Evaluation of WLP Dielectrics for High Voltage Applications

Due to the excellent mechanical and electrical properties together with a cost efficient processing, the thin film polymers are used in various applications as an interlayer dielectric (ILD) material where the device is generally working at CMOS like low voltages. The integration of high voltages in the redistribution layer becomes more attractive in future to realize higher system miniaturization and integration. Regarding to this, the power transformations could be integrated on wafer level for direct support of the CMOS device without an additional power transformation unit. Also high voltage III-V semiconductor dies will be integrated in Fan-Out packages. Thin film polymers have the potential to meet the demands for high voltage applications like high break down voltage and low leakage current. Well established thin film polymers like CYCLOTENE (Benzocyclotbutene - BCB) show a break down voltage in the range of 530 V/um. Compared with inorganic passivation such as thermal SiO2 with a breakdown voltages from 400 up to 1000 V/μm the BCB is quiet compatible regarding to the processing windows of layer thickness from BCB in the range from 5 to 20 um. BCB has been widely adopted in electronic applications, due to its low dielectric constant, excellent chemical resistance, and high thermal stability (glass transition temperature, T g > 350 °C) after hard-curing. The BCB has several advantages, especially the good process capabilities such as a photo structuring capability, the low temperature and the vacuum-less processing, might be interesting to use it in high voltage applications. This paper contains the profound investigation of the behavior of BCB at high voltages. Regarding the usability analyzing for the BCB as high voltage dielectric the break down voltage behavior was estimated and a time depending breakdown voltage was observed regarding the duration of the voltage application. This behavior is not yet published in the literature for BCB. MIM structures with 1 mm and 5 mm radius were manufactured on wafer level for this investigation. The leakage current of a few pA is measured and the time-dependent dielectric breakdown (TDDB) is extracted from Current-Time characteristics and shown in Weibull-plots. The measurement shows a spectrum from 350 V/μm to 450 V/μm, depending of the duration of the voltage and the level of the applied voltage. It is found that there is an exponential linkage between field strength and the time till the breakthrough occurs. The break through at 450 V/μm occurs after about 280 seconds, while the sample at 350 V/μm withstands longer by the factor of 13. These results will a very important milestone for future high power applications.