Effect of Temperature on Incubation Time for Spontaneous Morphology Change in Electrodeposited Copper Metallization
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Effect of Temperature on Incubation Time for Spontaneous Morphology Change in Electrodeposited Copper Metallization S. Ahmed, D.N. Buckley and S. Nakahara Dept. of Physics, Materials and Surface Science Institute, University of Limerick, Ireland Y. Kuo Dept. of Chemical Engineering, Texas A&M University, College Station, TX, USA ABSTRACT A systematic investigation of the effect of annealing time and temperature on the incubation period for spontaneous morphology change (SMC) in electrodeposited copper metallization is reported. The incubation time is greatly reduced at higher temperatures. At each temperature, the remaining incubation time at room temperature was found to decrease approximately linearly with increasing annealing time. An Arhennius plot of the measured rates of decrease showed good linearity and yielded a value of 0.48 eV for the activation energy. This is consistent with a vacancy diffusion mechanism for the process occurring during the incubation period and supports our proposed mechanism for SMC. INTRODUCTION Understanding and controlling the microstructure of electrodeposited copper1-8 metallization is increasingly important. It has been observed that the microstructure of both sputtered and electrodeposited copper can change considerably with time, even at room temperature.9-20 Scanning probe microscopy, including atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), has proved to be a useful tool to study copper electrodeposition,1,3,5-8,18,21,22 including mechanistic studies, the effect of additives and the influence of the substrate on microstructure. Studies reported to date have generally emphasized evolution of the structure to larger grain size (i.e. grain growth) with corresponding changes in film characteristics such as a decrease in resistivity. Recently, however, we reported19,20 the observation by atomic force microscopy (AFM) of a sudden spontaneous morphology change (SMC) at room temperature in electrodeposited copper metallization. We observed the appearance of new particles that were significantly smaller than the existing features. We have proposed23,24 a mechanism to explain the SMC phenomenon based on the outdiffusion of excess vacancies. An incubation period was observed before the onset of SMC. This paper presents results on the temperature dependance of incubation time from which a value for the activation energy of the corresponding incubation process is obtained. EXPERIMENTAL The substrates used consisted of polished silicon wafers on which a 40 nm layer of either TaN or Ti and a 100 nm layer of copper were deposited sequentially by sputtering. Films of copper were galvanostatically electrodeposited at 15 mA cm-2 on these substrates at room
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temperature from a stirred bath using a PAR EG&G 363 potentiostat/galvanostat. The plating bath contained 0.25 mol dm-3 CuSO4 and 1 mol dm-3 H2SO4 and no additives were used. The deposition time was 112 s and film thickness was estimated as 620 nm assuming 100% current efficiency and a density of 8.9
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