Effects of Cl-Based Ligand Structures on Atomic ... - ACS Publications

Article pubs.acs.org/JPCC

Effects of Cl-Based Ligand Structures on Atomic Layer Deposited HfO2 Bo-Eun Park,† Il-Kwon Oh,† Chang Wan Lee,† Gyeongho Lee,† Young-Han Shin,§ Clement Lansalot-Matras,‡ Wontae Noh,‡ Hyungjun Kim,*,† and Han-Bo-Ram Lee*,∥ †

School of Electrical and Electronic Engineering, Yonsei University, Seoul 03722, Korea Department of Physics, University of Ulsan, Ulsan 44610, Korea ‡ Air Liquide Korea Co., LTD., Seoul 03722, Korea ∥ Department of Material Science Engineering, Incheon National University, Incheon 22012, Korea §

S Supporting Information *

ABSTRACT: Atomic layer deposition (ALD) of HfO2 is a key technology for the application of high dielectric constant gate dielectrics ranging from conventional Si devices to novel nanodevices. The effects of the precursor on the growth characteristics and film properties of ALD HfO2 were investigated by using hafnium tetrachloride (HfCl4) and bis(ethylcyclopentadienyl)hafnium dichloride (Hf(EtCp)2Cl2, Hf(C2H5C5H4)2Cl2) with O2 plasma reactant. The growth characteristics were significantly affected even by simply changing the precursor. Theoretical calculations utilizing geometrical information on the precursor and density functional theory revealed that the steric demands of the precursor ligands have a dominant effect on the different growth characteristics rather than the reaction probability of the precursor on the surface. The chemical compositional analysis results showed that the Cl residue in the HfO2 films was reduced by using Hf(EtCp)2Cl2 due to the lower number of Cl atoms in each Hf precursor molecule and the relieved bridge formation of Hf−Cl−Hf bridge on the surface compared to HfCl4. The electrical property measurement results showed significantly improved insulating properties in HfO2 using Hf(EtCp)2Cl2 compared to HfCl4 due to the low concentration of Cl residue in the film. These results provide broad insights to researchers who are interested in the fabrication of high quality dielectric layers to achieve better device performance and overcome physical limitations in the nanoscale regime.

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INTRODUCTION As metal oxide semiconductor field effect transistor (MOSFET) devices have been scaled down, HfO2 has been widely used as a gate oxide material due to its superior properties, including a suitable band offset with Si (∼1.4 eV) and good dielectric properties with a dielectric constant (k) as high as 25.1−4 In addition, HfO2 has been applied to novel electronic devices as an insulating material such as dielectric layers for 2D material-based devices to improve their mobility.5 The structures of current electronic devices have evolved to 3D structures such as FinFET, such that gate dielectric materials are required to have good conformality on 3D structures as well as a high purity and large area uniformity.1 Atomic layer deposition (ALD) has many benefits for nanoscale device fabrications, such as atomic scale thickness control, excellent conformality, low impurity contamination, and deposition of pinhole-free films. Therefore, it is considered as a promising deposition method of HfO2.6,7 ALD films are deposited through chemical reactions of a precursor and a counter reactant which take place only on surfaces, such that the reactions strongly affect the film © XXXX American Chemical Society

properties and growth characteristics. Therefore, proper selection of the precursor is very important for potential applications of ALD HfO2. Until now, several Hf precursors including hafnium tetrachloride (HfCl 4),8−12 bis(cyclopentadienyl)hafnium dichloride (Cp2HfCl2),13 hafnium tetraiodide (HfI 4 ), 1 4 − 1 6 tetrakis(dimethylamino)hafnium (TDMAH), 17 and tetrakis(ethylmethylamino)hafnium (TEMAH)18,19 have been reported. Among them, chloride precursors have attracted great interest, since they have high vapor pressures, which are beneficial for precursor delivery. In addition, since the C impurity deteriorates film quality and device performance, using chloride precursors has advantages over C-containing precursors. Previous studies of ALD HfO2 using HfCl4 with H2O and/or O3 showed high dielectric constants (19 when using O3 and 21−23 when using H2O) and low leakage current densities of ∼10−7−10−9 A/cm2.8−10 However, since HfCl4 molecules form Received: June 3, 2015 Revised: February 29, 2016

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DOI: 10.1021/acs.jpcc.5b05286 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

window temperatures and the default process sequences (composed of precursor exposure time, purge, reactant exposure time, and purge) were 2 s−5 s−1 s−5 s and 3 s−5 s−1 s−5 s for HfCl4 and Hf(EtCp)2Cl2, respectively. The chemical composition and impurity contents were analyzed by ex situ X-ray photoelectron spectroscopy (XPS) (KRATOS, AXIS NOVA) and dynamic secondary ion mass spectrometry (SIMS) (CAMEACA, IMS 4FE7). For XPS, a monochromatic Al Kα source (beam energy: 1486.6 eV and analysis area: 100 μm2) was used and surface cleaning by sputtering with Ar ion bombardment (energy: 3 keV, beam current density: 22.2 mA/cm2, induced beam current: 2 mA, rastered over a 3 × 3 mm2 area) was performed prior to the XPS analysis. The surface C 1 s peak at 284.5 eV was used as a reference to calibrate the spectrum energy. For dynamic SIMS, we used a Cs+ ion gun (impact energy: 6 keV, current: 5 nA, raster size: 150 μm) and performed the measurements in the polarity-negative condition. The electrical properties based on the capacitive−voltage (C−V) and current−voltage (I−V) characteristics were evaluated using a Keithley 590 C−V analyzer and Agilent 4155C semiconductor parameter analyzer. Topological images of the films were analyzed by an ex situ atomic force microscope (AFM) (VEECO, multimode). The thicknesses of the films were measured by a spectroscopic ellipsometry (Ellipso Technology, Elli-SE-F). To evaluate the evaporation characteristics of the HfCl4 and Hf(EtCp)2Cl2 precursors, thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) measurements were performed in a nitrogen-filled glovebox on a Mettler Toledo device equipped with an evolved gas analysis furnace. The precursors were heated at a ramp rate of 10 °C/min in the standard open cup (OC) mode at atmospheric pressure. For the MOS capacitor fabrication, ALD HfO2 films were deposited on p-type Si(001) substrates that were cleaned at 70 °C for 10 min in a RCA solution (NH4OH:H2O2:H2O = 1:1:5 on a volume basis), followed by dipping in a buffered oxide etchant solution for 30 s to remove the native oxide. After HfO2 ALD, post-deposition annealing (PDA) was carried out in a rapid thermal annealing (RTA) system with N2 ambient at 1000 °C for 10 min. Ru was used as the metal electrode of the MOS structure and was deposited by dc magnetron sputtering with a plasma power of 30 W. The thickness of Ru was 100 nm, and a patterned shadow mask was used to define the contact area. To calculate the energy difference between each ALD reaction step, first-principles calculations of simplified molecular structures were performed using the Vienna Ab-initio Simulation Package (VASP). The exchange and correlation functional was treated by a generalized gradient approximation using the Perdew−Burke−Ernzerhof parametrization, and the interactions between ions and electrons were described by the projector augmented wave (PAW) potentials. A cutoff energy of 400 eV was used to calculate the total energies with a single k-point. The sizes of the supercell were set at 20 Å × 20 Å × 20 Å and 30 Å × 30 Å × 30 Å for the HfCl4 and Hf(EtCp)2Cl2 precursors, respectively, to make the shortest distance between neighboring images larger than 10 Å in the x, y, and z directions. Hf and four OH functional groups were fixed, and all all other atomic coordinates were fully relaxed until the energy difference was smaller than 10−5 eV/(unit cell) to mimic the HfO2 surface, in which only one OH group is available for reactions with the ligands of the precursor, and the remaining

Hf−Cl−Hf bridges between molecules on the substrate, byproduct molecules including Cl atom, such as HCl, are easily trapped in the complex structure of HfCl4.20 These residual Cl atoms from the ligands of HfCl4 are segregated at the grain boundaries of ALD HfO2 films, leading to deterioration of the film quality, such as void formation and high leakage currents at high temperatures.12 Since the use of chloride precursors deteriorates final device performance, the C-containing Hf precursor and the Cl-containing Hf precursor must be balanced to minimize C and Cl impurities and maximize precursor volatility and reactivity simultaneously. There have been several attempts to reduce Cl residue in ALD HfO2 films by using other Hf precursors which contain a smaller number of Cl atoms per each Hf atom. Among them, replacement of Cl ligand into cyclopentadienyl (Cp) rings is known to block the formation of the metal atom−Cl−metal atom bridge in CpTiCl2 system.21 In previous study with Cp2HfCl2, although ALD using Cp2HfCl2 with H2O and/or O3 produced a smooth HfO2 film with uniform layer-by-layer growth at 350 °C, there was still Cl contamination in the ALD HfO2 films.13 In addition, compared to HfCl4, Cp2HfCl2 showed a lower growth rate, indicating that the ligands of the precursor strongly affect the ALD growth characteristics. Therefore, a comparative study of different ligands of the precursors can provide critical clues to understand the ALD HfO2 reaction for the process design of high quality Cl-free ALD HfO2 films. However, there has been no systematic discussion on the effect of Cl ligand on the growth characteristics or film properties of ALD HfO2 to date. In this study, we investigated the growth characteristics and film properties of plasma-enhanced ALD (PE-ALD) HfO2 produced using two Cl-based Hf precursors, HfCl4 and bis(ethylcyclopentadienyl)hafnium dichloride (Hf(EtCp)2Cl2, Hf(C2H5C5H4)2Cl2), and O2 plasma counter reactant. The Hf(EtCp)2Cl2 precursor was newly synthesized by Air Liquide Company and designed as an alternative to the HfCl4 precursor with a similar molecular structure and low Cl contamination. The chemical compositions, microstructures, and surface morphologies of the two ALD HfO2 films were analyzed, and the results were correlated to the effects of the precursor ligands. Theoretical calculations were carried out by using the density functional theory (DFT) and geometrical information on the precursors to understand the different growth characteristics. Also, the electrical properties including the dielectric constant and leakage current were evaluated for potential applications. This study provides fundamental and practical insights of the fabrication of ALD HfO2-based electronic devices.

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EXPERIMENTAL AND/OR THEORETICAL METHODS We used a commercial showerhead type ALD chamber (NCD Co., Lucida M100-PL) which has an 8 in. wafer capacity. HfCl4 and Hf(EtCp)2Cl2 precursors were contained in individual stainless-steel bubblers and evaporated at 170 and 130 °C, respectively, in order to obtain a sufficient vapor pressure. The precursor vapors were transported into the reaction chamber by Ar carrier gas of 25 sccm which was controlled by a mass flow controller (MFC). Ar gas of 50 sccm was also used to purge excess gas molecules and byproducts between each precursor and reactant exposure step. O2 plasma was generated by applying RF power of 300 W connected to the showerhead and substrate during the O2 pulse of 200 sccm. The substrate temperature was changed from 80 to 200 °C to find ALD B


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Figure 1. Physical property analysis of HfCl4 and Hf(EtCp)2Cl2 precursors as a function of temperature: (a) TGA and (b) DSC.

Hf atom and three OH groups already have bonding with the neighboring atoms in the lattice.

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RESULTS AND DISCUSSION Growth Characteristics. To start, we evaluated the evaporation characteristics and melting points of the HfCl4 and Hf(EtCp)2Cl2 precursors by TGA and DSC, respectively, as shown in Figure 1. Figure 1a shows that the TGA curves of two precursors decrease with increasing temperature and the precursors are fully evaporated above 290 °C for HfCl4 and 320 °C for Hf(EtCp)2Cl2. For both precursors, a small amount of residue, as low as 1% of the total mass, was obtained, indicating that negligible decomposition occurred during evaporation of the precursors. These results demonstrate that the precursors have good thermal stability as an ALD precursor. Although the temperature of full evaporation for Hf(EtCp)2Cl2 is higher than that for HfCl4, the evaporation behaviors for both precursors are different in the low temperature region below 200 °C. As shown in the inset of Figure 1a, the amount of evaporated Hf(EtCp)2Cl2 is larger than that of HfCl4. As a result, a sufficient vapor pressure of Hf(EtCp)2Cl2 was obtained at a lower temperature (130 °C) than HfCl4 (170 °C). In Figure 1b, the DSC curves show that the melting points of HfCl4 and Hf(EtCp)2Cl2 are 409 and 99 °C, respectively. The substitution of two Cl atoms with two EtCp rings in the molecular structure significantly lowers the melting point. The low melting point of Hf(EtCp)2Cl2 can make it easier to maintain the flow rate of the vaporized precursor at a constant level and relieve particle generation in the chamber and precursor gas-line clogging, which are considered critical productivity issues when using HfCl4.

Figure 2. Growth per cycle of PE-ALD HfO2 on Si as a function of (a) the growth temperature, (b) the precursor exposure time, and (c) the reactant exposure time at 180 °C.

The ALD HfO2 process using the HfCl4 and Hf(EtCp)2Cl2 precursors with O2 plasma were investigated. Figure 2a shows the growth per cycle (GPC) of the PE-ALD HfO2 at various growth temperatures ranging from 80 to 200 °C. In the case of HfCl4, the GPC remains almost constant from 145 to 200 °C and increases with decreasing temperature below 145 °C. Because the precursor is condensed at the low temperature without self-saturated ALD reactions, the GPC increases.7 For Hf(EtCp)2Cl2, the GPC remains almost constant over all temperatures studied from 80 to 200 °C, indicating that selfsaturated ALD reactions occur. Interestingly, there was no increase in GPC from the condensation of the precursor down to 140 °C, which is lower than the precursor temperatures of both HfCl4 and Hf(EtCp)2Cl2; rather, the GPC of HfO2 remained almost constant. It is likely that the exposure times were insufficient for the condensation of precursors on the C


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Table 1. Molecular Structures, Calculated Projection Areas, Ratios, and Experimental GPCs of the HfCl4 and Hf(EtCp)2Cl2 Precursors

substrate due to the small difference between the precursor temperature and growth temperature. In other words, the amount of precursor exposed to the surface was small, so most of the precursor was pumped out during the purge time, and the small amount of precursor remaining was chemisorbed on the surface. Consistent with this, the HfCl4 precursor (whose vapor pressure is much higher than that of the Hf(EtCp)2Cl2 precursor) showed an increase of GPC at lower temperatures in Figure 2a due to the condensation from the exposure of a large amount of precursor, while the Hf(EtCp)2Cl2 precursor showed constant GPCs with decreasing temperatures. At 180 °C, the GPCs were measured while varying the Hf precursor exposure time and reactant exposure time, as shown in Figures 2b and 2c. The GPCs are saturated over 2 and 3 s for HfCl4 and Hf(EtCp)2Cl2 as Hf precursors and 1 s for O2 plasma as a reactant. The saturated GPCs were approximately 1.48 ± 0.16 Å/cycle for HfCl4 and 0.65 ± 0.11 Å/cycle for Hf(EtCp)2Cl2. Consistent with the results obtained at the various temperatures shown in Figure 2a, both precursors showed suitable saturation behavior for the ALD process. On the basis of these growth behaviors in ALD mode, we set the process conditions of the following experiments of ALD HfO2 to 2 s−5 s−1 s−5 s and 3 s−5 s−1 s−5 s for HfCl4 and Hf(EtCp)2Cl2, respectively, at 180 °C of growth temperature. The reported GPCs ALD HfO2 using HfCl4 and H2O are in the range of 0.5−1.0 Å/cycle.6,22−24 The wide distribution of GPCs was explained by the different working pressure which changes the reaction probability of precursor and reactant.25 In addition, a GPC of PE-ALD is higher than that of thermal ALD in many cases due to the higher reactivity of plasma reactant than gas reactant. In the current study, ALD HfO2 using O2 plasma as a reactant showed a higher growth rate of 1.48 Å/cycle than that using H2O due to the high reactivity of O2 plasma than H2O and O3, which is consistent with many previous reports.26

Interestingly, however, even by changing the precursor, the GPC increased by more than a factor of 2. Generally under same process conditions such as pressure and growth temperature, the GPCs of HfO2 by HfCl4 and Hf(EtCp2)Cl2 are mainly affected by two factors: The steric demands of the ligands and the reaction probability of the precursor on the surface.27−29 Hf(EtCp)2Cl2 molecules that are already adsorbed on the surface hinder near adsorption sites available for the following precursor adsorption. Since the molecular volume of Hf(EtCp)2Cl2 is larger than that of HfCl4, the effects of steric demands of ligands by Hf(EtCp)2Cl2 are more significant than those of HfCl4. If the interactions between Hf precursor molecules during exposure are ignored, the effects of steric demands of ligands on ALD growth can be simply estimated by geometrical calculations. We calculated the theoretical maximum projection areas of each precursor with error range below ∼1 Å2, which can be translated to the lateral area occupied by one precursor molecule on the surface. The ratio of the maximum projection areas of HfCl4 to Hf(EtCp)2Cl2 is 1:2.258 ± 0.091, as shown in Table 1. The geometrical calculations can be affected by the change of structure during the adsorption of precursor molecules on the surface; however, the differences are negligible (2.65% for HfCl4 and 1.52% for Hf(EtCp)2Cl2). The reciprocal of the maximum projection area is the number of precursor molecules adsorbed per unit area. Since HfCl4 and Hf(EtCp)2Cl2 have only one metal atom per precursor molecule, the reciprocal of the maximum projection area is also the number of metal atoms per unit area, which is proportional to the GPC.27 Interestingly, the ratio of the maximum projection areas is very similar to the reciprocal of the ratio of the two GPCs (HfCl4:Hf(EtCp)2Cl2 = 2.3 ± 0.7:1). This indicates that the difference of the GPCs can be attributed D


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of the OH species, as shown in Figure 3e, and the HCl byproduct is desorbed in the adsorption state in Figure 3f. The energy changes from the initial state to the transition state of HfCl4 and Hf(EtCp)2Cl2 are 76.9 and 74.1 kJ/mol, respectively, as shown in Figure 3g. The energy changes from the transition state to the adsorption state are also very similar. The results of the DFT calculation show that the activation energies required for complete adsorption of precursors are almost the same for HfCl4 and Hf(EtCp)2Cl2. This indicates that HfCl4 and Hf(EtCp)2Cl2 adsorb on the surface with almost same probability. These reaction pathways are consistent with previous reports on ALD HfO2 using HfCl4 and H2O on Hf or Si surfaces, in which HfCl4 undergoes transition states with energies lower than those of the final products and the adsorption energy levels are required to overcome the transition state.31,34 The adsorption energies of HfCl4 on HfOH* in previously reported results are the range of 65−105 kJ/ mol, which is comparable with our result.31,35,36 A recent study including the larger number of HfCl4 precursor molecules in their DFT calculation showed that HfCl4 molecules adsorb on the surface without any ligand removal due to the large activation energy (≥96 kJ/mol).20 Also, the literature showed that interactions between HfCl4 molecules form chains of HfCl4 by the formation of Hf−Cl−Hf bridges, resulting in further adsorption of HfCl4 over than the monolayer, that is, no selfsaturation, depending on the temperature. In the case of Hf(EtCp)2Cl2, EtCp groups play a role in blocking the bridge formation between Hf and Cl, which is expected to reduce the nonideal adsorption of precursor.21 The following reaction between the adsorbed precursors and O2 plasma reactant could affect the next adsorption of the precursor. However, theoretical prediction of the O2 plasma reaction with adsorbed precursors by DFT is not simple because of the variety of radical species of O2 plasma. However, from inference, we can expect that the surface is Hf-OH* after O2 plasma reactant exposure for both precursors. Previous reports showed the presence of hydrogen radicals in oxygen plasma from optical emission spectroscopy analysis, resulting in the formation of Hf-OH* on the surface by reaction of oxygen radicals with hydrogen which is likely to originate from the precursor or leaks in the ALD chamber.26,37 The additional DFT calculations show that the H2O reactant reacts the adsorbed precursors, resulting in the formation of Hf-OH*, as shown in Figure S1 of the Supporting Information, which is a general mechanism of precursor ligand exchange during the reactant exposure step in the ALD process.38 Also, it is known that the reactivity of O2 plasma is much higher than that of H2O.39 A previous report on the reaction pathways of PE-ALD HfO2 showed that, among various oxygen species generated during the O2 plasma pulse, 3O and 1O have much higher energies than the others and form stable reaction pathways to the products.40 On the basis of these studies, we assume that high reactivity of the 1O2, 3O, and 1O species in O2 plasma is enough to fully react with the adsorbed Hf precursor molecules and generate an Hf-OH* surface. As a result, the surface chemical species after O2 plasma exposure in both precursors are assumed to be the same toward precursor exposure. Therefore, the different GPCs of ALD using HfCl4 and Hf(EtCp)2Cl2 can be solely attributed to the effect of the steric demands of ligands. Chemical Composition. The chemical compositions of the 20 nm thick HfO2 films using PE-ALD were analyzed by XPS. Figures 4a−d show the Hf 4f, O 1s, Cl 2p, and C 1s core level

Figure 3. Molecular configuration and energy change during the first half-reaction of ALD calculated by DFT: initial state, initial physisorption (a, d), transition state (b, e), and final adsorption (c, f).

to the effect of the steric demands of ligands if reaction probabilities of the precursor on the surface are the same. Meanwhile, the GPC of ALD can also depend on the reaction probability of precursors on the surface. The reaction probability is a function of the activation energy for adsorption of precursor on the surface by the following definition: (initial reaction probability) = A × exp(−EA/(kBT)) where A, EA, kB, and T are the pre-exponential factor, activation energy, Boltzmann constant, and temperature in kelvin, respectively.30 Therefore, we calculated the changes of energy during adsorption of the Hf precursors by utilizing DFT calculations, as shown in Figure 3. For the calculation, the initial adsorption surface was assumed to be a complex of one Hf atom and four OH functional groups representing an OH-terminated HfO2 surface (Hf-OH*), based on the previous results of the reaction mechanism of HfO2 ALD.31 In a previous study, Hf−[O− Hf(OH)3]3−OH clusters were used to represent the Hf-OH* surface for DFT calculations of the ALD HfO2 reaction.31 In initial state, the HfCl4 molecule is introduced on the Hf-OH* surface. Then, the HfCl4 molecule adsorbs on the Hf-OH* surface through the generation of the bondings between Hf−O and Hf−Cl as shown in Figure 3a. According to previous reports, in the transition state in Figure 3b, the Cl ligand reacts with the hydrogen of the OH species to form HCl.31 The HCl is finally desorbed from the surface with the remainder of the Hf−O−Hf bonding in the adsorption state in Figure 3c. The bondings in the adsorption state of Hf(EtCp)2Cl2 are similar to those of HfCl4. It has been reported in previous studies that the Cl ligands react with the surface OH group and the Cp ligands remain attached on the metal atom because the metal−Cp bonding is more stable than the metal−Cl bonding in the Cpcontaining precursor.32,33 Thus, the Cl reacts with the H atom E


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Figure 4. XPS spectra of 20 nm PE-ALD HfO2 films before annealing: (a) Hf 4f, (b) O 1s, (c) Cl 2p, and (d) C 1s.

6 s for the interface between HfO2 and Si, indicating the formation of an HfO2 film and an interlayer, respectively. Then, the intensity of the O signal gradually decreases over 6 s for Si substrate. Figure 5d shows that the concentration of Cl in the HfO2 films using Hf(EtCp)2Cl2 is lower than that using HfCl4. During an ideal ALD process, precursors react with surface species and the following reactants, so that all Cl ligands are removed from precursors through the formation of a volatile byproduct, such as HCl. In a real case, however, Cl ligands can remain in the HfO2 film due to incomplete reactions with surface species and the following reactants and the readsorption of byproducts on the surface. Although these nonideal cases are quite difficult to predict with the current level of theoretical calculation, statistically, it is expected that the larger number of Cl ligands causes the larger number of residual Cl atoms through the incomplete reactions. In addition, HfCl4 and Hf(EtCp)2Cl2 molecules have a different structure on the surface. HfCl4 molecules form an oligomer structure through the formation of a Hf−Cl−Hf bridge between adjacent molecules on the substrate.20 In contrast, Hf(EtCp)2Cl2 molecules may not form an oligomer, since the Cp groups are known to block the formation of the metal atom−Cl−metal atom bridge.21 Therefore, the possibility of trapping byproduct molecules is much higher in a more complex structure of the HfCl4 oligomer than in that of the Hf(EtCp)2Cl2 monomer, leading to a higher concentration of residual Cl impurities in the films. Therefore, it is thought that the probability of incomplete reactions in the HfCl4 precursor is much larger than that in the Hf(EtCp)2Cl2 precursor. However, the amount of Cl for Hf(EtCp)2Cl2 increases at the interface between HfO2 and

XPS spectra of HfO2, respectively. The XPS spectrum of the Hf 4f core level in Figure 4a shows two main peaks at 18.7 and 16.9 eV corresponding to Hf 4f5/2 and 4f7/2, respectively. In the O 1s core level spectrum shown in Figure 4b, O peaks are observed at 530.5 and 532.3 eV, which correspond to Hf−O and Hf−O−Si bonding of HfO2, respectively, indicating the formation of HfO2 with a small amount of Hf silicate.41 The stoichiometric compositions of Hf to O in the PE-ALD HfO2 films using HfCl4 and Hf(EtCp)2Cl2 are 1:1.88 and 1:1.72, respectively, which is not a significant deference considering the error range of the XPS tool (∼1% of atomic ratio). In Figures 4c and 4d, the Cl and C peaks are negligible, indicating that the HfO2 films using both precursors have high purities with low impurity contamination. Although Cl was not observed in the XPS results, the small amount of residual Cl in the films below the XPS detection level can contribute to a high leakage current after annealing at 1030 °C.12 To trace a small amount of Cl residue in the films, dynamic SIMS analysis, which has a higher detection resolution than XPS, was employed. Because Si was used as the substrate, the SIMS results can be divided into three regions based on the changes of the Si signal in Figure 5a: 0−4 s for the HfO2 film, 4−6 s for the interface between HfO2 and Si, and over 6 s for the Si substrate, along the depth direction. Although Si signals were detected in the HfO2 film region, their intensities are very low and can be considered to be negligible. In Figure 5b, the intensity of Hf is very low because it is electropositive and not ionized very well by Cs+ ions from the gun.42 In contrast to the Si signal, the intensity of the O signal in Figure 5c remains high from 0 to 4 s for the HfO2 film and slightly increases from 4 to F


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Figure 6. (a) Gate leakage currents and (b) capacitance−voltage curves of MOS capacitors using 20 nm PE-ALD HfO2 after annealing at 1000 °C.

such as C and H atoms, are not a main factor in the different electrical properties. Electrical Properties. Gate oxides in gate first process should be stable at high temperatures as high as 1000 °C because the dopant activation process is performed at such temperatures during general CMOS fabrication processes.1 As mentioned earlier, residual Cl atoms segregated at the grain boundaries of ALD HfO2 films can result in void formation and a high leakage current at high temperatures.12 As a result, the electrical properties of ALD HfO2 after annealing at 1000 °C were evaluated by measuring the C−V and I−V characteristics with low error range below 5%, as shown in Figure 6a,b. In Figure 6a, the HfO2 film produced using Hf(EtCp)2Cl2 possessed much better insulating properties of over 2 orders of magnitude than that obtained using HfCl4. The leakage current densities at −1 MV/cm are 2.7 × 10−6 and 2.8 × 10−8 A/cm2 for HfO2 using HfCl4 and Hf(EtCp)2Cl2, respectively. In Figure 6b, the C−V characteristics of the two HfO2 films show typical trends of high k materials, and the dielectric constants extracted from the maximum capacitance values are almost the same for HfO2 obtained using HfCl4 and Hf(EtCp)2Cl2 with values of 19.8 and 19.7, respectively. The other physical properties obtained from the C−V measurements also reveal similar results between the two HfO2 films. As shown in Table 2, the interface state density (Dit) values determined using the conductance method are 1.22 × 1012 and 1.18 × 1012 eV−1 cm−2,46 and the trapped oxide charges (Not) calculated using the midgap charge separation method are −2.71 × 1011 and −1.82 × 1011 cm−2 for HfO2 produced using HfCl4 and Hf(EtCp)2Cl2, respectively.46,47

Figure 5. SIMS profiles of 20 nm PE-ALD HfO2 films before annealing: (a) Si, (b) Hf, (c) O, (d) Cl, (e) C, and (f) H.

Si. The GPC of ALD HfO2 using Hf(EtCp)2Cl2 is smaller than that using HfCl4, so that the time to cover the original Si surface with ALD HfO2 using Hf(EtCp)2Cl2 is longer than that using HfCl4. Therefore, the Hf(EtCp)2Cl2 has a higher probability of direct reaction with the original Si substrate, which can generate an interfacial layer, than the HfCl4 process. In fact, Cl is more stable at the interface between the HfO2 film and Si bonding than that in HfO2 film due to the incomplete bonding configuration.43 In addition, the heat of formation of SiCl4 is −657 kJ/mol, which is much lower than that of HfCl4 (−236.9 kJ/mol).44,45 Therefore, although the amount of residual Cl in the film region of ALD HfO2 using Hf(EtCp)2Cl2 is much smaller than that using HfCl4, the amount of Cl at the interface region shows the opposite tendency. Figures 5e and 5f show the depth profiles for C and H atoms, which are also important impurities as determinant factors for electrical properties. Though Hf(EtCp)2Cl2 contains C atoms in Cp ligands, the amount of C impurities in films was almost the same as that using HfCl4, as shown in Figure 5e. Likewise, significant difference was not observed in the H profiles in Figure 5f. As a result, it was concluded that other impurities, G


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The Journal of Physical Chemistry C Table 2. Electrical Properties of MOS Capacitors Using PE-ALD HfO2 after Annealing at 1000 °C definition unit

leakage currents density at −1 MV/cm (A/cm2)

dielectric constant

interface state density (Dit) (eV−1 cm−2)

trapped oxide charge (Not) (cm−2)

HfCl4 Hf(EtCp)2Cl2

2.7 × 10−6 2.8 × 10−8

19.8 19.7

1.22 × 1012 1.18 × 1012

−2.70 × 1011 −1.82 × 1011

increased where the thickness of films is relatively thin, a large amount of electron emissions at the rough interface results in a high leakage current of HfO2 using HfCl4.51,52

The HfO2 film using Hf(EtCp)2Cl2 showed much better insulating properties than that using HfCl4. The leakage current densities of HfO2 film using Hf(EtCp)2Cl2 are over 2 orders of magnitude smaller than those using HfCl4. Leakage current can be affected by various factors, such as Dit,48 Not,48 and microstructure.49 In our experiment, however, the Dit and Not values of HfO2 after annealing do not show a significant difference, so the Dit and Not values are ruled out from the factors affecting the different leakage current levels.

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CONCLUSIONS In this study, the effects of the precursor on the growth characteristics and electrical properties of PE-ALD HfO2 films were systematically investigated by using two Hf precursors, HfCl4 and Hf(EtCp)2Cl2. The saturated growth rates were 1.48 ± 0.16 Å/cycle for HfCl4 and 0.65 ± 0.11 Å/cycle for Hf(EtCp)2Cl2 with O2 plasma as a coreactant at 180 °C. Two effects on the growth rate difference were considered: the steric demands of ligands and the reaction probability of the precursor on the surface. The geometrical and theoretical DFT calculation results showed that the effect of the precursor the steric demands of ligands was much more dominant than the reaction probability of the precursor on the surface. The SIMS data showed a lower concentration of Cl impurities in the HfO2 films using Hf(EtCp)2Cl2 than in the films obtained using HfCl4 due to the different ligand structures. The leakage current density of HfO2 using Hf(EtCp)2Cl2 was 2 orders of magnitude lower than that using HfCl4. The superior electrical properties of ALD HfO2 using Hf(EtCp)2Cl2 compared to that using HfCl4 can be explained by the higher concentrations of Cl residue in the film region.

Figure 7. AFM images of 20 nm PE-ALD HfO2 films annealed at 1000 °C obtained using (a) HfCl4 and (b) Hf(EtCp)2Cl2.

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The microstructures contain interlayer, crystallinity, and roughness, which are strongly correlated with leakage currents. During the annealing at a high temperature, a hafnium silicate interlayer is easily formed with a decrease of HfO2 thickness, leading to high leakage currents.50 However, despite the much lower dielectric constant of hafnium silicate than HfO2, no degradation of dielectric constant was observed with the increase of annealing temperature from 400 to 1000 °C (see Figure S2). These results show no significant formation of hafnium silicate interlayers in both HfO2 films using HfCl4 and Hf(EtCp)2Cl2 after annealing at 1000 °C. However, polycrystalline HfO2 films using HfCl4 and Hf(EtCp)2Cl2 show a significant difference in their roughness. After annealing at 1000 °C, HfO2 forms a polycrystalline structure with a similar grain size in both the HfCl4 and Hf(EtCp)2Cl2 cases (see Figure S3). Since the grain boundary in polycrystalline films is an imperfect region, compared with grain, it is an easy diffusion path for Cl. The out-diffusion of Cl can cause the formation of a void during high-temperature annealing, leading to an increase of film roughness.12 Figure 7 shows AFM images of the 20 nm thick HfO2 films deposited by HfCl4 and Hf(EtCp)2Cl2 after annealing at 1000 °C. A much rougher surface morphology was observed in HfO2 using HfCl4 compared with that using Hf(EtCp)2Cl2. The root-mean-square (RMS) roughness value of HfO2 using HfCl4 was 0.787 ± 0.045 nm, which is much higher than for that using Hf(EtCp)2Cl2 (0.465 ± 0.030 nm). The difference in roughness can be attributed to the more aggressive void formation from Cl diffusion due to the higher concentration of Cl in the HfO2 films using HfCl4 than that using Hf(EtCp)2Cl2. The rough surface of HfO2 films using HfCl4 forms a rough interface with the Ru top electrode. Since the electric field is

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05286. Figure S1: molecular configuration and energy change during the full reaction of ALD calculated by DFT: initial state, precursor physisorption (a, g), transition state (b, h), final adsorption (c, i), reactant physisorption (d, j), transition state (e, k), and final product (f, l); Figure S2: (a) the gate leakage currents and (b) capacitance− voltage curves of MOS capacitors using 20 nm PE-ALD HfO2 after annealing at 400 °C; Figure S3: XRD results of PE-ALD HfO2 films after 1000 °C annealing (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.K.). *E-mail [email protected] (H.-B.-R.L.). Author Contributions

B.-E.P. and I.-K.O.contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (10041926, Development of high density plasma technologies for thin film deposition of nanoscale semiconductor and flexible display processing) funded by the Ministry of Knowledge Economy (MKE, H


Article


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Korea), Korea Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry and Energy (MOTIE) (Project No. 10050296, Large scale (Over 8“) synthesis and evaluation technology of 2-dimensional chalcogenides for next generation electronic devices), the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF2014R1A2A1A11052588), the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project. (CISS-2011-0031848), Global PH.D Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014021146) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (NRF-2014R1A2A1A11050893).

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