| 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 |
- Core Achievement: Demonstrated the use of the Nitrogen-Vacancy (NV) centerâs ground state electron spin as an embedded, atomic-scale sensor to measure localized mechanical strain induced by Focused Ion Beam (FIB) milling in bulk diamond.
- Strain Quantification: Strain fields perpendicular to the NV axis were quantified by measuring the splitting of the |±1,0) hyperfine states using pulsed Optically Detected Magnetic Resonance (ODMR).
- Sensitivity: The technique achieved high sensitivity, measuring strain fields corresponding to low megapascal stress levels (4 MPa to 20 MPa), with a minimum detectable equivalent electric field of 1.8 kV cm-1.
- Fabrication Optimization: Strain increased significantly as the FIB milled interface approached the NV center (from 1.5 ”m to 0.5 ”m), causing spectral broadening (4.7 kV cm-1 equivalent E-field) and eventual splitting (21.5 kV cm-1 equivalent E-field).
- Device Yield Improvement: The method successfully identified highly strained NV centers (unsuitable for quantum applications) and demonstrated recovery: high-temperature annealing reduced the splitting of a quenched NV center (NV05) by nearly half (from 770 kHz to 480 kHz).
- Application: This technique provides a critical tool for optimizing diamond fabrication processes (e.g., FIB milling, RIE, implantation) necessary for high-coherence quantum devices.
| Parameter | Value | Unit | Context |
|---|
| Diamond Material | Electronic Grade Bulk | N/A | Element6, [110] top surface |
| Nitrogen Impurity Concentration | 4 | ppb | Starting material specification |
| NV Center Zero Field Splitting (Dgs) | 2.88 | GHz | Unperturbed ground state |
| Gyromagnetic Ratio (Îł) | 28 | GHz T-1 | NV center property |
| Electric Field Susceptibility (kEgs) | 17 ± 3 | Hz cm V-1 | Literature value used for calculation |
| Strain Field Susceptibility (kÏgs) | 21.5 ± 1.2 | GHz (per unit strain) | Literature value used for calculation |
| Diamond Youngâs Modulus | 1200 | GPa | Used to convert strain to stress |
| Residual Magnetic Field (Bz) | ~0.3 ± 0.1 | ”T | Aligned perpendicular to NV axis |
| Strain Splitting (0.5 ”m distance) | 720 ± 4 | kHz | Maximum splitting observed from planar milling |
| Equivalent E-Field (0.5 ”m distance) | 21.5 ± 4.0 | kV cm-1 | Corresponds to 720 kHz splitting |
| Equivalent Stress (0.5 ”m distance) | ~20 | MPa | Calculated from strain field |
| SIL Milling Splitting (NV03) | 510 ± 3 | kHz | Splitting observed after Solid Immersion Lens fabrication |
| Annealed Splitting (NV05) | 480 ± 5 | kHz | Reduced splitting after 800 °C annealing |
| Minimum Detectable E-Field | 1.8 | kV cm-1 | Sensitivity measure of the technique |
The experiment relied on precise FIB milling followed by high-temperature annealing, with strain measured in-situ using pulsed ODMR spectroscopy.
- NV Center Characterization:
- NV centers (NV01, NV02, NV03) were located and characterized in bulk diamond using a home-built scanning confocal setup.
- An external magnetic field was precisely aligned perpendicular to the NV axis to minimize Zeeman splitting, ensuring that subsequent measured splitting was dominated by strain/electric fields.
- Focused Ion Beam (FIB) Milling (Planar Interface):
- A planar air-diamond interface was milled near NV centers using a Gallium beam (FEI Strata FIB-201 or Helios NanoLab 600).
- Coarse Cut: Initial milling used a 350 pA beam.
- Polishing Step: Followed by a polishing step using a 150 pA beam.
- The interface was brought progressively closer to the NV centers (1.5 ”m, then 0.5 ”m) to observe the strain gradient.
- FIB Milling (Solid Immersion Lens - SIL):
- SILs were milled around pre-characterized NV centers (e.g., NV03) with an alignment accuracy better than 100 nm.
- Post-Milling Cleaning and Measurement:
- After each milling step, the sample was cleaned using acid and plasma treatment to remove residual Gallium and surface defects.
- Pulsed ODMR spectroscopy was performed at room temperature to measure the frequency splitting (ÎÏsplit) of the central |±1,0) states, which is directly proportional to the perpendicular strain field.
- High-Temperature Annealing (Strain Recovery):
- The sample was annealed in a tube furnace (Lenton - LTF 12/50/300).
- Pre-treatment: Furnace heated to 400 °C for 12 h to minimize residual particles.
- Vacuum: Sample placed in the furnace center, pumped to a pressure less than 3 x 10-6 mbar.
- Soak: Heated to 600 °C for 30 min.
- Target Anneal: Temperature raised to 800 °C for 7.5 h.
- Post-Anneal: Cooled back to room temperature, followed by cleaning to remove residual surface graphitization.
This research directly supports the engineering and manufacturing of high-performance diamond-based quantum and photonic devices.
| Industry/Field | Application | Relevance to Research |
|---|
| Quantum Computing & Networks | Fabrication of high-coherence photonic structures (waveguides, cavities, SILs) coupled to single qubits. | Ensures NV centers retain long coherence times by minimizing fabrication-induced strain, which causes spectral broadening and quenching. |
| Quantum Sensing (Strain/E-Field) | Development of diamond mechanical resonators and hybrid opto-mechanical-spin control systems. | Provides a method to characterize and optimize the mechanical environment of the sensor, crucial for high-fidelity strain sensing. |
| Diamond Material Science | Optimization of post-processing recipes (annealing, cleaning) for implanted or grown NV centers. | Demonstrated that 800 °C annealing can recover quenched NV centers and reduce existing strain fields, improving material quality. |
| Semiconductor Device Characterization | In-situ mapping of internal electric fields and band bending in operating semiconductor devices. | The technique can be adapted to measure residual electric fields caused by doping or charge traps deep within the diamond lattice. |
| Advanced Manufacturing | Quality control and process feedback for high-precision milling techniques (FIB, RIE) used in nanoscale device fabrication. | Allows engineers to quantify the damage caused by specific beam currents and milling distances, leading to optimized, low-damage recipes. |