Mechanism of copper deposition on silicon from dilute hydrofluoric acid solution

Metal deposition on silicon from HF-based solutions is initiated by electrochemical reduction of metal ions, a process which is driven by the difference between the electron quasi Fermi energy in the silicon, E-Fn, and the redox energy level of the ions in solution, eE(Mz+/Mo). Mechanisms for metal ion reduction are elucidated by aligning the silicon bands with the redox levels of ions in solution. For copper, the reduction reaction occurs by capture of conduction band electrons, a process which requires nucleation of nanometer-sized precipitates on the silicon surface. As the concentration of electrons at the surface, n(s), is increased (e.g., by n-type doping, illumination, or cathodic biasing), the nucleation rate of precipitates, N, increases steeply. N is further dependent on the activity of the metal ion in solution and the amount of surface charge at the Si/HF interface. The growth rate, v, of copper nuclei is determined by the surface concentration of holes, p(s), and is increased by illumination and p-type doping. The predicted effects on N and v of illumination level, doping level, and substrate bias were verified by immersion tests on Czochralski wafers in 1:100 HF. Total reflection x-ray fluorescence was used to measure copper coverage; atomic force microscopy and surface-sensitive minority carrier lifetime measurements were used to determine the areal density of copper precipitates. Application of these findings to reduce copper deposition from HF in industrial wafer cleaning practice are discussed.