Ultra-thin Die Characterization for Stack-die Packaging

On both the process side and the reliability side, the mechanical strength of ultra-thin die is critical to ensure high assembly yield and good package reliability. To understand the mechanical strength and characteristics of ultra-thin die, comprehensive characterization work must be conducted. The current paper details the various ultra-thin die/wafer characterization and optimization work done by UTAC, where wafer is thinned down to 75μm and 50μm for stack-die application. At the wafer level, characterization tools like FSM's laser based system and Atomic Force Microscopy (AFM) were used to measure the finished wafer thickness, total thickness variation (TTV) and roughness. Two important dry polishing process parameters, namely applied load and stroke time, were investigated to understand their effects on those wafer characteristics. Subsequently, various types of dry polished wafers were diced into individual dies for die strength characterization study. Three test configurations, 3-point bending, 4-point bending and ring-on-ring, were used. Again, effect of dry polishing parameters on die strength was studied. Efforts were also paid to correlate die strength with wafer back surface roughness and finished wafer thickness.