Role of Phosphoric Acid in Copper Electrochemical Mechanical Planarization Slurries
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1157-E12-03
Role of Phosphoric Acid in Copper Electrochemical Mechanical Planarization Slurries
Serdar Aksu SoloPower Inc., 5981 Optical Court, San Jose, CA 95138, U.S.A.
ABSTRACT In this paper, the electrochemical behavior of copper in aqueous solutions containing phosphoric acid (H3PO4) is investigated to elucidate the role of H3PO4 in the Cu ECMP slurries. Aqueous solubility and potential-pH diagrams were constructed for copper-phosphate-water system. Good correlations were found between the diagrams and the experimental polarization data. It was found that H3PO4 might not able to sufficiently increase the solubility of copper alone. A complexing agent is needed to ensure the high solubility of copper, especially as the slurry pH and dissolved copper concentration increase. Specific conductance measurements revealed that phosphoric acid was the key constituent responsible for increasing the conductivity of the ECMP electrolyte. In situ electrochemical polarization experiments showed that the planarization mechanism during the ECMP process was similar to that observed in conventional copper CMP.
INTRODUCTION Planarization of copper damascene structures is becoming increasingly challenging due to the ongoing reduction of feature sizes, along with the associated high packing density, use of low-k dielectrics and growing number of interconnect layers in silicon integrated circuits [1, 2]. Electrochemical mechanical planarization, ECMP is a technique, which can increase the planarization efficiency and the throughput at lower down force and lower defectivity [3,4]. In contrast to conventional CMP process, where chemical oxidizers are primarily responsible for controlling the polishing rate, in the ECMP process applied current density is used to control the polishing rate. ECMP offers the high planarization efficiency, which is insensitive to the pattern density [5]. Unlike the conventional electropolishing process, which uses inorganic acids that dissolve copper uniformly from the entire wafer surface, ECMP electrolyte produces a soft protective layer that is removed easily by the pad abrasion from the protruded areas. On the other hand, the mechanical action leaves the soft protective film intact in the recessed areas, thus channeling more current to the abraded areas. Selection of correct pad stiffness is critical in ECMP process to ensure high planarization efficiency. A soft pad would bend and remove the protective surface layer in the recessed areas while a stiff pad would not respond well to the low pressure in the ECMP process [5]. Planarization of the copper film depends highly on the electrolyte chemistry in the ECMP process. For best results the protective surface film must be soft and easily abraded when contacted by the pad while it should remain intact in the recessed areas. The main chemical constituents of ECMP slurries could be listed as complexing agents, passivating agents
(corrosion inhibitors), pH buffers and inorganic acids [6]. Oxidizers, such as H2O2, are sometimes also added to enhance th
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