Dielectric Response of Tantalum Oxide Deposited on Polyethylene Terephthalate (PET) Film by Low-Temperature Pulsed-DC Sputtering for Wound Capacitors

Deposition of high-${\rm k}$ tantalum oxide thin films on thin polymer substrates was investigated, using low-temperature $({-100}^\circ {\rm C})$ pulsed-dc reactive sputtering. Degradation of two different polymers, polyethylene terephthalate (PET) and polypropylene (PP), were studied as a function of sputtering conditions. Two different deposition configurations have been explored for polymer films with aluminum electrode on one side. In one configuration, tantalum oxide was deposited on the nonelectroded side of the polymer, while in the other the deposition was on the electroded side of the polymer. The two fabricated structures have been characterized for dielectric permittivity, loss, and ac conductivity as a function of frequency and temperature. Sputtering tantalum oxide on the nonelectroded side of PET substrates results in a 37% higher permittivity for PET than the series model prediction of permittivity. Higher dielectric loss and ac conductivity accompany the higher permittivity. The ${\alpha}$ bulk relaxation in PET moves to slightly higher temperatures, indicating that there is an increase in the crystallinity of the bulk polymer. This observation is supported by the broader glass transition and an additional endothermic peak around 200$^\circ {\rm C}$ in ${\rm PET}/{\rm Ta}_{2}{\rm O}_{5}$ compared to neat PET. In addition, modifications of the space charge activation energy in PET from 1.35 eV to 1.82 eV and of dc conductivity in PET from 6 $\,\times {\rm 10}^{-15}~{\rm S/m}$ to 4 $\,\times {\rm 10}^{-14}~{\rm S/m}$ is observed. Sputtering ${\rm Ta}_{2}{\rm O}_{5}$ on the electroded side of the PET, under the same sputtering conditions, results in the formation of high-k tantalum oxide with dielectric permittivity, loss, and ac conductivity of 30, $≪ 5\%$ , and ${\rm 10}^{-7}~{\rm S/m}$ at 1 kHz, respectively.