Nanoindentation for reliability assessment of ULK films and interconnects structures

The structural integrity of interconnect structures containing ultra-low-k (ULK) dielectrics is highly dependent on the mechanical properties of the porous dielectrics, e.g. fracture toughness elastic modulus and adhesion as well. Four-point-bending (FPB) and double-cantilever-beam (DCB) methods for the evaluation of fracture properties require out-of-fab sample preparation and testing. The reliable characterization of interfacial adhesion is important for in-line/at-line process development and control in microelectronics manufacturing. The ability to detect an out-of-spec or defective ULK film at an early process step could potentially save processing and materials cost. Therefore, the development of quick turnaround experimental methodologies for monitoring in-line/at-line mechanical stability of ULK films and ULK-containing interconnects is of great interest for semiconductor industry. This study presents two novel experimental approaches for the evaluation of inter face adhesion and mechanical robustness of on-chip interconnects structures based on nanoindentation and nanoscratch, (a) wedge indentation and (b) bump assisted BEOL stability indentation (BABSI) tests, respectively. Wedge indentation tests on ULK films with increasing porosity show a decrease of adhesion values. Correspondingly, BABSI tests show increasing failure rates for Cu/ULK interconnect structures containing mechanically weaker dielectrics.