Comparative study of titanium nitride, tantalum nitride, and tungsten nitride as diffusion barriers between copper and silicon dioxide

As copper metallization in the microelectronics industry continues to evolve, the quest for copper diffusion barriers is still underway. Many new materials and processes are being studied, yet there are currently no clear standards by which these materials can be compared to each other. This research focuses on the use of TiN_x, TaN_x, and WN_x, films with thickness of 5 nm, 10 nm, and 20 nm as a diffusion barrier between copper and SiO₂. WN_x was investigated at two different compositions. The main contributions of this research to the field of diffusion barriers for copper are a detailed comparison of barrier materials, especially with respect to the thickness dependence of barrier reliability, and an in-depth evaluation of three electrical characterization methods for detecting copper diffusion. The experiments involve MOS capacitors subjected to bias temperature stress (BTS) for various periods of time at various temperatures and characterized by capacitance-voltage (C-V), current-voltage (I-V), and triangular voltage sweep (TVS) measurements.; All capacitors were pre-annealed at 350°C prior to any BTS, which caused no initial failures for WN_x barriers at all thicknesses. A few initial failures were seen for TaN_x barriers, while a high number of initial failures was observed for TiN_x barriers.; The non-failure data were evaluated using Weibull analysis to extract barrier lifetimes. Lifetimes were determined for each barrier type and thickness using the survival function, S(t) = exp(−t/C) m, where C is the characteristic life at which 63% of the samples fail and m is the shape parameter. TiNx barriers had much lower lifetimes than the other three barriers regardless of thickness. Lifetimes decrease rapidly with increasing temperature for all barriers. On the other hand, the effect of thickness on lifetime is inconsistent. TaN_x barriers have lifetimes consistently increasing with thickness, whereas 5 and 10 nm WN_x barriers have quite similar lifetimes.; Triangular Voltage Sweep (TVS) measurements were shown to be the most sensitive method for the detection of mobile charges in the oxide in the sense that TVS indicates charge motion, before a sample is considered to have failed (J > 10−6 A/cm2). Larger peaks were observed with increasing bias time and temperature. Mobile charge concentrations as low as 1010/cm2 in the oxide were detected.