The goal of this work was to study the influence of shrinking the gap width between the fingers of interdigitated tunable capacitors (IDCs). Voltage control of the capacitance was achieved with a 500-nm-thick Ba0.5Sr0.5TiO3 film which is in paraelectric state at room temperature. Eight devices with finger spacing ranging from 3 μm down to 0.25 μm were fabricated by the sol-gel deposition technique, electron beam patterning, and gold evaporation. The equivalent capacitance, quality factor, and tunability of the devices were measured subsequently by vector network analysis from 40 MHz to 40 GHz and for a dc bias voltage varying from -30 V to +30 V. This experimental study mainly shows that a decrease of the gap below 1 μm 1) introduces a frequency dependence of the capacitance caused by resonance effects with the finger inductance; 2) degrades the quality factor above 20 GHz, and 3) optimizes the tunability of the devices by enhancing the local electric field values. As a consequence, some trade-offs are pointed out related to the goal of ultra-thin ferroelectric film which can be voltage controlled by means of finger-shaped electrodes with deep submicrometer spacing.

We investigated the dielectric losses of doped and undoped BaSrTiO(3) (BST) from thermal noise measurements. The results are compared to impedance measurements. The value for the frequency independent loss angle is about tg delta=2 x 10(-2) in the range 100<f(Hz)<10(5). The thermal voltage noise of the BST capacitor with losses has a 1/f spectrum in agreement with 4kT R(Z) and a frequency independent tg delta. The detection limits due to the low noise voltage amplifier are investigated and experimentally verified. The frequency range f(high), f(low), where the "1/f thermal noise" is above the background noise is characterized by the ratio f(high)/f(low)=tg(2)delta(R(in)/R(eqw)), with R(in) the input resistance of the low noise voltage amplifier and R(eqw) the frequency independent part of its equivalent noise resistance at high frequencies. (C) 2010 American Institute of Physics. [doi:10.1063/1.3327446]

BST thin films with various dopants are growth by sol-gel method on Platonized silicon and MgO substrates. Thiers dielectric properties are investigated at low frequency (less than 1MHz) on silicon with plate capacitor and high frequency (up to 15GHz) by interdigital capacitor on MgO substrate. These results are discussed with the nature of dopant and show, Mg as very good candidate to reduce grain size and dielectric losses. In opposite K is good candidate as dopant of BST thin film to increase the drastically the tunability.

Lead in electronic components is prohibited by the RoHS directive since july 2006. Nevertheless, there are some exceptions for example PbZrxTi1-xO3 (PZT) materials because they present exceptional piezoelectric properties. PZT is used as sensors and actuators in numerous applications such in sonar, echography, alarm, etc. . .Up to now, neither in ceramic form nor in film form, it have been surpassed by lead free materials. At present, many studies are devoted to find a substitute to PZT as it will be probably banned in five years or so. Our work is in this objective and concerns the study of the electrical properties at low frequency, from DC to 1MHz, of ferroelectric films with thickness inferior or equal to 1 mu m. The starting material is BaTiO3 in which Ba is exchanged by Sr at different levels and also doped with various cations (monovalent, divalent, trivalent. . .). This permits us to improve the dielectric and ferroelectric properties: for example to decrease the losses and to increase the tunability. Another method to optimize the electrical properties is to study the annealing conditions: very high temperature applied a short time (950 degrees C@15mn) should be favoured. This leads to both higher dielectric constant and polarization. The pyroelectric and piezoelectric coefficients have also been improved but as concerns the piezoelectric performance, our lead free films are still inferior to the PZT one. Further optimization is needed.