Effect of Surface Roughness on the Electrical Performances of CPW Transmission Lines Used in Future Ultra-High Frequency Applications

The development of integrated circuits and packaging technology has led to smaller and smaller transmission line sizes and higher and higher operating frequencies up to nearly 100 GHz. However, the skinning depth of transmission lines due to eddy currents becomes smaller and smaller as the operating frequency of coplanar wave guide (CPW) transmission lines becomes higher and higher, while the reduction of device size makes the skinning depth consistent with the surface roughness of the device. In this paper, the concept of modified roughness coefficient was proposed based on the existing correlation factor. The concept of threshold modified roughness coefficient was proposed with a 20 dB reflection coefficient as the threshold value. The effect of surface roughness on transmission line transmission performance at frequencies above 100 GHz up to 1000 GHz was investigated. It was found that when the operating frequency of the signal was greater than the threshold roughness coefficient, the effect of surface roughness on the transmission line reflection coefficient should be considered. The modified roughness coefficient in this paper could quickly determine the effect of surface roughness on transmission line performance at different frequencies.