Development of an etchant for selectively etching TiWNx in the presence of electroplated 95%Pb-5%Sn solder

Shrinking die sizes and increasing I/O density is motivating the push towards flip chip packages. A flip chip interconnection system with a under bump metallurgy stack containing sputtered TiWNx/sputtered Cu/electroplated Cu stud/electroplated 95%Pb-5%Sn was developed. An important step in the above process is the selective etching of the sputtered Cu bus layer and the TiWN_x barrier layer, in the presence-of the Pb-Sn solder. The Cu bus layer was selectively etched using commercial etchants. However, no commercial etchants were available for selectively etching the TiWN_x layer. H₂ O₂-NH₄OH based etching systems, popularly known as Standard Clean-1 cleaning solutions, have been extensively used to clean silicon wafers in front end wafer fabrication where only trace metal contamination exists. Since metals like lead, copper, titanium, tin and tungsten catalyse the heterogeneous decomposition of the peroxide, the unstable H₂O₂-NH₄OH based etching systems are rarely used to etch metal films. In this paper the development of a H₂O₂-NH₄OH based etchant to selectively etch the sputtered TiWN_x films in the presence of electroplated 95%Pb-54%Sn solder bumps is discussed. A 2³ full factorial experiment with mid point was conducted to establish the etchant composition, as well as process temperature, that give satisfactory responses with respect to etch time, permissable undercut of the Cu stud (caused by the NH₄OH), and acceptable bump shape after reflow. Statistical analysis was used to understand the significant factors influencing the etch rate and undercut. An etchant containing 6% by volume of 30%-H₂O₂ and 0.75% by volume of 30%-NH₄OH operated at a temperature of 37°C was found to give satisfactory results