Study On The Thermal Stress Reliability Of Copper Interconnects In Ic

As the feature dimension of integrated circuits (IC) reaches the 65nm node and beyond, the copper metal becomes the dominant IC interconnect material. The thermal stress-related stress-induced void (SIV or stress migration (SM)) is the significant issue affecting interconnection reliability in the copper interconnects. This thesis attempts to study the thermal stress reliability of copper interconnects in IC with the assistance of finite element analysis based on kinematic strain hardening model of copper.The effect of the structure dimensions of the copper interconnect and used SiOx and SiCOH as inter-metal dielectric (IMD) on stress and stress distribution were studied in copper interconnects. It was found that the hydrostatic stress and equivalent plastic strain level increased with the barrier thickness in interconnects. In the case of different line widths, the maximum hydrostatic stress in copper interconnects occurred at 0.48μm line width (aspect ratio of 1). Used low-k dielectric SiCOH as IMD showed significant decreased hydrostatic tensile stress level compared to SiOx in via, however, it increased the stress gradient and plastic deformation in via and the surrounding region. The existence of rigid etch stop layer helped to reduce the thermal stress and plastic strain. SIV at via bottom and bottom metal beneath via was investigated in the aspects of variation of via height, gouging via depth and via bottom barrier layer thickness due to process variation. The vias with bigger aspect ratio exhibited severely SIV reliability; however, the existence of gouging via could enhance the SIV performance of Cu interconnects. Microstructure effects with and without dummy via were also simulated to evaluate their impacts on improving SIV immunity. Wide M1 inserted with dummy via could effectively reduce SIV and double dummy vias could further enhance SIV performance. The thermal stress analysis for the air-gap copper interconnects structures which the permittivity of air is close to 1 was also studied. It was found that three stress weak points were existed in M1 and via. For several possible air-gap type copper interconnects with air-gap in the metal or extending to via level, combination of fully dense CVD SiCOH and air-gap extension into the via level indicated better thermal stress performance.