Coplanar waveguides (CPWs) are fabricated by directly bonding 17-m Al foils to Si substrates with different resistivities and wet etching. Their RF characteristics are compared with those of CPWs fabricated on 0.8-m thick evaporated Al layers. A lower insertion loss is observed for Si substrates with higher resistivity irrespective of thickness of conductors because the substrate loss is decreased. For a 10000-Ω·cm Si substrate, Al-foil-based CPWs reveal a ∼0.7-dB/cm lower insertion loss in comparison with evaporated-Al-based CPWs at 1 GHz. These features agree with calculation, which implies the superiority of direct bonding of metal foils for realizing low-loss passive components.

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