Electrodeposition of copper from sulfate electrolytes - Effects of thiourea on resistivity and electrodeposition mechanism of copper

Copper was electrodeposited from sulfate bath onto copper substrate over silicon wafer with titanium adhesive film and the resistivity was measured using a four-point probe method. The measured resistivities increased as thiourea concentration increased from 10(-6) to 10(-3) M and as the current density was increased from 1 to 50 mA/cm(2). The deposition potentials shifted to more negative values with an increase in thiourea concentration and cathodic overpolarization was observed in presence of thiourea, thereby suggesting chemisorption of thiourea on cathodic sites. Scanning electron micrographs show the presence of sulfur in the deposit when thiourea is present in the solution. The presence of thiourea in the deposit is suggestive of the reaction H2NCSNH2 + 2H(+) + 2e(-) --> NH4CN + H2S at electrolyte-electrode interfaces. This reaction can further lead to deleterious formation of copper sulfide in the deposit and a consequent increase in resistivity of electrodeposited copper. (C) 2001 The Electrochemical Society.