Attenuation mechanisms of aluminum millimeter-wave coplanar waveguides on silicon

The loss mechanisms of silicon coplanar waveguides (CPW) with aluminum metallization are investigated up to 40 GHz. Three main parts contribute to the attenuation of coplanar waveguides (CPWs): the frequency-dependent conductor losses of the metallization, frequency-independent substrate losses, and the specifically investigated bias-dependent interface losses caused by free charges at the Si-SiO₂ interface. The minimum losses found in 50-Ω CPWs with 45-μm signal line width were 0.19 db/mm at 10 GHz and 0.33 dB/mm at 40 GHz. High-purity silicon from a float zone (FZ) process was used as substrate. Substrates with lower purity from a Czochralski (CZ) process (resistivity 50-100 Ωcm) resulted in somewhat higher (0.2-0.3 dB/mm) losses for the same CPW geometry.