Dielectric Microwave Characterization of the SU-8 Thick Resin Used in an Above-IC Process

International audienceA broadband technique for determining electrical properties of dielectric materials is presented, based on microstrip lines. Relative permittivity and Loss tangent are computed from S-parameter measurements and analytical equations. The analytical computation is either direct by using equations derived from Bahl formulas, or iterative by using Jensen-Hammerstad formulas coupled with the efficient secant algorithm. Thin film microstrip transmission lines have been fabricated for the extraction of dielectric electrical properties of SU-8 resin. A relative dielectric constant of 2.85 and a loss tangent of 0.04 were determined. These values are used in an EM simulator for the design of an SU-8 based High-Q inductor implemented on a low resistivity silicon substrate. The good agreement between measurements and simulations validates the characterization procedure and confirms the relevance of SU8 for applications up to 15 GHz. I. INTRODUCTION Current developments of mobile telecommunications are accompanied by an increasing demand for powerful, low cost and reliable new technologies. In addition, the increasing complexity of circuits implies the use of electric and electromagnetic (EM) simulators to carry out effective designs. However, simulations require a good knowledge of the electrical properties of involved materials, in particular of dielectric ones. In this paper, two simple yet accurate dielectric constant and loss tangent extraction methods are presented. These methods are applied to the measured S-parameters of microstrip lines. One method is based upon direct analytical calculations while the other relies on an iterative algorithm. These methods are extremely fast compared to previous ones [1] since they do not require direct and inverse problem solving as well as a comparison between calculated and measured S-parameters. After validation on an Alumina substrate, these methods were used to investigate the electrical properties of the epoxy based photoresist SU-8. The SU-8 appears as an attractive material for above IC process, because of its moderate mechanical strain and because it allows important thickness. This material has been previously characterized by using different techniques and methods. However, they either required a complex fabrication process [2] or showed inaccurate results [3]. In addition, several different types of SU-8 are available whose electrical properties are sensitive to processing conditions. For all these reasons, effective dielectric parameters extraction methods are needed, and hereby presented