Fabrication and characterization of ferro- and piezoelectric multilayer devices for high frequency applications

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Informaatio- ja luonnontieteiden tiedekunta | Doctoral thesis (article-based)
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By means of thin film technology a reduction of size, cost, and power consumption of electronic circuits can be achieved. The required specifications are attained by proper design and combinations of innovative materials and manufacturing technologies. This thesis focuses on the development and fabrication of low-loss ceramic thin film devices for radio and microwave frequency applications. The materials, growth conditions, and physical properties of the films and device structures are discussed in detail. Moreover, special emphasis is placed on the integration of highly conductive low-loss electrode materials into parallel-plate structures. The thin films were prepared by sequential magnetron sputtering from metallic and ceramic deposition targets. The devices under study include tunable ferroelectric barium strontium titanate and lead strontium titanate parallel-plate capacitors, and piezoelectric aluminum nitride thin film bulk acoustic wave resonators. Furthermore, tantalum pentoxide and tantalum nitride thin films were investigated for capacitor and resistor applications. As electrode material we used Au, Cu, Mo, and Pt. The use of highly conductive low-loss Cu electrodes was only possible after the development of a new layer transfer fabrication method for parallel-plate ceramic devices. This method, which was successfully used to fabricate tunable ferroelectric capacitors and AlN bulk acoustic wave resonators, allows for high-quality ceramic film growth on suitable substrate and seed layers and, most importantly, deposition of the bottom and top electrodes after high-temperature reactive sputtering of the ceramic material. Optimization of the ceramic growth conditions and the integration of these functional materials into low-loss parallel-plate structures resulted in state-of-the-art device performance. Key achievements include, device quality factors of more than 100 up to GHz frequency in ferroelectric parallel-plate capacitors, the tailoring of ferroelectric film properties using substrate bias during magnetron sputtering, and very efficient electro-acoustic coupling in Mo/AlN/Mo bulk acoustic wave resonators.
ferroelectric, piezoelectric, parallele-plate capacitator, dielectric tuning, dielectric loss, RF applications
  • [Publication 1]: Tommi Riekkinen, Tomi Mattila, Sebastiaan van Dijken, A. Lüker, Qi Zhang, Paul B. Kirby, and Ana M. Sánchez. 2007. Ferroelectric parallel-plate capacitors with copper electrodes for high-frequency applications. Applied Physics Letters, volume 91, number 25, 252902, 3 pages.
  • [Publication 2]: T. Riekkinen, J. Molarius, and M. Ylilammi. 2007. Electrode metallization for high permittivity oxide RF thin film capacitors. Journal of the European Ceramic Society, volume 27, numbers 8-9, pages 2983-2987.
  • [Publication 3]: T. Riekkinen and J. Molarius. 2003. Reactively sputtered tantalum pentoxide thin films for integrated capacitors. Microelectronic Engineering, volume 70, numbers 2-4, pages 392-397.
  • [Publication 4]: T. Riekkinen, J. Molarius, T. Laurila, A. Nurmela, I. Suni, and J. K. Kivilahti. 2002. Reactive sputter deposition and properties of TaxN thin films. Microelectronic Engineering, volume 64, numbers 1-4, pages 289-297.
  • [Publication 5]: T. Riekkinen, A. Nurmela, J. Molarius, T. Pensala, P. Kostamo, M. Ylilammi, and S. van Dijken. 2009. Influence of the seed layer on structural and electro-acoustic properties of sputter-deposited AlN resonators. Thin Solid Films, volume 517, number 24, pages 6588-6592.
  • [Publication 6]: Tommi Riekkinen, Jan Saijets, Pasi Kostamo, Timo Sajavaara, and Sebastiaan van Dijken. 2009. Influence of substrate bias on the structural and dielectric properties of magnetron-sputtered BaxSr1−xTiO3 thin films. arXiv:0909.1637v1 [cond-mat.mtrl-sci].