Potential-pH Diagrams of Interest to Chemical Mechanical Planarization of Copper Thin Films
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Potential-pH Diagrams of Interest to Chemical Mechanical Planarization of Copper Thin Films
Serdar Aksu Faculty of Engineering and Architecture Suleyman Demirel University 32260 Isparta, TURKEY
ABSTRACT Chemical mechanical planarization (CMP), which can globally planarize both silicon dioxide (the prevalent interlayer dielectric), and copper films, has become the key process in the damascene method used for producing integrated circuit (IC) devices with multilevel copper interconnects. Cu CMP is typically carried out with slurries containing oxidizing agents, complexing agents, and corrosion inhibitors as the principal chemical components. In such slurries, complexing agents enhance the solubility of copper and increase the dissolution rate of the abraded material in Cu CMP. They also assist achieving high copper removal rates during dynamic polishing conditions. The nature of the complexing agent used, the pH and the redox potential of the slurry system are among the main factors controlling the dissolution and passivation behaviors of copper during CMP. Consequently, these factors are intimately related to the key CMP performance metrics such as removal rate and planarity. In this paper, potentialpH diagrams of copper in aqueous systems containing a number of organic complexing agents including ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), oxalic acid and malonic acid are presented. The predominance regions of copper complexes under different copper and ligand activities and their implications on copper removal during CMP are discussed. INTRODUCTION Copper is currently replacing aluminum as the interconnect metal in integrated circuits due to its higher electrical conductivity and superior electromigration resistance (1). CMP is the critical enabling process for manufacturing IC devices with copper multilevel metallization in the damascene method (2,3). Inorganic and water-soluble organic complexing agents are among the major chemical components used in the preparation of Cu CMP slurries to enhance copper dissolution and increase the copper removal rate. Establishment of potential-pH diagrams or Pourbaix diagrams is a convenient way to examine the effect of a complexing agent on the copper complexation and the enhancement made in the copper solubility. These diagrams map the predominant species as a function of potential and pH, and their construction principles are given by Pourbaix (4). Potential-pH diagrams are often utilized to achieve a better understanding of the Cu CMP process as illustrated by Aksu and co-workers for Cu-glycine-H2O system (5, 6). In addition, potential-pH diagrams examining the effect of ammonia, ethylenediamine, citric acid, hydroxylamine, nitro salicylic acid, benzotriazole, and acetohydroxamic acid on the copper stability are available in the Cu CMP the literature (7, 8, 9). In the present paper, potential-pH
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diagrams are established for Cu- ethylenediaminetetraacetic acid (EDTA) -H2O, Cunitrilotriacetic acid (NTA)-H2O, Cu- oxalic acid-H2O and
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