RF losses, crosstalk and temperature dependence for SOI and Si/SiC hybrid substrates

• RF losses and crosstalk were studied for 6H-SiC, poly-SiC and silicon substrates.
• SiC substrates behave semi-insulating with low losses and no temperature dependence.
• Losses, crosstalk and temperature behavior in silicon depend strongly on resistivity.
• Poly-SiC shows complex high frequency behavior.
• Poly-SiC can replace 6H-SiC in Si/SiC hybrid substrates without performance loss.

Single- and polycrystalline silicon carbide (6H-SiC/poly-SiC) substrates were investigated regarding RF losses and crosstalk for their use in Si/SiC hybrid substrates. Such hybrid substrates would be ideal for silicon high power and high frequency applications. To get a relevant comparison to SOI substrates, silicon substrates with varying resistivity were also included in the study. Regarding the crosstalk, both 6H-SiC and poly-SiC are capacitive across the whole frequency range, and the level of crosstalk is dependent on geometry and frequency. The low resistivity (LR) silicon substrate shows low crosstalk compared to medium and high resistivity (MR/HR) substrates, which both suffer from high crosstalk due to the substrate resistivity and dielectric relaxation effects in the GHz range. From 1-port measurements of RF losses it was observed that 6H-SiC by far has the lowest losses. The poly-SiC has low losses in the same range as the LR substrate while the MR substrate showed the highest losses. The 6H-SiC and LR silicon substrates were unaffected at higher temperatures, while at these conditions, HR silicon behaves more like MR silicon. Overall, the poly-SiC substrate has complex behavior with frequency dependent components, but still has the advantages necessary for successful realization of low loss Si/SiC hybrid substrates.