Tunable interdigital capacitors at microwave frequency

As applications for integrated structures advance to higher frequencies, the electrical properties of materials must be well characterized. An interdigital capacitor (IDC) structure was designed, fabricated and tested for dielectric constant characterization of substrates and films in the 1 to 20 GHz range. A mathematical model based on the physical geometry of the capacitor was implemented and an experimental technique for measuring the actual capacitance was developed. Microwave characterization of the IDC with a vector network analyzer and simulations in CST Microwave Studio were compared with the analytical results for determining capacitance. The dielectric constant of the substrate was derived from the expression for capacitance and calculation of the properties of a dielectric layer on the surface of the substrate was performed by a similar procedure. IDCs lithographed on lanthanum aluminum oxide and magnesium oxide substrates were coated with a barium strontium titanate films and were tested under a DC voltage bias to measure the shift in capacitance due to the change in permittivity under the electric field. The capacitance change of IDCs as a function of bias voltage was investigated and the results were assessed for a final goal of creating tunable capacitors for microwave applications.