Copper and Copper Alloys:  Studies of Additives
Creators: Volov, Igor
Publisher: name: Columbia University
Year: 2013
Subjects: subject: Chemical engineering
subject: FOS: Chemical engineering

description: Electrodeposition of copper is used extensively for the fabrication of electrical interconnects in semiconductor device manufacturing and in printed circuit board production, as well as other industries. Copper is often plated from an acidic copper sulfate electrolyte with a number of inorganic and organic constituents. Electrolyte enables filling of complex surface geometries with desired internal and surface properties. Continuing miniaturization of modern microelectronics requires a highly controlled electrodeposition process and also requires interconnect materials with improved electromigration and stress migration resistances. Thus, current research deals with two avenues that have a potential to improve the process of copper electroplating and extend copper technology to meet the challenges of future device dimensions.
The improvement in the plating process of copper is being sought by the integration of an iron redox couple (Fe3+/Fe2+) to copper electrolytes. Certain benefits of incorporating Fe3+/Fe2+ subsystem in combination with inert anode to the electrolyte have been previously recognized, though without regards to the impact that Fe3+/Fe2+ can exert on the behavior of additives. Organic additives are essential constituents of all copper plating baths. Therefore, we studied how the presence of Fe3+/Fe2+ affects organics additives, with focus on two representative components: polyethylene glycol (PEG) and bis(3-sulfopropyl)-disulfide (SPS). 
Electrochemical studies on a rotating disk electrode (RDE) and microfluidic device showed that the behavior of PEG during copper deposition is not affected in the presence of Fe3+ and Fe2+ ions. Kinetics of adsorption and desorption of PEG on copper electrode were also unaffected. In contrast, the activity of SPS increased when Fe3+/Fe2+ were present in a copper-plating bath. By means of the electrochemical analysis and investigation by high performance liquid chromatography (HPLC), it was revealed that the Fe3+/Fe2+ redox couple reacts with SPS to form 3-mercaptopropyl sulfonate (MPS) in the bulk solution. The ratio of Fe3+/Fe2+ determined the reducing power of the electrolyte by changing the concentration of MPS derived from SPS. The estimates of the standard reduction potential of SPS to MPS reduction, based on equilibrium calculation with reference to HPLC results, put the reduction potential in the range between 0.3 - 0.4 V vs. standard hydrogen electrode (SHE).
To facilitate the study SPS/MPS equilibrium in the presence of ferrous and ferric ions, a new chromatographic method was developed for the detection of SPS, MPS, monoxide-of-SPS, and dioxide-of-SPS from a copper electrolyte. An HPLC tool was coupled with an electrochemical detector, which enabled concentration sampling in a range of just a few parts per billion (ppb). Due to its low limit of detection and effective separation of detectable compounds, this method can prove crucial for plating bath control, where very little amount of certain byproducts may significantly decrease performance of the electrolyte. 
As technology advances to create smaller microelectronics, copper interconnects are becoming more prone to failures by electromigration and stress migration effects. Copper can potentially be made less susceptible to these effects if alloyed with about one weight percent of another metal. Accordingly our research examined copper-silver (Cu-Ag) and copper-tin (Cu-Sn) alloys as two possible applications in interconnect technology.
The main challenge for depositing silver from copper plating electrolytes, which contain about 50 ppm of chloride, is the low solubility of silver ions with chloride. Together with other additives, chloride is a crucial component promoting defect-free filling of surface features. To overcome this challenge, it was shown that the application of pulse-plating instead of direct current plating enabled the use of chloride at a substantial concentration, while also allowing a wide range of Cu-Ag alloy compositions. Theoretical estimates of alloy compositions were in agreement with experimental values at various pulse frequencies and duty cycles, electrode rotation speeds, and electrolyte concentrations. However, organic additives decreased incorporation of Ag into the alloy due to a possible complexing effect on silver ion. It was also discovered that salt of copper (CuSO4*5H2O) from a number of major chemical suppliers contains Ag as an impurity. Pulsating conditions were responsible for an unwanted increased in surface roughness of the plated film alloys. However, the addition of a leveling agent polyvinylpyrrolidone (PVP) at 10 ppm of chloride improved roughness and surface quality of the deposited Cu-Ag thin films.	
Electrodeposits of copper-tin alloy thin films were prepared from acidic copper sulfate electrolytes by polarizing copper deposition into the region where Sn deposition became possible. The ability to polarize copper deposition demonstrated with the use of EPE, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), and bromide in comparison to several other chemistries. Sn content between 0 - 7 at% was obtained above the reduction potential of Sn2+. High Sn content of up to 20 at% was achieved when Cu deposition was suppressed below the deposition potential of Sn2+ by the combination of Br and EPE. A positive correlation between Sn content and concentration of Sn2+ in the electrolyte was observed. Higher tin content in the alloy was also correlated with low rotation speeds.