What substrate materials are suitable for circuits operating above 300 GHz?
Selecting Substrate Materials for Sub-THz and THz Circuits
Above 300 GHz, every aspect of substrate performance becomes critical. Dielectric loss that was acceptable at 30 GHz becomes prohibitive. Surface wave excitation on thick substrates creates parasitic coupling. Dimensional tolerances that were comfortable at microwave frequencies become a significant fraction of a wavelength.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
Why is InP the dominant technology for circuits above 300 GHz?
InP offers the best combination of electron mobility, breakdown voltage, and established MMIC fabrication processes. InP HEMT transistors hold the record for highest fmax (above 1.5 THz), providing usable gain at frequencies where other technologies have none.
What is a membrane-supported circuit?
A membrane circuit suspends the conductor pattern on a thin (1-3 micrometer) dielectric membrane with the substrate beneath removed by etching. The effective dielectric constant approaches 1.0, virtually eliminating substrate loss.
Can standard PCB materials be used above 300 GHz?
No. Even the best organic PCB laminates produce unacceptable dielectric loss above 200-300 GHz, and their dimensional tolerances represent a significant fraction of a wavelength.