Research Progresses of Cobalt Interconnect and Superfilling by Electroplating in Chips

Abstract

Copper interconnect using dual damascene technology has always been the main means for metallization in the back end of line process. However, with the size effect becoming more and more obvious due to feature size reduction, copper interconnect can no longer meet the demand for high circuit speed in Post-Moore era. Following copper interconnection, cobalt interconnection in chips attracts much attention as an interconnect technology by the next generation, which has been introduced in 7 nm node of integrated circuit manufacturing and below. The electron mean free path of cobalt (~10 nm) is much shorter than copper’s (39 nm), thus exhibiting the potential to further shrink the critical dimension without increasing line resistance and RC delay especially for contacts or local interconnects in the first few stack layers. Also, cobalt is considered as a suitable barrier/liner material, which means implementing cobalt interconnects needs no such layers and gives more space for conductive metal. Besides, higher melting point of cobalt makes it more favorable with good electromigration resistance compared with copper interconnects. Cobalt interconnection mainly adopts the wet electrodeposition method and the quality of the electrodeposite matters a lot to the reliability of the metal interconnects. For the reason of confidentiality and the limitation of research conditions, there are few research reports about cobalt interconnection. Based on existing patents and literature reports, this paper systematically introduces the advantages and current developments of cobalt interconnection. To better understand the behavior of the metal ions during electroplating process, this paper reviews the basic technology, bath composition and additives used in the electrolyte for cobalt electroplating from the point of view of solution chemistry and electrochemistry. For superconformal electroplating, there are several superfilling mechanisms for bottom-up electrodeposition with different emphasis, this paper gives a brief summary about three mechanisms and makes a comparison. Furthermore, this paper introduces the annealing control of cobalt deposition and the influence of impurities, since the evolution of grains and migration of impurities determine the sheet resistance. Finally, further study of cobalt interconnection technology is prospected. Cobalt interconnect is expected to be a proper alternative to extend Moore’s Law and promises to play a part in next advanced technology node. More researches about cobalt interconnection are worthwhile to be carried out in the future.

Keywords

cobalt / electrodeposition / superfilling / interconnect / bottom-up

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