In-situ measurements of fabrication induced strain in diamond photonic-structures using intrinsic colour centres

At a Glance

Metadata	Details
Publication Date	2020-06-02
Journal	npj Quantum Information
Authors	Sebastian Knauer, John P. Hadden, John Rarity
Institutions	Bristol Robotics Laboratory, University of Bristol
Citations	39
Analysis	Full AI Review Included

Technical Documentation & Analysis: Fabrication-Induced Strain in Diamond Photonic Structures

This document analyzes the research paper “In-situ measurements of fabrication induced strain in diamond photonic-structures using intrinsic colour centres” by Knauer et al. (npj Quantum Information (2020)6:50) and connects its findings and material requirements directly to the advanced Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) capabilities offered by 6CCVD (6ccvd.com).

Executive Summary

The research successfully demonstrates the use of the Nitrogen-Vacancy (NV) center ground state spin as an embedded, atomic-scale sensor to quantify localized strain fields induced by nanoscale fabrication processes in diamond.

Core Value Proposition: The NV center’s ground state spin splitting, measured via pulsed Optically Detected Magnetic Resonance (ODMR), provides a highly sensitive, room-temperature probe for lattice deformation.
Strain Quantification: Fabrication processes (Focused Ion Beam, FIB, milling) induce measurable strain fields corresponding to low megapascal stress levels (4 MPa to 20 MPa).
Fabrication Optimization: The technique allows for in-situ measurement of strain caused by milling nanoscale photonic structures (e.g., Solid Immersion Lenses, SILs), which is crucial for optimizing device yield.
Material Recovery: High-temperature annealing (800 °C) was proven effective in recovering quenched NV center fluorescence and significantly reducing fabrication-induced strain splitting (e.g., from 720 kHz down to 480 kHz).
Material Requirement: Successful replication and extension of this work requires ultra-high purity, electronic-grade Single Crystal Diamond (SCD) with precise control over nitrogen concentration (e.g., 4 ppb N).
6CCVD Relevance: 6CCVD specializes in providing the necessary high-purity SCD substrates, custom dimensions, and post-processing consultation required for strain-free quantum device engineering.

Technical Specifications

The following hard data points were extracted from the research paper, highlighting the critical material and measurement parameters.

Parameter	Value	Unit	Context
Diamond Grade (N Purity)	4	ppb	Electronic grade bulk SCD (Element6)
Surface Orientation	[110]	N/A	Used for sample preparation
NV Center Depth (Example)	4.3 to 5.6	µm	Depth of NV centers studied near milled structures
Zero Field Splitting (Dgs)	2.88	GHz	Unperturbed NV ground state
Maximum Measured Splitting	720 ± 4	kHz	Observed after 2nd FIB milling step
Equivalent Electric Field (Max)	21.5 ± 4.0	kV cm^-1	Corresponds to 720 kHz splitting
Equivalent Stress Range	4 to 20	MPa	Low megapascal stress levels
Polishing FIB Current	150	pA	Used for final FIB milling step
Annealing Temperature (Max)	800	°C	Used for strain recovery and NV center recovery
Annealing Hold Time	7.5	h	Duration at 800 °C
Post-Annealing Splitting (NV05)	480 ± 5	kHz	Reduced strain splitting after 800 °C anneal
Polishing Quality (SCD)	Ra < 1	nm	6CCVD standard for optical grade SCD

Key Methodologies

The experiment relied on precise material selection, nanoscale fabrication, and rigorous post-processing to control and measure strain.

Material Selection: Use of bulk electronic grade Single Crystal Diamond (SCD) with a low, controlled nitrogen impurity concentration (4 ppb) to ensure high-coherence NV centers.
Spin Measurement: Pulsed Optically Detected Magnetic Resonance (ODMR) was performed at room temperature to measure the ground state spin splitting, which is directly proportional to the perpendicular strain field (Stark effect).
Nanoscale Fabrication (FIB Milling): Focused Ion Beam (FIB) was used to create air-diamond interfaces and Solid Immersion Lenses (SILs) near NV centers. Milling involved a two-step process:
- Coarse cut (350 pA beam).
- Polishing step (150 pA beam).
Chemical Cleaning: Acid and plasma treatments were applied after each milling step to remove surface contaminants, residual Gallium doping, and mitigate charge traps.
Strain Recovery (High-Temperature Annealing): Samples were annealed in a tube furnace under vacuum (< 3 x 10^-6 mbar) following a specific thermal recipe:
- Pre-bake at 400 °C (12 h).
- Ramp to 600 °C (30 min hold).
- Ramp to 800 °C (7.5 h hold).
Post-Annealing Cleaning: A final cleaning step was performed to remove any residual surface graphitization caused by the high-temperature treatment.

6CCVD Solutions & Capabilities

This research underscores the critical need for ultra-high quality diamond substrates and precise post-processing control to achieve strain-free quantum devices. 6CCVD is uniquely positioned to supply the materials and engineering support necessary to replicate and advance this work.

Applicable Materials

To replicate the high-coherence NV centers studied, researchers require diamond with extremely low intrinsic strain and controlled nitrogen content.

Optical Grade SCD Wafers: 6CCVD supplies ultra-high purity Single Crystal Diamond (SCD) plates, essential for minimizing background defects and maximizing NV center coherence time (T₂). We offer controlled nitrogen incorporation (e.g., < 1 ppb or specific doping) to optimize NV creation yield.
Custom Substrate Thickness: The paper utilized NV centers several µm deep (up to 5.6 µm). 6CCVD provides SCD plates with custom thicknesses ranging from 0.1 µm up to 500 µm, and substrates up to 10 mm thick, ensuring compatibility with deep implantation or intrinsic NV studies.

Customization Potential

The success of the photonic structures (SILs) depends on precise alignment and surface quality, areas where 6CCVD excels.

Research Requirement / Challenge	6CCVD Solution & Capability	Technical Advantage
Surface Quality: Need for ultra-smooth surfaces to minimize initial strain and optimize optical coupling (SILs).	Precision Polishing: 6CCVD guarantees SCD surfaces polished to Ra < 1 nm (atomic smoothness) and inch-size PCD polished to Ra < 5 nm.	Minimizes surface defects and charge traps, reducing inhomogeneous broadening and initial strain fields.
Device Alignment: Requirement for high-accuracy alignment (< 100 nm) of FIB structures relative to the NV center.	Custom Metalization: We offer in-house deposition of alignment markers (Au, Pt, Pd, Ti, W, Cu) and custom patterning services.	Facilitates high-precision lithography and FIB targeting, crucial for coupling NV centers to nanoscale photonic waveguides or SILs.
Custom Dimensions: Need for specific plate sizes for FIB and annealing equipment.	Custom Dimensions: We supply plates and wafers up to 125 mm (PCD) and custom-cut SCD plates to exact specifications.	Ensures seamless integration into existing fabrication tools (e.g., FIB chambers, annealing furnaces).

Engineering Support

The paper highlights that post-processing (annealing and cleaning) is critical for strain mitigation and NV center recovery.

Strain Mitigation Consultation: 6CCVD’s in-house PhD team can assist with material selection and optimization of post-growth processing recipes (e.g., high-temperature annealing up to 1500 °C) to minimize residual strain and control the NV charge state (NV^- vs. NV⁰) for similar quantum sensing and photonic structure projects.
Global Logistics: We ensure reliable global shipping (DDU default, DDP available) of sensitive diamond materials, supporting international research collaborations.

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/s41534-020-0277-1