Dept. of Electrical and Computer Engineering
Oregon Health & Science University
Copper damascene is the most common technique used in ULSI device fabrication. To meet the challenges of continuing increase of device speed and shrinkage of device dimensions, one of the strategies is to use ultra low dielectric constant (k) materials with porosity as the interlayer dielectric (ILD) between the layers of copper interconnects. Unfortunately, these ultra low-k ILDs are too fragile to be integrated with conventional chemical mechanical polishing (CMP) technique. To explore the possibility of using electrochemical polishing/planarization (ECP) as an alternate polishing technique for replacement of CMP, this work studied: (1) copper anodic polarization behavior in various electrolyte solutions, (2) copper anodic layers in different solutions, (3) ECP effects of copper bulk material and films electroplated on trenched silicon substrates, (4) ECP mechanisms, and (5) correlation between anode surface profiles and ECP effects. Copper anodic polarization curves in phosphoric acid, sulphuric acid, sodium chloride, ethylene glycol, and hydroxyethylidenediphosphonic acid (HEDP), with or without organic and inorganic additives were measured with a computer-controlled potentiostat. The data indicated that a limiting current plateau existed in certain concentration ranges of the above electrolyte solutions. Steady-state data were collected to validate that the limiting current conditions were mass transport controlled, which is a prerequisite for ECP. Electrochemical impedance spectroscopy (EIS) and SEM observations of anodic layers suggested that an electrically resistive salt film formed on a copper anode in solutions of hydroxyethylidenediphosphonic acid (HEDP). Whereas, in solutions of phosphoric acid or phosphoric acid with copper oxide, ethylene glycol and sodium tripolyphosphate as additives, no salt film was detected. Analysis of EIS data further suggested that H2O molecules are the mass transport controlling species in solutions of phosphoric acid and of phosphoric acid with copper oxide, whereas Cu [superscript ++] ions are the mass transport controlling species in solutions of HEDP and of phosphoric acid with additives ethylene glycol or sodium tripolyphosphate. ECP effects of copper bulk material and film electroplated on trenched silicon substrate in various electrolyte solutions were evaluated with computer-controlled chronoamperometry and atomic force microscopy (AFM). The results indicated that excellent ECP effect (surfaces mean roughness Ra < 10 nm) on bulk copper could be obtained with .phosphoric acid, HEDP, and phosphoric acid solutions with ethylene glycol, sodium tripolyphosphate, and copper oxide as additives. However, for copper film electroplated on trenched silicon wafer, a good ECP effect was obtained only with HEDP solutions. ECP mechanisms have been studied and a detailed description of three possible ECP mechanisms is presented. It shows that ECP effect is strongly dependent on the surface profiles of the anode to be polished. Copper films electroplated on trenched silicon wafers consist of sine waves of short wavelength, which is more difficult (compared to copper bulk surface) to be planarized. Optimal ECP conditions for this case were determined, based on the results of this study.
OGI School of Science and Engineering
Huo, Jinshan, "Electrochemical planarization of copper for microelectronic applications" (2004). Scholar Archive. 209.