Development and application of an experimental concept for surface characterization of semiconductor based substrates using scanning electrochemical microscopy

In this work, evaluation of the applicability of electrochemical scanning microscopy (SECM) for semiconductor industry-relevant thin film materials was carried out. These investigations were focused on the local electrochemical characterization of electrodeposited copper layers and their growth behavior on a variety of barrier materials such as Pt, Ru, TiN, TaN, Ta, Ti, W, and TiW. A special holding device for wafer-based samples was developed to be able to handle this combined task on a laboratory scale. This multipurpose cell provided electrical contact for the thin films deposited on the silicon substrate while simultaneously sealing the sample with only a small exposed area without any complex sample preparation. Based on preliminary studies, it could be shown that local surface characterization by means of SECM as well as electrochemical copper deposition in a commercial laboratory tool on the aforementioned materials with this cell was feasible. Consequently, deposition protocols for electroplating of dense Cu films with good adhesion on various barrier materials were developed for surface characterization studies with SECM. It became apparent that conventional commercial acidic copper electrolytes were only suitable for deposition of dense and adherent films on Cu or platinoids such as Ru or Pt. In contrast, direct electroplating on Ta-, Ti- and W-based barrier thin films had shown that the deposition of dense and adherent layers was heavily dependent on potential, on electrolyte composition and process handling and was therefore inappropriate for galvanic coating on a wafer scale. On the other side, implementation of the multipurpose cell into the SECM setup revealed that the local surface characterization of semi-precious metals in conventional feedback mode was strongly restricted. It could be shown, that oxide formation and corrosion of the thin film surface on the basis of surface interaction effects in aqueous solution had a strong influence on the measurement results. Moreover, the local resolution of this technique is limited, since materials with similar conductivity cannot be distinguished as shown by approach curves studies on different metallic thin films such as Ru, Pt, Cu, TiN, TiW, W and TaN. Based on these results a non-destructive measuring concept was developed which would ensure a high electrochemical contrast between different metallic materials without mediator-based surface interferences on the measured signal. It was demonstrated that the hydrogen evolution reaction had the necessary material selectivity according to the results of chronoamperometric studies on different barrier thin films. Therefore, a mediatorless SECM concept in SG/TC mode was characterized from these findings, which showed that the aforementioned requirements were achieved. In the next step, the measurement concept was used for the electrochemical characterization of the growth of direct electroplated Cu on Ru thin films. This model system was selected since Ru did not restrict the composition of the Cu electrolyte. Therefore, it was possible to study the influence of a grain refiner such as citric acid on the early electrocrystallization stage of Cu on Ru. In a preliminary SEM-supported study, it was shown that citric acid had a strong impact on the nucleation since it effectively inhibited grain growth in the deposition process. Thus, nanocrystalline and adherent Cu layers with a grain radius of 10 nm could be formed on pure Ru surface. Since the local resolution of the SECM is dependent on the probe size, ultramicroelectrodes with a size of rtip < 10 nm are required. The fabrication of electrodes of this dimensions cannot be accomplished with existing methods. In order to electrochemically characterize the nucleation behavior of Cu on foreign substrates electrochemically, new manufacturing processes for ultramicroelectrodes in the lower nanometer range have to be developed