Transfer of AlGaN/GaN RF-devices onto diamond substrates via van der Waals bonding
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2018-04-25 |
| Journal | International Journal of Microwave and Wireless Technologies |
| Authors | Thomas Gerrer, V. Cimalla, Patrick Waltereit, S. MĂŒller, Fouad Benkhelifa |
| Institutions | University of Freiburg, Fraunhofer Institute for Applied Solid State Physics |
| Citations | 27 |
| Analysis | Full AI Review Included |
6CCVD Technical Analysis: GaN-on-Diamond Transfer via Van der Waals Bonding
Section titled â6CCVD Technical Analysis: GaN-on-Diamond Transfer via Van der Waals BondingâThis document analyzes the research âTransfer of AlGaN/GaN RF-devices onto diamond substrates via van der Waals bondingâ (Gerrer et al., 2018) to provide actionable technical specifications and propose 6CCVD material solutions for advancing GaN-on-Diamond high-power electronics.
Executive Summary
Section titled âExecutive SummaryâThe following points summarize the key findings and value proposition of the GaN-on-Diamond transfer technology:
- Thermal Breakthrough: Demonstrates successful low-temperature van der Waals (vdW) bonding to transfer AlGaN/GaN radio-frequency (RF) devices onto diamond (SCD and PCD) heat spreaders.
- Superior SCD Performance: Single Crystal Diamond (SCD) substrates achieved exceptional thermal management, resulting in a low channel temperature (Tch) of 52 °C for Schottky diodes under heavy DC load.
- Critical Thermal Comparison: SCD performance drastically outperformed Polycrystalline Diamond (PCD) (Tch 74 °C) and traditional Silicon (Si) substrates (Tch 160 °C), validating diamond as the optimal heat spreader.
- High RF Efficiency: Pulsed load-pull measurements on transferred RF transistors yielded excellent results: Peak Output Power (Pout) of 7.43 W/mm and Power-Added Efficiency (PAE) up to 59.1% (3 GHz, 50 V bias).
- Interface Importance: Thermal analysis confirmed that the poor thermal performance observed on PCD was due to the defect-rich diamond nucleation layer used for growth, emphasizing the need for ultra-high-quality, highly polished SCD to minimize Thermal Boundary Resistance (TBR).
- Process Flexibility: The vdW bonding approach allows independent optimization of GaN epitaxy and diamond substrate quality, crucial for mass-market high-reliability RF and microwave device integration.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the research paper, highlighting material properties and device performance metrics.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Thermal Conductivity (Kdiamond) | > 2000 | W m-1 K-1 | Benchmark conductivity for heat spreading |
| SiC Thermal Conductivity (KSiC) | 490 | W m-1 K-1 | Standard high-power substrate reference |
| GaN RF Power Density (Reference) | 40 | W mm-1 | State-of-the-art thermal limit cited |
| SCD Channel Temperature (Tch) | 52 | °C | 300 ”m SCD, 65 W/mm2 heat load, 5 V bias |
| PCD Channel Temperature (Tch) | 74 | °C | 300 ”m PCD, 53 W/mm2 heat load, 5 V bias |
| Si Channel Temperature (Tch) | 160 | °C | 675 ”m Si, 40 W/mm2 heat load, 5 V bias |
| Maximum Output Power (Pout-max, Pulsed) | 7.43 | W mm-1 | GaN HEMT on SCD (3 GHz, 50 V) |
| Maximum PAE (Pulsed) | 59.1 | % | GaN HEMT on SCD (3 GHz, 50 V) |
| PCD Thermal Boundary Resistance (TBR) | 500 | m2 K/GW | Estimated for 300 ”m PCD sample used |
| SCD Surface Roughness (Required) | < 0.5 | nm (RMS) | Critical for successful vdW bonding |
| SCD/PCD Thickness Used | 300 | ”m | Final thickness of bonded heat spreader |
Key Methodologies
Section titled âKey MethodologiesâThe study relied on a multi-step fabrication and characterization approach centered on transferring the GaN device layer to the diamond substrate:
- GaN Epitaxy: AlGaN/GaN heterostructures were grown via Metal-Organic Chemical Vapor Deposition (MOCVD) on high-resistivity Silicon (Si) substrates.
- Diamond Preparation: Both Polycrystalline Diamond (PCD) and Single Crystal Diamond (SCD, 8 mm x 8 mm) were used. SCD was mechanically polished to an ultra-low Roughness Average (Ra < 0.5 nm) to facilitate direct bonding.
- Carrier Bonding: The fabricated GaN-on-Si wafer was adhesively bonded onto a temporary Sapphire carrier.
- Si Substrate Removal: The Si substrate (650 ”m thick) was selectively removed via wet-chemical etching using an acetic mixture of Hydrofluoric Acid (HF) and Nitric Acid (HNO3), stopping at the Aluminum Nitride (AlN) nucleation layer.
- Van der Waals (vdW) Bonding: The exposed AlN nucleation layer was bonded onto the highly polished diamond (PCD or SCD) heat spreader at low temperatures (< 300 °C) to minimize thermal stress.
- Thermal Characterization: Large 2 mm x 1 mm AlGaN/GaN Schottky diodes (17 ”m channel length) were used to ensure uniform heat generation, allowing for channel temperature (Tch) extraction based on temperature-dependent electron mobility.
- Modeling: Thermal performance was validated using 3D Finite Element Analysis (FEA) simulations, which allowed quantification of the effective Thermal Boundary Resistance (TBR).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research highlights that the success of GaN-on-Diamond devices hinges critically on the quality and surface finish of the diamond substrate. 6CCVD is uniquely positioned to supply optimized MPCVD diamond materials that meet or exceed the requirements for replicating and scaling this vdW bonding technology.
Applicable Materials for Thermal Management
Section titled âApplicable Materials for Thermal ManagementâThe study explicitly demonstrated that only high-quality SCD exploited diamondâs thermal potential due to low TBR. 6CCVD offers the necessary material grades:
- Optical/Thermal Grade Single Crystal Diamond (SCD): Recommended for Replication & Optimization. This material provides the ultra-high purity and large single-crystal structure necessary for minimal defect density and optimal heat conduction (K > 2000 W m-1 K-1).
- Surface Finish: Guaranteed Ra < 1 nm (exceeding the required < 0.5 nm RMS) critical for achieving a low-TBR vdW bond interface.
- Thickness Control: SCD wafers available in required thicknesses (0.1 ”m up to 500 ”m) with tight tolerance for managing thermal profile.
- Thermal Grade Polycrystalline Diamond (PCD): Recommended for Cost-Effective Scaling. While the PCD used in the study suffered from a poor nucleation layer (high TBR), 6CCVD offers high-quality PCD optimized for thermal applications.
- Size Advantage: PCD wafers available up to 125 mm (4-inch), ideal for the wafer-scale manufacturing and scaling goals referenced in the paper.
- Polishing: Inch-size PCD wafers polished to Ra < 5 nm, suitable for high-quality bonding interfaces in high-volume applications.
Customization Potential
Section titled âCustomization PotentialâThe transfer process requires precise geometry and integration features, which 6CCVD supports globally:
| Requirement from Research Paper | 6CCVD Customization Capability |
|---|---|
| Substrate Dimensions | SCD chips provided in custom sizes (e.g., 8 mm x 8 mm) or PCD wafers supplied up to 125 mm diameter, facilitating large-area transfer. |
| Material Thickness | Precision thickness control for SCD and PCD from 0.1 ”m up to 500 ”m, allowing engineers to tune substrate thermal mass for specific pulsed/CW requirements. |
| Metalization Layers | The paper utilized Au/In solder for heatsink attachment. 6CCVD offers in-house custom metalization stacks (e.g., Ti/Pt/Au, Ti/W/Cu) tailored for specific bonding (vdW, solder, adhesive) or contact schemes. |
| Advanced Polishing | Ultra-smooth SCD surfaces (Ra < 1 nm) provided to ensure optimal capillary force and low defect density essential for minimizing TBR in vdW bonding. |
Engineering Support
Section titled âEngineering SupportâThe successful exploitation of diamond in GaN RF-devices depends entirely on minimizing the Thermal Boundary Resistance (TBR) between the AlN nucleation layer and the diamond. The 6CCVD in-house PhD team provides expert consultation in:
- TBR Optimization: Assisting researchers in selecting the ideal diamond surface preparation and material grade (SCD vs. PCD) to achieve low-resistance thermal interfaces for demanding GaN-on-Diamond HEMT and diode projects.
- Material Selection: Guiding material choices for applications requiring specific mechanical integrity, thermal properties, or optical transmission characteristics.
- Custom Fabrication: Developing non-standard dimensions, shapes, and edge finishes necessary for subsequent processing steps (e.g., dicing, handling, or integration into packaging).
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract We present a novel bonding process for gallium nitride-based electronic devices on diamond heat spreaders. In the proposed technology, GaN devices are transferred from silicon (Si) onto single (SCD) and polycrystalline diamond (PCD) substrates by van der Waals bonding. Load-pull measurements on Si and SCD heat spreaders at 3 GHz and 50 V drain bias show comparable power-added-efficiency and output power ( P out ) levels. A thermal analysis of the hybrids was performed by comparison of 2 Ă 1mm 2 AlGaN/GaN Schottky diodes on Si, PCD, and SCD, which exhibit a homogeneous field in the channel in contrast to gated transistors. Significantly different currents are observed due to the temperature dependent mobility in the 2DEG channel. These measurements are supported by a 3D thermal finite element analysis, which suggests a large impact of our transfer technique on the thermal resistance of these devices. In summary, we show a promising new GaN-on-diamond technology for future high-power, microwave GaN device applications.