Study of Ta as a Diffusion Barrier in Cu/SiO 2 Structure
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Study of Ta as a Diffusion Barrier in Cu/SiO2 Structure J. S. Pan1, A. T. S. Wee2, C. H. A. Huan2, J. W. Chai1, J. H. Zhang3 Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602 2 Department of Physics, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 3 School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798
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Abstract Tantalum (Ta) thin films of 35 nm thickness were investigated as diffusion barriers as well as adhesion-promoting layers between Cu and SiO2 using X-ray diffractometry (XRD), Scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). After annealing at 600°C for 1h in vacuum, no evidence of interdiffusion was observed. However, XPS depth profiling indicates that elemental Si appears at the Ta/SiO2 interface after annealing. In-situ XPS studies show that the Ta/SiO2 interface was stable until 500oC, but about 32% of the interfacial SiO2 was reduced to elemental Si at 600oC. Upon cooling to room temperature, some elemental Si recombined to form SiO2 again, leaving only 6.5% elemental Si. Comparative studies on the interface chemical states of Cu/SiO2 and Ta/SiO2 indicate that the stability of the Cu/Ta/SiO2/Si system may be ascribed to the strong bonding of Ta and SiO2, due to the reduction of SiO2 through Ta oxide formation. 1. Introduction Copper is an attractive material for ULSI metallization due to its lower resistivity (1.7 µΩcm) and higher electromigration resistance compared to Al and Al-based alloys [1-4]. However, the implementation of Cu metallization requires the use of a barrier layer since Cu diffuses easily into Si and SiO2 even at quite modest temperatures, which leads to degradation of device reliability. Also, copper lacks the ability to adhere to SiO2 and most insulating substrates [4]. This is due to the inability of copper to reduce SiO2, which results in a purely mechanical bond at the interface. Thus thin film layers should be used between Cu and SiO2 to improve the adhesion of Cu to the SiO2 layer [5-7]. The characteristics of the adhesion promoter must have good adhesion with copper and the oxide layer. This means that it should have some limited interaction with both of these layers. A good diffusion barrier should have minimal interaction with copper so that it does not affect the resistance of the copper interconnect. Another problem with metallization is the stress that is developed in the thin metal lines during annealing. Excessive stresses can lead to failure of interconnect lines. Bilayer metallization is especially susceptible to the stress problem because any interaction between the layers tends to alter the stress state of the metallization as a whole. There has been considerable effort to identify a suitable diffusion barrier for Cu metallization. Ta and Ta(N) materials are among the most promising barrier materials for Cu metallization [8-13], since Ta has relatively high melting (2996 oC) and silic
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