Phase stability of hafnium oxide and zirconium oxide on silicon substrate

Phase stabilities of Hf-Si-O and Zr-Si-O have been studied with first-principles and thermodynamic modeling. From the obtained thermodynamic descriptions, phase diagrams pertinent to thin film processing were calculated. We found that the relative stability of the metal silicates with respect to their binary oxides plays a critical role in silicide formation. It was observed that both the HfO2/Si and ZrO2/Si interfaces are stable in a wide temperature range and silicide may form at low temperatures, partially at the HfO2/Si interface. c © 2008 Elsevier Ltd. All rights reserved. thin films; Silicides; thermodynamics; CALPHAD; first-principle electron theory The thickness of SiO2 as a gate oxide material in advanced complementary metal oxide semiconductor (CMOS) integrated circuits has continuously decreased and reached the current processing limits[1]. Alternative materials with higher dielectric constants, such as HfO2 and ZrO2, are considered as candidates to replace SiO2 for further improvement of their performance[2]. However, during the thin film deposition or the subsequent rapid thermal annealing, oxides, silicates, and silicides may form at the interface since most high-k materials are metal oxides[3, 4]. Among those interfacial phases, silicides are detrimental to capacitor performance due to their metallic behavior[5]. In this regard, thermodynamic stability calculations and experimental results have shown that the interface between HfO2 and Si is found to be stable with respect to the formation of silicides[4]. On the other hand, the ZrO2/Si interface was found to be unstable around 1000K, which is in contradiction to the calculation by Hubbard and Schlom [2]. It was also observed that the Hf-silicide forms upon decomposition of HfO2 in low oxygen partial pressures [5, 6, 7, 8] and HfSiO4 suppresses Hf-silicide formation[9]. Although the phase stabilities in the Hf-Si-O and Zr-SiO systems are important, comprehensive thermodynamic explanations are not yet available. In this paper, based on the recently developed thermodynamic descriptions of the Hf-Si-O[10] and Zr-Si-O systems with first-principles calculations and thermodynamic CALculation of PHAse Dia∗ Corresponding author. Corresponding author. Email address: [email protected] (Dongwon Shin). grams (CALPHAD)modeling[11], various phase diagrams pertinent to thin film processing are investigated. In the CALPHAD approach, the Gibbs energies of individual phases in a system are evaluated from the existing experimental data with the so-called sublattice model based on the crystal structures. The Gibbs energies of a higher-order system can be readily extrapolated from the lower-order systems, and any new phases of the higherorder system can be introduced. However, it is not always possible to have enough experimental data for thermodynamic modeling of a system[2] so that theoretical calculations, such as first-principles calculation results, can be used as supplementary experimental data. The Hf-Si-O system was recently modeled with first-principles calculations and the CALPHAD approach[10]. The formation enthalpy for HfSiO4 is calculated from first-principles calculations since no experimental measurement is reported. The reference states of the formation enthalpy for HfSiO4 are derived from the two binary metal oxides as shown in Eqn. 1, where E represents the total energy of each phase. The formation entropy of HfSiO4 was evaluated from the temperature of peritectic reaction, HfO2 + Liquid → HfSiO4, in the HfO2-SiO2 pseudo-binary system. The thermodynamic description of the Zr-Si-O system was obtained by combining the previous modelings[12, 13, 14] and first-principles calculation of ZrSiO4 in the present work. ∆H4 f = E(HfSiO4)− 1 2 E(HfO2)− 1 2 E(SiO2) (1) The highly efficient Vienna Ab initio Simulation Package (VASP)[15] was used to perform the density functional theory (DFT) electronic structure calculations. The projector 0304-8853/08/$ see frontmatter c © 2008 Elsevier Ltd. All rights reserved. Dongwon Shin & Zi-Kui Liu / Scripta Materialia 0 (2008) 1–0 2 augmented wave (PAW) method[16] was chosen, and the generalized gradient approximation (GGA)[17] was used to take into account exchange and correlation contributions. An energy cutoff was constantly set as 500 eV for all the structures, and the Monkhorst-Pack scheme was used for the Brillouin-zone integrations. For the k-point sampling, authors aimed all the structures to have the k-point meshes as close as (# of atoms in a structure) × kx × ky × kz ≃ 5000 k-points. Thus, HfSiO4 and ZrSiO4, for example, have 8 × 8 × 8 k-point meshes. The calculated results of metal oxides and silicates are listed in Table 1. From the constructed thermodynamic databases of the Hf-Si-O and Zr-Si-O systems, the isopleths of HfO2-Si and ZrO2-Si are calculated in order to investigate the stability range of silicides at the metal oxides/silicon interface and are given in Figure 1. Calculated results show that HfSi2 is stable up to 544K based on the formation enthalpy of HfSiO4 from first-principles calculations. It is generally accepted that the uncertainty of the formation enthalpy of intermetallic compounds, which originates from the density functional theory itself, is about ±1 kJ/mol-atom[18, 19]. Thus the associated decomposition temperature of HfSi2 at the HfO2/Si interface ranges from 382 to 670K when the formation enthalpy of HfSiO4 is adjusted within its uncertainty range from –0.769 to –2.769 kJ/mol-atom. The formation entropy of HfSiO4 with respect to the binary oxides was evaluated correspondingly to reproduce its peritectic reaction at 2023K. It should be noted that the phase stability range of HfSi2 in the HfO2-Si isopleth is not directly correlated with the first-principles calculation of HfSiO4, but predicted from the Gibbs energies of other phases, including the HfSiO4 phase, in the Hf-Si-O system. Even with the uncertainty of formation enthalpy for HfSiO4, the temperature range for the HfO2 and Si coexistent phase region in the isopleth is fairly wide from 670 to 1700K. For ZrSiO4, besides the uncertainty of formation enthalpy from first-principles in the present work, the peritectic reaction (ZrO2 + Liquid → ZrSiO4) temperature in the ZrO2-SiO2 pseudo-binary is also uncertain from 1910 to 1949K. Thus, the formation entropy of ZrSiO4 varies accordingly. The Gibbs energy of ZrSiO4 at 1000K evaluated from the formation enthalpy derived from first-principles and formation entropy evaluated from the peritectic temperature of 1949K (listed in Table 1) is almost identical to the value used by Hubbard and Schlom [2]. With these formation enthalpy and entropy values of ZrSiO4, ZrSi2 is completely suppressed by ZrSiO4 and does not show up in the ZrO2-Si isopleth. To make ZrSi2 appear in the ZrO2-Si isopleth, the formation enthalpy of ZrSiO4 should be more negative than the first-principles calculation result within the uncertainty of formation enthalpy and peritectic temperature for ZrSiO4. When formation enthalpy of ZrSiO4 with respect to the binary metal oxides is set to its lowest limit from the uncertainty of first-principles calculations, ∆H4 f = −3.358 kJ/mol-atom, and entropy of formation is evaluated as ∆S4 f = 0.788 J/mol-atom·K, ZrSi2 is stable up to 879K in the ZrO2-Si isopleth. Then formation enthalpy of ZrSiO4 is ∆H ZrSiO4 f = −338.568 kJ/molatom with respect to SER (Standard Element Reference) and this agrees well with the experimental measurement, –339.033 kJ/mol-atom from Ellison and Navrotsky [20]. Consequently, the safe temperature range for ZrO2 to be stablewith Si is between 879 and 1630K, narrower than that of HfO2 and Si. However, even with these uncertainties, both metal oxides are stable with Si approximately above 900K as summarized by Hubbard and Schlom [2](1000K). T em pe ra tu re ( K ) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Atomic Fraction of Silicon 0 50