ALD and PVD Tantalum Nitride Barrier Resistivity and Their Significance in via Resistance Trends

As dual damascene interconnects are scaled down to sub-30nm critical dimension (CD), the patterning processes required are highly complex, and metallization processes, beyond providing void-free Cu fill, need to address the resistance challenges encountered [1]. In terms of scaled barriers, suitable PVD Ta(N) and ALD TaN processes need to be identified for acceptable barrier performance as well as for their impact on interconnect resistance. For example, enhancing nitrogen content in PVD TaN may enable Cu barrier thickness scaling without a reduction in its barrier properties.

In order to evaluate the impact of nitrogen content in TaN on resistivity, various PVD TaN processes were developed based on Ar/N₂ DC sputtering from a Ta target. While maintaining a fixed Ar flow rate, both DC sputtering power and N₂ flow rate were varied to allow deposition of TaN films with a particular nitrogen content. Four-point probe sheet resistance measurements were performed to determine TaN resistivity, while the composition of TaN was determined by Rutherford back-scattering spectrometry (RBS). For comparisons to PVD TaN, PEALD TaN processes, based on the reduction of pentakis(dimethylamino)tantalum (PDMAT) with either a H₂ plasma or an NH₃ plasma, were utilized in this study.

Limiting the N₂ flow rate while sputtering TaN is one of the most commonly adopted strategies to avoid N contamination of PVD Ta, resulting in typical PVD TaN resistivity values of approximately 200-250 μΩcm. When the N₂ flow rate during PVD of TaN is significantly increased, a stark trend in TaN resistivity is observed [2]. For example, under fixed Ar flow rate and DC sputtering conditions at 15kW, TaN resistivity is observed to change from ~270 μΩcm to more than 1600 μΩcm as the N₂ flow rate is increased from 40 to 50 sccm (Figure 1). For comparison, typical resistivity values of ALD TaN are on the order of ~5000 μΩcm or higher, although high-power plasma reduction or ion bombardment steps can reduce ALD TaN resistivity to ~500 μΩcm [3].

Sputtering at a reduced TaN deposition rate (e.g., by applying low DC power) is expected to allow more nitrogen incorporation into TaN.  Composition analysis of the PVD TaN films studied indeed confirms that, at fixed N₂ flow rate, 15kW DC TaN with a resistivity of ~240 μΩcm contains only ~26 at.% N, while 6kW DC TaN with a resistivity of ~550 μΩcm contains ~48 at.% N (Figure 2). Typical N content in ALD TaN based on PDMAT is ~50 at. % or higher, which is likely related to the Ta-N bond structure in the precursor molecule.

Both ALD TaN and PVD TaN barrier processes were compared in terms of their impact on Cu dual damascene via resistance. A test structure with 40nm-wide vias was constructed for this purpose. For an ALD TaN barrier at 3nm thickness, the via resistance measured was ~28% lower than its PVD counterpart (Figure 3). Given the blanket ALD TaN resistivity (~5000 μΩcm) and PVD TaN resistivity (~240 μΩcm) values for these barrier processes, it is apparent that other factors besides blanket TaN resistivity play a role in the total via resistance observed. It is likely that TaN nucleation differences, interface cleanliness, via test structure geometry, a.o. also need to be taken into account.

This work was performed by the Research and Development Alliance Teams at various IBM Research and Development Facilities.

REFERENCES

1. S.T. Chen, T.-S. Kim, S. Nam, N. Lafferty, C.-S. Koay, N. Saulnier, H. Shobha, W. Wang, Y. Xu, B. Duclaux, Y. Mignot, M. Beard, Y. Yin, H. Shobha, O. Van der Straten, M. He, J. Kelly, M. Colburn, T. Spooner, Proc. IEEE International Interconnect Technology Conference (IITC) 2013, pp. 1-3 (2013).
2. S.-M. Na, I.-S. Park, S.-Y. Park, G.-H. Jeong, S.-J. Suh, Thin Solid Films 516, 5465 (2008).
3. P. Ma, J. Lu, J. Aubuchon, T.-J. Gung, M. Chang, ECS Trans. 33 (2), 169 (2010).