Thermal Decomposition of Tungsten Nitrido Precursors for Low Temperature MOCVD of WNxCy
Authors: Kim Seo Young , Koley Arijit , Bonsu Richard , Nolan Michelle , McElwee-White Lisa , Anderson Tim 
Published: February 2020

As the feature size of electronic devices continues to shrink, the integrated circuit device density increases. This not only modifies the RC time delay to severely limit the device performance, but also places increased demand on Cu metallization effectiveness (1). The extremely high diffusivity of Cu in Si and its ‘killer’ effect on devices has led to the requirement of a barrier layer to prevent Cu transport into the underlying Si. Among various candidates, W-based nitrides have shown promise as replacements of current barrier layers due to their good thermal stability, low resistivity, and the possibility of one-step CMP with Cu slurry (2, 3). In addition, a single barrier layer process is simpler than the Ta-based barrier layer, which needs a double layer of TaN and Ta. The performance of W-based nitrides increases when they are alloyed with carbide to make WNxCy due to its higher crystallization temperature and possibility to adjust film resistivity to lower values. The tungsten nitrido complex WN(NMe2)3 was demonstrated to serve as a single-source precursor for metal organic chemical vapor deposition (MOCVD) of WNxCy films at temperatures as low as 125 ˚C (4). The goal of this study is to understand decomposition mechanisms and their kinetics for the WN(NMe2)3 precursor, and furthermore, to optimize conditions for deposition of the WNxCy film. To elucidate the mechanism and kinetics of thermal decomposition of WN(NMe2)3, in-situ Raman spectroscopy was used to quantitatively measure the disappearance of precursor and appearance of products. An up-flow, cold-wall MOCVD reactor was designed to perform Raman scattering experiments along the centerline of the reactor (5).
The system design allows the CVD reactor to translate along three-dimensionally, to permit the laser to focus on any spot in the reactor. In-situ Raman experiments were performed using a heater set point temperature of 650 ˚C. Axial centerline temperature profiles in the reactor are measured by analysis of the rotational bands of the diatomic N2 carrier gas. Due to the low volatility of the precursor, it was dissolved in liquid pyridine and delivered to the reactor as an aerosol with N2 carrier gas (6). The 532 nm line of a Nd:YVO4 solid-state laser at 1.5 W was used as the light source to excite the molecules in the sample. Raman spectra were measured at various distances below the heated susceptor surface and thus various temperatures from 159 ˚C to 474 ˚C.
Density Functional Theory (DFT) calculations (using the B3LYP functional and LanL2DZ basis sets) were performed to assess possible pathways of thermal decomposition of WN(NMe2)3 and to assign observed Raman peaks to the decomposition products. The results of this study are consistent with proposed mechanistic steps for the thermal decomposition of the precursor, including dimerization of the precursor and loss of the amine. In addition to gas phase data, liquid phase Raman experiments were performed to better define additional species that are Raman active but not detected in the lower density gas phase experiments and to find vibrational frequencies before the molecule is decomposed.

References

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Figure 1