1. Introduction

With the rapid iteration of 6G communication, millimeter-wave radar and terahertz devices, operating frequencies have extended to the 100–325 GHz sub-millimeter band. Traditional substrates such as silicon, ceramic and ordinary glass suffer from high dielectric loss, severe dispersion and poor processing compatibility, limiting the performance of ultra-high-frequency circuits.
As an excellent dielectric material, fused silica features ultra-low loss, stable dielectric constant, high surface flatness and great mechanical stability. It has become a preferred substrate for next-generation 6G millimeter-wave RF chips.
Previously, most high-frequency dielectric data of fused silica above 110 GHz were obtained via optical measurement, which cannot match the actual quasi-TEM operating mode of RF circuits and fails to guide chip design effectively. To fill this technical gap, NIST released a full-band wafer-level electrical characterization covering 320 MHz–325 GHz based on commercial JGS2 fused silica wafers, providing authoritative engineering data for 6G industrial applications.

2. Core Advantages of Fused Silica for Millimeter-Wave Scenarios

2.1 Lower Dielectric Constant Reduces Processing Difficulty

The guided wavelength of CPW on fused silica is significantly longer than that on silicon substrates. With a stable relative dielectric constant (εr=3.87), fused silica enlarges the layout feature size of millimeter-wave devices at 300 GHz+ bands. This effectively relaxes lithography and probing precision requirements, improving production yield and reducing manufacturing costs.

[Figure 1 CPW Structure Schematic]
CPW top view, cross-section structure, equivalent RLC circuit and electric field distribution diagram, showing the RF transmission mechanism on fused silica substrate.
 

2.2 Ultra-Broadband Low Loss Improves RF System Efficiency

Measured results show that the loss tangent (tanδ) of JGS2 fused silica remains below 0.005 across 320 MHz–325 GHz, much lower than silicon and ordinary glass. In millimeter-wave phased arrays, transceivers and on-chip interconnections, fused silica greatly reduces insertion loss, enhances signal sensitivity and communication distance, and lowers overall module power consumption.
 

[Figure 2(b) Tanδ Curve of Fused Silica (320MHz–325GHz)]
Full-band loss tangent test curve; minor fluctuations at 10 GHz and 160 GHz are system errors rather than material characteristics.

2.3 High Environmental Stability and Mass-Production Compatibility

Fused silica has an extremely low thermal expansion coefficient, ensuring stable dielectric performance under wide temperature variation. Its nano-level surface roughness ensures uniform metal film adhesion and suppresses additional high-frequency loss.
By adopting mature temporary bonding technology with silicon carrier wafers, thin fused silica substrates can be fully compatible with standard semiconductor production lines, including lithography, evaporation, probing and dicing, enabling large-scale commercial manufacturing.

2.4 Cost-Effective for Large-Scale 6G Deployment

The commercial-grade JGS2 fused silica used in the test meets 325 GHz ultra-high-frequency performance requirements without high-purity optical-grade materials. Compared with ceramic and special glass substrates, JGS2 features lower cost and a more mature supply chain, perfectly matching large-volume 6G terminal and base station production.

3. Industry Technical Pain Points

For a long time, high-frequency dielectric data of fused silica above 110 GHz is insufficient and inaccurate: discrete low-frequency test data cannot cover 220–325 GHz key bands; traditional optical testing cannot simulate real chip quasi-TEM transmission, radiation and parasitic effects; there is no unified wafer-level ultra-high-frequency measurement standard in the industry, resulting in inconsistent material evaluation criteria.

4. Key Measured Conclusions (320MHz–325GHz)

The test system covers three core millimeter-wave bands with coaxial and waveguide modules (1mm / WR5 / WR3) and GSG wafer probes. The key conclusions are as follows:
1. The relative dielectric constant of fused silica is stable at εr=3.87±0.03 across the full band with no obvious dielectric dispersion.
 

[Figure 2(a) Relative Dielectric Constant Frequency Response Curve]
The dielectric constant remains stable at 3.87 from 0 to 325 GHz, supporting constant-value modeling for RF simulation.
2. The full-band loss tangent tanδ < 0.005, realizing ultra-low dielectric loss.
3. No engineering-level dispersion exists within 325 GHz, simplifying circuit design and simulation modeling.
4. The main transmission loss comes from conductor ohmic loss, while dielectric loss is negligible.

5. Industrial Guidance Value

For RF designers: The authoritative measured εr and tanδ data can be directly applied to electromagnetic simulation to reduce design iteration.
For semiconductor manufacturers: Bonded fused silica wafers can replace silicon substrates as standardized millimeter-wave RF wafers for mass production.
For 6G terminal and base station manufacturers: Fused silica-based millimeter-wave modules feature lower loss, longer transmission distance and lower power consumption, optimizing equipment miniaturization and stability.

6. Summary

With full-band 325 GHz wafer-level electrical verification, fused silica has proven its superior performance of low loss, zero dispersion, high stability and low cost. It is evolving from traditional optical materials into a core substrate for 6G millimeter-wave RF chips, becoming the mainstream solution for future ultra-high-frequency communication hardware.

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