Buried Tantalate-Niobate Microwave Varactors

We present characteristics of microwave variable capacitors (varactors) buried in 2.5 mum thick AgTa_0.5Nb_0.5O₃ (ATN) film pulsed laser deposited on sapphire single crystal. 2 mum gap interdigital capacitors (IDC) were fabricated by photolithographic, dry etching and lift-off processes. For comparison, similar IDCs were also defined on top of ATN film. Capacitance and loss tangent have been determined using a modified de-embedding technique in the microwave range 25 MHz -40 GHz. Buried structures show higher values of capacitance and tunability, keeping the same level of losses compared to standard topped devices and resulting in an increased K-factor = tunability/tan delta. Experimental results are explained within equivalent circuit model. Besides the increased performance, the new design avoids the need of a successive planarization step, which could be required in an integration process.