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

摘要

Shrinking die sizes and increasing I/O density is motivating thepush towards flip chip packages. A flip chip interconnection system witha under bump metallurgy stack containing sputtered TiWNx/sputteredCu/electroplated Cu stud/electroplated 95%Pb-5%Sn was developed. Animportant step in the above process is the selective etching of thesputtered Cu bus layer and the TiWN_x barrier layer, in thepresence-of the Pb-Sn solder. The Cu bus layer was selectively etchedusing commercial etchants. However, no commercial etchants wereavailable for selectively etching the TiWN_x layer. H₂O₂-NH₄OH based etching systems, popularlyknown as Standard Clean-1 cleaning solutions, have been extensively usedto clean silicon wafers in front end wafer fabrication where only tracemetal contamination exists. Since metals like lead, copper, titanium,tin and tungsten catalyse the heterogeneous decomposition of theperoxide, the unstable H₂O₂-NH₄OH basedetching systems are rarely used to etch metal films. In this paper thedevelopment of a H₂O₂-NH₄OH basedetchant to selectively etch the sputtered TiWN_x films in thepresence of electroplated 95%Pb-54%Sn solder bumps is discussed. A23 full factorial experiment with mid point was conducted toestablish the etchant composition, as well as process temperature, thatgive satisfactory responses with respect to etch time, permissableundercut of the Cu stud (caused by the NH₄OH), and acceptablebump shape after reflow. Statistical analysis was used to understand thesignificant factors influencing the etch rate and undercut. An etchantcontaining 6% by volume of 30%-H₂O₂ and 0.75% byvolume of 30%-NH₄OH operated at a temperature of 37° C wasfound to give satisfactory results