A TRL-Like Calibration for Tunable Interdigitated BST Varactors

We report on a de-embedding technique for measuring the reflection coefficients of comblike capacitors patterned on barium-strontium-titanate (BST) ferroelectric films. The technique is derived from the thru-reflect-line calibration technique and is combined with conformal mapping to retrieve the complex permittivity of BST thin films. The accuracy of the method is demonstrated over a broad frequency for 0.3-mum-thick Ba_xSr_1-xTiO₃, with x = 0.5, in a paraelectric state, grown by a sol-gel technique. The samples are characterized up to 35 GHz and exhibit a constant permittivity of 245 plusmn 12%, which is in good agreement with the values deduced from propagation wave constants of BST transmission lines.