JINSHAN (JASON) HUO
10.1 Introduction
Electrochemical polishing, or electropolishing, is conventionally used for producing shiny surfaces where mechanical polishing is difficult to perform. Examples include components with complicated surfaces, decorative items, and other special applications. For microelectronic fabrication, planarization is emphasized in addition to surface smoothness. Hence, the term electrochemical planarization (ECP) is used throughout this chapter=book.
Surface polishing can be achieved under certain conditions of electrochemical dissolution, which is a reverse process of electroplating (EP). A simple electrochemical cell is shown in Fig. 10.1. Two metal (e.g., Cu) bars are immersed in an electrolyte. A voltage is applied between the two bars. The one connected to the positive pole of the power supply is anode. The other one is cathode. The positive potential applied to the anode may pump out electrons from copper atoms on the anode surface. As a result, copper dissolution may occur in certain electrolytes. Conversely, copper deposition may occur on the cathode. That is, copper electroplating results when the working electrode is chosen to be cathode, and copper dissolution is accomplished when the working electrode is chosen to be the anode.
For EP, the biggest challenge is void-free gap filling. This is realized by adding additives such as accelerator, suppressor, and leveler into a plating solution. Among them, accelerators have a function of catalyzing charge transfer. These rapidly diffusing small electroactive molecules accumulate onto the trench bottom as the copper deposition process goes on. As a result, the current density on the trench bottom becomes higher and higher than that on