Parabolic Diamond Scanning Probes for Single-Spin Magnetic Field Imaging

At a Glance

Metadata	Details
Publication Date	2020-12-02
Journal	Physical Review Applied
Authors	Natascha Hedrich, Dominik Rohner, Marietta Batzer, Patrick Maletinsky, Brendan J. Shields
Institutions	University of Basel
Citations	40
Analysis	Full AI Review Included

Technical Documentation & Analysis: Parabolic Diamond Scanning Probes

Reference: Hedrich et al., Parabolic diamond scanning probes for single spin magnetic field imaging (arXiv:2003.01733v1, 2020).

Executive Summary

This research successfully demonstrates a significant advancement in nanoscale quantum sensing by engineering truncated parabolic single crystal diamond (SCD) scanning probes. The core value proposition lies in the enhanced collection efficiency of Nitrogen-Vacancy (NV) center photoluminescence (PL), leading to superior magnetic sensitivity.

Record Sensitivity: Achieved a median saturation count rate of (2.1 ± 0.2) MHz, the highest reported to date for single NV centers in scanning probes.
Performance Gain: Demonstrated a 5-fold improvement in measurement signal compared to previous state-of-the-art cylindrical scanning probe designs.
Optimized Geometry: The truncated parabolic profile acts as a highly efficient photonic structure, collimating NV emission into a narrow numerical aperture (NA = 0.46).
Scalable Fabrication: Developed a reliable, two-stage ICP-RIE dry etching process utilizing a flowable oxide mask (FOX-16) to precisely control the parabolic tip curvature.
Nanoscale Resolution: The truncated apex minimizes the NV-sample separation to (40 ± 5) nm, enabling high-resolution magnetic imaging with spatial resolution better than 50 nm.
Material Requirement: The success relies on high-purity Type-IIa Single Crystal Diamond (SCD) optimized for hosting stable, near-surface NV centers.

Technical Specifications

The following hard data points were extracted from the research paper, detailing the performance and fabrication parameters of the parabolic diamond probes.

Parameter	Value	Unit	Context
Median Saturation Count Rate	2.1 ± 0.2	MHz	Highest reported for single NV scanning probes.
Measurement Signal Improvement	5-fold	N/A	Relative to state-of-the-art cylindrical NV sensors.
Spatial Resolution	< 50	nm	Demonstrated imaging of ferromagnetic stripes (CoFeB).
Effective NV-Ta Surface Separation	40 ± 5	nm	Achieved via truncated parabolic tip geometry.
Effective Numerical Aperture (NA_eff)	0.46	N/A	Emission contained within the NA_eff cone.
Device Collection Efficiency (η_dev)	0.57	N/A	Measured efficiency, approaching simulated maximum (0.68).
Paraboloid Facet Diameter	~200	nm	Minimal diameter supporting strong optical mode confinement.
Diamond Material Type	Type-IIa	N/A	High-purity SCD used for NV center hosting.
Etch Stage 1 Time (Waveguide)	240	s	O₂ etch steps alternating with O₂/CF₄ clean steps.
Etch Stage 2 Pressure	0.5	Pa	Used during variable CF₄ flow for curvature control.

Key Methodologies

The fabrication of the parabolic diamond scanning probes relies on a highly controlled, two-stage Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) process.

Material Selection and Preparation: High-purity Type-IIa Single Crystal Diamond (SCD) was used, pre-patterned with cantilevers etched to a depth of 2 µm. NV centers were introduced via implantation.
Masking: A ~300-nm thick layer of flowable oxide (FOX-16) was applied and patterned via electron beam lithography into 1-µm diameter discs to serve as the etch mask.
Stage 1 Etch (Tapered Waveguide): A 240s O₂ etch chemistry (50 sccm O₂) was alternated with 4s O₂/CF₄ steps (50 sccm O₂, 2 sccm CF₄) to clean off resputtered material. This cycle was repeated nine times to achieve a ~6 µm tapered pillar.
Stage 2 Etch (Parabolic Curvature): CF₄ gas flow was introduced throughout the etch at increasing flow rates (2-10 sccm) while maintaining O₂ flow (50 sccm) and low pressure (0.5 Pa). This procedure controllably eroded the FOX mask relative to the diamond, tuning the wall angle and resulting in the desired parabolic profile.
Final Device Geometry: The resulting pillar featured a parabolic tip section with a ~200-nm flat end facet and a long tapered waveguide section.
Release: A deep etch from the back side of the diamond released the cantilevers for integration into scanning probes.

6CCVD Solutions & Capabilities

The successful replication and extension of this high-impact quantum sensing research requires diamond materials and fabrication support that meet stringent purity, dimensional, and surface quality requirements. 6CCVD is uniquely positioned to supply the necessary components.

Applicable Materials

To replicate the high-performance NV center probes, researchers require the highest quality diamond available.

Optical Grade Single Crystal Diamond (SCD): This material is essential for minimizing background fluorescence and maximizing NV center coherence time. 6CCVD supplies high-purity Type-IIa SCD plates, ideal for subsequent NV implantation and annealing processes.
Boron-Doped Diamond (BDD) Potential: While not used in this specific paper, BDD thin films (PCD or SCD) are often required for integrated microwave delivery (ODMR). 6CCVD offers custom BDD films for integration into advanced quantum devices.

Customization Potential

The fabrication relies on precise control over starting material dimensions and the ability to integrate functional layers. 6CCVD provides the following custom services:

Research Requirement	6CCVD Capability	Technical Advantage
Custom Wafer Dimensions (Optimizing for microfabrication yield)	Plates/Wafers up to 125mm (PCD) and 10x10 mm (SCD). We provide custom laser cutting services to match specific tool requirements (e.g., cantilever arrays).	Maximizes throughput and minimizes material waste during complex ICP-RIE processing runs.
Precise Thickness Control (Required for 6 µm pillar height)	SCD Thicknesses from 0.1 µm to 500 µm. We guarantee tight thickness tolerances necessary for deep etching and uniform cantilever release.	Ensures consistent starting material for high-aspect ratio structures and reliable device yield.
Ultra-Low Surface Roughness (Critical for optical coupling)	SCD Polishing to Ra < 1 nm. Our proprietary polishing techniques ensure minimal scattering losses at the diamond-air interface.	Essential for achieving the high collection efficiency (η_dev = 0.57) demonstrated by the parabolic geometry.
Integrated Functional Layers (Future Antireflection/ODMR)	In-House Metalization Services. We offer deposition of Au, Pt, Pd, Ti, W, and Cu.	Allows researchers to integrate microwave striplines (for ODMR) or anti-reflection coatings (as suggested in the conclusion) directly onto the diamond substrate, streamlining device assembly.

Engineering Support

The complexity of integrating high-purity diamond with advanced photonic structures demands expert consultation.

6CCVD’s in-house PhD team specializes in MPCVD growth parameters and material selection for quantum applications. We offer dedicated engineering support to assist researchers in selecting the optimal SCD grade, crystal orientation (e.g., (111) orientation, which is noted in the paper as a potential improvement for optimal mode overlap), and thickness required for similar NV-based Nanoscale Magnetic Field Imaging projects.

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

View Original Abstract

Enhancing the measurement signal from solid state quantum sensors such as the\nnitrogen-vacancy (NV) center in diamond is an important problem for sensing and\nimaging of condensed matter systems. Here we engineer diamond scanning probes\nwith a truncated parabolic profile that optimizes the photonic signal from\nsingle embedded NV centers, forming a high-sensitivity probe for nanoscale\nmagnetic field imaging. We develop a scalable fabrication procedure based on\ndry etching with a flowable oxide mask to reliably produce a controlled tip\ncurvature. The resulting parabolic tip shape yields a median saturation count\nrate of 2.1 $\pm$ 0.2 MHz, the highest reported for single NVs in scanning\nprobes to date. Furthermore, the structures operate across the full NV\nphotoluminescence spectrum, emitting into a numerical aperture of 0.46 and the\nend-facet of the truncated tip, located near the focus of the parabola, allows\nfor small NV-sample spacings and nanoscale imaging. We demonstrate the\nexcellent properties of these diamond scanning probes by imaging ferromagnetic\nstripes with a spatial resolution better than 50 nm. Our results mark a 5-fold\nimprovement in measurement signal over the state-of-the art in scanning-probe\nbased NV sensors.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.14.064007