Silicon substrate integrated high Q-factor parallel-plate ferroelectric varactors for microwave/millimeterwave applications

A. Vorobiev
Chalmers University of Technology, Sweden

P. Rundqvist
Chalmers University of Technology, Sweden

K. Khamchane
Chalmers University of Technology, Sweden

S. Gevorgian
Chalmers University of Technology, Sweden \ Microwave and High Speed Electronics Research Center, Ericsson AB, Sweden

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Ingår i: GigaHertz 2003. Proceedings from the Seventh Symposium

Linköping Electronic Conference Proceedings 8:69, s.

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Publicerad: 2003-11-06


ISSN: 1650-3686 (tryckt), 1650-3740 (online)


Parallel-plate Ba0.25Sr0.75TiO3 (BST) varactors with record high Q-factor are fabricated on Si substrate. At 45 GHz the Q-factor is about 40 and tuneability at 25 V is more than 40 % in the measured frequency range 0.045-45 GHz. These parameters are comparable or far better than corresponding parameters of best Si and GaAs analogues. The improvement in Q-factor is achieved by using thick bottom electrode consisting of Pt(50 nm)/Au(0.5 µm) allowing to reduce the microwave losses in metal layers. However; the measured loss tangent of varactor is about one order of magnitude more than SrTiO3 single crystal; indicating that there is a room for further improvement of the Q-factor. The analysis of dielectric dispersion of BST film allows to assume that the dielectric loss is associated with charged defects. The dc current through varactor is found to be controlled by Poole-Frenkel mechanism associated with the field enhanced thermal excitation of charge carriers from internal traps; possibly ascribed to be oxygen vacancies. The varactors can be used above 10 GHz to develop different high performance tuneable microwave devices integrated with Si substrates.

Thin BaxSr1-xTiO3 (BST) films have been extensively considered for applications in voltage controlled microwave devices (e.g. varactors). However; relatively high losses have limited large-scale industrial applications of these devices. Additionally; from industrial respective; it is important to integrate these devices with Si substrate and reduce required control voltages. In comparison with co-planar design the devices with parallel-plate electrodes offer lower control voltage and higher tuneability. The Q-factor of BST varactors on Pt/Si substrates is limited mainly by the losses in bottom Pt electrodes which is usually 100-200 nm thick. To reduce these losses bottom electrodes made of thick Pt (up to 2 µm); Au; and Cu have been proposed. However; no Q-factor or/and tuneability of these varactors at microwave frequencies are reported explicitly. In this paper; we present the results of fabrication and microwave characterization of record high Q-factor parallel-plate BST varactors on Si substrate with thick Pt/Au bottom electrode. The Qfactor and the tuneability of these varactors are comparable or far better than the corresponding parameters of best GaAs and Si analogues. The simultaneous analysis of the microwave dielectric response and dc conduction mechanism of Au/BST/Au varactor allows to suggest the predominant mechanism of the extrinsic microwave loss and corresponding BST film microstructure imperfections.

Oxidized n-type silicon (100)Si ( ? = 5 kOm) with adhesive TiO2 (15 nm) and Pt (100 nm) layers are used as substrate. To complete the bottom electrode a 0.5 µm thick Au film; and a 50 nm Pt layer are deposited by e-beam evaporation at room temperature. This structure forms BST/Pt/Au varactors. For comparison; Pt(200 nm)/TiO2/SiO2/Si structures are used as substrate to form BST/Pt varactors. BST films (300 nm thick) are grown by laser ablation of Ba0.25Sr0.75TiO3 target. To keep the parallel-plate structure symmetric a 50 nm Pt and a 0.5 µm Au layers are deposited on BST film by e-beam evaporation. Top electrodes are patterned by lift-off process.


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