Parameters affecting the DC breakdown strength of parylene F thin films

The effect of some experimental parameters (electrode area, thickness, temperature and voltage rising rate) on the breakdown strength of films of poly(α,α, α´,α´-tetrafluoro-p-xylylene), a fluorinated parylene (PA-F), has been studied. The measurements were performed in air on 20 capacitor structures for each condition and the two parameters of the Weibull distribution (α and β) were fitted to the data. The influences of electrode area (0.28, 4.5 and 18 mm²), film thickness (1.4, 5.0, 9.8, 21.5 and 49.4 μm), for a temperature range from 25°C to 350°C and voltage rising rate between 10 V/s and 100 V/s, have been investigated and discussed for the thinnest films. The thickness increases up to 5 μm leads to an increase in the dielectric strength with increasing the thickness whereas a continuous decrease is observed for higher thicknesses. Those results are correlated to the thickness dependent crystallinity of parylene films. The effect of electrode area, measured on the thinnest films, on the PA-F dielectric strength does not impact the Weibull parameters at 25 °C for high dielectric breakdown field values, whereas it induces a decrease in the β values at low field with increasing electrode area. This highlights the presence of randomly distributed defects in the tested structures. The temperature dependence of the dielectric strength was also investigated between 25°C and 350°C for two different thicknesses (1.4 and 5 μm) and shows negative temperature dependence in both cases. Finally, the effect of the rate of the applied field rising between 0.07 and 0.7 MV/cm.s on the thin films was studied at 25°C and 300°C and does not show any remarkable effect on the Weibull parameters.