Ru-Ta and Ru thin films ͑ϳ15 nm in thickness͒ as diffusion barrier for Cu metallization on SiO 2 /Si and Si substrates are studied. Experimental results show that the Ru-Ta film exhibits amorphous structure until annealing at 700°C, whereas the Ru film crystallizes in as-deposited and annealed states. Sheet resistances of Cu/Ru and Cu/Ru-Ta stacking layers increase after annealing at 500 and 700°C, respectively, regardless of whether the substrate is SiO 2 /Si or Si. Increase in resistance is concurrent with the formation of Ru 2 Si 3 and Cu 3 Si when Cu/barrier stacks are deposited on Si substrate. Increase in resistance for Cu/barrier stacks deposited on SiO 2 /Si substrate is related to the diffusion of Cu through the crystallized barrier to the underlayers. Furthermore, current-voltage measurement also reveals that the Cu/Ru-Ta metallization has a lower leakage current than the Cu/Ru system. The failure mechanism and the effectiveness of Ta addition on barrier performance are discussed.Copper ͑Cu͒ interconnection has been widely used in ultralargescale integration devices because of its favorable electrical conductivity ͑1.67 ⍀ cm͒ and superior resistance to electromigration. 1 However, the major drawback of Cu interconnection is that Cu is a fast diffusion species in Si 2 as well as SiO 2 . 3 It is necessary to interpose a diffusion barrier between Cu and its underlayer to prevent devices from deteriorating interdiffusion and/or reaction. After the barrier deposition, a Cu-seed layer is deposited to ensure the bottom-up filling of electrochemical plating Cu for dual-damascene structure. However, the conventional copper liner ͑Cu-seed/Ta/TaN trilayer configuration͒ encounters scaling difficulty at the 45 or 32 nm node, where an ultrathin diffusion barrier ͑Ͻ5 nm͒ is needed to scale down the interconnection dimensions and maintain a low effective resistivity ͑ eff Ϸ 2.2 ⍀ cm͒. 4 Therefore, direct Cu electrodeposition on the diffusion barrier is extremely desirable to optimize the overall integration by consolidating the Cu-seed/Ta/TaN trilayer liner.Ruthenium ͑Ru͒ is a potential candidate for the Cu diffusion barrier and may also serve as a seed layer for Cu electroplating. Ru is an inert metal with a lower electrical resistivity ͑bulk = 7.6 ⍀ cm͒ than that of Ta ͑bulk = 13 ⍀ cm͒ or TaN film ͑ = 229 ⍀ cm͒ 5 and shows negligible solubility with Cu even after annealing at 900°C. 6 More importantly, it has been demonstrated that conformal Cu electroplating can be done directly on Ru. 7 Kim et al. 8 reported that the Ru film with preferred ͑001͒ crystal orientation enhances Cu wettability, because it has a low lattice misfit with the Cu͑111͒ plane. Josell et al. 9 demonstrated that superfilling of fine trenches by direct copper electrodeposition onto a ruthenium barrier could be achieved. However, pure Ru may not be a good diffusion barrier, because a 15 nm Ru film can prevent Cu diffusion in the Cu/Ru/Si system after annealing at 450°C for 10 min but fails after annealing at 550°C. 10 In order to improve the bar...
Sputtered Ru and Ru-C films 5 nm thick are employed in the Cu/barrier/SiO 2 /Si system, and their performances as the diffusion barrier as well as the seed layer for direct Cu electroplating are investigated in parallel. Based on the sheet resistance measurement and energy dispersive X-ray line profiles, the 5 nm Ru-C film can retard the diffusion of Cu after a prolonged ͑30 min͒ annealing up to 700°C, while the Ru film is an effective barrier up to only 400°C. Direct electroplating of Cu is successfully carried out on both Ru and Ru-C films, which proves that Ru-C is a Cu seed layer in addition to being a robust diffusion barrier. The microstructural characteristics of ultrathin Ru and Ru-C films are also examined, indicating that the superior barrier performance of the 5 nm Ru-C film is associated with its inferior crystallinity and resistance to agglomeration at elevated temperatures.
The electroforming and resistive switching behaviors in the Ag/TaOx/Pt trilayer structure are investigated under a continual change of temperatures between 300 K and 100 K to distinguish the contributions of Ag ions and oxygen vacancies in developing of conducting filaments. For either electroforming or resistive switching, a significantly higher forming/set voltages is needed as the device is operated at 100 K, as compared to that observed when operating at 300 K. The disparity in forming/set voltages of Ag/TaOx/Pt operating at 300 K and 100 K is attributed to the contribution of oxygen vacancies, in addition to Ag atoms, in formation of conducting filament at 100 K since the mobilities of oxygen vacancies and Ag ions become comparable at low temperature. The presence of oxygen vacancy segment in the conducting filament also modifies the reset current from a gradually descending behavior (at 300 K) to a sharp drop (at 100 K). Furthermore, the characteristic set voltage and reset current are irreversible as the operation temperature is brought from 100 K back to 300 K, indicating the critical role of filament constituents on the switching behaviors of Ag/oxide/Pt system.
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