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

The paper discusses the use of copper direct bonding to create low-loss multilayer quartz substrates for microwave components. A substrate-integrated-waveguide (SIW) based two-plane hybrid coupler, made from five quartz layers, demonstrated low loss characteristics at the 60 GHz band. The copper direct bonding method allows for low-loss multilayer substrates without using bonding materials like solder, which typically increase transmission losses.

Key findings include:

1. Low-loss SIW lines: The straight SIW lines showed an attenuation constant of 0.013 dB/mm from 57 GHz to 64 GHz, with a low insertion loss of 0.12 dB from an external waveguide to the inner quartz layer between 58 GHz and 63 GHz.

2. Two-plane hybrid coupler performance: The two-plane hybrid coupler achieved low amplitude imbalance (0.75 dB) and a low insertion loss of 1.1 dB. These results are consistent with the design goals, showing that copper direct bonding enables low-loss, multilayer microwave components.

3. Scalability: The fabrication technique is scalable, allowing for an increase in the number of quartz layers, making it suitable for use in beam-switching circuits and other low-loss, compact microwave components.

4. Fabrication process: The process involves five steps: wafer polishing, drilling, metallization, patterning, and lamination. The copper direct bonding method was used to bond the quartz layers without the need for adhesive bonding materials, ensuring a high-quality bond.

In comparison to other quartz-based microwave components, the proposed system outperforms them in terms of both insertion loss and attenuation, highlighting the potential for these multilayer quartz substrates in advanced microwave and millimeter-wave applications.