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Microwave Characterization of Flow Coated SU-8 Thick Film for Printed Circuit Applications
Keywords: Flow Coating, SU-8, Ring Resonator
This paper presents a unique flow coating process for depositing a uniform polymer thick film which is crucial for next-generation printed circuits. The newly-developed flow coating apparatus consists of a stationary blade placed above a motorized linear stage which holds the substrate to be coated. The vertical position, the inclination, and planarity of the blade with respect to the substrate can be precisely controlled by other positioners. The polymer is deposited between the blade and the substrate using an automatic syringe, followed by the lateral motion of the linear stage underneath the blade to spread the polymer and form a wet film with thickness determined by the velocity. The coating process is a competition between the capillary forces that hold the polymer solution with substrate, and the frictional drag forces generated as the polymer solution is pulled across the substrate. Flow coating technique offers several advantages over traditional spin coating such as a greater range of achievable thickness, tolerance to flexible and irregular-shape substrates, elimination of edge bead, and efficient use of coating solution. To evaluate the new coating method, SU-8, an epoxy-type photoresist, was coated over the microstrip ring resonator test structures followed by extraction of the microwave properties from its measured frequency response. Particularly, several microstrip ring resonators with a fundamental resonance frequency of 1GHz were designed and fabricated on a variety of printed circuit boards. A 200m-thick SU-8 layer was flow coated over these microstrip ring resonators. The dielectric constant of SU-8 was accurately extracted by monitoring the shift in resonance frequencies induced by SU-8 coating at the fundamental and higher modes. This paper presents the first ever demonstration of microwave characterization of dielectric properties of the flow-coated SU-8 polymer by microstrip ring resonator operating at its fundamental and higher modes with discrete frequencies up to 6GHz.
Jing Wang, Assistant Professor
University of South Florida
Tampa, FL

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