Measurement of radio transparency of polycrystalline CVD-diamond in millimeter-wave range by free-space method
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-01-01 |
| Journal | Письма в журнал технической физики |
| Authors | D. L. Gnatyuk, А. В. Зуев, D. V. Krapukhin, P. P. Maltsev, Д. Н. Совык |
| Institutions | Institute of Superhigh-Frequency Semiconductor Electronics of the Russian Academy of Sciences, MIREA - Russian Technological University |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Millimeter-Wave Radio Transparency of MPCVD Diamond
Section titled “Technical Documentation & Analysis: Millimeter-Wave Radio Transparency of MPCVD Diamond”This document analyzes the findings of the research paper “Measurement of radio transparency of polycrystalline CVD-diamond in millimeter-wave range by free-space method” and aligns them with the advanced manufacturing capabilities of 6CCVD.
Executive Summary
Section titled “Executive Summary”This research confirms the potential of Polycrystalline CVD (PCD) diamond as a material for high-speed millimeter-wave applications (5G, gyrotron windows) but highlights the critical role of material quality and post-processing in minimizing dielectric loss.
- Application Focus: PCD diamond disks were evaluated for radio transparency in the 50-67 GHz range, relevant for 5G communication systems (66-71 GHz) and high-power gyrotron windows (up to 170 GHz).
- Material Dimensions: Large-diameter PCD disks (up to 73 mm) were successfully synthesized using MPCVD, demonstrating scalability.
- Dielectric Performance: The dielectric constant ($\epsilon’$) was consistently measured at approximately 5.7.
- Loss Tangent (tan $\delta$): Measured tan $\delta$ values ranged from $7.5 \times 10^{-3}$ to $8.0 \times 10^{-2}$, increasing monotonically with frequency.
- Defect Correlation: High dielectric losses were strongly correlated with defects, specifically amorphous carbon inclusions and the fine-grained, high-defect layer adjacent to the silicon substrate (Raman FWHM up to 9.0 cm-1).
- Post-Processing Necessity: The authors suggest that mechanical or laser polishing to remove the defective substrate-side layer (tens to hundreds of µm thick) is necessary to achieve the ultra-low tan $\delta$ values ($10^{-5}$) required for high-power applications.
- Transmission Loss: Despite the relatively high tan $\delta$, transmission loss due to absorption was low ($\sim 1%$), making the material acceptable for low-power microwave windows.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the analysis of Sample A (57 mm diameter, 366 µm thickness) and Sample B (73 mm diameter, 450 µm thickness).
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Frequency Range Tested | 50 - 67 | GHz | Millimeter-wave range |
| Dielectric Constant ($\epsilon’$ ) | $\sim 5.7$ | Dimensionless | Calculated via NIST Precision method |
| Minimum tan $\delta$ (Sample A) | $7.5 \times 10^{-3}$ | Dimensionless | Measured at 50 GHz |
| Maximum tan $\delta$ (Sample A) | $2.2 \times 10^{-2}$ | Dimensionless | Measured at 67 GHz |
| Transmission Loss (Absorption) | $\sim 1$ | % | Acceptable for low-power windows |
| Absorption Coefficient ($\alpha$) | 0.52 | cm-1 | Calculated at 58 GHz (Sample A) |
| Substrate Side Roughness ($R_{rms}$) | $\sim 10$ | nm | Smooth side, adjacent to Si substrate |
| Growth Side Roughness ($R_{rms}$) | 3 - 8 | µm | Decreasing towards the disk edge |
| Crystallite Size (Center) | $\sim 130$ | µm | Sample A, large-grained growth surface |
| Crystallite Size (Edge) | $\sim 50$ | µm | Sample A, near disk edge |
Key Methodologies
Section titled “Key Methodologies”The following parameters and techniques were used for the synthesis and characterization of the PCD diamond disks:
- Synthesis Method: Microwave Plasma Chemical Vapor Deposition (MPCVD) using an ARDIS-100 reactor (2.45 GHz frequency).
- Gas Mixture: Methane (CH4) and Hydrogen (H2).
- Synthesis Parameters (Sample A):
- CH4 Concentration: 2.5%
- Pressure: 100 Torr
- Substrate Temperature: 820 °C
- Deposition Rate: $\sim 3$ µm/h
- Synthesis Parameters (Sample B):
- CH4 Concentration: 5%
- Pressure: 50 Torr
- Substrate Temperature: 720 °C
- Deposition Rate: $\sim 1.5$ µm/h
- Substrate Removal: Chemical etching of the 3 mm thick single-crystalline silicon substrate using HF+HNO3 mixture.
- Dielectric Measurement: Free-space method using WR-15 horn antennas and a Keysight vector network analyzer (VNA) (10 MHz to 67 GHz range).
- Data Calculation: Reflection/Transmission Epsilon Precision (NIST Precision) method applied to measured S-parameters, utilizing GRL calibration.
- Structural Characterization: Raman spectroscopy (473 nm laser) to assess diamond quality (FWHM of 1332 cm-1 peak) and detect amorphous carbon (1540 cm-1 band).
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The research clearly demonstrates that achieving ultra-low loss in PCD diamond for high-power millimeter-wave applications (e.g., 170 GHz gyrotrons) requires precise control over material purity and the removal of the defective, fine-grained layer formed near the substrate. 6CCVD is uniquely positioned to meet these requirements through custom material synthesis and advanced post-processing.
Applicable Materials
Section titled “Applicable Materials”To replicate or extend this research, particularly targeting lower tan $\delta$ values, 6CCVD recommends the following materials:
| Material Grade | Description & Application | 6CCVD Capability Match |
|---|---|---|
| Optical Grade PCD | Required for high-transparency, low-loss windows in the millimeter-wave range. Our synthesis process is optimized to minimize non-diamond carbon phases (amorphous carbon) and reduce grain boundary defects. | PCD Wafers (0.1 µm - 500 µm) |
| High Purity SCD | For applications demanding the absolute lowest possible tan $\delta$ (approaching $10^{-5}$), Single Crystal Diamond (SCD) offers superior structural perfection, eliminating grain boundaries and associated amorphous carbon losses. | SCD Plates (up to 500 µm) |
Customization Potential
Section titled “Customization Potential”The paper utilized large-diameter disks (57 mm and 73 mm) and specific thicknesses (366 µm, 450 µm). 6CCVD’s capabilities directly address these dimensional requirements:
- Large Diameter Synthesis: 6CCVD offers Polycrystalline Diamond (PCD) plates and wafers up to 125 mm in diameter, significantly exceeding the dimensions tested in this study.
- Custom Thickness Control: We provide precise thickness control for both SCD and PCD materials, ranging from 0.1 µm to 500 µm for wafers, and substrates up to 10 mm thick.
- Critical Post-Processing (Defect Removal): The research identified the fine-grained substrate layer as the primary source of high tan $\delta$. 6CCVD offers high-precision polishing services to mechanically or laser-remove this defective layer, ensuring the final component utilizes only the high-quality, large-grained diamond material.
- Polishing Specification: We guarantee surface roughness Ra < 5 nm for inch-size PCD wafers, ensuring minimal microwave dispersion ($R_{rms}/\lambda < 0.05$).
- Metalization Services: While not the focus of this paper, 6CCVD offers in-house metalization (Au, Pt, Pd, Ti, W, Cu) for integration into complex microwave or electronic packaging designs.
Engineering Support
Section titled “Engineering Support”The relationship between CVD growth parameters (CH4 concentration, pressure, temperature) and resulting dielectric loss is complex and application-specific. 6CCVD’s in-house PhD team specializes in tailoring MPCVD recipes to optimize material properties for specific frequency ranges.
We offer consultation services to assist engineers and scientists in:
- Material Selection: Determining the optimal balance between cost (PCD) and performance (SCD) for specific millimeter-wave or high-power gyrotron projects.
- Recipe Optimization: Adjusting growth parameters to maximize crystallite size and minimize non-diamond carbon inclusions, thereby reducing tan $\delta$.
- Post-Processing Strategy: Defining the necessary polishing depth and surface finish to eliminate the high-defect substrate layer and meet required roughness specifications ($R_{rms} < 10$ nm).
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Radio transparency of polycrystalline CVD-diamond disks with diameter up to 75 mm in millimeter-wave range was measured by free-space method. The structure of the disks was characterized by Raman spectroscopy and scanning electron microscopy. Dielectric loss tangent of the samples in the frequency range of 50-67 GHz was found to be in the range of 7.5· 10 -3 -8· 10 -2 , increasing with frequency. The transmission loss due to the radiation absorption is about 1%. Keywords: millimeter-waves, polycrystalline diamond, radio transparency.