Diamond in the D Band: The Low-Loss RF Substrate for Next-Generation mmWave

As wireless systems move toward D-band frequencies from 110 to 170 GHz, the choice of substrate is becoming increasingly important.

At these frequencies, even extremely small dielectric losses can translate into significant RF attenuation, insertion loss, heating, and reduced resonator or antenna efficiency. Materials that perform well at conventional microwave frequencies must therefore be reconsidered for the mmWave and sub-THz regime.

Recent measurements on inch-scale CVD diamond have revealed a particularly interesting behavior: rather than increasing with frequency, the measured dielectric loss tangent appears to decrease substantially across the D band, reaching the 10⁻⁶ level near 170 GHz in one high-quality single-crystal sample. At the same time, the relative permittivity remains remarkably stable at approximately 5.666 across 110–170 GHz.

This combination—stable dielectric constant and extremely low RF loss—makes diamond an intriguing candidate for future D-band RF platforms.

Why RF Loss Matters More at D Band

At 110–170 GHz, electromagnetic wavelengths become extremely short. Consequently, the substrate is no longer simply a mechanical support; it becomes an active part of the RF design.

Dielectric loss directly affects:

• Insertion loss

• Resonator Q factor

• Filter selectivity

• Antenna efficiency

• Transmission-line attenuation

• RF heating

• Power handling

For high-Q filters and resonators, even a small increase in dielectric loss can significantly degrade system performance.

This is where diamond becomes particularly interesting.

Unlike conventional semiconductor substrates, diamond combines very high thermal conductivity with potentially extremely low microwave/mmWave dielectric loss. The material therefore offers a unique possibility: an RF substrate that can simultaneously transport heat away from the active device while introducing very little electromagnetic loss.

Diamond's D-Band Advantage

The reported D-band measurements show two characteristics that are particularly important for RF engineers.

1. Extremely Low Loss

For the best single-crystal sample, the measured loss tangent reportedly decreases from approximately 10⁻⁴ near 110 GHz toward the 10⁻⁶ range near 170 GHz.

If independently reproduced across different diamond materials and measurement systems, this would place high-quality diamond among the most attractive low-loss substrates for D-band applications.

The significance is not simply the absolute value.

A lower loss tangent means that less electromagnetic energy is converted into heat inside the substrate. For high-Q structures, this can translate into:

Lower insertion loss → higher Q → sharper filtering → lower RF heating → higher system efficiency

2. Stable Permittivity

The measured dielectric constant remains close to:

εᵣ ≈ 5.666

throughout the 110–170 GHz range.

This stability is equally important.

A material with strong dielectric dispersion can shift the electrical dimensions of an RF structure as frequency changes. Stable permittivity makes electromagnetic modeling more predictable and helps maintain accurate impedance matching, resonant frequency, phase response, and filter characteristics.

For D-band circuit designers, the combination is therefore highly attractive:

Low loss + low dispersion + high thermal conductivity

Why Could Diamond Become a D-Band RF Material?

The most interesting question is not simply why diamond has low loss, but what controls its residual loss.

Perfect crystalline diamond has no free-carrier conduction mechanism comparable to conventional semiconductors. However, real CVD diamond contains various imperfections, including:

• Nitrogen-related defects

• Vacancy-related defects

• NV centers

• SiV centers

• Dislocations

• Grain boundaries in polycrystalline material

• Surface and interface states

• Residual impurities

These defects can interact with an electromagnetic field through localized polarization or dipolar mechanisms.

The implication is important:

At D-band frequencies, RF loss may become a sensitive indicator of crystal quality rather than simply a fundamental property of diamond itself.

This provides a new way to think about diamond materials engineering.

Instead of asking only:

“How high is the thermal conductivity?”

RF applications increasingly require asking:

“How low is the dielectric loss at the actual operating frequency?”

Single-Crystal Diamond vs. Polycrystalline Diamond

The reported measurements also highlight an important distinction between single-crystal and polycrystalline diamond.

The measured permittivity of the polycrystalline sample was close to that of the single-crystal samples, around 5.670, suggesting that the average dielectric response can remain remarkably similar.

However, the loss behavior was less ideal.

This distinction is critical for RF applications.

For thermal management, polycrystalline diamond can already provide extremely high thermal conductivity and excellent heat spreading.

For D-band RF substrates, filters, resonators and transmission structures, however, the requirements become much stricter.

The target is no longer simply:

High thermal conductivity

but:

High thermal conductivity + low RF loss + low dispersion + low defect density + excellent thickness uniformity + low surface roughness

This is a much higher material-quality threshold.

D Band Turns Diamond Quality Into an RF Parameter

The D band may therefore introduce a new method of evaluating diamond.

Traditional diamond characterization relies heavily on Raman spectroscopy, X-ray diffraction, optical spectroscopy, defect density, thermal conductivity and surface metrology.

These measurements remain essential, but they do not necessarily reveal how a diamond wafer behaves inside a 140-GHz electromagnetic field.

A D-band loss measurement provides another dimension:

Crystal defects → electromagnetic interaction → dielectric loss

This creates the possibility of using high-frequency dielectric characterization as an additional quality metric for advanced diamond substrates.

DIASEMI: Diamond Inside the D-Band RF Platform

For DIASEMI, this represents an important evolution of the Diamond Inside technology concept.

Diamond is traditionally viewed as a thermal material placed underneath or behind an RF device.

At D-band frequencies, diamond can potentially become much more than a heat spreader.

It can become part of the RF electromagnetic architecture itself.

A diamond substrate could simultaneously provide:

RF function
Low dielectric loss and stable permittivity

Thermal function
Ultra-high thermal conductivity for rapid heat spreading

Electrical isolation
Extremely wide bandgap and high electrical resistivity

Mechanical function
High stiffness, strength and dimensional stability

This combination is particularly attractive for:

• D-band filters

• mmWave antennas

• Low-loss transmission lines

• High-Q resonators

• D-band front-end modules

• High-power mmWave electronics

• Sub-THz communication systems

• Advanced radar

• 6G RF platforms

The DIASEMI Perspective

The most important opportunity is not simply to claim that “diamond has low dielectric loss.”

The larger opportunity is to engineer diamond specifically for the D-band electromagnetic environment.

That means controlling the entire material stack:

Diamond crystal quality → defect concentration → wafer thickness → surface roughness → metallization/interface → RF loss

For next-generation mmWave and sub-THz electronics, this integrated approach could turn diamond from a passive heat spreader into a low-loss RF and thermal platform.

Diamond Inside.

Low RF Loss. High Thermal Conductivity. D-Band Ready.

As operating frequencies move beyond 100 GHz, the substrate itself becomes part of the RF system.

DIASEMI is developing Diamond Inside technology with the goal of making diamond not only the material that removes heat—but the material that helps preserve the RF signal.