Nucleation and Growth kinetics of Cu 2 O During Reduction of CuO Thin Films
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Nucleation and Growth kinetics of Cu 2 0 During Reduction of CuO Thin Films
Jian Li*, K.N.Tu** J.W. Mayer* *Dept. of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 **IBM, T.J. Watson Research Center, Yorktown Heights, NY 10598
Abstract: The combination of 160(a, a)16O oxygen resonance measurement and transmission electron microscopy (TEM) provides an unique and effective method to study the kinetics of nucleation and growth of Cu 20 phase during reduction. In situ TEM observation showed that isolated and large Cu 20 grains emerge from the small CuO grain matrix and the growth of Cu 2 0 grains is linear with time. We propose that the discontinuous morphology of grain growth of Cu 2 0 is due to the migration of the Cu 2 0-CuO phase boundary induced by oxygen out-diffusion along the moving phase boundary. Based on the classical analysis of phase transformation by Johnson, Mehl and Avrami, the activation enthalpy of nucleation of Cu 20 phase in the CuO matrix has been deduced as AE,,=2.3 eV. The specific interfacial energy between CuO and Cu 20 phases has been estimated as 0.5 eV/atom. I. Introduction: Copper is widely used in electronic packaging and in integrated circuit technology [1]. Cu is readily oxidized at low temperature, leading to poor adhesion between Cu and a solder pad. Thus, the degradation of the package may occur. Studies on the phase transformation and stability of copper oxide thin films are of importance for device fabrication. The investigation of oxygen in and out diffusion induced phenomena in pure copper oxides can contribute to our understanding of adhesion failure mechanism, e.g. in tape automate bonding (TAB) technology. Oxygen diffusion in the copper oxides leads not only to oxygen concentration variations but also to microstructural changes. In this study, we combine the transmission electron microscopy (TEM) and oxygen resonance, an extended Rutherford backscattering technique to study the nucleation and growth kinetics of Cu 20 phase during reduction of CuO thin films in vacuum. II. Experimental: Thin films of CuO were prepared by reactive cathodic sputtering of Cu in oxygen atmosphere onto Si(100) substrate and thin layer of Si0 2 covered NaCl substrate, followed by an annealing at 300'C for 30 minutes in ambient oxygen. The base pressure in the sputtering chamber was 2X10-7 Torr. A Rutherford backscattering (RBS) was employed to measure the film thickness at 70 nm and the composition as CuO. Self-supporting CuO films were floated off the NaCl substrates and were placed on transmission electron microscopic grids for in situ observation. A thin Si0 2 membrane acts as a passivation layer to prevent oxygen loss from one side. A hot stage was used in the electron microscopy upon vacuum annealing. CuO thin films on Si(100) substrate were isothermally annealed 7 at different temperatures in a quartz tube furnace with a vacuum level 2X10- Torr. A Tandetron accelerator with a 1.7 MV terminal was used to conduct 160(e, t)"60 scattering resonance measurements for quantif
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