Studies of Copper Surfaces modified by Thermal and Plasma Treatments
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Studies of Copper Surfaces modified by Thermal and Plasma Treatments G.P. Beyer, M. Baklanov, T. Conard, and K. Maex Imec Kapeldreef 75 3001 Leuven BELGIUM ABSTRACT It was found that copper surfaces, which had been exposed to a clean room atmosphere, were covered by a layer, whose chemical composition can be described by Cu(OH)2·CuCO3. This layer can effectively be removed by either a short thermal treatment in vacuum at 350°C, a hydrogen plasma treatment, or a combination of both. Ex-situ photoelectron spectroscopy measurements show little difference of the chemical composition of the surface after the respective treatments. The thermal treatment, however, gives rise to re-crystallisation of the copper film due to the difference in temperature of deposition and the anneal. Ex-situ ellipsometry measurements indicate that the hydrogen plasma not only removes Cu(OH)2·CuCO3 but also passivates the copper surface. INTRODUCTION Advanced interconnect structures of integrated circuits incorporate copper for the transmission of the electrical signal. Although copper has a lower resistivity than aluminium it has been introduced into integrated circuit production only lately because of concerns of copper diffusion into the active region of the electronic devices. In order to suppress the copper diffusion the copper interconnect is usually encapsulated by metallic and dielectric diffusion barriers. This encapsulation scheme raises a number of issues such as interface diffusion in electromigration [1] and interface reactions during the deposition of the dielectric diffusion barrier on top of copper [2]. In the latter study it was demonstrated that the reaction between the silicon nitride and the copper was retarded by the presence of a native passivation layer on the copper surface. The disadvantage of this passivation layer, however, is that it is not self-limiting and stable as in the case of aluminium. The exposure of the copper surface to wet chemicals has been described by Apen et al. [3]. In this study the influence of a thermal treatment or hydrogen plasma on the copper surface has been studied. EXPERIMENTAL DETAILS Copper layers were deposited by sputtering on substrates consisting of a silicon wafer, covered by oxide and tantalum nitride. The temperature during deposition was approximately 50°C. The thickness of the layer was 150 nm. The wafers were stored in a clean room ambient for about 6 months. Then the copper layers were subjected to a thermal treatment, a hydrogen plasma treatment, or a combination of both. The experimental details of the various treatments are as follows. The wafers were heated in a vacuum chamber to a temperature of 350°C for two minutes. The back ground pressure of the chamber is 10-7 Torr. During processing the pressure rises to about 1 mTorr due to the presence of argon, which couples the wafer thermally to the heated chuck. The plasma treatment occurs in a mixture consisting of 5% hydrogen and 95% helium at pressures below 100 mTorr and lasts for one minute. D9.17.1
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