||Compared with other varactor technologies, the ferroelectric varactor possesses the advantages of high capacitance density, high tunability, low bias voltage, and low fabrication complexity. Because ferroelectric thin film often undergoes high-temperature process during either the deposition or the post-annealing, the bottom electrode of a parallel-plate-based ferroelectric thin-film varactor must be thin enough to avoid being roughened under such high temperature, which would degrade the quality of the ferroelectric thin film. However, the thin bottom electrode, which causes higher radio-frequency (rf) loss, often limits the quality factor of the ferroelectric varactors. To solve this problem, we propose to expose the bottom electrode by making a via-hole from the backside of the substrate after the high-temperature process is done and then thicken the bottom electrode by electroplating.|
In this work, we develop a through-substrate-via fabrication process on silicon to implement ferroelectric varactors with thick electrodes. In Chapter 2, the structure and the fabrication process of the proposed ferroelectric varactor with thick electrodes are described, whereas the measurement results are presented in Chapter 3. Measurement results shown that, the fabricated ferroelectric varactor exhibits a tunability of 1.3:1 under 10-V bias and its quality factor at 2.4 GHz is about 10. The measured quality factor unfortunately does not see improvement. Nevertheless, by the measurement result of the GSG (ground–signal–ground) open-circuit test structure, we find that it is because the rf loss of the high-resistivity silicon substrate in use is not as small as expected. If the effect of GSG pad is de-embedded, the quality factor of the ferroelectric varactor is increased from the original 10 to about 40.
To conclude, we have successfully developed the fabrication process of ferroelectric varactors with through substrate vias on silicon. Though the measurement results have not yet demonstrated the expected improvement of the quality factor, we have found that it is due to the high rf loss caused by the high-resistivity silicon substrates we use. In the future, we plan to incorporate into our fabrication process the procedures that stabilize the high-resistivity silicon, making the substrate to exhibit the expected low-loss property.
|| T. Riekkinen, J. Molarius, and M. Ylilammi, “Electrode metallization for high permittivity oxide RF thin film capacitors, ”J. Eur. Ceram. Soc., 27, 2983–2987, 2007.|
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