Effect on Two-Step Polishing Process of Electrochemical Mechanical Planarization and Chemical–Mechanical Planarization on Planarization

Abstract

Chemical–mechanical planarization (CMP) is a technique used for planarizing an overburden film in the fabrication of semiconductor devices by chemical treatment and mechanical abrasion. However, a variety of defects such as dishing of metal interconnects, erosion, delamination, and metal layer peeling are generated by a high down force in CMP. A high down force is required to generate a high material removal rate (MRR), which results in greater defects. To minimize these defects, a new planarization process is used, known as electrochemical mechanical planarization (ECMP), which requires electrochemical and mechanical energies. ECMP first involves using an electrochemical reaction to change the surface on the target material into a passivation film. Then, the passivation film is worn down using a polishing pad or abrasives on the contacted areas of the metal film with the polishing pad under a low down force. The electrochemical energy dissolves the copper solid into copper ions in an aqueous electrolyte on the contacted areas of the metal film and the polishing pad. Therefore, the low-down-force ECMP reduces the defects such as dishing, erosion, delamination and metal layer peeling to a greater degree than a conventional high-down-force CMP. Also, the MRR of the ECMP process is higher than that of the low-down-force CMP process because the MRR of the ECMP process is proportional to current density. However, some residual metal between the dielectric material was generated through the use of a nonconductive polishing pad in the ECMP process. Therefore, the CMP process is required for the final process to remove residual metals. In this research, we investigated a two-step polishing method that consists of ECMP with a nonconductive polishing pad and a conventional CMP process to planarize a micro-patterned wafer for microelectromechanical systems (MEMS). First, the ECMP process using a nonconductive polishing pad removed several tens of micrometers (µm) of bulk copper on the patterned wafer over a shorter process time than the copper CMP process only. Then, the residual copper was completely removed through the low-down-force copper CMP process. The total process time and the amount of dishing defects were reduced by applying the two-step polishing method.