Study of Tunability of BST with Different Structures for Microwave Applications
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Study of Tunability of BST with Different Structures for Microwave Applications Heba B. El-Shaarawy1, 2, Sébastien Pacchini1, 2, Badreddine Ouagague1, 2, Sandrine Payan3, Anthony Rousseau3, Mario Maglione3, Robert Plana1, 2 1
CNRS ; LAAS ; 7 avenue du colonel Roche, F-31077 Toulouse, France.
2
Université de Toulouse ; UPS, INSA, INP, ISAE ; LAAS ; F-31077 Toulouse, France.
3
ICMCB ; CNRS; Université de Bordeaux, 87 Avenue du Docteur Albert Schweitzer, 33608, PESSAC, France. ABSTRACT This paper addresses the characterization of the ferroelectric Ba0.6Sr0.4TiO3 on different substrates using three different microwave components. First, at low frequencies, metalinsulator-metal (MIM) capacitors are used to investigate the variation of the BST dielectric constant and loss tangent for different biasing voltages. BST shows a variation for the dielectric constant from 380 to 130, recording a tunability range of 66 %, and loss tangent of 0.027 to 0.005. In the range of frequency from 1 to 40 GHz, coplanar waveguides (CPW) are used to investigate the effective dielectric constant of BST on four different substrates, HR silicon substrates covered by silicon dioxide, silicon covered by silicon dioxide and silicon nitride, magnesium oxide (MgO (100)), and R-plane sapphire (Al2O3) substrate, all covered with 350 nm BST layer. The effective dielectric constant over silicon substrates covered by silica and BST is 7.2, 6.3 for Al2O3 substrates and 5.8 for MgO; and for the loss tangent, Al2O3 and MgO give about 0.03, while silicon substrates suffer higher values of 0.08 to 0.25. Finally, to study the tunability of microwave structures on the investigated substrates, interdigital capacitors (IDC) are fabricated and measured for different biasing voltages ranging from 0 to 55 V. IDCs over MgO show a tunability of 8.3%, while IDCs over sapphire show 20%.
INTRODUCTION Nowadays, the development of wireless communication systems concentrates on two main aspects; enhancement of performance with respect to losses, reliability and temperature stability, and high integration density that is achieved through the miniaturization of microwave components and the introduction of multi standard functionalities. Ferroelectric materials are materials able to produce internal electric polarization that changes with an externally applied electric field, changing the material electrical characteristics with respect to effective dielectric constant and loss tangent [1]. Since the late 1960’s, ferroelectric materials have attracted the interest of many researchers and have been extensively studied for the implementation of tunable microwave and millimeter wave applications, such as, tunable bandpass and bandstop filters, impedance matching networks, tunable antennas and antenna arrays, voltage controlled oscillators (VCOs), etc. [1]-[5]. Owing to its interesting characteristics with respect to its high dielectric constant, high tunability in applied electric field and low-loss tangent, Barium strontium titanate (BST) has showed a great pr
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