Metal interconnections having a small cross-section and short length can be subjected to very large mass transport due to the passing of high current densities. As a result, nonlinear diffusion and electromigration effects which may result in device failure and electrical instabilities may be manifested. Various thicknesses of Pd were deposited over SrTiO3 substrate. Residual stress of the deposited film was evaluated by measuring the variation of d-spacing versus sin2ψ through conventional X-ray diffraction method. It has been found that the lattice misfit within film and substrate might be relaxed because of mass transport. Besides, the relation between residual intrinsic stress and oxygen diffusion through deposited film has been expressed. Consequently, appearance of oxide intermediate layer may adjust interfacial characteristics and suppress electrical conductivity by increasing electron scattering through metallic films.
KeywordsMass transport Residual stress Interfacial properties Relaxation process
There is hardly an area related to thin film formation, properties, and performance that is uninfluenced by mass transport phenomena. This is especially true in microelectronic applications where very small lateral as well as depth dimensions of device features and film structures are involved. When these characteristic dimensions become comparable in magnitude to atomic diffusion lengths, compositional change could be expected. New phases in the form of precipitates, layered compounds, or even voids may be formed from ensuing reactions. These new phases can alter the initial film integrity through, for example, generation of stress or decreasing the adhesion. This, in turn, frequently leads to component and device malfunction and electrical instabilities manifested by decreases in conductivity as well as short- or even open-circuiting of conductors.
Residual stresses generally appear during the deposition of the thin films. Such stresses may lead to problems such as film detachment, bending of the system, and varying the interfacial properties. Therefore, evaluation of such interfacial stresses is of fundamental characteristics for metallization in microelectronic devices. The residual stress mainly consists of three kinds of stress; intrinsic, thermal, and external stress. Basically, residual stresses might be varied via substrate relaxation process , and essentially, substrate relaxation process occurs due to lattice misfit between the film and the substrate . For that reason, it would be essential to concentrate on the variation of residual stress in different thicknesses of Pd film. Besides, by studying the variation of oxygen concentration on the interface of Pd/SrTiO3, it might be possible to reveal the correlation between residual stress and oxygen diffusion in the interface of Pd and SrTiO3.
The materials used in this study were 101B grade, both sides polished (100), oriented SrTiO3(STO) substrate, and 99.99% pure palladium. Electron beam evaporator was used for palladium evaporation. Graphite crucibles were employed for resistive evaporation. The evaporation chamber was pumped with mechanical and turbo molecular vacuum pump, while the base pressure of the chamber was 4.7 × 10−6 torr [1, 3].
Palladium was evaporated at 1.6 kV accelerating potential and 200 mA emission intensity for 20 min. Different STO substrates were positioned on the heater with different distances to evaporation source in order to obtain different thickness of Pd thin film . Afterward, thicknesses varying from 10 to 100 nm with around 10-nm increment have been obtained with the substrate temperature kept on 300 °C.
Figure 1 represents XPS in depth analysis of Pd thin film deposited over STO. Accordingly, thin Pd film with 20 nm thickness exhibited negligible amount of oxygen atoms concentrated in the interface of the Pd films (Fig. 1a). In fact, the oxygen and Ti concentration increased similarly by increasing the sputtering time. This reveals the normal variation of elements from film toward substrate. From 50-nm Pd film shown in Fig. 1b, one can distinguish that when sputtering time reaches to 10 min (where it is essentially supposed to be empty from substrate elements) oxygen concentration is even higher than its concentration at substrate. Since the base pressure of the chamber was 4.7 × 10−6 torr, oxygen may not diffuse from the ambient. This anomalous behavior has been previously observed in Nazarpour et al. [1, 3, 6]. The top atomic layer of used STO substrate in this study consists of mainly Sr–O bonds. However, when the top atomic layer of STO is made of Ti–O bonds, presence of Oxygen in the interface has not been detected in any thickness of Pd thin film. Figure 1c and 1d correspond to variation of oxygen 1 s spectrum of 30 and 50 nm Pd thin films, respectively. High presence of oxygen in the interface of Pd and STO is significant. This oxygen has bounded to Pd and has different binding energies in the substrate. This is detected by the existence of two peaks in the interfacial region. This presence of oxygen is in agreement with Fig. 1a, 1b which show the oxygen in the interface of Pd and STO.
Initially, it is found that oxygen diffusion from substrate into the Pd thin films occurs which leads to an intermediate oxide layer between Pd and STO. Besides, relation between the residual stresses and oxygen concentration in the interface reveals that oxygen penetrates into Pd film to compensate lattice misfit between film and substrate. Therefore, it could be concluded that mass transport could even appear because of residual stresses in the thin films. This, in turn, drastically influences desired properties in the microelectronic technology. Mass transport frequently leads to component and device malfunction and electrical instabilities manifested by decreases in conductivity as well as short- or even open-circuiting of conductors. This study reveals the importance of deep and cutting edge further studies on mass transport caused by residual stresses.
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