Study of direct Cu electrodeposition on ultra-thin Mo for copper interconnect

Abstract

Direct electrodeposition (ECD) process of copper film on ultra-thin molybdenum film is investigated in both copper sulfate - sulfuric acid (CuSO4 - H2SO4) acidic bath and copper sulfate - ethylenediamine (CuSO4 - En) neutral bath (pH=7.5). The neutral electrolyte containing 0.03M CuSO4 and 0.06M En was chosen to avoid excessive corrosion of Mo and introduction of other ions. The complexing effect of En affects the electrodeposition process of Cu on Mo. Potentiodynamic polarization curves and scanning electron microscopic characterization show that Cu can be electroplated in CuSO4 - En bath with lower corrosion rate, greater nucleation density and shorter fully-covering time than that in CuSO4 - H2SO4 bath. The fitting results of current-time transients using Scharifker and Hill's model support the instantaneous nucleation and growth of Cu on Mo in both CuSO4 - H2SO4 bath and CuSO4 - En bath (with background electrolyte) at the initial stage of electrodeposition. By using an optimized two-step plating process (at the current density of 8mA/cm2 in the first step for a short time and 4mA/cm2 in the second step), the resistivity of direct ECD Cu on Mo was almost the same to that of ECD Cu on Cu/Mo in the same plating bath.