SrTiO 3 Thin Film Varactors on Si for Microwave Applications
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SrTiO3 Thin Film Varactors on Si for Microwave Applications Andrei Vorobiev1,2, Par Rundqvist1, Khaled Khamchane3, Spartak Gevorgian1,4 1 Department of Microelectronics, Chalmers University of Technology, SE-41296 Gothenburg, Sweden 2 Institute for Physics of Microstructures RAS, N. Novgorod, GSP-105, 603600, Russia 3 Department of Microelectronics and Nanoscience, Chalmers University of Technology and Gothenburgs University, SE-41296 Gothenburg, Sweden 4 Microwave and High Speed research Center, Ericsson Microwave Systems, 431 84 Molndal, Sweden ABSTRACT
Dielectric properties of SrTiO3 (STO) thin films in Au/Pt/STO/Pt/TiO2/SiO2/Si structure are studied at microwave frequencies. STO films with thickness of 300 nm were grown by laser ablation at different temperatures on templates of Pt/TiO2/SiO2 on Si with resistivity of 5 kΩ⋅cm. XRD measurements reveal highly textured (111)STO films with small amount of (100) and (110) domains. The full with at half maximum of the rocking curve of (111)STO domains is about 2 degrees. Circular Au/Pt top electrodes with diameters in the range of 10-60 µm were formed by e-beam evaporation and lift-off process. The on chip microwave impedance of devices was measured in the frequency range of 45 MHz-45 GHz as a function of DC electric field in the range of 0-800 kV/cm. The loss tangent is less than 0.06 in the whole frequency range. Application of DC bias results in resonant absorption of microwave power at frequencies of 1.3, 3.0, 4.7, 6.4, and 9.7 GHz. The relative change of the dielectric permittivity under 800 kV/cm DC field is more than 20% in the whole frequency range. The results indicate that the varactors based on these STO films are suitable for tunable microwave applications.
INTRODUCTION
Strontium-titanate, SrTiO3 (STO), or a solid solution of Ba1-xSrxTiO3 (BSTO), are being investigated as dielectric material for tunable microwave applications. The electric fielddependent dielectric permittivity can be used to utilize thin film devices such as varactors, tunable oscillators, phase shifters, etc. Usually these devices require high tunability of permittivity and low dielectric loss. It is important, from industrial perspective, to integrate these devices with Si substrate since microwave IC’s are going to heavily rely on Si technology. The integrated parallel-plate electrode devices are especially attractive for most microwave applications due to lower control voltage and higher tunabilities in comparison with co-planar configuration. The vertical configuration places heavy demands on properties of bottom electrode. First, it should have oriented microstructure to enable the oriented growth of ferroelectric film. On the other hand, the Si industry would like to introduce the least number of new metals in their fabrication line, and preferably only noble metals. It has been demonstrated that Pt is suitable candidate as bottom electrode in STO (BSTO) incorporated parallel-plate
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structures on Si substrates [1]. However, the reported quality factors and tunabi
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