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 | MIREA - Russian Technological University, Institute of Superhigh-Frequency Semiconductor Electronics of the Russian Academy of Sciences |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study investigates the radio transparency of polycrystalline Chemical Vapor Deposition (CVD) diamond disks in the millimeter-wave range (50-67 GHz) for high-frequency engineering applications.
- Core Achievement: Measured the dielectric loss tangent (tan δ) of CVD diamond disks using the free-space method, confirming its suitability for millimeter-wave components.
- Performance Metrics: The measured tan δ values ranged from 7.5 · 10-3 to 8.0 · 10-2 across the 50-67 GHz spectrum, showing a monotonic increase with frequency.
- Low Absorption Loss: Transmission loss due to radiation absorption was determined to be approximately 1%, which is acceptable for low Microwave-power windows and covers.
- Material Quality: Raman spectroscopy confirmed the high quality of the diamond lattice (1332 cm-1 peak), although amorphous carbon inclusions (1540 cm-1 peak) were detected on the fine-grained substrate side, which contributes to increased loss.
- Key Application Relevance: The material’s high thermal conductivity and wide spectral window make it a promising candidate for output windows in 1 MW continuous-action gyrotrons and components for 5G mobile communication systems (66-71 GHz).
- Methodology: Measurements were performed using the Reflection/Transmission Epsilon Precision (NIST Precision) method based on S-parameters measured by a vector network analyzer in a free-space setup.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Frequency Range Tested | 50 - 67 | GHz | Millimeter-wave range |
| Dielectric Loss Tangent (tan δ) Range | 7.5 · 10-3 - 8.0 · 10-2 | N/A | Measured range across 50-67 GHz |
| Transmission Loss (Absorption) | ~1 | % | Acceptable for low Microwave-power applications |
| Relative Permittivity (ε’) | ~5.7 | N/A | Calculated value |
| Absorption Coefficient (α) | 0.52 | cm-1 | Sample A, measured at 58 GHz |
| Sample A Diameter / Thickness | 57 / 366 | mm / µm | Polycrystalline CVD-diamond disk |
| Sample B Diameter / Thickness | 73 / 450 | mm / µm | Polycrystalline CVD-diamond disk |
| Average Crystallite Size (Sample A Center) | ~130 | µm | Measured by Raster Electron Microscope (REM) |
| RMS Roughness (Growth Side, Rrms) | 3 - 8 | µm | Sample A |
| RMS Roughness (Substrate Side, Rrms) | ~10 | nm | Sample A (Smooth side) |
| Diamond Raman Peak Frequency | 1332 | cm-1 | Indicates diamond lattice quality |
| Amorphous Carbon Peak Frequency | 1540 | cm-1 | Detected on fine-grained substrate side |
| Synthesis Reactor Frequency | 2.45 | GHz | ARDIS-100 Microwave plasma-chemical reactor |
Key Methodologies
Section titled “Key Methodologies”The radio transparency and structural integrity of the CVD diamond disks were assessed using a combination of synthesis control, microscopy, spectroscopy, and free-space microwave testing.
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Material Synthesis:
- Reactor: ARDIS-100 Microwave plasma-chemical reactor (2.45 GHz).
- Substrate: Single-crystalline silicon (3 mm thick).
- Gas Flow: Total gas flow rate was 400 sccm (standard cubic centimeters per minute).
- Sample A Parameters (High Rate): CH4 concentration 2.5%, Pressure 100 Torr, Substrate Temperature 820°C, Deposition Rate ~3 µm/h.
- Sample B Parameters (Low Rate): CH4 concentration 5%, Pressure 50 Torr, Substrate Temperature 720°C, Deposition Rate ~1.5 µm/h.
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Structural Characterization:
- Microscopy: Raster Electron Microscopy (REM) was used to image the growth surface topography and measure crystallite size (130 µm center to 50 µm edge).
- Roughness: NewView 5000 optical profilometer (ZYGO) measured root-mean-square roughness (Rrms). The ratio Rrms/λ was < 0.0017, confirming negligible dispersion effects.
- Spectroscopy: Raman-scattering spectroscopy (LabRam HR800, 473 nm laser) analyzed carbon phases, confirming the diamond peak (1332 cm-1) and identifying amorphous carbon inclusions (1540 cm-1) on the substrate side.
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Microwave Measurement (Free-Space Method, FS):
- Setup: The sample was placed between WR-15 flanged horn antennas, 143 cm apart, ensuring the sample was in the far zone. A radiopaque shield was used to match the beam size to the disk diameter.
- Instrumentation: Keysight vector network analyzer (10 MHz to 67 GHz).
- Calculation: The N1500A software application used the Reflection/Transmission Epsilon Precision (NIST Precision) method, based on measured S-parameters, to determine ε’ and tan δ.
Commercial Applications
Section titled “Commercial Applications”The unique combination of high thermal conductivity, wide spectral transparency, and low absorption loss makes polycrystalline CVD diamond highly valuable for demanding RF and thermal management applications.
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High-Power Microwave Devices:
- Output windows (covers) for superpower (1 MW class) continuous-action gyrotrons, typically operating around 170 GHz.
- Heat-conducting enclosures for high-power RF components where thermal management is critical.
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Telecommunications and Radar:
- Components for 5G mobile communication systems, specifically in the 66-71 GHz band.
- Radio-transparent radiator covers and radomes requiring minimal signal loss in the millimeter-wave range.
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Broadband Data Transmission:
- Materials for high-speed, broad-band transmission systems operating in the five-millimeter wavelength range (50-70 GHz).
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Dielectric Substrates:
- Use as low-loss dielectric substrates in high-frequency circuits, provided the defective fine-grained substrate layer is removed (e.g., by polishing) to minimize conductivity losses.
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.