Low-loss characteristics of coplanar waveguides fabricated by directly bonding metal foils to high-resistivity Si substrates

Coplanar waveguides (CPWs) are fabricated by directly bonding 17 μm Al foils to Si substrates with different resistivities and wet etching. Their RF characteristics are compared with those of CPWs fabricated on 0.8 μm thick evaporated Al layers. A lower insertion loss is observed for Si substrates with higher resistivity irrespective of the thickness of conductors because the substrate loss is decreased. For a 10000-ω · cm Si substrate, Al-foil-based CPWs reveal a a1/40.7 dB cm-1 lower insertion loss in comparison with evaporated-Al-based CPWs at 1 GHz. These features agree with the calculation, which implies the superiority of direct bonding of metal foils for realizing low-loss passive components.