Low-temperature direct bonding of InP and diamond substrates under atmospheric conditions

At a Glance

Metadata	Details
Publication Date	2021-05-27
Journal	Scientific Reports
Authors	Takashi Matsumae, Ryo Takigawa, Yuichi Kurashima, Hideki Takagi, Eiji Higurashi
Institutions	Kyushu University, National Institute of Advanced Industrial Science and Technology
Citations	15
Analysis	Full AI Review Included

Technical Documentation & Analysis: Low-Temperature Direct Bonding of InP and Diamond

Executive Summary

This research demonstrates a critical advancement in thermal management for high-power Indium Phosphide (InP) electronic devices by achieving robust, low-temperature direct bonding between InP and diamond heat spreaders.

Core Achievement: Successful direct bonding of InP and diamond substrates under atmospheric conditions via low-temperature annealing (250 °C).
Thermal Management Solution: This technique leverages diamond’s superior thermal conductivity (2200 W/m/K) to mitigate severe heat dissipation issues inherent in high-frequency InP devices (k = 68 W/m/K).
Interface Quality: The bonding interface is characterized by an ultra-thin (3 nm) amorphous intermediate layer (composed of In, P, O, and C), which minimizes thermal resistance compared to conventional thick metal bonding layers (2-4 µm).
Process Reliability: The resulting bond exhibits a high shear strength of 9.3 MPa, meeting industrial reliability standards (MIL STD 883E die shear strength requirements).
Methodology: Bonding relies on hydrophilic surface activation—oxygen plasma for InP and NH₃/H₂O₂ cleaning for diamond—to generate reactive OH groups, followed by thermal dehydration at 250 °C.
Material Requirement: Successful direct bonding is highly dependent on atomically smooth surfaces, requiring RMS roughness preferably less than 5 Å.

Technical Specifications

The following hard data points were extracted from the research paper detailing the physical and process parameters of the InP/Diamond direct bonding experiment.

Parameter	Value	Unit	Context
Diamond Thermal Conductivity (k)	2200	W/m/K	Highest k material used for heat spreading
InP Thermal Conductivity (k)	68	W/m/K	Device substrate suffering from heat issues
Annealing Temperature	250	°C	Low-temperature bonding process
Annealing Time	24	h	Duration under applied load
Applied Bonding Load	1	MPa	Pressure applied during annealing
Bond Shear Strength	9.3	MPa	Measured on 3 x 3 mm sample
Intermediate Layer Thickness	3	nm	Amorphous layer (In, P, O, C) at interface
Diamond Substrate Thickness	100	µm	Thickness used for TEM analysis
InP RMS Roughness (Pre-Plasma)	2.76 ± 0.3	Å	Surface smoothness required for direct bonding
InP RMS Roughness (Post-Plasma)	3.03 ± 0.3	Å	Surface smoothness maintained after activation
Conventional Metal Layer Thickness	2-4	µm	Thickness of layers avoided by direct bonding

Key Methodologies

The direct bonding process relies on precise surface preparation and controlled low-temperature annealing to achieve atomic bonding between the InP and diamond substrates.

Diamond Surface Preparation (Hydrophilic Activation):
- Diamond (111) substrates were cleaned using an NH₃/H₂O₂ mixture (1:1:4 ratio) for 10 minutes.
- This process generates C-OH groups, resulting in a hydroxyl-terminated (hydrophilic) diamond surface.
InP Surface Preparation (Plasma Activation):
- InP substrates were activated using oxygen plasma irradiation (200 W power) for 30 seconds.
- This activation generates In-OH and P-OH groups on the InP surface.
Pre-Contact Cooling:
- The diamond substrate was cooled to 14 °C using a Peltier cooler for approximately 30 seconds. This step was hypothesized to promote hydrogen bond networks via condensed water molecules.
Contacting:
- The activated InP and diamond surfaces were brought into contact in ambient air (23 °C, 40% RH).
Annealing and Bonding:
- The contacted specimen was annealed at 250 °C for 24 hours under a load of 1 MPa.
- Thermal dehydration during annealing facilitates the formation of atomic bonds between the OH-terminated surfaces.
Interface Analysis:
- The bonded specimen was thinned (InP reduced to 10 µm) and analyzed using TEM and EDX to confirm the 3 nm amorphous intermediate layer and atomic composition (In, P, O, C).

6CCVD Solutions & Capabilities

6CCVD provides the high-quality MPCVD diamond materials and precision engineering services necessary to replicate and scale the advanced thermal management solution demonstrated in this research.

Applicable Materials

To achieve the 2200 W/m/K thermal conductivity and the ultra-smooth surface finish required for hydrophilic direct bonding, Optical Grade Single Crystal Diamond (SCD) is the ideal material solution.

Material Recommendation: Optical Grade SCD (High Purity, High Thermal Conductivity).
Thickness Control: 6CCVD offers SCD thickness control from 0.1 µm up to 500 µm, allowing engineers to precisely tailor the thermal path thickness (the paper used 100 µm).

Precision Polishing for Direct Bonding

The success of this low-temperature direct bonding technique is critically dependent on achieving an atomically smooth surface (RMS roughness < 5 Å). 6CCVD guarantees the required surface quality.

SCD Polishing Capability: 6CCVD provides SCD polishing achieving Ra < 1 nm (10 Å). For specific, high-end thermal management applications like this, our specialized polishing processes can achieve sub-nanometer roughness necessary for successful hydrophilic bonding.
PCD Polishing Capability: For larger area integration, our Polycrystalline Diamond (PCD) wafers (up to 125 mm) can be polished to Ra < 5 nm.

Customization Potential & Scaling

The research utilized small, three-square-millimeter diamond substrates. 6CCVD’s capabilities enable immediate scaling for industrial and research applications.

Requirement from Paper	6CCVD Capability	Benefit to Client
Small 3x3 mm samples	Custom Dimensions up to 125 mm (PCD)	Enables large-scale integration of InP devices onto diamond wafers.
Diamond Substrate	SCD and PCD Substrates up to 10 mm thick	Provides robust mechanical and thermal support for high-power modules.
No Metalization Used	Custom Metalization Available (Au, Pt, Pd, Ti, W, Cu)	If alternative bonding methods (e.g., flip-chip) are required, 6CCVD offers in-house metalization services for ohmic contacts or bonding layers.
Specific Geometry	Laser Cutting and Shaping Services	We can provide diamond heat spreaders cut to precise geometries required for specific InP device layouts (e.g., HEMT/HBT arrays).

Engineering Support

The successful implementation of this InP/Diamond integration technique requires expertise in both material science and surface chemistry.

Application Focus: 6CCVD’s in-house PhD team specializes in material selection and optimization for high-frequency and high-power applications, including InP HEMT/HBT integration and THz monolithic integrated circuits (TMIC).
Process Consultation: We offer technical consultation on surface preparation protocols (e.g., optimizing cleaning mixtures or plasma parameters) to ensure the diamond surface is optimally terminated for hydrophilic direct bonding.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-021-90634-4