The influences of pulse and pulse-reverse electrodeposition on the properties of Cu thin films and superfilling for the fabrication of Cu interconnection

Abstract

The scale-down of Cu interconnection raises the issues such as the reduction of interconnection property and the difficulty in superfilling using electrodeposition. In the aspect of the interconnection property, the electrical resistivity and the electromigration resistance are the major concerns. In this study, it was attempted to improve the properties of Cu by means of pulse and pulse-reverse electrodeposition. For the purpose of enhancing the superfilling performance, the pulse-reverse electrodeposition was employed. Pulse electrodeposition consisted of on-time during which Cu reduction takes place and off-time where open circuit potential is applied. The peak potential and the lengths of on- and off-times were the important variables determining the film properties. In the results of experiments with varying the off-time, it was confirmed that the grain growth took place during the off-time, resulting in the enhancement of crystallinity and the reduction of resistivity as well. This grain growth seemed to be related to the differences of energies originated from the crystal orientation and grain size. With the enough off-time, Cu film deposited by pulse electrodeposition exhibited 68% higher Cu (111) peak intensity and 22% lower resistivity as compared to constant potential deposition when the film thickness was 260 nm including 60 nm Cu seed layer. However, when the organic additives generally used for Cu superfilling, the grain growth during the off-time was significantly retarded because the organic additives, which strongly adsorbed on Cu surface, changed the energy differences related to the orientation and grain size. Therefore, it was decided to additionally apply the anodic step to pulse electrodeposition, i.e. pulse-reverse electrodeposition, in order to further improve the properties of Cu films. Pulse-reverse electrodeposition was performed without and with organic additives to clarify the impacts of anodic step and organic additives on the film properties. From the variation in the film property according to the anodic conditions in the absence of organic additives, it was observed that the selective dissolution took place, originated from the energy differences related to the orientation and grain size. The selectivity of dissolution depended on the anodic potential which determined the rate of change in the film properties. That is, the application of more positive anodic potential reduced the selectivity, and it slowly increased the grain size and surface roughness. Regarding the electrical resistivity, the impacts of surface roughness and grain size competed with each other, and it resulted in the optimum anodic charge showing the lowest resistivity. As compared to the pulse electrodeposition, pulse-reverse electrodeposition reduced 9% of resistivity in the absence of additives. On the contrary, when the organic additives used for Cu superfilling are introduced, the selectivity of dissolution was determined by the species of the adsorbates. It was found that Cu covered by SPS was much easily dissolved compared to that covered by PEG-Cl-. In this case, the resistivity was found to be strongly determined by the surface roughness, which also exhibited the optimum point at the relatively low dissolution ratio. Pulse-reverse electrodeposition in the presence of organic additives also showed the advantage on the electrical resistivity, which reduced 14% of resistivity compared to the pulse electrodeposition. Therefore, it can be concluded that the pulse-reverse electrodeposition has the merit in the aspect of electrical conductivity. Superfilling performance was strongly determined by the adsorption of organic additives and their accumulation at the trench bottom produced by the area reduction. Prior to applying the pulse-reverse electrodeposition to superfilling, the impact of anodic step on the competitive adsorption between SPS and PEG-Cl- was investigated. As the results, it was clarified that the anodic step accelerated the displacement of preadsorbed PEG-Cl- by SPS, and the extent of displacement was increased with longer reverse time and more positive anodic potential. This acceleration of displacement has the potential to affect the superfilling, therefore, superfilling performance was assessed with various anodic conditions. As compared to the gap-filling result of constant potential deposition, the pulse-reverse electrodeposition exhibited better bottom-up performance at the trench with 55 nm of width and 300 nm of depth. At the corner of low-aspect-ratio trench, the rapid growth of Cu was observed with pulse-reverse electrodeposition, implying more accumulation of SPS at the corner. These results were understood by the acceleration of displacement with the anodic step. Considering the improvement of film properties as well as the superfilling performance, the modification of potential waveform, i.e. pulse and pulse-reverse electrodeposition, can be a candidate for resolving the current issues. This enables us to achieve Cu interconnection in the electronic devices with high speed and superior reliability.