Improving TDDB reliability in Cu damascene by modulating ESL structure

Low k time-dependent dielectric breakdown (TDDB) is increasing becoming a major issue at 28 nm and beyond. Although TDDB models, such as E model, the √E model and the 1/E model, have been extensively explored, determining the BEOL processing direction for TDDB warrants further study. This study attempts to determine whether the thickness of the etching stop layer film influences the electron conduction mechanism. Cu damascene structures were designed following three approaches with ESL in various thicknesses. They were Co/ESL=0A-550A (LK: SiCO k= 3.1), Cu/ESL=0A-275A (ELK: SiCO k=2.5) and Co/ESL=0A-275A (ELK: SiCO k=2.5). Analytical results demonstrated superior breakdown fields of 8.5 MV/cm, 7.5 MV/cm and 7.5 MV/cm for Co/ESL=0A (SiCO k=3.1), Cu/ESL = 0A (SiCO k=2.5) and Co/ESL = 0A (SiCO k=2.5), respectively. TDDB results further reveal that the ESL=0A structure is essential to a long TDDB lifetime, because electrons are conducted through the ESL bulk film. The mechanism of TDDB improvement is considered to be the absence of ESL, the modified metal/LK electrical potential and the negligibility of interfacial LK/LK surface defects.