Theoretical and Experimental Investigation of Thermal Stability of HfO 2 /Si and HfO 2 /SiO 2 Interfaces
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Theoretical and Experimental Investigation of Thermal Stability of HfO2/Si and HfO2/SiO2 Interfaces Chun-Li Liu and Matt Stoker, Rama I. Hegde, Raghaw S. Rai, and Philip J. Tobin Advanced Process Development and External Research Laboratory, Motorola, Mesa, AZ 85202
ABSTRACT The assessment of the thermal stability across HfO2/Si and HfO2/SiO2 interfaces has been difficult due to lack of thermodynamic data. In this paper, we present the results of thermodynamic calculations intended to fill this gap. A thermodynamic model was developed by assuming that HfSiO4 is an ideal solution of HfO2 and SiO2 to a first order approximation. The theoretical results predict that the HfO2/Si interface is thermodynamically stable up to 1100 °C, while the HfO2/SiO2 interface is thermodynamically unstable even at room temperature. Our experimental results from TEM and XPS analysis are consistent with these modeling predictions. The thermal stability of HfO2/Si and HfO2/SiO2 interfaces is a critical aspect of the reliability of integrated gate stacks using HfO2 as a potential candidate for replacement of SiO2 for future generations of CMOS (complimentary metal oxide semiconductor) devices. However, the assessment of the thermal stability across these interfaces has been difficult due to lack of thermodynamic data [1, 2] for the following possible chemical reactions: Si + HfO2 2Si + HfO2 2Si + 2HfO2 Si + 2HfO2 SiO2 + HfO2
Hf + SiO HfSi + SiO HfSi + HfSiO Hf + HfSiO HfSiO 2
2
4
4
4
(reduction of HfO2) (formation of Hf monosilicides) (formation of Hf monosilicides and silicates) (formation of silicates) (formation of silicates)
(1) (2) (3) (4) (5)
Thermodynamic data for HfSiO4 do not exist in literature and there is also lack of data for HfSix. Many attempts were made in the past to estimate the thermodynamic data for silicates [3-7]. Thus, it has been almost impossible to calculate the Gibbs free energy changes for these reactions without the above thermodynamic data, which are the most important criteria to assess the likelihood that the reactions will proceed. In order to calculate the Gibbs free energy changes at finite temperatures for a given material, three sets of data are needed, i.e., the heat capacity as a function of temperature, Cp(T), standard enthalpy, ∆H°, and standard entropy, ∆S°. Then, one can use the following standard thermodynamic relationships to calculate the Gibbs free energy change at finite temperatures: ∆H (T) =∆H˚ + ∫ Cp dT , ∆S (T) =∆S˚ + ∫ Cp / T dT , and ∆G (T) = ∆H° (T) -T∆S° (T). We assume that HfSiO4 can be considered to a first order approximation as an ideal solution of HfO2 and SiO2 and thus the needed thermodynamic data for HfSiO4 can be obtained in the following way since the data for HfO2 and SiO2 are readily available: Cp, HfSiO4 = Cp, HfO2 + Cp,SiO2 ∆HHfSiO4 = ∆H˚HfO2 + ∆H˚SiO2+ ∆Hmix ∆SHfSiO4 = ∆S˚HfO2 + ∆S˚SiO2+ ∆Smix where the mixing enthalpy ∆Hmix = 0 and the mixing entropy ∆Smix= - R (XAlnXA + XBlnXB) where XA =XB=0.5 in this case. Such an assumption is supported by the ato
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