Phase formation behavior and diffusion barrier property of reactively sputtered tantalum-based thin films used in semiconductor metallization

Tantalum (Ta) and nitrogen-contained tantalum (Ta-N) thin films an sputter deposited at different argon/nitrogen flow ratios onto (001) silicon-based substrates with and without a titanium adhesion layer. The impact of varying the nitrogen flow rate and the underlying titanium on the phase formation process is also investigated using X-ray diffractometry, resistivity measurement and scanning electron microscopy. In contrast to previous works on bare silicon and thermally oxidized silicon wafers, our results indicate that a thin titanium adhesion layer inhibits the formation of high-resistivity (200 mu Ohm cm) tetragonal Ta over a wide range of realistic deposition conditions. The titanium layer leads to the deposition of a low-resistivity (29 mu Ohm cm) body-centered cubic alpha-Ta arising from its epitaxial orientation on the underlying titanium. The thresholds of nitrogen flow rates for depositing nitrogen-saturated alpha-Ta, amorphous Ta2N (a-Ta2N) and stoichiometric NaCl-type TaN on silicon are 0.25, 1.0 and 2.0 seem, respectively. However, the underlying titanium can increase the thresholds fur forming nitrogen-saturated alpha-Ta, a-Ta2N and stoichiometric TaN to 1.0, 1.5 and 2.5 seem, respectively. Consequently, the electrical properties and microstructures for Ta and Ta-N thin films on Ti are significantly changed. Moreover, the barrier properties of 40-nm-thick stoichiometric a-Ta2N (Ta67N33) and nitrogen over-saturated a-Ta2N thin films are evaluated. According to X-ray diffraction analyses and sheet resistance measurements, all of the a-Ta2N barrier layers degrade in a similar manner, triggered mainly by an entire crystallization of the amorphous barrier layers. This is followed by a phase transformation process, sequentially forming Cu3Si and TaSi2. Cross-sectional transmission electron microscopy reveals that copper can penetrate through the crystallized films either alone grain boundaries or thermal-induced crevices to react with silicon, subsequently forming Cu3Si precipitates. As adequately doping nitrogen into stoichiometric a-Ta2N can dramatically increase the crystallization temperature by approximately 150 degrees C, the effectiveness of the nitrogen over-eloped a-Ta2N barrier layers can be greatly improved, subsequently elevating the degrading temperature by at least 100 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.