We present comparative characteristics of microwave variable capacitors (varactors) fabricated on Na0.5K0.5NbO3 (NKN), AgTa0.5Nb0.5O3 (ATN), and Ba0.5Sr0.5TiO3 (BST) ferroelectric films grown by rf-magnetron sputtering (NKN) and pulsed laser deposition (ATN and BST) techniques on the sapphire. Two port 2μm finger gap coplanar waveguide interdigital capacitors (CPWIDCs) were defined on ferroelectric films surface by photolithographic lift-off technique. Deembedding method was employed to extract properties of CPWIDC from the S parameters measured in microwave range up to 40GHz. BST films on sapphire substrates show superior tunability of 26% (20GHz, 200kV∕cm), whereas ATN films possess the lowest tanδ=0.06 at 20GHz and extremely low dispersion of 4.3% in a whole frequency range of 45MHz–40GHz.
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Kim et al. (2006) studied this question.
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