FERROELECTRIC FILMS

BaSrTiO₃ ferroelectric films for microwave applications

Ferroelectrics: Advantages:
• Advantage in loss level at frequency higher than 10 GHz
• Potentially low cost of ferroelectric devices production (Only 2-3 litho masks required)

Field of microwave applications:
• Electrically tunable microwave devices (tunable filters, phase shifters, varactors, channel switches, limiters, etc.)

Requirements:
• Tunability of ferroelectric films ≥2
• Loss tangent less than 0.01 at 1 GHz
• Loss tangent less than 0.04 at 30 GHz
• Temperature stability of capacitance in operating temperature range

Structure properties of BSTO films

Composition of as-deposited films is investigated by secondary ion mass-spectrometry (SIMS). According to SIMS the stehiometry of all deposited films corresponds to the sputtered target composition ±5 %. Depth distribution of Ba, Sr and Ti is uniform ±5 % as well.

The crystal structure of BSTO films on sapphire and alumina substrates is investigated by X-ray diffraction. All films deposited under deposition conditions mentioned above are polycrystalline and contain grains of (hk0), (hkl) and (h00) orientations regardless of substrate material. Film intrinsic tension decreases with the deposition temperature increase.

Electrophysical properties of (Ba,Sr)TiO₃ capacitors vs. deposition temperature

For all capacitors prepared from targets of different compositions, there is an obvious increase both of capacitor tunability and loss tangent with an increase of film deposition temperature (Ts). The increase of Ts higher 800ºC does not result in any significant increase both tunability and loss level. Dielectric permittivity increases with the deposition temperature as well.

Electrophysical properties of (Ba,Sr)TiO₃ capacitors vs. target composition

• The best tunability can be obtained in the case of BSTO films deposited from the 50% Ba content target. These films, however, exhibit an intolerably high loss factor for microwave applications (see the red oval)
• According to maximum value of commutation quality factor, the best result has been achieved by using the target with 30% Ba content. Only 30% Ba / 70% Sr films satisfy microwave requirements (see the green oval)

Electrophysical properties of (Ba,Sr)TiO₃ capacitors vs. film thickness

The variations of tunability of BSTO capacitors with film thickness can be explained by parallel connection of untunable capacitance of substrate and tunable capacitance of ferroelectric (FE) layer. At starting section of dependence contribution of FE layer tunable capacitance in total capacitance is low. At further increasing of film thickness FE layer capacitance becomes significantly higher than capacitance of substrate. For film thickness above 1 μm there is a phenomenon of electrical field strength decreasing in the gap of capacitor. Thus, optimum value of BSTO film thickness in planar structures was found as 1 μm and optimum composition of BSTO films was found as 30% Ba from points of view of high tunability and low loss.

Electrophysical properties of (Ba,Sr)TiO₃ capacitors vs. frequency

The low level of dielectric losses at frequency range 1-60 GHz and rather high tunability (n ≈ 2) makes it possible to design low cost tunable ferroelectric microwave devices with promising figures of merit.