The Nucleation and Growth of Cu Nanoclusters on Silicon Surfaces

Due to the recent adoption of copper interconnect technology by the semiconductor industry, there has been great interest in understanding the kinetics and mechanisms of copper metal deposition on silicon wafer surfaces in ultra pure water (UPW) solutions. To study the kinetics of the copper deposition mechanism on silicon surfaces, silicon [100] samples were immersed in non-deoxygenated and deoxygenated UPW solutions contaminated with a copper concentration of 100 ppb with dipping times ranging from 5 to 300 seconds and then measured using total reflection x-ray fluorescence (TXRF) at the Stanford Synchrotron Radiation Laboratory (SSRL). By measuring the Cu fluorescence signal as function of angle of incidence of the incoming x-rays, it was possible to ascertain whether the deposited copper was atomically dispersed or particle-like in nature. It was established that in non-deoxygenated UPW, the copper is incorporated atomically into the silicon surface oxide as a copper oxide, while in deoxygenated UPW, copper is deposited on the silicon surface in the form of nanoparticles. The heights of these particles were determined by performing quantitative fits to the angle scans using a spherical cap model to describe the Cu clusters. The results were consistent with measurements conducted with atomic force microscopy (AFM). Finally, the surface density of the metallic copper nanoparticles deposited in deoxygenated UPW was determined for the whole range of dipping times from the AFM measurements, indicating that Ostwald Ripening mechanisms, where large particles grow at the expense of smaller, less thermodynamically stable particles, describe the growth of Cu nanoclusters in deoxygenated UPW solutions.