The electrodeposition of Cu-Ag alloys was studied as a possible application for interconnect technology, where Cu-Ag alloys may be less susceptible to electromigration than Cu alone. The presence of chloride in state-of-the-art copper plating electrolytes limited the solubility of Ag. However, pulse-plating approach enabled a wide range of Cu-Ag alloy compositions at substantial chloride concentration levels. The deposition of Ag was driven by the displacement reactions between the metallic copper and ionic silver during the off-time. Measured alloy compositions were consistent with theoretical estimates at various electrolyte concentrations, electrode rotation speeds, pulse frequencies and duty cycles. However, organic additives decreased incorporation of Ag into the alloy. It was also discovered that CuSO 4 · 5H 2 O from a number of major chemical suppliers contained Ag as an impurity. The roughness of the films was significant when produced by pulsed plating, but was shown to be substantially reduced in the presence of a leveling agent. Additionally, the concentration of chloride in the electrolyte was shown to significantly affect surface quality of the deposited Cu-Ag thin films. With the continuing miniaturization of microelectronics, electromigration effects in copper interconnect systems are becoming a major factor in determining device lifetime and reliability.1-3 Accordingly, there is a need for interconnect materials with improved electromigration resistance, while maintaining adequate electrical resistivity. It has been shown that co-deposits of Cu with small amount of other metals (such as Ag, Sn, Co, and Mg) can potentially mitigate both electro-and stress-migration.4-8 However, the resistivity of the interconnect increases with the addition of foreign metals. The International Technology Roadmap of Semiconductors (ITRS) has set the resistivity criterion at ρ < 2.2 μ · cm, 9 only slightly above the bulk resistivity of pure copper at 1.68 μ · cm. Alloying copper with silver (ρ Ag = 1.59 μ · cm) was shown to increase the resistivity of electrochemically deposited Cu-Ag films the least when compared to other copper alloys.10 For Ag content between 0.17 and 3.2 wt% the resistivity ranges 1.8 to 3.1 μ · cm.10 For this reason Cu-Ag alloys, especially at the lower Ag weight percentages, are potential candidates for the fabrication of interconnects in microelectronic devices.Acidic copper-sulfate electrolytes containing chloride have been successfully applied for many years to the electrochemical fabrication of copper interconnects. [11][12][13] Chloride in these electrolytes is known to be one of the critical constituents enabling defect-free filling of surface features.14-20 The main challenge for depositing silver from copper-plating electrolytes, which contain about 50 ppm of chloride, is the low solubility of silver in the presence of chloride ions (the solubility product of AgCl in water at 25• C is 1.8×10 We demonstrate that the application of a pulsating current instead of a direct current permits ...

The impact of the ferric/ferrous (Fe3+/Fe2+) redox couple on the behavior of polyethylene glycol (PEG) and bis-(3-sulfopropyl) disulfide (SPS) during copper electrodeposition is investigated. The Fe3+/Fe2+ couple is used in plating of copper onto printed circuit boards and may have advantages when implemented for on-chip metallization of copper. Experiments show that in the presence of Fe3+/Fe2+ the suppressing behavior of PEG does not change; in contrast, the accelerating activity of SPS increases when Fe3+/Fe2+ is present in a copper-plating bath. Furthermore, potentiostatic experiments suggest that SPS interacts with Fe2+ ions in the bulk electrolyte, probably to produce 3-mercapto-1-propane sulfonic acid (MPS) at low ppb levels.

No abstract

The ferric/ferrous redox couple interacts with bis-(3-sulfopropyl) disulfide (SPS), leading to increased amounts of 3-mercaptopropyl sulfonate (MPS) in solution. SPS is an essential organic additive in the electrochemical deposition of copper from acidic cupric sulfate electrolytes onto printed circuit boards and integrated circuits. Electrochemical studies show that the accelerating action of SPS is dependent on the ferric/ferrous concentration ratio, with weakening dependence at increasing cupric sulfate concentration. HPLC and electrochemical results suggest that an increased concentration of MPS in the presence of the redox couple is the primary cause of the increased acceleration. Equilibrium estimates of MPS generation by the introduced redox chemistry partially explain these observations; the standard reduction potential of SPS to MPS reduction is estimated to be in a range between 0.3 and 0.4 V vs. SHE. However, the amount of acceleration goes through a maximum at an intermediate ferric-ferrous ratio, possibly because SPS may be converted to MPS via both an oxidative and a reductive pathway.

A chromatographic method for the detection of bis-(3-sulfopropyl) disulfide (SPS), a common additive in acidic copper plating baths, and its breakdown products is demonstrated. The detection scheme involves a combination of solid-phase extraction for sample pre-treatment, C(18) reversed-phase high-performance liquid chromatography column for separation, and electrochemical sensor for detection of all non-fully oxidized sulfur-containing compounds. We were able to achieve an effective separation and accurately assign chromatographic peaks to all detectable species. Owing to a high sensitivity of the utilized electrochemical detector, detection in low parts per billion range was possible. This can prove crucial for plating bath control, since minute amounts of certain by-products significantly affect the bath performance.