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

Abstract

Tantalum (Ta) and nitrogen-contained tantalum (Ta–N) thin films are 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 μΩ cm) tetragonal Ta over a wide range of realistic deposition conditions. The titanium layer leads to the deposition of a low-resistivity (29 μΩ cm) body-centered cubic α-Ta arising from its epitaxial orientation on the underlying titanium. The thresholds of nitrogen flow rates for depositing nitrogen-saturated α-Ta, amorphous Ta 2N ( a-Ta 2N) and stoichiometric NaCl-type TaN on silicon are 0.25, 1.0 and 2.0 sccm, respectively. However, the underlying titanium can increase the thresholds for forming nitrogen-saturated α-Ta, a-Ta 2N and stoichiometric TaN to 1.0, 1.5 and 2.5 sccm, 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-Ta 2N (Ta 67N 33) and nitrogen over-saturated a-Ta 2N thin films are evaluated. According to X-ray diffraction analyses and sheet resistance measurements, all of the a-Ta 2N 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 Cu 3Si and TaSi 2. Cross-sectional transmission electron microscopy reveals that copper can penetrate through the crystallized films either along grain boundaries or thermal-induced crevices to react with silicon, subsequently forming Cu 3Si precipitates. As adequately doping nitrogen into stoichiometric a-Ta 2N can dramatically increase the crystallization temperature by approximately 150°C, the effectiveness of the nitrogen over-doped a-Ta 2N barrier layers can be greatly improved, subsequently elevating the degrading temperature by at least 100°C.