What are the packaging challenges for circuits operating above 200 GHz?
THz Circuit Packaging Challenges
Packaging is often the performance-limiting factor for circuits above 200 GHz. A state-of-the-art MMIC that achieves excellent on-wafer performance can lose 3-10 dB of gain and 2-5 dB of noise figure degradation through packaging parasitics.
- 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
Can standard IC packaging be used above 200 GHz?
No. Standard IC packages (QFN, BGA, QFP) have: wire bond interconnects with inductance that creates excessive reactance above 50 GHz, package cavities that resonate below 100 GHz, lead frame or ball grid array connections with uncontrolled impedance at THz frequencies, and molding compound that adds dielectric loading and loss. For 200+ GHz: custom packaging with waveguide interfaces, flip-chip assembly, and micromachined structures is required. The only exception: some research groups have demonstrated chip-scale packaging using through-silicon vias (TSVs) and silicon interposers that can extend to 300+ GHz.
How is the MMIC grounded?
Grounding is critical because even a few pH of ground inductance creates significant impedance at 200+ GHz. Approaches: through-substrate vias in the MMIC (InP MMICs typically use via holes from the front-side ground pads to the backside ground plane; the backside is then soldered to the package ground), multiple parallel through-vias to reduce the total inductance, and flip-chip assembly (the MMIC ground pads are bumped directly to the package ground through short, inductance bumps). The total ground inductance should be less than 5 pH for acceptable performance at 300 GHz.
What about on-wafer testing instead of packaging?
Many 200+ GHz circuits are characterized on-wafer using probe stations with WR-X waveguide probes (e.g., from GGB Industries, Cascade Microtech). On-wafer testing avoids all packaging parasitics and provides the true MMIC performance. However: for any deployed system, the MMIC must be packaged. The challenge is to achieve packaged performance within 1-2 dB of the on-wafer performance. This requires co-design of the MMIC and package from the beginning (designing the MMIC input/output matching to include the package transition parasitics).