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 ¬Ķm); 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 ¬Ķm 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 ¬Ķm Au layers are deposited on BST film by e-beam evaporation. Top electrodes are patterned by lift-off process.
GigaHertz 2003. Proceedings from the Seventh Symposium
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