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Miedema's plot is used to select the Cu/metal barrier for Cu metallization. The Cu/metal barrier system selected should have positive heat of formation (Hf) so that there is no intermixing between the two layers. In this case, Ru is chosen as a potential candidate, and then the barrier properties of sputtered Cu/Ru thin films on thermally grown SiO2 substrates are investigated by Rutherford backscattering spectrometry (RBS), X-ray diffractometry (XRD), and electrical resistivity measurement. The Cu/Ru/SiO2 samples are analyzed prior to and after vacuum annealing at various temperatures of 400, 500, and 600 oC and at different interval of times of 0.5, 1 and 2 hrs for each temperature. Backscattering analysis indicate that both the copper and ruthenium thin films are thermally stable at high temperature of 600 oC, without any interdiffusion and chemical reaction between Cu and Ru thin films. No new phase formation is observed in any of the Cu/Ru/SiO2 samples. The XRD data indicate no new phase formation in any of the annealed Cu/Ru/SiO2 samples and confirmed excellent thermal stability of Cu on Ru layer. The electrical resistivity measurement indicated that the electrical resistivity value of the copper thin films annealed at 400, 500, and 600 oC is essentially constant and the copper films are thermally stable on Ru, no reaction occurs between copper films and Ru the layer. Cu/Ru/SiO2 multilayered thin film samples have been shown to possess good mechanical strength and adhesion between the Cu and Ru layers compared to the Cu/SiO2 thin film samples. The strength evaluation is carried out under static loading conditions such as nanoindentation testing. In this study, evaluation and comparison is donebased on the dynamic deformation behavior of Cu/Ru/SiO2 and Cu/SiO2 samples under scratch loading condition as a measure of tribological properties. Finally, the deformation behavior under static and dynamic loading conditions is understood using the scanning electron microscope (SEM) and the focused ionbeam imaging microscope (FIB) for topographical and cross-sectional imaging respectively.
"As technology progressed to ultra - large scale integration leading to smaller and smaller devices, there are continuous challenges in the fields of materials, processes and circuit designs. Copper is the interconnect material of choice because of its low electrical resistivity and high electromigration resistance. However, copper is quite mobile in silicon at elevated temperatures. Therefore, to prevent the diffusion of copper into silicon, a diffusion barrier layer that has fewer grain boundaries, good adhesion to Si and Si02, high thermal and electrical stability with respect to Cu is necessary. Tantalum nitride compounds have been investigated as potential barrier materials. TaN has a very high melting point of 2950C. It is thermodynamically stable with respect to Cu and has good adhesion to the substrate. It has a dense microstructure and shows good resistance to heavy mobility of Cu in Si and has electrical stability at temperatures upto 750 C. The diffusion barrier properties of Ta and its nitrides for copper metallization at RIT have been investigated. The TaNx films were reactively sputter deposited on Si02 substrates at various N2/AJ- ratios. The influence of nitrogen partial pressure on the electrical and structural properties of the films is studied. It has been observed that as deposited pure Ta is tetragonal, which becomes bcc-Ta with small increase in N2 flow to 5% of the sputtering gas mixture. When the nitrogen flow is increased from 12 up to 20%, amorphous and a mixture of amorphous and crystalline Ta2N phase is formed. The amorphous phase crystallizes when annealed to higher temperatures. An fee- TaN phase is formed at N2 flow of 30%. At higher concentrations of N2; nitrogen rich compounds like Ta5N6, Ta3N5 are formed. During backend semiconductor processing, both Cu and TaN films are subjected to various annealing treatments in N2, 02, and Ar at relatively high temperatures. Since these treatments influence the stability of the metallization it was important to establish the effect of the ambients on the integrity of the copper interconnect. The Cu/TaN/Si02 films were annealed to various temperatures up to 600 C in N2, Ar ambients for 20 min and the thermal stability and barrier effectiveness of the films was studied. Annealing the films to temperatures above 500 C cause de-lamination of films at the Cu/TaN interface, which is attributed to the formation of copper oxides with a high density of voids. This was observed by XRD analyis and SEM. RBS spectra showed diffusion of tantalum into the surface of copper at temperatures ~ 500 to 600 C. Therefore we can conclude that cubic TaN films act as stable barrier films up to 500 C in an inert ambient."--Abstract.
By extending our previous investigations of the ternary thin-film barriers of Ta-Si-N to Mo-Si-N and W-Si-N, we have confirmed that the reason for the success of these thin-film barrier layers between Si and Cu rests on two basic features: (1) their thermodynamic stability with Cu, and (2) their amorphous (or near-amorphous) structure. The dependence of the electrical resistivity of these layers on the nitrogen composition further proves that at the atomic level, local order exists that concurs with that of the equilibrium phases. Structural investigations including small-angle x-ray scattering and high resolution transmission electron diffraction have established beyond doubt that the alloys, as they are formed by our reactive spattering process, are amorphous over large ranges of composition. Differences between the stability of various transition metal (Tm)-Si-N alloys can be attributed to the variation of the strength with which nitrogen is bonded in the alloy. The barriers have been successfully applied to enhance the stability of n(+)p shallow silicon junction diodes with a Cu metallization, as to suppress the diffusion of Cu into SiO2. A very complete investigation of Ti-Si-N has been initiated because this alloy has the best chance of being accepted by the industry, TiN being already an industry standard for thin-film diffusion barrier applications.