Integrated Lumped Element and Ferroelectrically Tunable Microwave Components

Doctoral thesis, 2007

This thesis treats the development of passive circuits for increased integration level in microwave front-ends. The subject is approached from two different directions. On the one hand, emerging materials for microwave integrated circuits are studied. On the other hand, novel circuit topologies are synthesized. Ferroelectric Ba0.25Sr0.75TiO3 (BSTO) varactors are considered for reconfigurable microwave circuits. Tunable compact delay lines and metamaterial phase shifters are integrated on highly resistive silicon (HRS) substrate. The circuits are implemented in coplanar configurations, coplanar strip (CPS) lines and coplanar waveguide (CPW), respectively. The problem of charge accumulation at the Si/SiO2 interface is addressed. Surface passivation techniques for HRS substrates covered by ferroelectric films are studied. It is found that established techniques for surface passivation, i.e., bombardment with Ar-ions and deposition of poly-Si, are compatible with the growth of BSTO films. It is also indicated that for millimeter-wave frequencies, multilayer structures consisting of two metal layers in parallel coupled through a high permittivity BSTO film exhibit lower loss than a single metal layer of the same thickness as the two metal layers together. Circuit topologies for generation of differential and quadrature signals are developed. The second order lattice balun (SOLB), a novel lumpedelement balun topology based on cross-coupled low-pass and high-pass filters, is synthesized. The circuit provides excellent balancing performance over bandwidths larger than one octave. For generation of quadrature signals, lumped quadrature power splitters (LQPSs) based on right/lefthanded transmission lines are designed. The LQPSs generate well quadraturebalanced signals with negligible amplitude error. Their bandwidth is restricted by the phase balance and the input matching. Depending on specifications, up to one octave bandwidth may be reached. Closed-form design equations for component values are derived for both the SOLB and the LQPSs. Both circuits are also validated by experiments. The LQPSs are implemented at a commercial GaAs MMIC foundry. The SOLB is manufactured at the same MMIC foundry as well as with surface mounted components soldered on a printed circuit board.