Electrochemical investigation of copper deposition onto silicon from hydrofluoric acid and buffered hydrofluoric acid solutions

Abstract

For the fabrication of ULSI circuits, the silicon surface should be free of metallic and particulate contamination and be atomically smooth. Heavy metal contaminants have detrimental effects on the yield of microelectronic processing and reliability of solid state devices by increasing the junction leakage current, degrading the integrity of gate oxide and reducing the oxide breakdown voltage. It has been demonstrated that the contamination level for most metals has to be controlled below 10¹⁰ atoms/cm² for sub-quarter micron processing. Some transition and noble metal ions such as copper, silver and gold are known to deposit on silicon surfaces in acidic fluoride based solutions. Among them, copper has been the focus of most research due to the widespread use of copper in microelectronic industry. To investigate the mechanisms by which copper deposits on silicon surfaces in HF and BHF solutions, different electrochemical techniques were used, including dc polarization and ac impedance spectroscopy (EIS). The results of electrochemical measurements, in conjunction with various surface characterization techniques, such as TXRF, AFM and TEM, reveal the rates of reactions that occur at silicon/solution interface can be affected by many factors, such as silicon dopant type and doping levels, ionic strength of electrolytes, illumination conditions and the use of additives. For example, under darkness, the amount of copper deposition is nearly one order of magnitude less than that under illumination for p-type silicon in both HF and BHF solutions and n-type silicon in HF solutions. In BHF solutions, n-type silicon is very close to its flat band condition where illumination plays little role. The addition of copper ions in BHF solutions not only causes metal contamination, but also generates severe surface roughness due to the masking effect of copper on the anisotropic etching of silicon in BHF solutions. Local etching rate of 1nm/min was measured on the silicon sample prepared in 100 ppb Cu²⁻ BHF solutions. Some additives such as surfactants and hydrochloric acid can effectively reduce the reaction rates of silicon corrosion and copper deposition by either blocking the active reaction sites or form copper ion complexes.