Thin film barium strontium titanate capacitors for tunable RF front-end applications


Tiggelman, Markus Petrus Josephus (2009) Thin film barium strontium titanate capacitors for tunable RF front-end applications. thesis.

Abstract:In this thesis, the results of intensive electrical characterization, modeling
and the design of hardware with thin film tunable capacitors, i.e., dielectric
varactors, has been presented and discussed. Especially the quality factor
Q and the tuning ratio of the tunable capacitors have been studied, since
these are crucial parameters for reconfigurable RF front-end applications
for mobile phones.
In Chapter 2, a literature survey was summarized describing the state of the
art Q and for different tunable capacitor technologies based on varactor
diodes, dielectric varactors, semiconductor-switched capacitors, and micromachined
electro mechanical systems. The Q() relations were described
separately by using basic physical models. However, the relations could be
described with a single Q() formula. The study concludes that RF microelectro-
mechanical switches have the highest tuning potential with a high
Q, but are relatively large in size. For continuous, moderate tuning ranges
highly doped GaAs varactor diodes could offer the best performance. Dielectric
varactors will be most attractive for small physical size and low cost
In Chapter 4, it was discussed how RF tunable capacitors based on thin film
barium strontium titanate (BST) were designed, fabricated, and measured.
In addition, a dedicated analysis technique was developed to separate the
electrical dissipation in the resistive electrodes from the dissipation in the
dielectric oxide films.
In Chapter 5 it was shown how voltage-dependent impedance measurement
errors, using a commercial wide-band bias-tee, could be prevented in RF C(Vdc) measurements by an additional open calibration or by employing a
custom-made wide-band bias-tee.
In Chapter 6, the Q and were studied as a function of barium to strontium
composition and doping in the BST films. A higher Q and a lower was
measured for a lower barium content in the BST films. Cation doping in the
BST films can have a similar effect. The electrical characterization results
were utilized as an input to further improve the capacitors RF performance.
In Chapter 7, a ferroelectric capacitor model has been implemented as a
new component in the design environment ADS. The model can describe a
C(Vdc) measurement by incorporating only 4 fit parameters.
In Chapter 8, a 2nd order bandpass transmit filter and a low-pass impedance
matching network was designed and simulated in ADS, to obtain the Q()
requirements for ferroelectric capacitors. A very high Q was required in the
filter for the inductor and capacitor components to fulfill the customer specifications.
Furthermore, an adaptive antenna impedance matching network
was designed and simulated for a user, which covers the antenna with his
hand resulting in a de-tuned antenna. Two cascaded low-pass L-sections
improved the antenna impedance matching. The simulation results, using
practical feasible values for the components, for the latter networks pointed
out that tunable ferroelectric capacitors are promising components.
Summarizing this thesis, the simulations and device characterization provided
the link between the applications and the performance requirements
of tunable capacitors. This could be the basis to optimize the ferroelectric
materials and devices for RF applications.
Item Type:Thesis
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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