TLP analysis of variation in ESD performance by co-firing with copper electrodes in SrCoO₃ doped-ZnO based multilayer varistors

抄録

This would be the first report that transmission line pulse (TLP) analysis for stability against electrostatic discharge (ESD) of Cu-MLCVs reveals the effect of ZnO grain resistivity under ESD zapping. The influence has been evident from results over a wide range of V–I characteristics provided by TLP measurement. ESD stability is found to be closely related to variations of ZnO grain resistivity, greatly depending on oxygen partial pressure (P_O2) during sintering. The lowering of ZnO resistance leads to high stability against ESD. Thus, the stability is demonstrated to be drastically improved with decreasing ohmic resistance down to the lowest levels by sintering below P_O2 of 1.6 × 10⁻⁹ MPa. Then, influence of Cu diffusion on their properties of MLCVs was investigated in bulk-disks of SrCoO₃ doped-ZnO with various CuO additions. The enhancement of ZnO grain resistivity is caused by the slight diffusion of Cu⁺ during the sintering in a relatively weak reducing P_O2 above 2.2 × 10⁻⁹ MPa, resulting in low ESD stability. However, ZnO grain resistivity is not influenced by Cu diffusion below 0.005 mol % as CuO, thus Cu-MLCVs containing below the affected level have been considered bringing high ESD performance. It was confirmed experimentally that sintering of Cu-MLCVs in the optimum reducing atmosphere provides high ESD stability and significant suppression (about 1100 to 150 V) for ESD zapping at 8 kV under IEC61000-4-2. Cu-MLCVs obtained are able to have high protection performance as well as ESD stability at the same level as a device in practical use. Lowering of ZnO resistivity by a decrease of Cu diffusion is believed to give benefits to overall performance of Cu-MLCVs.