Analysis of the composition of tantalum nitride in CMOS metallization trenches using parallel angle‐resolved XPS

We demonstrate the utilization of parallel angle‐resolved X‐ray photoelectron spectroscopy (pAR‐XPS) as a useful tool to analyze ultrathin sputtered tantalum nitride (TaN) thin films in complementary metal‐oxide‐semiconductor (CMOS) trenches. The chemical composition of TaN was estimated by pAR‐XPS using a Theta 300i from Thermo Fischer. An improved lateral resolution was achieved by analyzing periodic structures. The XPS data was correlated with transmission electron microscopy (TEM) in combination with energy‐dispersive X‐ray spectroscopy (EDX) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) data. The results show that the nitrogen content in the TaN thin films was about 6% higher at the sidewall compared with the top and bottom of the trench.


| INTRODUCTION
The progress in Moore's law and the need for higher performance in nanoelectronic technologies resulted in the reduction of feature sizes in the back end of line (BEoL) of microchips. This shrinkage of interconnect structures is accompanied by an increase in the resistancecapacitance (RC) delay, which reduces the performance of the entire device. Hence, with advancing technology nodes, research is aimed toward improving BEoL materials and their properties.
This work focuses on the characterization of barrier layers in metallization trenches. Current industry standard uses a tantalum nitride (TaN)/tantalum (Ta) bilayer as barrier layer. The layer is typically deposited by physical vapor deposition (PVD), which can result in a poor sidewall coverage and elemental composition differences over the trench. 1,2 The nitrogen content in TaN thin films affects the diffusion barrier performance and the sheet resistance of the thin film, which has a noticeable impact on the RC delay. 3,4 Furthermore, it influences the crystallinity of the Ta layer deposited on top of it. Several publications describe a change from high resistive tetragonal β-Ta to low resistive body-centered cubic α-Ta with increasing nitrogen content in the TaN interlayer. [5][6][7] For future technology nodes, it is therefore important to develop analysis methods that can be used to quantify current and future materials in small structures. In this paper, we show that parallel angle-resolved X-ray photoelectron spectroscopy (pAR-XPS) as a nondestructive analyzing method is suitable to analyze the sidewall of narrow CMOS trenches. Already in 1990, Oehrlein et al. used

| RESULTS AND DISCUSSION
All acquired spectra were quantified, and the ratio of nitrogen to tantalum was calculated for each angle and plotted in Figure 4. It can be seen that with decreasing angle, the N/Ta ratio increases. By applying the aforementioned model, it has been derived that lower angles represent the XPS signals from the sidewall and the top of the trench, whereas with increasing angle, the influence of the sidewall on the overall signal decreases. Keeping that in mind, from the data trend in Figure 4, it can be concluded that the sidewall of 50 nm and 100 nm trenches has a higher nitrogen content than the top of the trench. In comparison with EDX where the measurement data can only be acquired from certain points, ToF-SIMS is able to provide the analysis from the entire trenches (top-down analysis) and/or crosssectional analysis. 10 The sputter area of the ToF-SIMS analysis was 300 × 300 μm and the acquisition spot was 100 × 100 μm to avoid effects caused by the sidewalls of the sputtered area. The resulting profile is shown in Figure 7.

| CONCLUSION AND OUTLOOK
In this study, we were able to show that the composition of TaN in CMOS trenches can be analyzed by pAR-XPS. XPS has the advantage to be a nondestructive analysis method, where no sample preparation is necessary. The data were verified by EDX and ToF-SIMS.
For future work, the applied model should be improved regarding the depth of the detected electrons depending on the analysis angle.
This work shows that our used model is sufficient to estimate the composition, as it has been verified with other analysis methods. In order to get more accurate results and have a model that can be used for other trench sizes as well, further improvements are required.
Furthermore, this analysis method has its limitation due to shadowing effects in trenches smaller than 50 nm. Some electrons from the sidewall might not be detected when reducing the dimensions.
Our future work will focus on correlating the composition results of TaN with XRD data of the overlaying Ta thin film in trenches and resistivity measurements. Also, the chemical composition of other barrier materials can be evaluated. This opens up opportunities for further improvement of barrier materials regarding their resistivity and will lead to the development of interconnects of future technology nodes.