Permittivity and Loss Characterization of SU-8 Films for mmW and Terahertz Applications

The dielectric permittivity measurement of thick SU-8 film is presented for the entire frequency band of 1 GHz to 1 THz. SU-8 is a high-resolution UV-patternable photoresist that can be used for fabrication of high-aspect-ratio 3-D structures for millimeter-wave and terahertz devices. Here, we report the measured dielectric constant and loss tangent of SU-8 films using terahertz time-domain spectroscopy. A quadratic polynomial model is established for the accurate calculation of complex permittivity up to 1 THz. The loss tangent of fully cross-linked 430- $\mu\hbox{m}$ -thick SU-8 film was measured to be 0.015, 0.027, and 0.055 at 1, 200, and 1000 GHz, respectively. Similarly, relative permittivity was found to be 3.24, 3.23, and 2.92. The fabrication process and level of cross-linking were demonstrated to have significant impact on the loss behavior of this material and the impact of cross-linking on dielectric permittivity is quantified across a wide frequency band. The characterization results reported in this letter are a platform for developing next-generation millimeter-wave and terahertz devices.