Coplanar Waveguides With Nanometer Thick Gold Films

Coplanar waveguides (CPWs) with 8 nm to 50 nm thick gold films are fabricated and characterized up to 40 GHz. Such film thicknesses are comparable with or thinner than electron mean-free-path (MFP) in gold. In this size effect regime, skin effect is negligible. The measured results show that the CPWs have less dispersion, but higher loss when compared with thick metal CPWs. A thin film resistivity model, which includes the effects of electron scatterings from metal surfaces, grain boundaries and surface roughness, is proposed to describe the microwave resistance of such CPWs. For an 8 nm CPW, it has much higher resistance and inductance given that 8 nm is around the percolation threshold of gold films. Furthermore, its inductance and resistance decrease with frequencies due to enhanced coupling among metallic islands. The quality factors of nanometer-CPWs are low, especially at low frequencies. Nevertheless, these CPWs have potential applications in local and regional interconnect, biomedical sensors, microwave nanofluidic channels, and disordered-electronic-system studies. Further work is needed to understand and accurately model nanometer CPWs.