398 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 18, NO. 6, JUNE 2008 Investigations of Indium Tin Oxide—Barium Strontium Titanate–Indium Tin Oxide Heterostructure for Tunability Mahmoud Al Ahmad, Member, IEEE, Ludovic Salvagnac, Dominique Michau, Mario Maglione, and Robert Plana, Member, IEEE Abstract—This letter reports for the first time the interesting be- havior of the interface between indium tin oxide (ITO) as a high resistive electrode and barium strontium titanate tunable paraelec- tric thin film material. The interface is consisting of barium stron- tium titanate (BST) thin film dielectric material sandwiched be- tween two ITO high resistive layers, all are integrated above glass substrate. When dc field is applied between the ITO layers, the BST thin film material properties are tuned. It is found that the ITO/BST/ITO heterostructure exhibits a tunable resistor perfor- mance. To our knowledge; these results are never reported. Index Terms—Agile components, barium strontium titanate (BST), characterization, distributed tuning, indium tin oxide (ITO), matching circuits, material parameters, metamaterial, thin film, tunability, tunable circuit, tunable resistor. I. INTRODUCTION A DAPTIVE and/or tunable microwave components in- cluding filters, variable radio frequency (RF) attenuators and RF variable-gain amplifiers are of great importance for future systems [1]. Tunability is being driven by a number of very interesting enabling technologies [2]. Among the different tuning technologies, barium strontium titanate (BST) has an advantage of tunability at room temperature and finds great potential applications in tunable devices and circuits designs [3]–[14]. The quality of the BST thin film depends on its pro- cessing conditions such as temperature, thickness and substrate [15]. For BST tuning, highly conductive electrodes are often used to apply bias either in parallel plate topology [16] or interdigited topology [17]. Moreover, the characterization of BST thin films has been carried out using these conductive electrodes. Integrated circuits having a variable RF resistor commonly use PIN diodes for signal path switching and controlled signal attenuation. metal oxide semiconductor field effect transistors (MOSFETs) are often used as electronically tunable resistors, by taking advantage of the resistive nature of their channel in Manuscript received December 26, 2007; revised January 30, 2008. M. Al Ahmad, L. Salvagnac, and R. Plana are with LAAS CNRS, Univer- sity of Touluse, Toulouse 31077, France (e-mail: al-ahmad.mahmoud@ieee.org; m_al_ahmad@hotmail.com). D. Michau and M. Maglione are with ICMCB CNRS, University of Bor- deaux, Pessac 33608, France (e-mail: maglione@icmcb-bordeaux.cnrs.fr). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2008.922631 the triode region of operation [18]. The key to the linearisation achieved is the application of voltages across the gate and across the body, in such a way that the gate-channel and body-channel potentials remain constant all along the channel [19]. A high resistive thin layer such as indium tin oxide has been used as bias electrodes to decouple the DC bias from the RF signal path and to reduce the tuning voltage as demonstrated by Yoon et al. [20]. Leng et al. [21] have prepared BST thin films on indium-doped tin oxide coated quartz substrates for optimization purposes. In this work, the feasibility of the BST tunability employing high resistive indium tin oxide (ITO) layers is explored. The in- terface of ITO/BST/ITO shows a tunable resistor when voltage bias is applied between the ITO layers which are separated by the BST thin film material. II. FABRICATION AND MEASUREMENTS The fabricated sample under investigations and its cross sec- tion are shown in Fig. 1(a) and Fig. 1(b), respectively. The func- tionality description of each layer is detailed in Table I. A glass substrate of thickness 500 m has been coated with ITO a (90:10 wt%) thin oxide layer. The ITO was sputtered with 10% of the 90% of the . The first ITO is deposited at room tem- perature. Next, the first BST thin dielectric layer is deposited by RF sputtering techniques as described in [17]. The second ITO oxide layer deposited at room temperature above the BST layer and finally the second BST layer is deposited. Layer 4, the second BST layer is not required for the tunable resistor but for further device processing and integration issues. DC measurements were performed to check for the electrical connection of the high resistive ITO layers using the FLUKE 75 III multimeter. The device has been measured using an Agilent 4294A impedance analyzer. The DC bias was superimposed on the same RF probes. The measurements have been performed from 10 kHz up to 110 MHz. The probes were placed on the surface of ITO layers as shown in Fig. 1(b). The measured per- formance as a function of applied voltage is shown in Fig. 2. III. RESULTS ANALYSIS The heterostructure exhibits a resistor that could be tuned from 22 k to 1 k with the application of 10 V. The tuning ratio is shown in Fig. 3. The tunability is sensitive for low bias values, i.e., 7.5:1 at 1 V, where as small sensitivity for higher ap- plied voltages. Tunability versus voltage for smaller values with step of 30 mV is shown in Fig. 4. 1531-1309/$25.00 © 2008 IEEE