Damage risk assessment of under-pad structures in vertical wafer probe technology

Due to the demand of the industry for an increase of the number of I/Os, while decreasing the die size, the bond pads had to shrink and design restrictions for the active structures underneath had to fall. This leads to new challenges for the electrical probing and the mechanical robustness of the under-pad structures. This paper presents analytical and numerical simulation approaches for predicting loading conditions, estimating stress states and assessing associated damage risks for the back-end-of-line (BEOL) interconnect system underneath a probe pad. For this purpose we investigate, first, the elastic stability of the probe needle according to large deflection theory of buckled bars. Micro-spring and buckling beam probe technologies are compared. Second, we determine probe forces as functions of the probe card overdrive. By using finite element analysis we then determine the stress and deformation state in the probe pad and underneath. Various stress criteria are used to assess and rank fracture risk in brittle and ductile material members of the BEOL stack.