Thermal oxidation mechanism and stress evolution in Ta thin films
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Jae Yong Songa) Division of Industrial Metrology, Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea; and School of Science, University of Science and Technology, Daejeon 305-333, South Korea
Soo-Hwan Jeongb) Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, South Korea
Jeong Won Kim Division of Industrial Metrology, Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea
Tae Geol Lee Division of Convergence Technology, Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea
Ju Hwang Kim and Junhee Hahn Division of Industrial Metrology, Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea (Received 15 December 2009; accepted 23 March 2010)
Oxidation-induced stress evolutions in Ta thin films were investigated using ex situ microstructure analyses and in situ wafer curvature measurements. It was revealed that Ta thin films are oxidized to a crystalline TaO2 layer, which is subsequently oxidized to an amorphous tantalum pentoxide (a-Ta2O5) layer. Initial layered oxidation from Ta to TaO2 phases abruptly induces high compressive stress up to about 3.5 GPa with fast diffusion of oxygen through the Ta layer. Subsequently, it is followed by stress relaxation with the oxidation time, which is related to the slow oxidation from TaO2 to Ta2O5 phases. The initial compressive stress originates from the molar volume expansion during the layered formation of TaO2 from the Ta layer, while the relaxation of the compressive stresses is ascribed to the amorphous character of the a-Ta2O5 layer. According to Kissinger’s analysis of the stress evolution during an isochronic heating process, the oxygen diffusion process through the a-Ta2O5 layer is the rate-controlling stage in the layered oxidation process of forming a a-Ta2O5/TaO2/Ta multilayer and has an activation energy of about 190.8 kJ/mol.
I. INTRODUCTION
Ta thin films have received much attention due to their high melting point (3019 C), good workability, and good corrosion resistance. Tantalum has two different crystal structures at room temperature, a stable bodycentered cubic a-phase and a metastable tetragonal b-phase. The a-Ta thin films with a low resistivity of about 13 mOcm have been used as a diffusion barrier between copper and silicon in microelectronic devices,1,2 while b-Ta films with a higher resistivity (180 mOcm) have been used as thin film resistors and heaters.3,4 Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2010.0157 1080
http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 6, Jun 2010 Downloaded: 14 Mar 2015
The physical and electrical properties of Ta are known to be easily affected by a small amount of impurities such as oxygen, nitrogen, and methane.5–7 For instance, when Ta thin films are annealed under a vacuum of about 2 102 Torr, they are easily oxidized,6 and the incorporation of oxygen into b-Ta films can retard the
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