Ivoyl P. Koutsaroff

Effect of the thermal expansion matching on the dielectric tunability of (100)-one-axis-oriented (Ba0.5Sr0.5)TiO3 thin films

The impact of the residual strain induced by the thermal strain on the dielectric tunability was systematically studied for rf sputtered (100)-one-axis-oriented polycrystalline (Ba0.5Sr0.5)TiO3 films. These films were grown on various substrates with different thermal expansion coefficients [α(sub)] covered with a stack of (100)cSrRuO3∕(100)cLaNiO3∕(111)Pt layers. The residual strain was ascertained to linearly increase with the increase in α(sub) by enhancement of the surface-normal lattice spacing of (Ba0.5Sr0.5)TiO3 and Pt. Dielectric tunability of the films also linearly increased with the increase in α(sub). These results clearly demonstrate that dielectric tunability tailoring of the (Ba0.5Sr0.5)TiO3 films can be achieved by using residual thermal strain.

Orientation Dependence of Epitaxial and One-Axis-Oriented (Ba0.5Sr0.5)TiO3 Films Prepared by RF Magnetron Sputtering

The dependences of electrical properties on (111) and (100) orientations were investigated for (Ba0.5Sr0.5)TiO3 films with in-plane random (one-axis-oriented films) and aligned orientations (epitaxial films) prepared by RF magnetron sputtering. Epitaxial films with (100) and (111) orientations were grown on (100)cSrRuO3∥(100)SrTiO3 and (111)cSrRuO3∥(111)SrTiO3 substrates, while one-axis-oriented films were grown on (100)cSrRuO3/(100)cLaNiO3/(111)Pt/TiO2/SiO2/Al2O3 and (111)cSrRuO3/(111)Pt/TiO2/SiO2/Al2O3 substrates, respectively. Films with the (111) orientation had larger relative dielectric constants, er, measured at an oscillation of 20 mV and 100 kHz under an applied dc bias electric field of 0 kV/cm and a larger tunability against the dc bias electric field than (100)-oriented films for both epitaxial and one-axis orientations. These data show the dependences of er and tunability on orientation were existed, irrespective of the in-plane orientation and the thermal strain caused in the film by the substrates.

Effect of bottom electrode on dielectric property of sputtered-(Ba,Sr)TiO3 films

(Ba0.5Sr0.5)TiO3 (BST) films were deposited on (111)Pt/TiO2/SiO2/Al2O3 substrates by rf sputtering. By inserting a thin layer of SrRuO3 in between BST film and (111)Pt electrode, the BST films grew fully (111)-oriented without any other orientations. In addition, it enables us to reduce the growth temperature of BST films while keeping the dielectric constant and tunability as high as those of BST films directly deposited on Pt at higher temperatures. The dielectric loss of the films on SrRuO3-top substrates was comparable to that on Pt-top substrates for the same level of dielectric constant. The results suggest that the SrRuO3 thin layer on (111)Pt electrode is an effective approach to growing highly crystalline BST films with (111) orientation at lower deposition temperatures.

Characterization of Thin-Film Decoupling and High-Frequency (Ba,Sr)TiO3 Capacitors on Al2O3 Ceramic Substrates

In this paper we present the results of the characterization of parallel-plate thin-film (Ba1-x,Srx)TiO3 (BST) capacitors, to demonstrate their suitability for use as decoupling capacitors (a capacitance as high as 0.34 µF and a capacitance density of up to 70 fF/µm2) and as tunable RF components (a small capacitance from 0.5 pF to 16 pF, a high tunability of 4.22:1 at 10 V and a capacitance density of up to 34 fF/µm2). BST films of different compositions, (Ba0.7Sr0.3)TiO3 and (Ba0.5Sr0.5)TiO3, were grown by metal-organic decomposition (MOD) and RF magnetron reactive sputtering on Pt/TiOx/SiO2/Al2O3 ceramic substrates. For large capacitors (2.25 mm2), capacitance and tan δ were measured at low frequencies (1 kHz) using an LCR meter. Smaller capacitors (16 µm2 to 961 µm2) were characterized in the frequency range of 0.01–20 GHz. Capacitance, tan δ and equivalent series resistance (ESR) were extracted from two port scattering parameters obtained using a vector network analyzer (VNA). The relationships between dielectric loss, tunability and commutation quality factor (CQF) vs BST composition and deposition conditions were outlined.

Effect of the Strain from the Substrate on Tunability of(100) One-axis Oriented (Ba 0.5 Sr 0.5 )TiO 3 Thin Films

The impact of the residual strain induced by the thermal strain on the dielectric tunability was systematically studied for (100)-one-axis-oriented polycrystalline (Ba_0.5Sr_0.5)TiO₃ films. These films were grown on various substrates with different thermal expansion coefficients [a_(sub.)] covered with a stack of (100)_cSrRuO₃/(100) _cLaNiO₃/(111)Pt layers. The residual strain was ascertained to linearly increase with the increase in a_(sub.) by enhancement of the surface-normal lattice spacing of (Ba_0.5Sr_0.5)TiO₃ and Pt. Dielectric tunability of the films also linearly increased with the increase in a_(sub.). These results clearly demonstrate that dielectric tunability tailoring of the (Ba_0.5Sr_0.5)TiO₃ films can be achieved by using residual thermal strain.