Dielectric Breakdown of Cu Interconnect under Cycling Eelctric Field and Temperature

Abstract

As integrated circuits scale down in size, their interconnects are placed under harsher conditions such as high electric fields, high current density, and high temperature. These conditions can cause a significant reliability issue in a Cu interconnect. Cu is well-known to migrate into the dielectric. This migration is accelerated under a high electric field and high temperature. A time-dependent dielectric breakdown (TDDB) is one of the most significant reliability issues in a Cu interconnect. A conventional TDDB test is conducted under a constant electric field and temperature. However, under real operating conditions, both the polarity of the electric field and the temperature is alternated. Therefore, this thesis concentrates on a time-dependent dielectric breakdown in a Cu interconnect under alternating polarity bias and after thermal cycle stress. First, the bias polarity effect of a Cu-migration-induced failure was revealed using a simple metal-insulator-semiconductor (MIS) structure. A Cu migrating system (Cu/SiO2) and non-migrating system (Al/SiO2) were compared. In the Cu/SiO2, the leakage current increased and decreased as the polarity of the applied electric field alternated during forward and reverse pre-BTS (bias temperature stress), whereas the current in the Al/SiO2 did not change. The leakage current increase in the Cu/SiO2 was due to the increase in the number of PF traps from the Cu migration under forward bias. The leakage current decrease under reverse bias was due to a recovery from backward migration, which was revealed through a voltage ramp test. The polarity effect of Cu migration was confirmed. The dielectric contamination due to Cu migration can be recovered through the polarity effect. Next, this study considered the real device operational conditions of a real device. The mechanism of Cu-ion migration was investigated in damascene Cu/SiO2 interconnects. The lifetime was investigated as the most useful factor for estimating reliability. The polarity effect differed from the MIS structure, because a damascene structure has both sides of a Cu electrode. Cu ions were injected at opposite sides of the electrode while reverse bias was applied. Backward migration and recovery cannot achive the during remove the application of reverse bias. Backward Cu-ion migration was investigated using TDDB tests under an alternating polarity. When alternating-polarity bias stress was applied, the Cu-ion migration could not be completely recovered because the diffusional and drift fluxes were in opposite directions. Therefore, the TDDB lifetime increases under the alternating-polarity bias stress than under DC. Longer TTFs were obtained with an increase in frequency. Finally, the stress effect was considered. In this chapter, a time-dependent dielectric breakdown after a thermal cycle is investigated. Although the samples suffered thermal stress, the TDDB reliability was enhanced. An increase in lifetime and decrease in leakage current were observed in both wafer-level and package-level TDDB tests. There are two possible explanations for this reliability enhancement, the annihilation of reactant residue in the inter-metal-dielectric (IMD), and thermo-mechanical considerations. The reliability enhancement mechanism originates from the thermo-mechanical effect rather than the annihilation of reactant residue. This was confirmed through an application of heat. Because the TC condition used in this study is a tensile-to-tensile stress cycle, the Cu interconnect tends to shrink. Therefore, the Cu interconnect pull down the capping layer, which enhances the interface between this layer and the IMD. It is expected that low temperature aging can enhance the TDDB reliability