Min-Hwan Kwak

Microwave properties of compositionally graded (Ba, Sr)TiO3 thin films according to the direction of the composition gradient for tunable microwave applications

Compositionally graded (Bax, Sr1−x)TiO3 (BST) thin films were deposited on MgO substrates by pulsed laser ablation. The microwave properties of the graded BST thin films were investigated at microwave frequencies with coplanar waveguide (CPW) meander-line phase shifters as a function of the direction of the composition gradient with respect to the substrate at room temperature. CPW phase shifters using graded BaTiO3(BT)→SrTiO3(ST) and ST→BT thin films exhibited a differential phase shift of 73° and 22° at 10 GHz with a dc bias voltage of 150 V, respectively. The figure of merit of the phase shifters at 10 GHz was 14.6 deg/dB for the graded BT→ST film, and 10 deg/dB for the graded ST→BT film. The graded BT→ST thin films showed a larger phase tuning than the ST→BT thin films. The microwave properties of the graded BST films depended strongly on the crystalline structure and the direction of the composition gradient with respect to the substrate.

High Dielectric Tunability of (Ba, Sr)TiO3 Thin Films and Their Coplanar Waveguide Phase Shifter Applications

In the present work, we report the epitaxial growth and dielectric properties of barium strontium titanate (BST) thin film which can be used to make frequency and phase agile tunable microwave (MW) components with a high performance, such as tunable filters, tunable oscillators, and phase shifters for application in phased array antennas. Epitaxial BST thin films were deposited on a MgO substrate by the insertion of a thin BaTiO3 (BT) seed layer between the BST films and the MgO substrate using pulsed laser ablation. The crystalline structure, dielectric tunability and microwave properties of the epitaxial BST thin films depended strongly on the thickness of the BT seed layer. The dielectric tunability and microwave phase tuning increased with increasing thickness of the BT seed layer. An interdigitated (IDT) capacitor based on the epitaxial BST/BT(12 nm) film on the MgO substrate exhibited a large capacitance tunability of 78% at a frequency of 100 kHz and an applied electric field of 133 kV/cm. Also, a coplanar waveguide (CPW)-type phase shifter fabricated on the BST/BT(12 nm) film showed a phase shift of 153° at 10 GHz and 40 V. These results indicate that the thin BT layer with an intermediate lattice constant between those of the BST film and MgO substrate can be used as an excellent seed layer for improving the crystallization and dielectric properties of BST films, and the high microwave performance of ferroelectric MW devices.

DIELECTRIC PROPERTIES OF PARAELECTRIC Ba(Zr,Ti)O 3 THIN FILMS FOR TUNABLE MICROWAVE APPLICATIONS

ABSTRACT Paraelectric Ba(Zr,Ti)O3 [BZT] thin films were prepared on MgO substrates by pulsed laser ablation and their dielectric characteristics and tunabiltiy were investigated as a function of the Zr content. Dielectric properties of BZT thin films were measured under dc bias using planar interdigitated capacitor. The excellent tunable dielectric characteristics were achieved for x = 0.25 at room temperature; tunability ∼74.5%, tanδ ∼0.0535 at 100 kHz, indicating that it is a promising canditate for voltage tunable dielectric working at room temperature.

A K-Band Distributed Analog Phase Shifter Using Etched Ba0.6Sr0.4TiO3 Thin Films

This work presents the design, fabrication and microwave performance of distributed analog phase shifter (DAPS) fabricated on etched Ba0.6Sr0.4TiO3 (BST) thin films for K-band applications. Single phase Ba0.6Sr0.4TiO3 thin films for interdigitated capacitors (IDC) were deposited by the pulsed laser deposition (PLD) on (001) MgO substrates. The DAPS design consists of periodically loading high impedance coplanar waveguide (CPW) with tunable BST IDC, and the phase shift of the loaded line can be controlled by varying the applied bias voltage to the BST thin films. BST thin films except IDC areas were etched, which means that BST layers locally exist beneath IDC and are removed from the CPW by photolithography and RF-ion milling, to reduce the insertion loss of DAPS and to eliminate the alteration of unloaded CPW properties according to an applied dc bias voltage. The DAPS can be accurately and easily modeled using a combination of full-wave electromagnetic and microwave circuit analysis. Experimental results are shown to agree very well with the simulated ones at the frequencies of interest. The fabricated DAPS showed that the return loss was better than -13 dB through the whole K-band frequency range. The measured differential phase shift based on BST thin films was 179° and the insertion loss is -5.6 dB–-2.1 dB with increasing the bias voltage from 0 to 200 V at 20 GHz.

ENHANCED DIELECTRIC PROPERTIES OF (Ba, Sr)TiO3 THIN FILMS FOR HIGH-PERFORMANCE MICROWAVE PHASE SHIFTERS

ABSTRACT Barium strontium titanate (BST) thin films have been intensively studied as a frequency and phase agile materials for the tunable microwave devices such as tunable filter, phase shifter and phased array antennas. The dielectric properties of BST thin films, including the dielectric constant, dielectric loss and tunability of capacitance are affected by many factors, such as Ba/Sr ratio, growth temperature, crystallographic orientation, grain size, defect chemistry, oxygen vacancies, strain and stress, and dopants. In this work, we report on the enhanced dielectric properties of (Ba0.6, Sr0.4)TiO3 (BST) thin films for high-performance microwave tunable device applications. Epitaxial BST thin films were deposited on MgO substrates by the insertion of thin BaTiO3 [BT] seed layers with intermediate lattice mismatch between the BST films and the MgO substrate using a pulsed laser ablation. Dielectric measurements were carried out using planar interdigitated (IDT) capacitor in ranges of RF/microwave frequency. IDT capacitor based on epitaxial BST film exhibited the large capacitance tunability of 77% at a frequency of 100 kHz and applied electric field of 80 kV/cm. CPW phase shifter showed the large phase tuning of 153° at 10 GHz and 40 V. These results indicate that it is a promising candidate for high-performance microwave phase shifters at room temperature. †Originally presented at 2004 ISIF, Kyung Ju, Korea, April 5-8, 2004.

HTS microstrip antenna array for circular polarization with cryostat

We report the comparison results of an HTS antenna array for circular polarization with a self cryostat and comparison with a gold counterpart. The HTS antenna array designed in this work was fabricated using a YBCO superconducting thin film on an MgO substrate, and the designed resonant frequency was 11.85 GHz. We used square patches with truncated corners for circular polarization and employed a corporate feed network using sequential rotation techniques for enhancement of axial ratio. A cryostat was constructed for the measurement of each antenna, and liquid nitrogen was used as a coolant. Measurements from HTS antennas, including return loss, bandwidth, gain, efficiency and axial ratio are presented and compared with those of gold counterparts. The measured results show that useful antennas can be made using the proposed architecture for satellite communication systems.

Planar and Parallel Dielectric Properties of Compositionally Graded (Ba, Sr)TiO3 Thin Films for Tunable Microwave Applications

Compositionally graded (Bax,Sr1 − x)TiO3 [BST] ferroelectric thin films have been received much attention in graded ferroelectric devices due to their unique properties, such as large pyroelectric coefficients, large polarization offset and small temperature coefficient of dielectric constant for microwave tunable devices. Compositionally graded BST thin films were deposited epitaxially on LaAlO3 [LAO] and Nb-doped SrTiO3 [STO:Nb] substrates by pulsed laser deposition. The planar and parallel dielectric properties of compositionally graded BST epitaxial thin films ware investigated in the frequency ranges of 100 Hz ∼ 1 MHz as a function of the direction of the composition gradient with respect to the substrate at room temperature. The dielectric properties of the graded BST films depended strongly on the direction of the composition gradient with respect to the substrate. The graded ST → BT films grown on LAO and STO:Nb substrates exhibited a excellent dielectric properties than the graded BT → ST films.

Low-Frequency Dielectric Responses of Barium Strontium Titanate Thin Films with Conducting Perovskite LaNiO3 Electrode

Highly (h00)-oriented (Ba,Sr)TiO3 [BST] thin films were deposited by pulsed laser depositi on on the perovskite LaNiO3 metallic oxide layer as a bottom electrode. The LaNiO3 films were deposited on SiO2/Si substrates by the rf-magnetron sputtering method. The crystal line phases of the BST film were characterized by X-ray θ–2θ, ω-rocking curve and Φ-scan diffraction measurements. The surface microstructure observed by scanning electron mi croscopy was very dense and smooth. The low-frequency dielectric responses of the BST films grown at various substrate temperatures were measured as a function of frequency in the frequency range from 0.1 Hz to 10 MHz. The BST films have the dielectric constant of 265 at 1 kHz and showed multiple dielectric relaxations in the measured frequency region. The origins of these low-frequency dielectric relaxations are attributed to ionized space charge carriers such as the oxygen vacancies and defects in the BST film, the interfacial polarization in the grain boundary region and the electrode polarization. We also studied the capacitance-voltage characteristics of BST films.

Microwave Performance of Distributed Analog Phase Shifter Using Ferroelectric (Ba,Sr)TiO3 Thin Films

This work presents the design, fabrication and microwave performance of distributed analog phase shifter (DAPS) fabricated on (Ba,Sr)TiO3 (BST) thin films for X-band applications. Ferroelectric BST thin films were deposited on MgO substrates by pulsed laser deposition. The DAPS consists of high impedance coplanar waveguide (CPW) and periodically loaded tunable BST interdigitated capacitors (IDC). In order to reduce the insertion loss of DAPS and to remove the alteration of unloaded CPW properties according to an applied dc bias voltage, BST layer under transmission lines were removed by photolithography and RF-ion milling. The measured results are in good agreement with the simulated results at the frequencies of interest. The measured differential phase shift based on BST thin films was 24° and the insertion loss decreased from −1.1 dB to −0.7 dB with increasing the bias voltage from 0 to 40 V at 10 GHz.

Optimum Reflection-Type Phase Shifter Using (Ba,Sr)TiO3 Thin Film

In this paper, in order to obtain a large differential phase shift with a little change in applied voltage, a ferroelectric reflective load circuit has been designed on top of barium strontium titanate (Ba,Sr)TiO3 [BST] thin film. The design of the ferroelectric reflection-type phase shifter is based on a reflection theory of terminating circuit, which has a reflection-type analogue phase shifter with two ports terminated in symmetric phase-controllable reflective networks. To achieve large amounts of phase shift in low bias-voltage range, the effects of change of capacitance and transmission line connected with two coupled ports of a 3-dB 90° branch-line hybrid coupler have been investigated. A large phase shift with a small capacitance change in the parallel terminating circuit has been demonstrated in the paper.