Characterization the microwave properties of SU-8 based on microstrip ring resonator

A novel method for characterizing the microwave properties of SU-8, an epoxy photoresist, using a microstrip ring resonator (MRR) is presented in this paper. In the presence of SU-8 overlay on the surface of the MRR, a deviation in the effective dielectric constant can be accurately detected by measuring the resonance frequency shift. In addition, the loss tangent of the SU-8 can also be extracted based on the fractional decrease in the Q-factor of the device induced by additional losses associated with the SU-8 coating. S-parameter measurements are conducted using a vector network analyzer in order to explore the variation in the frequency responses (e.g., resonance frequency, bandwidth and insertion loss) of the MRR device in its fundamental and higher resonance modes affected by the SU-8 coating. The dielectric constant and loss tangent of the overlay can be extracted precisely at the fundamental and higher-mode resonance frequencies. The design procedure, fabrication process and the parameter extraction method are detailed in this paper. The measured frequency-dependent dielectric constant of SU-8 is also reported up to 6 GHz at discrete resonance frequencies of the MRR device for the first time.