Low Loss Quartz Multilayer Substrates and Substrate Integrated Two-Plane Hybrid Couplers Enabled by Copper Direct Bonding

This paper presents a fused silica glass (quartz) multi-layer substrate using the copper direct bonding method. A fabricated substrate-integrated-waveguide (SIW) based two-plane hybrid coupler composed of five quartz layers showed low loss characteristics at the 60 GHz band. Quartz has low loss characteristics even at higher frequency bands but suffers from the difficulty of multilayer. We propose a copper direct bonding method to realize low-loss multilayer quartz substrates and characterize straight SIW lines and the two-plane hybrid coupler composed of a five-quartz-layer substrate. The straight SIW line shows low loss characteristics of 0.013 dB/mm at most from 57 GHz to 64 GHz with estimated $\epsilon _{r}=3.75$ and $\tan \delta =0.00034$ for the quartz and $\sigma = 4.0\times 10^{7}$ S/m for the copper. The low insertion loss between an external waveguide and the SIW located in the inner quartz layer is confirmed as 0.12 dB at most, from 58 GHz to 63 GHz. The two-plane hybrid shows a low amplitude imbalance of 0.75 dB with a low insertion loss of 1.1 dB. These results strongly show that the quartz multi-layer substrate using the copper direct bonding method enables low-loss multilayer microwave components. Because this fabrication technique is scalable and the number of quartz layers can be increased, it can be applied to beam-switching circuits and feeding circuits with low loss and compact volume.