  This paper describes a method to determine the complex permittivity of a thin
dielectric film from finite element analysis and microstrip line measurements.
Two transmission line equivalent circuit models were used for the cases of an
air-filled line and a lossless line, whose distributed elements can be
calculated from full wave finite element simulations. With these calculated
distributed elements and microstrip line measurements, the complex permittivity
was extracted. The technique utilizes a simple way to separate the dielectric
loss from the measured total loss and the complex permittivity was extracted
using the measured propagation constant. A rational dielectric model was
employed to fit the extracted complex permittivity, which ensures causality of
the final solution. The best fitting results obtained through this procedure
are considered as the final permittivity results, which have shown excellent
match to the data sheet values. Moreover, simulations using the fitted
permittivity exhibit good agreement with the experimental propagation constant
data of microstrip lines up to 60 GHz. The proposed method was demonstrated on
polyimide, and it can be applied to other thin film materials.
