Metal-organic chemical vapor deposition of NbxTa(1−x)NyOmCn films as diffusion barriers for Cu metallization
Introduction
Copper has drawn much attention as an interconnect material for deep sub-micron circuits due to its low resistivity, high electromigration and superior stress migration resistance as compared to Al and Al alloy based interconnects. However, to incorporate copper into interconnection structures, it is necessary to introduce a highly reliable diffusion barrier to prevent Cu diffusion into the silicon substrate and thus prevent formation of deep level traps. In addition, Cu has poor adhesion to typical dielectric materials and requires a base layer that acts as an adhesion promoter as well as a diffusion barrier.
As the feature size of devices continues to decrease below 350 nm, the associated increase in aspect ratio for smaller contacts and via holes adversely affects the barrier performance of diffusion barriers. Reactive-sputtering processes are reaching their limit of usefulness, because of shadowing effect which causes poor step coverage. Recently, metal-organic chemical vapor deposition (MOCVD) of barrier films has been developed to provide better step coverage, good uniformity, low temperature and selective processing for barrier layer deposition [1], [2], [3], [4], [5].
Current research on diffusion barriers for Cu metallization has been directed to the improvement of the barrier property of thin film using the extra process steps such as stuffing and development of the new barrier materials, especially amorphous structures thin films [6], [7], [8]. Amorphous films have been reported as more effective diffusion barriers than polycrystalline films due to the absence of grain boundary, which can act as a fast diffusion path for Cu. The films reported to date have focused on (Ta, W, Mo, Ti)–Si–N systems [9], [10], [11], [12], [13].
TaN films were generally found to be of polycrystalline structure. On the other hand, low temperature chemical vapor deposition (CVD) process for amorphous-like NbN barriers have been reported [14], [15]. An attempt to prepare amorphous NbxTa(1−x)Ny films was therefore made. The process involves the reaction of ammonia gas with the vapor of niobium dialkylamide compounds such as niobium (V) dimethylamide, Nb(NMe2)5, niobium (IV) diethylamide, or Nb(NEt2)4. In the present work, low temperature CVD deposited NbxTa(1−x)NyOmCn barrier film prepared with a mixture of metallic-organic sources: tetrakis-diethylamido-niobium (TDEAN) and pentakis-diethylamido-tantalum (PDEAT) have been investigated.
Section snippets
Experimental
The starting materials were 100 mm, (1 0 0)-oriented p-type single crystal silicon wafers with a resistivity of 25 Ω cm. Prior to the deposition, the substrate was first cleaned by the standard RCA method in order to remove the native oxide layer on the silicon surface. Then, a 120-nm-thick SiO2 was thermally grown on the Si substrate. All samples were in situ cleaned under Ar plasma atmosphere by controlling the radio frequency power density in the CVD chamber. The etching was carried out with a
Results and discussion
Fig. 1 shows an Arrhenius plot of the deposition rate of NbxTa(1−x)NyOmCn films. The deposition rate was found to increase with the deposition temperature in the temperature range of 500–570 °C. The steep rise in rate at these temperatures is attributed to a surface reaction-limiting growth with an activation energy of 79.1±4.8 kJ/mol. A sharp drop in deposition rate appeared at 600 °C. In addition, powdery particles were found to form on the wafer and near the shower head. It implies that
Conclusions
NbxTa(1−x)NyOmCn diffusion barriers were deposited at 375 °C with two MO sources: TDEAN and PDEAN. The concentration of carbon in diffusion barrier films can be further reduced by the addition of NH3 gas into the MO source gas. The deposition rate of MOCVD NbxTa(1−x)NyOmCn films was also increased with NH3 flow rate (for <15 sccm). This led to the reduction in resistivity and deposition temperature of MOCVD NbxTa(1−x)NyOmCn films. Finally, the NH3 plasma post-treatment was implemented to reduce
Acknowledgements
The authors would like to thank Nanmat Co. (Taiwan) for providing MO source. The work was supported by the Republic of China National Science Council through a Grant No. NSC89-2218-E-007-070.
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