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Development of an Improved Process for Integrated Passive Capacitors
Keywords: Ta2O5 capacitors, diffusion barrier, oxygen vacancies
Planar Ta2O5 capacitors fabricated on silicon or flex could find a wide variety of applications, due to their high levels of capacitance per unit area. Oxygen vacancies in the dielectric are believed to cause these capacitors to fail prematurely, impacting overall reliability. Ta2O5 dielectric films formed by reactive sputtering show low yield and give large leakage currents due to the high defect density of the film. Defect density was lowered by the densification of the dielectric by anodization after sputtering. Dielectric films produced by this combination technique yielded almost perfect oxide dielectric. Though these capacitors showed very high yield and low leakage right after fabrication, they tend to degrade over time, failing at a low breakdown voltage. This failure was probably caused by the dielectric degradation or diffusion of oxygen in the dielectric over time. Diffusion of oxygen can be prevented by depositing a diffusion barrier above and below the oxide dielectric, forming a ‘sandwiched’ structure. TaN is a proven diffusion barrier which is used here for sandwiching the dielectric. Wafers were tested in the as-deposited state and also after thermal annealing at 100°C and 150°C for 7 days. TaN sandwiched capacitors showed 99.99% yield, proving its ability to act as an excellent diffusion barrier. Thermal annealing did not change the defect density, indicating high stability of capacitors. These capacitors showed low leakage, 0.1 µA for voltages as high as 20 V. Thus, high yield, defect-free, and low-leakage capacitors were fabricated by this sandwich structure. A similar method was extended to Nb2O5 capacitors with a NbN diffusion barrier, and the results are compared.
Susan Jacob, Graduate Student
University of Arkansas
Fayetteville, AR

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