Effect of oxygen on the reactions in the Si/Ta/Cu metallization system
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Jyriki Molarius and Ilkka Suni VTT Microelectronics, FIN-02044 VTT, Espoo, Finland (Received 12 February 2001; accepted 26 July 2001)
The effect of oxygen on the reaction mechanisms in the Si/Ta/Cu metallization system was studied experimentally and by utilizing the thermodynamically assessed Ta–O binary system. It was presented that an interfacial tantalum oxide was formed between Cu and Ta and that it established an additional barrier layer for Cu diffusion. The formation of additional barrier layer was supported by the following observations: (i) No detectable amount of Cu was found from the Ta layer with the combined transmission electron microscopy and energy dispersive spectroscopy at temperatures as high as 650 °C. (ii) Secondary ion mass spectrometry measurements indicated that significant amount of oxygen was incorporated into the films already after the sputtering stage. (iii) 181Ta16O molecular ion signals were detected from the Ta/Cu interface, indicating that the additional layer was in fact some form of tantalum oxide. The diffusion of Cu through the Ta layer could not proceed until the interfacial oxide had been dissolved by the Ta matrix. Since the oxygen solubility in Ta matrix is high in the temperature range of interest, the interfacial oxide dissolution was kinetically controlled. It is to be noted that the threshold temperature range of the dissolution reaction was found to coincide with that of the –Ta to the bcc-Ta transition, which was anticipated to enhance the kinetics of the dissolution.
I. INTRODUCTION
Cu interconnections have been attracting more attention since the introduction of the first functional microprocessors fabricated with this technology.1 Although Cu offers many advantages in comparison to Al-based materials, it requires a capping layer to prevent diffusion of Cu into Si and into dielectric materials used. Tantalum and its compounds are among the most widely investigated diffusion barriers for Cu metallization.2–10 The effect of oxygen on the stability of Ta diffusion barrier layers has been previously shown to be substantial.9 However, the reason for that is not yet clear. It has been argued, for example, that oxygen is present at the grain boundaries and is blocking there the fast grain boundary diffusion of Cu. This is not probably the correct explanation, since the solubility of oxygen in Ta is large in the temperature range of interest, which does not favor strong segregation of oxygen into grain boundaries of Ta.11 On the other hand, on the basis of the thermodynamics as well as of the results from the literature (e.g., Ref. 12), the formation of an amorphous TaOx layer at the Ta/Cu interface is more likely.10 The prediction is supported by the experimental observation that no J. Mater. Res., Vol. 16, No. 10, Oct 2001
extensive diffusion of Cu into Ta layer was detected with the energy dispersive spectroscopy (EDS) up to 650 °C. Furthermore, the abrupt nature of the reaction, after only a relatively small temperature increase (35 °C up from 650 to 685°C), suggest
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