Entering the Millimeter-Wave Frontier

Millimeter-wave (mmWave) frequencies — defined as the spectrum between 30 GHz and 300 GHz, though often extended to include the 24 to 30 GHz bands used by 5G NR — represent the new frontier of wireless communication and sensing. At these frequencies, enormous bandwidths are available: a single 5G NR channel at 28 GHz can be 400 MHz wide, compared to just 20 MHz for a typical sub-6 GHz channel. This bandwidth enables multi-gigabit data rates that approach wired network speeds.

However, working at mmWave frequencies introduces engineering challenges across every aspect of the design and manufacturing process. Signal wavelengths shrink to single-digit millimeters, making physical dimensions that were negligible at lower frequencies suddenly significant. Atmospheric absorption, material losses, and packaging parasitics all become critical design constraints.

At INDNIX Technology, our RF division has developed specialized fabrication processes and design methodologies to address these mmWave-specific challenges.

Propagation Physics at mmWave

Radio waves at mmWave frequencies behave fundamentally differently from signals at sub-6 GHz bands:

Free-Space Path Loss: Path loss increases with the square of frequency. A 28 GHz signal experiences 20 dB more path loss than a 2.8 GHz signal over the same distance. This limits the range of mmWave links and necessitates high-gain antenna arrays.

Atmospheric Absorption: Oxygen molecules absorb electromagnetic energy around 60 GHz (approximately 15 dB/km absorption), and water vapor causes absorption peaks near 22 GHz and 183 GHz. System designers must account for these frequency-dependent losses when selecting operating bands.

Foliage and Building Penetration: mmWave signals are severely attenuated by building materials, vegetation, and even the human body. A hand blocking a 28 GHz signal can cause 25 to 35 dB of additional loss. This necessitates beam steering and non-line-of-sight (NLOS) relay strategies.

Rain Fade: Rainfall causes significant attenuation at mmWave frequencies — approximately 7 dB/km at 28 GHz in heavy rain (25 mm/hour). Link budgets for outdoor mmWave systems must include rain margin.

MMIC Design Challenges

Monolithic microwave integrated circuits (MMICs) at mmWave frequencies face several design challenges not encountered at lower frequencies:

Transistor Parasitics

At mmWave frequencies, the parasitic capacitances and inductances associated with transistor layout become comparable to the intrinsic device elements. The gate-drain feedback capacitance (Cgd) that causes negligible gain degradation at 5 GHz can dominate transistor behavior at 50 GHz. Our compact device layouts minimize interconnect lengths and use aggressive layout optimization to reduce parasitics.

Transmission Line Effects

Every interconnect on a mmWave MMIC is a transmission line. A 200-micrometer-long wire that behaves as a simple wire at 5 GHz becomes an electrically significant quarter-wavelength stub at 80 GHz. Our electromagnetic (EM) simulation tools model every metal trace, via, and ground plane opening as a distributed element to ensure accurate performance prediction.

Grounding

Inadequate grounding is the number one cause of mmWave MMIC performance degradation. Substrate via-holes connecting the front-side ground plane to the backside metallization introduce inductance that creates resonances within the operating band. Our 100-micrometer thin substrates with via-hole pitches as small as 75 micrometers provide the low-inductance grounding essential for mmWave stability.

Coupling and Isolation

Circuit elements that are physically close on a mmWave MMIC can couple strongly through electromagnetic fields in the substrate. A power amplifier and an LNA on the same die must be carefully isolated using ground fences, deep trench isolation, and physical separation to prevent oscillation.

Fabrication Solutions

Thin-Wafer Processing

mmWave MMICs require thin substrates (typically 50 to 100 micrometers) to minimize via-hole inductance and suppress substrate modes that cause resonances and coupling. Our thin-wafer processing includes temporary wafer bonding to rigid carriers, controlled thinning by mechanical grinding followed by chemical polishing, via-hole etching and metallization at the thinned state, and carrier debonding using thermal or UV-release adhesives.

Fine-Feature Lithography

Gate lengths of 100 to 150 nanometers are required for transistors operating above 60 GHz. We use electron-beam direct-write lithography for gate definition, achieving consistent gate profiles across the full wafer. Our T-gate (mushroom gate) process creates a narrow footprint at the semiconductor surface for high-frequency performance with a wider top for low resistance.

Air Bridge and Membrane Technology

At mmWave frequencies, even thin dielectric layers between crossing conductors introduce unacceptable parasitic capacitance. We use air bridges for all signal crossovers, eliminating dielectric loading. For particularly sensitive circuits, we employ membrane technology — removing the substrate beneath critical transmission lines to create freestanding metal conductors surrounded only by air.

Gold Metallization

Unlike silicon CMOS which uses copper metallization, our mmWave MMICs use gold. Gold offers superior conductivity at microwave frequencies (where skin effect limits current to the conductor surface), excellent wire bonding compatibility, and complete resistance to oxidation — critical for circuits that may operate for decades without hermetic sealing.

Packaging Considerations

The MMIC die is only half the challenge; the package must preserve mmWave performance. Package transitions, bond wire inductances, and cavity resonances can all degrade performance at mmWave frequencies.

Our packaging solutions include:

• Flip-chip on laminate: Direct bump attachment to low-loss PCB substrates eliminates wire bond inductance. Bump heights of 50 to 75 micrometers minimize the transition discontinuity.
• Embedded wafer-level ball grid array (eWLB): Fan-out packaging with redistributed I/Os and integrated antenna elements for single-chip radar sensors.
• Waveguide transitions: For E-band (60-90 GHz) and W-band (75-110 GHz) applications, we design probe transitions from microstrip MMIC output to rectangular waveguide with insertion loss below 0.5 dB.

Testing at mmWave

On-wafer testing at mmWave frequencies requires specialized probe stations with coplanar waveguide (CPW) probes rated to 110 GHz or higher. Our test infrastructure includes:

• Vector network analyzers calibrated to 110 GHz for S-parameter measurements
• Noise figure measurement systems with calibrated noise sources to 67 GHz
• Load-pull systems for power amplifier characterization to 40 GHz
• Automated wafer probing with probe placement accuracy of plus or minus 2 micrometers

Conclusion

Millimeter-wave RF design and fabrication demands a holistic approach that addresses challenges from transistor-level parasitics through MMIC layout, packaging, and testing. At INDNIX Technology, our RF division combines advanced III-V fabrication processes with deep mmWave design expertise to deliver MMICs that meet the demanding performance requirements of 5G, automotive radar, satellite communication, and emerging sensing applications.