While the spacing of interconnect structures has continued to scale with technology generation, operating voltages have not scaled accordingly leading to inevitable higher electric fields across BEOL dielectrics. Moreover, the introduction of ULK materials with lower dielectric constant is necessary to scale capacitance, however, this comes at a significant increase in integration complexity and reliability challenges due to the degraded thermo-mechanical properties and lower electrical breakdown strength of the dielectric. As a result of these combined factors, time dependent dielectric breakdown (TDDB) has become a critical aspect of the reliability of interconnect structure for sub-80nm pitch technology. In many of the models for TDDB, the electron flux through the dielectric directly determines the lifetime of a structure. Hence, in order to determine how to continue to scale ULK materials, a fundamental understanding of the critical ULK material properties needed to improve TDDB reliability behavior is needed. This paper will review our approach to tailoring porous ULK films with varying porosity, pore structure, and carbon bonding, and reveal the impact of these properties on plasma damage, mechanical properties, and reliability. Results showing the impact of dielectric structure and extrinsic agents such as moisture and copper ions on the breakdown process will also be presented and used to discuss factors that limit TDDB lifetime and the desirable material properties that can lead to a highly reliable future CMOS technology. © 2012 IEEE.