Copper-Sulfide Passivation Capping for Cu Interconnects

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Copper-Sulfide Passivation Capping for Cu Interconnects Uri Cohen UC Consulting, 4147 Dake Avenue, Palo Alto, CA 94306 ABSTRACT Copper-sulfide is proposed [1-2] as a selective self-aligned passivation capping film on Cu lines, to improve the lines immunity against electromigration. Copper-sulfide is tenacious, having excellent adhesion (chemically bonded) to the underlying copper lines and strong adhesion to the overlying dielectric capping barrier. Contrary to other passivation schemes, Cu sulfidation is 100% selective. The sulfidation can be done by a wet or a dry process. A very thin copper-sulfide film (of about 25-100Å) will be required, and no significant increase of RC delay is expected. INTRODUCTION Electromigration (EM) in Cu interconnects is mostly related to poor adhesion and bonding between Cu lines and the capping dielectric barrier. The poor adhesion results in fast diffusion (channel) path for the top copper atoms along the interface. In order to reduce EM, the top copper atoms must be immobilized by binding them strongly to another layer. Several techniques are being developed in order to reduce EM of Cu lines, including electroless plating of CoWP or CoWB; siliciding the top surface of Cu lines; and "selective" or blanket deposition of thin Ta(N) or W layer. The leading candidate is "selective" electroless deposition of a CoWP or CoWB film on the Cu lines, with reported EM reduction by a factor of about 10-100X [3-9]. However, electroless deposition technique has several serious drawbacks, and is problematic, because: (a) electroless deposition has (i) an erratic incubation (or initiation) period during which there is no deposition, and (ii) an erratic deposition rate due to inherent bath instability and aging; (b) non-selective spontaneous deposition on surrounding dielectrics which may result in bridging and/or increasing leakage current between conductor lines; (c) spontaneous deposition due to bath instability (i) on chamber walls, and (ii) in-situ in the bath; (d) contamination of exposed dielectric; (e) high sensitivity to CMP residues and surface preparation; and (f) the metallic CoWP or CoWB increases Cu line resistance, RC delays, and leakage current [9]. Selective siliciding of copper lines was shown to improve EM by a factor of about 2-10X, at the expense of increased line resistance (10%), RC delays, leakage current (10X), and process complexity [9-11]. Ta(N) or W capping can improve EM by a factor of up to about 10X, at the expense of increased RC delays and leakage current [9]. Unlike copper-oxides, copper-sulfides have excellent adhesion and tenacity to Cu metal (chemical bonding). Therefore, copper-sulfide should be an excellent passivation films for Cu lines. The copper-sulfide film should reduce EM by chemically binding and immobilizing the top surface Cu atoms. In addition, copper-sulfides also have strong adhesion to dielectrics and polymers, which is why they are frequently used for metallization of dielectrics and plastics. Therefore, copper-sulfides are expected t