CPW line-to-line coupling on glass and low resistivity silicon

This paper presents experimental results for line-to-line coupling between adjacent coplanar waveguide (CPW) transmission lines. The CPW lines are fabricated on glass and low resistivity CMOS grade Si substrates coated with a 20/spl mu/m cyclotene polymer. From the measurement results it is found that the coupling increases with frequency on low resistivity silicon substrate while the coupling on glass shows relatively constant peaks. A spacing of 600/spl mu/m between CPW lines is required to avoid parasitic coupling less than 30dB for frequencies up to 30GHz when fabricated on low resistivity silicon, while a spacing of 200/spl mu/m is required when fabricated on glass. The conductor width (W) and the slot width (S) for low resistivity silicon is 80/spl mu/m and 45/spl mu/m, and on the glass substrate W = 150/spl mu/m, S = 30/spl mu/m. Based on full-wave EM simulation results, it is argued that the primary coupling mechanism is through the interaction of higher-order modes.