reliability 52 (212) Contents lists available at SciVerse ScienceDirect Microelectronics Reliability journal homepage: Physical properties and electrical characteristics of H 2 O-based and O 3 -based HfO 2 films deposited by ALD Jibin Fan, Hongxia Liu, Qianwei Kuang, Bo Gao, Fei Ma, Yue Hao School of Microelectronics, Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, Xi an 7171, China article info abstract Article history: Received 16 September 211 Received in revised form 14 January 212 Accepted 17 January 212 Available online 21 February 212 Ozone (O 3 ) and H 2 O are used as the oxidant to deposit hafnium oxide (HfO 2 ) thin films on p-type Si (1) wafers by atomic layer deposition (ALD). The physical properties and electrical characteristics of HfO 2 films change greatly for different oxidants and deposition temperature. Compared with O 3 as the oxidant, HfO 2 films grown with H 2 O as the oxidant are more consistent in composition and growth rate. The O 3 - based HfO 2 films have lower C impurity and higher concentration N impurity than the H 2 O-based HfO 2 films. The impact of the annealing process on the electrical properties and stability of HfO 2 films are also investigated. A width step is observed in the O 3 -based HfO 2 C V curves, which disappears after annealing process. It is because the unstable Hf O N and Hf N bonds in O 3 -based HfO 2 films are re-bonded with the non-hfo 2 oxygen after annealing process, and the binding energy of N 1s shifts. Ó 212 Elsevier Ltd. All rights reserved. 1. Introduction 2. Experiments To meet future static power dissipation constraints, Hf-based high-j dielectrics in combination with the metal gate electrodes are currently being implemented for transistor production into sub-3 nm CMOS technology nodes [1,2]. Among them, the hafnium oxide has been extensively studied as a potential alternative to silicon dioxide. H 2 O and tetrakis (ethylmethylamino) hafnium (TEMAH) are used as the oxidant and the hafnium precursor, respectively [3,4]. However, one of the disadvantages of H 2 O-based ALD is the high concentration hydroxyl groups in the films, which degrades the dielectric interface during the post-deposition annealing process. Meanwhile, the purge time is needed sufficiently due to H 2 O tends to physisorb on the surface strongly, especially at low temperature [5,6]. To solve this problem, O 3 is used as one of the most promising alternative oxidants in ALD process, due to its strong oxidization and high volatility [7]. However, the mechanism of O 3 -based ALD HfO 2 has not been understood completely because its by-products cannot be accurately determined. In this work, the HfO 2 films are deposited using different oxidants at different deposition temperature. TEMAH is used as the hafnium precursor, H 2 O and O 3 are used as the different oxidants, respectively. The HfO 2 films are annealed with thermal treatments in nitrogen atmosphere. The impurities and composition of the films are analyzed by X-ray photoelectron spectrometer (XPS). The flat-band voltage and the hysteresis of the layers are analyzed by the mercury-probe C V testing system. Corresponding author. Tel.: ; fax: address: jbfan@mail.xidian.edu.cn (J. Fan). The wafers are obtained after RCA cleaning and 3 s dip in diluted HF solution to remove the native oxide, which followed by 6 s rinse in deionized water. Liquid TEMAH is used as the hafnium precursor. The container of Hf precursor is heated to 95 C, corresponding to the vapor pressure of 1 15 hpa. When H 2 Ois used as the oxidant, the container is set at the room temperature, corresponding to a vapor pressure of 7 hpa. The ozone generator uses the ultra pure O 2 (99.995%) to obtain O 3, the concentration is 2 g/nm 3. HfO 2 films are grown on p-type Si (1) wafers by ALD for different oxidants (O 3,H 2 O) and deposition temperatures (15 C, 2 C, and 3 C). In order to decrease the interface traps and the fixed charges, the as-deposited films are annealed in nitrogen atmosphere. 3. Results and discussion 3.1. Comparison of physical properties of HfO Thickness of HfO 2 films Table 1 shows the thickness of HfO 2 films (T ox ) measured by Woollam M2D Spectroscopic Ellipsometer. T ox and growthper-cycle (GPC) of H 2 O-based HfO 2 films depend on the deposition temperature greatly. T ox of H 2 O-based HfO 2 films increases with increasing temperature. The same dependent relation is observed for the O 3 -based HfO 2 deposited at 15 C and 2 C respectively. However, T ox and GPC suddenly decrease for O 3 -based HfO 2 films deposited at 3 C. There is the strong decomposition of O 3 in the direction of gas flow when the temperature is higher than 25 C, and GPC does not increase by increasing O 3 or TEMAH dose /$ - see front matter Ó 212 Elsevier Ltd. All rights reserved. doi:1.116/j.microrel

2 144 J. Fan et al. / Microelectronics Reliability 52 (212) Table 1 Thickness of H 2 O-based and O 3 -based HfO 2 films deposited at different temperatures. Temp. ( C) H 2 O-based HfO 2 O 3 -based HfO 2 T ox (nm) GPC EOT (nm) Permittivity T ox (nm) GPC EOT (nm) Permittivity [8]. Furthermore, the GPC of H 2 O-based HfO 2 is more than.1 nm per cycle, and it is higher than that of O 3 -based HfO 2. In order to obtain the accurate values of permittivity and EOT of the HfO 2, the interlayer (T SiOx ) is measured by Woollam M2D Spectroscopic Ellipsometer and subtracted in the calculation, as shown in Eq. (2). The EOT and permittivity are determined as follows: EOT total ¼ A e e SiO2 C OX EOT HfO2 ¼ EOT total T SiOx ð2þ e HfO2 ¼ T HfO 2 EOT HfO2 e SiO2 where A is the area, C OX is the accumulation capacitance of HfO 2 films, e HfO2 and e SiO2 are the permittivity of HfO 2 and SiO 2 respectively. Table 1 shows the thickness of H 2 O-based and O 3 -based HfO 2 films deposited at different temperatures. The equivalent oxide thickness (EOT) decreases greatly with increasing deposition temperature and the permittivity increases obviously for O 3 -based HfO 2 films. While the EOT and permittivity of H 2 O-based HfO 2 films almost do not depend on the deposition temperature. Meanwhile, the permittivity of H 2 O-based and O 3 -based HfO 2 deposited at the same temperature shows significant difference. In order to investigate the impact of the thermal treatments on the physical properties and electrical characteristics of HfO 2 films, the samples are annealed in nitrogen. The annealing time is 1 min and annealing temperature is 5 C and 7 C respectively. The completely different behaviors are observed after different annealing conditions. Fig. 1 shows the thickness of HfO 2 after annealing process. After 5 C thermal annealing, the thickness of H 2 O- based HfO 2 films deposited at 15 C, 2 C and 3 C increase.2 nm,.1 nm and.1 nm, respectively. The increasing magnitude can be negligible because it is within the experimental error. However, the thickness increases significantly after 7 C thermal annealing due to the growth of interface layer. As shown in Fig. 1a, the films deposited at 15 C, 2 C and 3 C increase.8 nm,.4 nm and.3 nm, respectively. For the O 3 -based HfO 2 films, the thickness of the films deposited at 15 C decreases about.5 nm after 5 C thermal annealing, and the thickness of ð1þ ð3þ the films deposited at 2 C decreases about.6 nm after 7 C thermal annealing. At lower deposition temperature, there is not enough thermal energy for surface reactions, and some by-products or intermediates are generated in the layers. As a result, the decomposition and re-composition of by-products take place during the thermal annealing, which causes the decrease of the thickness. However, the thickness of film deposited at 3 C increase slightly after the thermal annealing process Composition of HfO 2 films The physical mechanism of H 2 O-based ALD HfO 2 is clear. However, the reaction mechanism of O 3 -based ALD HfO 2 is complicated due to the strong oxidization and lability of O 3. During the deposition process, O 3 can split the N C bond and C H bonds of by-products and ligands that are still attached to Hf. Different by-products can be formed in the HfO 2 films at different deposition temperature. The unstable by-products can be decomposed or re-composited, and the final by-products can be CO 2,H 2 O, CH 2 O, and NO 2 /NO, etc. [9 11]. As some nitrogen oxides are the strong oxidants, particularly in NO 2 (or its dimer N 2 O 4 ) and N 2 O. They can participate in the ALD reactions. Meanwhile, NO x can deactivate active sites of O 3 catalysts [12,13]. It means that nitrogen oxides have the priority to react with the TEMAH in the deposition process. In order to investigate the different physical properties of H 2 O-based HfO 2 and O 3 -based HfO 2, the films deposited at different temperatures are analyzed by XPS. Fig. 2 shows the effect of 7 C annealing on the XPS spectra for H 2 O-based and O 3 -based HfO 2 films deposited at 3 C. Fig. 2 shows that there is almost no significant difference for H 2 O-based HfO 2 before and after the thermal annealing. The main peaks are Hf and O, subordinate peak is C. The peak of N is so weak that it can be neglected. The percentage composition of C changes from 6.22% to 2.34% after 7 C thermal annealing. However, for the O 3 -based HfO 2 films, the thermal annealing impacts the XPS spectra obviously. The main peaks are Hf and O, subordinate peaks are N and C (.69%). The percentage composition of N decreases from 7.5% to 6.78% after the thermal treatment. Further XPS spectra of O 3 -based HfO 2 films deposited at different temperature shows that the impurities (C and N) in the HfO 2 films increase with decreasing deposition temperature, as shown in Table 2. Tox (nm) HfO 2 (H 2 O) Tox (nm) HfO 2(O 3 ) 7 As-deposited As-deposited 5 7 Fig. 1. The thickness of HfO 2 films after annealing process H 2 O-based, O 3 -based.

3 J. Fan et al. / Microelectronics Reliability 52 (212) Counts (s) HfO2(H2O) Hf4f Hf5p Hf4d Hf4p C1s N1s O1s located at ev and ev, respectively. The O 1 peak and O 2 peak are attributed to O as HfO 2 and non-hfo 2 oxygen, respectively. The proportion of O 1 peak and O 2 peak is 92.65% and 7.35%, respectively. However, after annealing process, the proportion of O 3 peak and O 4 peak located at ev and ev, which change to 95% and 5%, respectively. The results indicate that the unstable Hf O N and Hf N bonds (N 1 and N 2 ) are re-bonded with the non-hfo 2 oxygen. Therefore, the N 1 and N 2 peaks shift to high binding energy and the proportion of O as HfO 2 increases Comparison of electrical characteristics of HfO 2 films Binding Energy (ev) Fig. 2. XPS spectra for H 2 O-based and O 3 -based HfO 2 films deposited at 3 C. The percentage composition of C in HfO 2 films deposited at different temperature is shown in Fig. 3a. It is obtained that the percentage composition of C in H 2 O-based HfO 2 decreases from 7.52% to 6.22% as the deposition temperature increases from 15 C to 3 C. However, for O 3 -based HfO 2, the percentage composition of C decreases from 5.15% to.69%. The results indicate that the effect of deposition temperature on the impurity C concentration is more serious for the O 3 -based HfO 2 films. Fig. 3b shows the percentage composition of Hf and O in H 2 O- based and O 3 -based HfO 2 films deposited at 15 C, 2 C and 3 C respectively. For H 2 O-based HfO 2, the atomic ratio of Hf/O only decreases from 2.3 to 1.89 as the deposition temperature increases from 15 C to 3 C. It seems that the deposition temperature has no significant influence on the atomic ratio. However, for O 3 -based HfO 2, the atomic ratio of Hf/O increases from 1.74 to 2.19 when the deposition temperature increases from 15 C to 3 C. Besides, Hf/O atomic ratio of H 2 O-based and O 3 -based HfO 2 decreases after the thermal annealing, which cause the increase of percentage composition of O atom. Fig. 4 shows N 1s peaks of O 3 -based HfO 2 films deposited at 3 C. The fitting curves indicate that the N 1s spectra mainly consist of two different components, denoted as N 1 and N 2. They are located at ev and ev, respectively. The N 1 peak and N 2 peak are caused by Hf N bonds and Hf O N bonds [14]. It is confirmed that the nitrogen oxide species participate in the ALD chemisorption reactions. On the other hand, the peaks of the fitting curves shift after 7 C thermal annealing. The N 3 and N 4 peaks are ev and ev, which correspond to N (Hf,O) and Hf O N bonds [15]. The N 1 and N 2 peaks shift to high binding energy, which indicates that the Hf O N and Hf N bonds are very unstable, and re-composition takes place after the annealing process. Meanwhile, the intensity of N 1 and N 2 peaks decrease apparently. N species diffuse to the subsurface region after the annealing process, which causes the concentration of N in HfO 2 films decreasing. Fig. 5 shows the O 1s peaks of O 3 -based HfO 2 films deposited at 3 C. The fitting curves indicate that the O 1s spectra mainly consists of two different components, denoted as O 1 and O 2. They are Fig. 6 shows the C V characteristics of H 2 O-based and O 3 -based HfO 2 deposited at 15 C, 2 C and 3 C respectively. Fig. 6a shows the C V curves of H 2 O-based HfO 2 films (f = 1 khz). The gate voltage (V G ) is swept from accumulation to inversion (symbol) and then swept back (solid line). The C V curve of films deposited at lower temperature shows a hysteresis loop and the hysteresis diminishes with increasing deposition temperature. The stretch out appears for deposition temperature of 15 C and 2 C, which corresponds the interface states. There is no enough thermal energy supplied for the surface reactions and traps produced at lower growth temperature. Fig. 6b shows the C V curves of O 3 -based HfO 2. Compared with the C V curves of H 2 O-based HfO 2, the hysteresis loop of O 3 -based samples is larger at higher temperature. There is a width step appears in C V curves, and accumulation capacitance value increases with increasing deposition temperature. The width step and difference of capacitance values can be explained by XPS, as shown in Fig. 3. At lower deposition temperature, the higher impurities level of O 3 -based films such as C and N will lower the refractive index and the dielectric constant, which contributes to the difference capacitance in C V curves. The width step is caused by the trapped holes injected from HfO 2 layer (unpaired bonding nitrogen oxide units) into the depletion layer. The width of the depletion layer in Si substrate grows with increasing V G, which decreases the total capacitance of the capacitor. However, if all the trapped holes in the HfO 2 /SiO 2 layer are injected into the depletion layer, the growth of the depletion layer stops and the capacitance becomes constant. At the beginning of depositing O 3 -based HfO 2 films, TEMAH is pulsed into the chamber by carrier gas, and then O 3 is pulsed. In reaction, the nitrogen oxides (NO x, etc.) are generated. Nitrogen oxides deactivate the active sites of O 3 and have the chance to react with the TEMAH in the deposition process, N O covalence bonds are destroyed. Dangling nitrogen bonds can form the new covalence bonds with other species or ligands before they are purged by nitrogen. Because N has the valence states of 3, +1, +2 and +4, it will destroy the structure of the intermediate precursor, the dangling bonds and unpaired bonding traps are produced in HfO 2. Therefore, the width step appears in C V curves Impact of thermal annealing on the electrical characteristics of HfO 2 films Fig. 7 shows the influence of the annealing temperature (T = 5 C, 7 C) on the C V characteristics of H 2 O-based HfO 2. Table 2 Percentage composition of H 2 O-based and O 3 -based HfO 2 films. Temp. ( C) H 2 O-based HfO 2 O 3 -based HfO 2 Hf 4f (a.t.%) O 1s (a.t.%) C 1s (a.t.%) N 1s (a.t.%) Hf 4f (a.t.%) O 1s (a.t.%) C 1s (a.t.%) N 1s (a.t.%) 15 As-deposited Annealed As-deposited Annealed As-deposited Annealed

4 146 J. Fan et al. / Microelectronics Reliability 52 (212) C1s 7 Atomic% O 3 Oxidant H 2O Atomic% O 3 Hf () O () Hf () O () Hf () O () Oxidant H 2O Fig. 3. The percentage composition of C, Hf and O atoms in H 2 O-based and O 3 -based HfO 2 films deposited at 15 C, 2 C and 3 C C, Hf and O. Counts (s) Counts (s) Annealed N1 N3 N2 N Binding Energy (ev) N1s experimental data N1s curve fitting Backgnd. Fig. 4. N 1s peaks of O 3 -based HfO 2 deposited at 3 C. O as HfO2 Annealed O1 O3 O2 O1s experimental data O1s fitting curve Backgnd. non-hfo 2 oxygen O Binding Energy (ev) Fig. 5. O 1s peaks of O 3 -based HfO 2 films deposited at 3 C. We can see that the flat-band voltage shifts to the positive voltage direction and the hysteresis almost disappears after 5 C annealing. Meanwhile, the stretch out of HfO 2 deposited at 15 C and 2 C also disappears, as shown in Fig. 7a and b. It indicates the interface traps at SiO x /Si and the excessive hydroxyl/hydrogen groups in the layers decrease after 5 C thermal treatment. However, the flat-band voltage shifts to the negative voltage direction after 7 C annealing. The interface layer of the silicate grows between the silicon and the high-j dielectric after 7 C annealing, and the thickness of silicate increases about.3 nm. Because of oxygen ions penetrate into the silicate layer, vacancies generated in the HfO 2 layer, which contribute to the negative shift of the flat-band voltage. Fig. 8 shows the influence of the annealing temperature on the C V characteristics of O 3 -based HfO 2 deposited at different deposition temperature. The flat-band voltage shifts to the positive direction after high temperature annealing. Fig. 8a and b shows that the width step in the O 3 -based HfO 2 films deposited at 15 C and 2 C diminishes after 5 C annealing and it disappears after 7 C annealing. The C V curves of O 3 -based HfO 2 films deposited at 3 C present overlap after 5 C and 7 C annealing, which is shown in Fig. 8c. The decrease of positive charges is the dominant mechanism of flat-band voltage shift after the thermal treatments. The kinetic energy of the N-containing species is close to the activation energy of desorption and the activation energy of oxidation. Furthermore, the interdiffusion of the corresponding N to the substrate region decreases the traps related to the N species. So, the traps in the HfO 2 films decrease and the wide step in the C V curves diminishes after annealing. In summery, thermal treatments decrease the traps in the HfO 2 films, and the interface layer grows after 7 C annealing for H 2 O- based HfO 2 films. However, for O 3 -based HfO 2 films, great numbers Acc. to Inv. Inv. to Acc HfO 2(H2O) Acc. to Inv. Inv. to Acc. Fig. 6. C V characteristics of as-deposited HfO 2 films H 2 O-based, O 3 -based.

5 J. Fan et al. / Microelectronics Reliability 52 (212) HfO2(H 2 O) 5 C HfO2(H2O) 5 C HfO 2(H2O) 5 C (c) Fig. 7. C V characteristics of H 2 O-based ALD HfO 2 films deposited at different temperature 15 C, 2 C, (c) 3 C. 5 C HfO2 (O3) 5 C V G (V) V G (V) 5 C (c) V G (V) Fig. 8. C V characteristics of O 3 -based ALD HfO 2 films deposited at different temperature 15 C, 2 C, (c) 3 C. of N-containing species traps play a major role for the shifts of C V characteristics. Thermal treatment makes the N-containing species decompose and decreases the traps in the O 3 -based HfO 2 films. The phenomenon is obvious for the O 3 -based HfO 2 films deposited at lower temperature. Furthermore, the decrease of the accumulation capacitance for O 3 -based HfO 2 deposited at 15 C is observed after 5 C annealing. It is attributed to the increasing of leakage current, which is discussed in the following section Flat-band voltage (V FB ) and hysteresis voltage Figs. 9 and 1 shows the extracted V FB values and oxide charge density of HfO 2 films for different process. It can be obtained from

6 148 J. Fan et al. / Microelectronics Reliability 52 (212) HfO 2(H 2O) HfO2 (O 3) VFB (V).6 VFB (V) As-deposited 5 7 As-deposited 5 7 Fig. 9. Flat-band voltage versus the annealing temperature for HfO 2 films H 2 O-based, O 3 -based. Oxide charge density (cm -2 ) 3x1 12 2x1 12 1x1 12-1x1 12 HfO 2 (H2O) 2x1 12 1x1 12-1x1 12-2x1 12-3x1 12-2x1 12-4x1 12 As-deposited 5 7 As-deposited 5 7 Oxide charge density (cm -2 ) Fig. 1. Density of oxide charge versus the annealing temperature for HfO 2 films H 2 O-based, O 3 -based. Hysteresis (V).1 5 HfO2 (H 2O) Hysteresis (V) As-deposited 5 7 As-deposited HfO 2 (O3) Fig. 11. Hysteresis versus the annealing temperature for HfO 2 films H 2 O-based, O 3 -based. Fig. 9, higher deposition temperatures produces higher V FB values for the as-deposited H 2 O-based HfO 2 and O 3 -based HfO 2 films. After thermal annealing, for H 2 O-based HfO 2 films, the increasing V FB of positive direction shows that negative oxide charges increase after 5 C annealing, whereas the increasing V FB of negative direction shows positive oxide charges increase after 7 C annealing. It is because the decreasing impurity CH x and the excessive hydroxyl/hydrogen groups after 5 C annealing [16]. However, after 7 C annealing, the formation of oxygen vacancies in HfO 2 films increases the positive oxide charge due to the growth of silicate interfacial layer. As shown in Fig. 1a, negative oxide charge density of all HfO 2 films increases after 5 C annealing, whereas positive oxide charge density increases about cm 2 after 7 C annealing. However, for O 3 -based HfO 2 films, the positive increasing of V FB values indicates that negative charges increase after 5 C and 7 C annealing. In analysis meanwhile, the decreasing intensity of N 1s spectra, as shown in Fig. 4, indicates that the indiffusion of N species exists after annealing. Since oxygen atoms in HfO 2 have similar size as nitrogen, the indiffusion of N can increase negative charges due to N atoms can substitute oxygen atoms and generate oxygen ions in HfO 2 films [17]. As shown in Fig. 1b, the negative oxide charge density for O 3 -based HfO 2 films minimum increase about cm 2. Figs. 11 and 12 shows the extracted hysteresis voltage and slow state density of HfO 2 films for different process. It is observed that

7 J. Fan et al. / Microelectronics Reliability 52 (212) Slow state density (cm -2 ) 8.x1 11 HfO2 (H2O) 6.x x x1 11 As-deposited 5 7 Slow state density (cm -2 ) 8.x x x x1 11 HfO 2(O3) As-deposited 5 7 Fig. 12. Density of slow state versus the annealing temperature for HfO 2 films H 2 O-based, O 3 -based. the hysteresis of as-deposited H 2 O-based HfO 2 decreases with increasing deposition temperature, whereas the adverse characteristic is observed for O 3 -based HfO 2. Furthermore, the hysteresis voltage of H 2 O-based HfO 2 and O 3 -based HfO 2 decreases after annealing and the acceptable hysteresis voltage level (<5 mv) is obtained. However, the hysteresis voltage and the slow interface states of O 3 -based HfO 2 film deposited at 15 C increase after 5 C annealing, as shown in Fig. 11b. The increasing slow interface states causes increasing leakage current, which agrees well with the reference [18].The neutral species are generated by electron trapping and diffuse to the SiO 2 /Si interface, which also causes slow interface states to increase [19]. There is no enough thermal energy for surface reaction, the poor quality O 3 -based HfO 2 film is obtained at 15 C. The neutral species proposed are the decomposition of the by-products or unstable intermediate. As it can be better observed in Fig. 12, the slow state density for H 2 O-based HfO 2 films is less than cm 2 after 5 C annealing, while for O 3 -based HfO 2 films is less than cm 2 after 7 C annealing. This implies that more defects exist in O 3 -based HfO 2 films and some of them cannot be eliminated through annealing in N Conclusions HfO 2 thin films are deposited by ALD for different oxidants and deposition temperature. Compared with the O 3 -based HfO 2, the H 2 O-based HfO 2 films have advantages in the GPC and permittivity. Meanwhile, low C impurity and high concentration N impurity are observed in O 3 -based HfO 2 while only high C impurity is observed in H 2 O-based HfO 2 films. The physical properties and electrical characteristics of H 2 O-based and O 3 -based HfO 2 are also investigated after 5 C and 7 C thermal treatment in nitrogen atmosphere. The XPS and ellipsometry testing results indicate that the change of O 3 -based HfO 2 films after annealing process is more complicated due to the existence of N-containing species. The C V testing results show that, the flat-band voltage shifts and hysteresis voltages are different for different oxidants, however, the annealing process can improve the electrical characteristics. Acknowledgements This work was supported National Natural Science Foundation of China (Grant Nos , ) and the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China Program (Grant No. 7883). References [1] Campabadal F, Zabala M, Rafi JM, Acero MC, Sainchez A, Sainchez J, Sainchez S, Andreu R. Thin high-j dielectric layers deposited by ALD. In: Proceedings of the 29 Spanish Conference on Electron Devices; 29. p [2] George SM. Atomic layer deposition: an overview. Chem Rev 21;11: [3] Kukli K, Ritala M, Sajavaara T, Keinonen J, Leskela M. Atomic layer deposition of hafnium dioxide films from hafnium tetrakis(ethylmethylamide) and water. Chem Vap Deposition 22;8: [4] Cho MH, Roh YS, Whang CN, Jeong K. Thermal stability and structural characteristics of HfO 2 films on Si (1) grown by atomic layer deposition. Appl Phys Lett 22;81: [5] Rafí JM, Zabala M, Beldarrain O, Campabadal F. 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