Free Space Dielectric Techniques for Diamond Composite Characterization

At a Glance

Metadata	Details
Publication Date	2023-12-19
Journal	IEEE Journal of Microwaves
Authors	Shu-Ming Chang, Chelsea Swank, Andrew C. Kummel, James F. Buckwalter
Institutions	University of California, Santa Barbara, University of California, San Diego
Citations	3
Analysis	Full AI Review Included

Technical Documentation & Analysis: Free Space Dielectric Characterization of Diamond Composites

Executive Summary

This research investigates novel packaging solutions for compact millimeter-wave (mm-wave) arrays operating in the D-band (110-170 GHz), focusing on dielectric materials that combine high thermal conductivity (TC) with low dielectric loss.

Core Challenge: Achieving a low-cost dielectric material with TC > 100 W/m·K and ultra-low loss tangent (LT) for high-power amplifier (PA) heat dissipation and signal integrity at D-band.
Material Tested: Ultradense Diamond Composites (UDC) formed from 10 µm synthetic diamond particles embedded in polymer matrices (TMPTA and PDMS).
Key Measurement Technique: Free-space focused beam system utilizing TRL calibration, the NIST iterative method, and Time-Domain Gating (TDG) to accurately characterize small samples (120-160 GHz).
Performance Achieved (TMPTA UDC): Relative permittivity (ε_r) ranged from 3.63 to 3.98, with a loss tangent (LT) as low as 0.027 at 140 GHz.
Limitation Identified: While the diamond filler contributes to the desired permittivity (roughly 50% of bulk diamond’s ε_r of 5.7), the loss tangent is dominated and limited by the high-loss polymer matrix (LT 0.027-0.067), failing to meet the intrinsic low-loss performance of bulk diamond (LT 5 x 10^-4).
6CCVD Value Proposition: 6CCVD supplies high-purity, bulk MPCVD Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) which intrinsically possess the required ultra-high thermal conductivity (up to 2000 W/m·K) and ultra-low loss tangent (LT < 1 x 10^-4), eliminating the need for lossy polymer matrices.

Technical Specifications

The following data points were extracted from the characterization of the diamond composite materials and the measurement setup:

Parameter	Value	Unit	Context
Operating Frequency Band	110 - 170	GHz	D-band Millimeter-wave
Reliable Measurement Range	120 - 160	GHz	Range after Time-Domain Gating
Target Thermal Conductivity (PA)	> 100	W/m·K	Required for acceptable temperature rise
Bulk Diamond Thermal Conductivity	~2000	W/m·K	Ideal reference material
Bulk Diamond Relative Permittivity (ε_r)	5.7	N/A	Reference value
Bulk Diamond Loss Tangent (LT)	5 x 10^-4	N/A	Reference value
UDC Diamond Particle Size	10	µm	Synthetic microdiamonds used in composite
TMPTA-Diamond Composite ε_r (140 GHz)	3.63 - 3.98	N/A	Range across four samples
TMPTA-Diamond Composite LT (140 GHz)	0.027 - 0.048	N/A	Lowest LT achieved by TMPTA UDC
PDMS-Diamond Composite LT (140 GHz)	0.061 - 0.067	N/A	Loss tangent limited by PDMS polymer
Measurement Floor (LT)	5 x 10^-3	N/A	Limit of the D-band free-space setup
Quartz Wafer Thickness	0.5	mm	Calibration standard
Sapphire Wafer Thickness	0.43	mm	Calibration standard

Key Methodologies

The characterization relied on advanced material processing and precise free-space measurement techniques suitable for D-band frequencies:

Ultradense Diamond Composite (UDC) Fabrication: 10 µm synthetic diamond particles were mixed into TMPTA or PDMS polymer slurry.
Low-Temperature Densification: Pressure was applied to the slurry in a 20-mm Al mold inside an ultrasonicator, processed at 60 °C to increase packing density.
Curing Process: The diamond matrix was cured at 120 °C for 1 hour, followed by TMPTA deposition and final curing at a maximum of 140 °C for 20 minutes.
Free-Space Focused Beam Setup: D-band (110-170 GHz) measurements were conducted using a Keysight PNA, D-band frequency extenders, 25 dBi horn antennas, and bi-convex PTFE lenses to focus the beam onto small samples.
Calibration Standard: Free-space Through/Reflect/Line (TRL) calibration was performed to establish accurate reference planes.
Permittivity Extraction: The NIST iterative algorithm was employed, utilizing all four S-parameters (S₁₁, S₂₂, S₁₂, S₂₁) to determine permittivity and loss tangent, mitigating sample position uncertainty.
Artifact Mitigation: Time-Domain Gating (TDG) was applied using a Kaiser-Bessel Window (order 6, span 0.2 ns) to filter out non-line-of-sight (NLOS) path components and multiple reflections, improving the accuracy of the extracted dielectric properties.

6CCVD Solutions & Capabilities

This research confirms that diamond is the ideal filler material for high-frequency, high-thermal-conductivity dielectrics. However, the performance is severely limited by the polymer matrix. 6CCVD provides bulk MPCVD diamond materials that offer the intrinsic properties sought by the researchers, enabling superior performance for D-band packaging.

Research Requirement / Challenge	6CCVD Solution & Capability	Technical Advantage
Ultra-High Thermal Conductivity (Target: 2000 W/m·K)	Optical Grade Single Crystal Diamond (SCD)	SCD provides intrinsic thermal conductivity up to 2000 W/m·K, ensuring maximum heat spreading efficiency for high-power MMICs, far surpassing the composite target of 100 W/m·K.
Ultra-Low Loss Tangent (Target: 1 x 10^-4)	High-Purity SCD and PCD Wafers	Our MPCVD diamond eliminates the need for lossy polymer matrices (LT 0.027-0.067). High-purity SCD/PCD retains the intrinsic LT < 1 x 10^-4, critical for minimizing signal loss in D-band applications.
Custom Dimensions for R&D and Production	Custom Plates/Wafers up to 125mm (PCD)	We provide custom dimensions for both small R&D samples (e.g., 20 mm diameter) and production-scale wafers up to 125 mm (PCD), supporting immediate replication and future scaling of this research.
Precision Thickness Control	SCD/PCD Thickness: 0.1 µm to 500 µm	We offer precise thickness control for thin dielectric layers (0.1 µm to 500 µm) and robust substrates (up to 10 mm), essential for impedance matching and thermal management stack design.
Integration and Packaging (Metalization)	Custom Metalization Services (Au, Pt, Pd, Ti, W, Cu)	To facilitate direct integration with III-V PAs and CMOS beamformers, 6CCVD provides in-house metalization capabilities, allowing engineers to define custom contact pads or transmission lines directly on the diamond surface.
Surface Quality for Low Conductor Loss	Ultra-Smooth Polishing (Ra < 1 nm SCD)	Our advanced polishing services ensure surface roughness (Ra) < 1 nm for SCD and < 5 nm for inch-size PCD, minimizing surface roughness effects that increase conductive loss at mm-wave frequencies.

Engineering Support

The 6CCVD in-house PhD team specializes in optimizing MPCVD diamond properties for extreme RF and thermal environments. We offer consultation on material selection (SCD vs. PCD), doping (BDD), and surface preparation to maximize performance for D-band packaging and high-frequency dielectric projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Compact millimeter-wave arrays demand novel packaging solutions that feature low-cost dielectric materials with significant thermal conductivity (<inline-formula><tex-math notation=“LaTeX”>$\sim$</tex-math></inline-formula>100 W/m ⋅ K). To characterize the permittivity and loss tangent of the dielectric materials above 100 GHz, free-space characterization is proposed to avoid de-embedding conductor losses. We review current approaches for characterization to investigate the properties of ultradense diamond composite materials at D-band. We compare free-space calibration multiple methods to extract the permittivity and loss tangent. Time-domain gating is employed to reduce the uncertainty in the free space characterization. Material characterizations of the dielectric constant and loss tangent include pure polymer TMPTA, PDMS, TMPTA-based, PDMS-based diamond composites as well as quartz and sapphire wafers for calibration from 120–160 GHz. To the author's knowledge, this is the first characterization of diamond composites for thermally conductive dielectric packaging requirements at D-band.

Tech Support

Original Source

DOI: https://doi.org/10.1109/jmw.2023.3339255

References

2021 - Fabrication of extreme density microdiamond composites for RF and logic heat spreaders
**** - The CVD diamond booklet