Pulse-Plating of Copper-Silver Alloys for Interconnect Applications

Abstract

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 Effect of Cu:Ag Atomic Ratio on the Properties of Sputtered Cu–Ag Alloy Thin Films

“…However, alloying normally causes higher electrical resistivity. Fortunately, it is also found that alloying Cu with Ag would cause the increase in the Cu–Ag resistivity the least, when compared to other copper alloys [ 7 , 8 ].…”

Electrodeposition and Microstructure Characterization of Bimetallic Copper-Silver Films from the Methanesulfonic Acid Baths

“…In order to circumvent the precipitation issue of AgCl in this methanesulfonic acid bath, thiourea (TU) was adopted as a chelating agent in the electroplating bath. 21 Since the concentration of Ag ion in the electrolyte should be one of the key factors in controlling the composition of Cu-Ag deposits, [22][23][24][25] silver ions in various concentrations were added into the methanesulfonic acid copper electrolyte to extend the composition of Cu-Ag deposits. Meanwhile, several concentration ratios between TU and Ag + (denoted as [TU]/[Ag + ]) were set to evaluate the effect of TU on the microstructure and composition of Cu-Ag deposits.…”

Flow Injection Electrochemical Quartz Crystal Microbalance with ICP-OES Detection: Recovery of Silver by Electrodeposition with Redox Replacement in a Flow Cell

“…31 In some of these studies, the elemental composition and total deposited amounts were calculated by removing the working electrode from the cell, and stripping off the deposited layer ex situ to analyze it through ICP-MS or ICP-OES. 25,27,29,32 Alternatively, metal recovery calculations were performed by analyzing the solution with ICP-MS before and after EDRR. 26 Static EQCM cells have also been used in EDRR and surface limited redox replacement (SLRR) studies to describe the electric potential and mass transients involved in deposition and redox replacement processes.…”