Optimized Planar Microwave Antenna for Nitrogen Vacancy Center Based Sensing Applications

At a Glance

Metadata	Details
Publication Date	2021-08-19
Journal	Nanomaterials
Authors	Oliver Opaluch, Nimba Oshnik, Richard Nelz, Elke Neu
Institutions	University of Kaiserslautern
Citations	32
Analysis	Full AI Review Included

Technical Documentation & Analysis: Optimized Planar Microwave Antenna for NV Center Sensing

This document analyzes the requirements and findings of the research paper “Optimized Planar Microwave Antenna for Nitrogen Vacancy Center Based Sensing Applications” and aligns them with the advanced Single Crystal Diamond (SCD) and fabrication capabilities offered by 6CCVD.

Executive Summary

This research successfully optimized a planar, Q-shaped microstripline microwave (MW) antenna design for high-performance Nitrogen Vacancy (NV) center quantum sensing applications, demonstrating key advancements critical for wide-field magnetometry and dynamical decoupling protocols.

High Performance: Achieved high Rabi frequencies ($\Omega_R$) up to 4.9 MHz (simulated for 50 µm Electronic Grade SCD) and 2.4 MHz (experimental for 300 µm IIa SCD) using 1 W input power.
Macroscopic Homogeneity: Demonstrated highly uniform MW fields over a large area (approximately 400 µm x 400 µm), crucial for ensemble NV sensing and enhanced signal-to-noise ratio.
Wide Bandwidth: The optimized design exhibited a wide bandwidth (up to 8.2 GHz for 50 µm SCD), enabling reliable NV spin manipulation even with high magnetic bias fields (up to 190 mT).
Material Optimization: Confirmed that the quality of the conductive layer is critical; thermally evaporated gold layers significantly outperformed sputtered layers, yielding higher ODMR contrast (21% ± 2%) and Rabi frequency (2.4 MHz ± 0.5 MHz).
Advanced Protocols: The antenna proved suitable for high-power applications, successfully enabling advanced dynamical decoupling protocols (CPMG-8), resulting in a fourfold increase in coherence time ($T_2$) up to 638.1 µs.
Design Robustness: The planar design offers easy integration into confocal/AFM setups and allows for risk-free sample handling by mounting the diamond sample directly on the antenna.

Technical Specifications

The following table summarizes the critical material and performance parameters extracted from the study.

Parameter	Value	Unit	Context
Diamond Orientation	(100)	N/A	Used for both Electronic Grade and IIa samples.
Electronic Grade SCD Thickness	50	µm	Used in simulations for optimal performance.
IIa SCD Thickness (Experimental)	300	µm	Used for experimental characterization.
Substitutional N Content (Electronic Grade)	< 5	ppb	Required for single NV center experiments.
Substitutional N Content (IIa)	< 1	ppm	Used for dense NV ensemble testing.
MW Frequency Range	2.5 - 3.5	GHz	Typical range for NV spin control.
Antenna Resonance Frequency	2.87	GHz	Optimized zero-field splitting (ZFS).
Achieved Rabi Frequency ($\Omega_R$)	Up to 2.4 ± 0.5	MHz	Experimental result (thermally evaporated Au).
Simulated Rabi Frequency ($\Omega_R$)	4.9 (50 µm SCD) / 2.5 (300 µm IIa)	MHz	Calculated at 1 W input power.
MW Field Homogeneity Area	400 x 400	µm²	Area of highly uniform MW field.
Antenna Bandwidth (50 µm SCD)	8.2	GHz	Enables bias fields up to 190 mT.
Antenna Bandwidth (300 µm IIa)	6.3	GHz	Limits feasible bias fields to 120 mT.
Gold Conductor Thickness	100	nm	Deposited layer thickness.
Chromium Adhesion Layer Thickness	20	nm	Standard adhesion layer.
Enhanced Coherence Time ($T_2$)	638.1	µs	Achieved using CPMG-8 protocol.

Key Methodologies

The planar Q-shaped antenna fabrication relied on precise microfabrication techniques, with material deposition being a critical factor influencing final performance.

Substrate Preparation: Borosilicate glass substrates ($\epsilon_r = 4.82$, 1 mm thickness) were cleaned using acetone and isopropanol in an ultrasonic bath, followed by heating (120 °C, 10 min) to remove absorbed water.
Thin Film Deposition (Critical Step):
- A 20 nm Chromium (Cr) adhesion layer was deposited.
- A 100 nm Gold (Au) conductive layer was deposited.
- Key Finding: Thermal evaporation of the gold layer resulted in significantly superior antenna performance (higher $\Omega_R$ and ODMR contrast) compared to sputtering, attributed to lower impurities and crystal defects.
Lithography and Patterning: Spin coating was used to apply an adhesion promoter (TI Prime) and photoresist (AZ1518). Contact UV lithography was performed using a laser-written binary intensity amplitude chromium photomask.
Wet Chemical Etching: The antenna structure was formed by stirring the substrate in gold etchant, followed by chromium etchant.
Final Assembly: SMT ultra-miniature coaxial connectors (U.FL-R-SMT(01)) were attached using electrically conductive epoxy adhesives (EPO-TEK H20E).
Diamond Sample Preparation: The (100)-oriented IIa CVD diamond (3 mm x 3 mm x 300 µm) was cleaned in a tri-acid mixture (H₂SO₄, HClO₄, HNO₃) at 500 °C for 1 hour.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality SCD materials and custom fabrication services necessary to replicate, optimize, and scale this advanced NV sensing technology.

Applicable Materials

The success of this research hinges on the quality and specific dimensions of the diamond material. 6CCVD offers both required grades:

Material Requirement (Paper)	6CCVD Solution	Key Specification Match
Electronic Grade SCD (50 µm thickness, N < 5 ppb)	Optical Grade SCD (High Purity)	Ideal for shallow NV centers and single-spin applications requiring maximum coherence time ($T_2$).
IIa SCD (300 µm thickness, N < 1 ppm)	Standard SCD (Low Nitrogen)	Perfect for dense NV ensemble sensing and high-sensitivity magnetometry.
Orientation	Standard (100) or Custom (111)	We provide precise crystal orientation control, essential for aligning the NV axis with the magnetic field.
Thickness	Custom SCD Thickness (0.1 µm to 500 µm)	We can supply the exact 50 µm or 300 µm wafers used, or custom thicknesses up to 500 µm for specialized applications.

Customization Potential

6CCVD’s in-house fabrication capabilities directly address the critical requirements identified in the paper, particularly concerning metalization and dimensional control.

Optimized Metalization Services: The paper demonstrated that thermal evaporation yields superior performance (2.4 MHz $\Omega_R$) compared to sputtering. 6CCVD offers both PVD methods (Sputtering and Thermal Evaporation) and can guarantee the high-quality, low-defect Au/Cr thin films required.
- Available Metals: We offer custom deposition of Au, Cr, Ti, Pt, Pd, W, and Cu, allowing researchers to test alternative adhesion layers or conductor materials.
Precision Dimensional Control: The paper used 3 mm x 3 mm samples. 6CCVD provides:
- Custom Dimensions: Plates/wafers up to 125 mm (PCD) and custom-cut SCD pieces.
- Laser Cutting/Dicing: Precise laser cutting services to achieve the exact lateral dimensions (e.g., 3 mm x 3 mm) required for integration into specific microscope setups (e.g., piezo scanners).
Surface Quality: NV sensing, especially with shallow NV centers, demands ultra-low surface roughness.
- Polishing Guarantee: 6CCVD guarantees surface roughness (Ra) < 1 nm for SCD, ensuring minimal decoherence caused by surface defects and compatibility with high-resolution imaging.

Engineering Support

The successful implementation of this optimized antenna design requires careful selection of diamond material purity and thickness based on the target application (single NV vs. ensemble, shallow vs. deep NV).

6CCVD’s in-house PhD team specializes in MPCVD growth and quantum material science. We offer consultation services to assist researchers in selecting the optimal diamond specifications (e.g., N concentration, ¹²C enrichment, thickness) for similar NV-based Quantum Sensing and AC Magnetometry projects, ensuring maximum $T_2$ coherence time and $\Omega_R$ amplitude.

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

View Original Abstract

Individual nitrogen vacancy (NV) color centers in diamond are versatile, spin-based quantum sensors. Coherently controlling the spin of NV centers using microwaves in a typical frequency range between 2.5 and 3.5 GHz is necessary for sensing applications. In this work, we present a stripline-based, planar, Ω-shaped microwave antenna that enables one to reliably manipulate NV spins. We found an optimal antenna design using finite integral simulations. We fabricated our antennas on low-cost, transparent glass substrate. We created highly uniform microwave fields in areas of roughly 400 × 400 μm2 while realizing high Rabi frequencies of up to 10 MHz in an ensemble of NV centers.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano11082108

References

2017 - Superresolution optical magnetic imaging and spectroscopy using individual electronic spins in diamond [Crossref]
2017 - Nanomechanical sensing using spins in diamond [Crossref]
2016 - NMR technique for determining the depth of shallow nitrogen-vacancy centers in diamond [Crossref]
2015 - Fourier magnetic imaging with nanoscale resolution and compressed sensing speed-up using electronic spins in diamond [Crossref]
2014 - Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins [Crossref]
2014 - Electronic properties and metrology applications of the diamond NV- center under pressure [Crossref]
2014 - All-optical thermometry and thermal properties of the optically detected spin resonances of the NV-center in nanodiamond [Crossref]
2013 - Nanometre-scale thermometry in a living cell [Crossref]
2013 - The nitrogen-vacancy colour centre in diamond [Crossref]
2015 - State-selective intersystem crossing in nitrogen-vacancy centers [Crossref]