Strain Effects in a Directly Bonded Diamond‐on‐Insulator Substrate
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-07-22 |
| Journal | physica status solidi (a) |
| Authors | Ioannis Varveris, Gianni D. Aliberti, Tianyin Chen, Filip A. Sfetcu, Diederik J. W. Dekker |
| Institutions | QuTech, Delft University of Technology |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Strain Effects in Directly Bonded Diamond-on-Insulator Substrates
Section titled “Technical Documentation & Analysis: Strain Effects in Directly Bonded Diamond-on-Insulator Substrates”Executive Summary
Section titled “Executive Summary”This research validates the feasibility of Single Crystal Diamond (SCD) for scalable Diamond-on-Insulator (DOI) substrates, a critical step for integrated quantum photonics. 6CCVD is uniquely positioned to supply the advanced SCD materials required to scale and optimize this technology.
- Quantum Integration Validated: The study confirms that direct bonding of SCD to SiO2/Si enables monolithic integration of NV centers, crucial for improving photon collection efficiency and entanglement generation rates.
- Strain Characterization: Depth-resolved Optically Detected Magnetic Resonance (ODMR) successfully quantified strain induced by the thermal expansion mismatch during cooling from the 200 °C annealing step.
- Quantified Strain Effects: Volumetric strain (Mz) increased by ≈0.45 MHz and shear strain (Mxy) increased by ≈0.71 MHz when moving from the top surface to the DOI interface.
- Emitter Quality Maintained: Despite the measurable strain increase, the optical properties of the NV centers remained largely unaffected, with ODMR contrast and peak linewidth showing only minor changes (contrast deterioration ≈0.36%).
- Methodology Established: The combination of Photoluminescence (PL) mapping (for bonding quality assessment) and ODMR (for detailed strain analysis) provides a robust framework for quality control in DOI fabrication.
- Future Material Requirement: The research highlights the need for ultra-thin SCD layers (micrometer to nanometer range) for future scalable DOI substrates, a core capability of 6CCVD’s MPCVD technology.
- 6CCVD Advantage: We provide the high-quality, ultra-smooth SCD substrates (Ra < 1 nm) and custom thin-film dimensions (down to 0.1 µm) necessary to achieve high-fidelity, void-free hydrophilic direct bonding on large-area wafers.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the research paper detailing the material properties and measured strain effects in the DOI substrate.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Material | Type Ib SCD | N/A | (100)-oriented, double-side polished |
| Diamond Thickness (Used) | ≈300 | µm | Bulk substrate investigated |
| Diamond Surface Roughness (Ra) | < 2 | nm | Initial roughness, critical for hydrophilic bonding |
| Insulator Layer | 300 | nm | PECVD SiO2 on Si |
| SiO2 RMS Roughness (Sq) | 4.44 | nm | Measured prior to bonding |
| Annealing Temperature | 200 | °C | Low-temperature bond stabilization |
| Annealing Duration | 24 | h | Performed under N2 flow |
| Volumetric Strain Increase (Mz) | ≈0.45 | MHz | Increase from top surface to DOI interface |
| Shear Strain Increase (Mxy) | ≈0.71 | MHz | Increase from top surface to DOI interface |
| NV Center Concentration | Roughly 15 | ppb | Randomly distributed ensembles |
| External Bias Magnetic Field (Bz) | 15.7 | G | Used during ODMR measurements |
| ODMR Contrast Deterioration | ≈0.36 | % | Observed near the interface |
| FWHM Decrease (Interface) | 0.38 | MHz | Drop from 6.08 MHz (top) to 5.70 MHz (interface) |
Key Methodologies
Section titled “Key Methodologies”The DOI substrate was fabricated using a hydrophilic direct bonding process, relying on surface chemical activation and low-temperature annealing to achieve robust interfacial adhesion.
- Diamond Surface Activation (Hydrophilicity):
- Cleaning: Piranha solution (3:1 H2SO4:H2O2) applied at 75 °C for 30 min.
- Result: Removes organic contamination and terminates the diamond surface with hydroxyl groups (C-OH).
- Insulator Surface Activation (Hydrophilicity):
- Substrate: 300 nm PECVD SiO2 layer on a Si wafer.
- Treatment: O2 plasma (1000 W, 5 min, 400 sccm O2 flow).
- Result: Increases silanol (Si-OH) group density, promoting hydrophilicity.
- Initial Contacting:
- Conditions: Room temperature (20 °C) and ambient humidity (≈40%).
- Mechanism: Van der Waals forces and hydrogen bonding across a thin interfacial water layer provide initial adhesion.
- Bond Stabilization (Annealing):
- Conditions: Low-temperature anneal at 200 °C for 24 h under N2 flow.
- Mechanism: Induces interfacial dehydration, leading to the formation of strong, irreversible covalent Si-O-C bonds.
- Strain Characterization:
- Techniques: Confocal ODMR (Optically Detected Magnetic Resonance) and PL (Photoluminescence) mapping.
- Purpose: PL mapping identifies bonding irregularities (interference fringes); ODMR quantifies volumetric (Mz) and shear (Mxy) strain amplitudes via frequency shifts and splittings.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The successful replication and scaling of this DOI technology—especially the transition to ultra-thin films—requires specialized diamond substrates that meet stringent purity, thickness, and surface quality standards. 6CCVD is the ideal partner to advance this research into scalable integrated quantum circuits.
Applicable Materials for Replication and Scaling
Section titled “Applicable Materials for Replication and Scaling”| Research Requirement | 6CCVD Material Solution | Technical Advantage |
|---|---|---|
| High-Purity SCD Substrate | Optical Grade Single Crystal Diamond (SCD) | High-purity MPCVD growth ensures minimal intrinsic strain and low defect density, crucial for high-fidelity NV centers. |
| Ultra-Thin Diamond Films | SCD (0.1 µm - 50 µm) | We supply pre-thinned SCD layers, eliminating the time-consuming and potentially damaging O2 plasma etching step used in the paper (which etched ≈50 µm). |
| Alternative Color Centers | SCD or PCD | Our MPCVD process allows for controlled incorporation of Si or Ge during growth, enabling the study of SiV or GeV centers in DOI structures, which are often preferred for integrated photonics. |
| Large-Area Substrates | PCD Wafers up to 125 mm | While the paper used 25 × 25 mm2 chips, 6CCVD offers inch-size PCD wafers, enabling industrial scalability for integrated photonic circuits. |
Customization Potential for Advanced DOI Structures
Section titled “Customization Potential for Advanced DOI Structures”The paper noted that bonding quality suffered on larger samples (10 × 10 mm2) and emphasized the need for superior surface preparation. 6CCVD directly addresses these challenges:
- Superior Polishing for Hydrophilic Bonding: The research used diamond with Ra < 2 nm. 6CCVD guarantees Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD. This ultra-low roughness is essential for achieving the uniform, void-free interfacial contact required for high-strength hydrophilic direct bonding across large areas.
- Custom Dimensions and Thickness Control: We provide SCD and PCD plates/wafers in custom dimensions up to 125 mm. Our precise MPCVD control allows engineers to specify SCD thickness from 0.1 µm up to 500 µm, perfectly matching the requirements for next-generation, thin-film DOI substrates.
- Integrated Metalization Services: For researchers integrating electrodes or waveguides, 6CCVD offers in-house metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu deposition, allowing for the creation of fully functional integrated quantum devices on the DOI platform.
Engineering Support
Section titled “Engineering Support”6CCVD’s in-house PhD team specializes in the material science of diamond quantum systems. We can assist researchers in:
- Strain Mitigation Strategies: Consulting on material selection (e.g., specific crystal orientation or doping levels) to minimize intrinsic strain and optimize the thermal expansion coefficient match for robust DOI structures.
- Material Selection for Quantum Projects: Providing expert guidance on selecting the optimal SCD or PCD grade, thickness, and surface finish for similar NV Center Quantum Sensing and Integrated Photonic projects, ensuring high ODMR contrast and long coherence times.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
The direct bonding process of a diamond‐on‐insulator (DOI) substrate enables monolithic integration of diamond photonic structures for quantum computing by improving photon collection efficiency and entanglement generation rate between emitters. It also addresses key fabrication challenges, such as robustness, bonding strength, and scalability. This study investigates strain effects in DOI substrates following direct bonding. Strain generation is expected near the diamond-SiO 2 /Si interface due to the thermal expansion coefficient mismatch between the bonded materials. Strain‐induced lattice distortions are characterized using nitrogen‐vacancy (NV) centers in diamond via optically detected magnetic resonance (ODMR) and photoluminescence (PL) mapping. PL mapping reveals interference fringes in unbonded regions, indicating bonding irregularities. Depth‐resolved ODMR measurements show a volumetric strain component increase of ≈0.45 MHz and a shear component increase of ≈0.71 MHz between the top surface and the DOI interface. However, ODMR signal contrast and peak linewidth remain largely unaffected, suggesting no visible deterioration in the optical properties of the emitters. By combining ODMR and PL mapping, this work establishes a robust methodology for assessing bonding quality and strain impact on NV centers, an essential step toward advancing scalable quantum technologies and integrated photonic circuits.