Inversion for Thermal Properties with Frequency Domain Thermoreflectance
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-01-09 |
| Journal | ACS Applied Materials & Interfaces |
| Authors | Benjamin Treweek, Volkan Akçelik, Wyatt Hodges, Amun Jarzembski, Matthew Bahr |
| Institutions | Sandia National Laboratories |
| Citations | 10 |
| Analysis | Full AI Review Included |
Technical Documentation and Analysis: High-Performance Diamond for Heterogeneous Integration
Section titled âTechnical Documentation and Analysis: High-Performance Diamond for Heterogeneous IntegrationâThis document analyzes the research paper âInversion for Thermal Properties with Frequency Domain Thermoreflectanceâ and outlines how 6CCVDâs advanced MPCVD diamond materials and processing capabilities directly support and enable the replication and extension of this critical work in heterogeneous integration (HI).
Executive Summary
Section titled âExecutive SummaryâThe research successfully demonstrates a sophisticated methodology for non-destructively characterizing the thermal interface quality in GaN-on-Diamond microelectronic devices, a critical step for improving reliability and power density.
- Core Achievement: Development of a gradient-based optimization technique coupled with Finite Element Method (FEM) simulations to invert Frequency Domain Thermoreflectance (FDTR) data, enabling spatial mapping of Thermal Boundary Conductance (TBC).
- Application Focus: Assessing bond quality in GaN-Diamond heterogeneously integrated systems, crucial for managing high heat fluxes and preventing thermomechanical failure.
- Key Findings: TBC values were successfully mapped, distinguishing between Poor Bond (< 10 MW/m2K), Partially Bonded (10-100 MW/m2K), and Good Bond (> 100 MW/m2K) regions.
- Material Stack: The analyzed structure consisted of a multilayer stack: Au transducer / GaN device layer / Unknown Bonding Layer / High-Conductivity Diamond Substrate.
- Thermal Requirement: The study confirms the necessity of high-thermal-conductivity diamond (k â 2500 W/m·K) as the heat spreader to effectively dissipate heat from the GaN device layer.
- 6CCVD Value Proposition: 6CCVD provides the necessary high-purity Single Crystal Diamond (SCD) substrates, custom dimensions, and integrated metalization services required to fabricate and optimize these high-performance GaN-on-Diamond devices.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the research, focusing on material properties and experimental parameters relevant to thermal transport.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Thermal Conductivity (k) | 2500 | W/m·K | Bulk Diamond (i=3) |
| GaN Thermal Conductivity (k) | 119 ± 9.5 to 124 ± 9.3 | W/m·K | SCD/PCD layer (i=2) |
| Diamond Specific Heat (Ïcp) | 1.73 | MJ/m3·K | Bulk Diamond (i=3) |
| GaN Device Layer Thickness (d2) | 4840 ± 180 | nm | Thinned GaN layer |
| Au Transducer Layer Thickness (d1) | 134 | nm | Fitted value for FDTR signal |
| Good Bond TBC (G2) | > 100 | MW/m2K | Target thermal performance |
| Poor Bond TBC (G2) | < 10 | MW/m2K | Unbonded region |
| Pump Beam Radius (wpump) | 3.46 | ”m | FDTR Setup |
| Probe Beam Radius (wprobe) | 2.75 | ”m | FDTR Setup |
| Measurement Frequency Range (fpump) | 1 kHz to 60 | MHz | Wide-bandwidth FDTR system |
| Sample Lateral Dimensions | 200 x 200 | ”m | Area of measurement |
Key Methodologies
Section titled âKey MethodologiesâThe experiment combined advanced material fabrication with high-performance computational modeling to achieve spatial mapping of thermal properties.
- Substrate Preparation: Bulk Gallium Nitride (GaN) and commercial diamond were used as starting materials.
- Metalization: Both GaN and diamond surfaces were coated with a Ti/Au film (5 nm Ti / 120 nm Au) via e-beam evaporation to facilitate bonding and serve as the FDTR transducer layer.
- Surface Cleaning: Surfaces were cleaned using Argon (Ar) plasma in a RIE chamber (100 W, 30 seconds).
- Thermal Compression Bonding: The die pair was subjected to a 2 kN force bond for 15 seconds at room temperature to achieve a cold weld of the gold surfaces.
- Device Thinning: The GaN layer was thinned and polished to a thickness of approximately 5 ”m (4840 nm).
- FDTR Measurement: A wide-bandwidth FDTR system was used to raster-scan the sample, obtaining hyperspectral data cubes across 16 frequencies (1 kHz to 60 MHz).
- Computational Modeling: Finite Element Method (FEM) simulations were performed using the Sierra/SD structural dynamics code on HPC clusters, utilizing a 3D stadium geometry mesh (231,537 nodes).
- Inverse Optimization: A gradient-based optimization technique was applied to the phase-only objective function to determine unknown thermal properties (thermal conductivity and TBC) in the discretized bonding layer domain.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & Capabilitiesâ6CCVD is uniquely positioned to supply the high-quality diamond materials and custom processing required to replicate and advance this research in high-power GaN-on-Diamond devices.
Applicable Materials
Section titled âApplicable MaterialsâThe research relies on diamond with k â 2500 W/m·K. 6CCVD offers materials that meet or exceed this requirement, ensuring maximum thermal performance for HI systems.
- Thermal Grade Single Crystal Diamond (SCD): Recommended material for replicating this work. Our SCD is grown via MPCVD, offering the highest purity and thermal conductivity (k > 2000 W/m·K) necessary for optimal heat spreading in GaN-on-Diamond devices.
- Polycrystalline Diamond (PCD): Available for applications where larger area or lower cost is prioritized, with thermal conductivities tailored to specific requirements.
- Boron-Doped Diamond (BDD): Available for integrated electrochemical or sensing applications, offering high conductivity while maintaining diamondâs superior mechanical properties.
Customization Potential
Section titled âCustomization PotentialâThe complexity of HI systems demands precise material dimensions and specific interface engineering, which are core capabilities of 6CCVD.
| Research Requirement | 6CCVD Customization Capability | Value Proposition |
|---|---|---|
| Substrate Dimensions | Paper used a 0.5 mm thick diamond substrate. | 6CCVD supplies SCD and PCD substrates up to 10 mm thick. We offer plates/wafers up to 125 mm (PCD) and custom laser cutting for precise die sizes (e.g., 200 ”m x 200 ”m). |
| Device Layer Thickness | GaN layer thinned to 4.84 ”m (4840 nm). | We provide SCD layers from 0.1 ”m up to 500 ”m, allowing researchers to specify the exact thickness needed for subsequent device growth or thinning processes. |
| Metalization Stack | Required Ti (5 nm) / Au (120 nm) transducer layer. | In-House Metalization: We offer custom deposition of Au, Pt, Pd, Ti, W, and Cu stacks directly onto the diamond surface, ensuring excellent adhesion and precise thickness control for FDTR transducer layers. |
| Surface Quality | High-quality interface required for thermal compression bonding and accurate FDTR. | Ultra-Precision Polishing: SCD surfaces are polished to Ra < 1 nm, and inch-size PCD to Ra < 5 nm, minimizing interfacial defects and maximizing TBC (> 100 MW/m2K). |
Engineering Support
Section titled âEngineering SupportâThe successful interpretation of FDTR data relies heavily on accurate thermal modeling (FEM, gradient-based optimization) and deep material knowledge.
- Thermal Transport Expertise: 6CCVDâs in-house PhD team specializes in thermal transport physics and TBC optimization. We can assist researchers in selecting the optimal diamond material and surface preparation to achieve target TBC values for similar GaN-on-Diamond projects.
- Modeling Consultation: We provide technical consultation on how diamond material parameters (e.g., defect density, surface termination) influence the input variables for complex inverse modeling techniques like those demonstrated in this paper.
- Global Logistics: We offer global shipping (DDU default, DDP available) to ensure timely delivery of custom materials worldwide.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
3D integration of multiple microelectronic devices improves size, weight, and power while increasing the number of interconnections between components. One integration method involves the use of metal bump bonds to connect devices and components on a common interposer platform. Significant variations in the coefficient of thermal expansion in such systems lead to stresses that can cause thermomechanical and electrical failures. More advanced characterization and failure analysis techniques are necessary to assess the bond quality between components. Frequency domain thermoreflectance (FDTR) is a nondestructive, noncontact testing method used to determine thermal properties in a sample by fitting the phase lag between an applied heat flux and the surface temperature response. The typical use of FDTR data involves fitting for thermal properties in geometries with a high degree of symmetry. In this work, finite element method simulations are performed using high performance computing codes to facilitate the modeling of samples with arbitrary geometric complexity. A gradient-based optimization technique is also presented to determine unknown thermal properties in a discretized domain. Using experimental FDTR data from a GaN-diamond sample, thermal conductivity is then determined in an unknown layer to provide a spatial map of bond quality at various points in the sample.