A physically unclonable function using NV diamond magnetometry and micromagnet arrays

At a Glance

Metadata	Details
Publication Date	2020-05-27
Journal	Journal of Applied Physics
Authors	Pauli Kehayias, Ezra Bussmann, Tzu-Ming Lu, Andrew M Mounce
Institutions	Center for Integrated Nanotechnologies, Sandia National Laboratories
Citations	13
Analysis	Full AI Review Included

Technical Documentation & Analysis: NV Diamond Magnetometry for Physically Unclonable Functions (PUFs)

This document analyzes the research demonstrating a robust Physically Unclonable Function (PUF) based on micromagnet arrays read out by Nitrogen-Vacancy (NV) centers in MPCVD diamond. The findings highlight critical material specifications—specifically NV layer thickness, nitrogen doping, and isotopic purity—where 6CCVD’s advanced capabilities provide direct solutions for optimizing device performance and scalability.

Executive Summary

Hardware Security Application: Demonstrated a robust, non-contact Physically Unclonable Function (PUF) utilizing the random magnetic polarities of micron-sized ferromagnetic arrays.
Sensing Technology: Employed widefield Nitrogen-Vacancy (NV) diamond magnetometry for rapid, parallel readout of 10⁴ micromagnets simultaneously, suitable for hardware security and trust validation.
Performance Metrics: Achieved a high bit areal density of 10⁴ bits/mm² and a fast, optimized readout rate of 5800 bits/s, positioning the technology for compact magnetic tags.
Spatial Resolution Limitation: Identified the NV layer thickness (4 µm) and the diamond-sample air gap (3.6 µm mean standoff distance) as the primary factors limiting spatial resolution and maximum micromagnet density.
Material Optimization Path: Confirmed that reducing the NV layer thickness (down to 0.15 µm) significantly reduces the standoff distance (to 1.9 µm), directly improving magnetic field strength and spatial resolution (scaling as 1/distance³).
Robustness: The NV readout method allows the micromagnet PUF to be isolated beneath opaque protective layers (e.g., Al₂O₃), making it robust against oxidation and difficult for counterfeiters to access or copy.

Technical Specifications

The following table summarizes the critical material and performance parameters extracted from the research, focusing on the diamond sensor properties and PUF metrics.

Parameter	Value	Unit	Context
NV Layer Thickness (Sample A)	4	µm	Used for main 100x100 array experiment
NV Layer Thickness (Sample B)	0.15	µm	Used to measure minimal air gap
Nitrogen Concentration (Sample A)	20	ppm	¹⁴N grown in
Nitrogen Concentration (Sample B)	45	ppm	¹⁵N implant
Carbon Isotope Purity (Sample A)	0.001	%	¹³C abundance (Isotopically enriched)
Mean Standoff Distance (Sample A)	3.6	µm	Effective altitude of NV layer above micromagnets
Mean Standoff Distance (Sample B)	1.9	µm	Primarily air gap measurement (minimal practical separation)
Magnetic Noise Floor (Sample A)	7	µT	Per 1x1 µm² area, 1s averaging
Micromagnet Dimensions	1x4	µm²	Bar-shaped Nickel (Ni)
Micromagnet Array Density	10⁴	bits/mm²	Achieved with 10 µm pitch
Optimized Bit Readout Rate	5800	bits/s	Calculated rate for SNR = 10
Diffraction-Limited Resolution	1.4	µm	Set by NA = 0.25 objective and 700 nm fluorescence

Key Methodologies

The experiment successfully integrated advanced MPCVD diamond material with standard semiconductor fabrication techniques to create a functional magnetic PUF.

Micromagnet Fabrication: Nickel (Ni) micromagnets (50 nm thick, 1x4 µm²) were patterned on a silicon substrate using 30 keV electron-beam lithography and lift-off.
Protective Layer Deposition: A 20 nm Al₂O₃ top layer was deposited to protect the nickel micromagnets from oxidation and enhance robustness.
NV Diamond Integration: A Single Crystal Diamond (SCD) chip containing the NV layer (4 µm or 0.15 µm thick, isotopically enriched ¹²C) was placed NV-side down onto the micromagnet array to minimize the critical standoff distance.
Optical Excitation: The NV layer was illuminated using a 532 nm pump laser, causing NV centers to fluoresce red light (650 nm long-pass filtered).
Magnetic Interrogation: A probe microwave field was applied to interrogate the magnetic-field-dependent transition frequencies between NV ground-state magnetic sublevels.
Field Mapping: The magnetic field projection along the NV [111] crystallographic direction (B₁₁₁) was measured simultaneously across the array.
Data Conversion: The B₁₁₁ map was computationally converted to the simpler B_z map (z-axis component) using 2D Fourier transform techniques to simplify image analysis and improve spatial separation.
Bit String Extraction: Image analysis (including Canny edge detection and dipole moment summation/difference) was used to reliably convert the B_z map into a binary string (0 or 1) representing the magnetic polarity of each micromagnet.

6CCVD Solutions & Capabilities

The research clearly defines the need for highly customized, high-quality MPCVD diamond to push the limits of NV magnetic sensing for hardware security applications. 6CCVD is uniquely positioned to supply the required materials and engineering support.

Research Requirement / Optimization Goal	6CCVD Solution & Capability	Technical Advantage
High Spatial Resolution / Low Standoff	Custom SCD Thickness (0.1 µm - 500 µm)	We provide precise control over the NV layer thickness, enabling the use of ultra-thin layers (e.g., 0.15 µm) to minimize the standoff distance and maximize the measured magnetic field strength (which scales as 1/distance³).
Enhanced Magnetic Sensitivity (SNR)	Isotopically Enriched Optical Grade SCD	Our capability to grow high-purity ¹²C diamond (low ¹³C abundance) minimizes spin bath decoherence, resulting in a lower magnetic noise floor (e.g., < 7 µT) and faster readout rates for PUF characterization.
Optimized NV Density	Custom Nitrogen Doping (¹⁴N or ¹⁵N)	We offer precise control over nitrogen concentration (ppm level) during MPCVD growth, allowing researchers to tune the NV center density for optimal fluorescence signal strength without sacrificing coherence.
Large-Area PUF Fabrication	PCD Wafers up to 125 mm	For scalable, high-volume manufacturing of PUF arrays, 6CCVD supplies large-area Polycrystalline Diamond (PCD) substrates up to 125 mm in diameter, compatible with CMOS processes.
Integrated Device Stacks	Custom Metalization Services (Ti, Ni, Au, Pt, W)	We offer in-house metalization capabilities, including the deposition of ferromagnetic materials (like Ni) and protective dielectric layers (like Al₂O₃), allowing for the integration of the magnetic elements directly onto the diamond surface, eliminating the air gap contribution.
Surface Quality for Minimal Air Gap	Precision Polishing (Ra < 1 nm SCD)	Our advanced polishing services achieve ultra-low surface roughness (Ra < 1 nm for SCD), critical for minimizing the physical air gap between the NV layer and the micromagnet array, thereby maximizing magnetic coupling.

Applicable Materials

To replicate and extend this research, 6CCVD recommends:

Optical Grade Single Crystal Diamond (SCD): Required for high-coherence NV centers and minimal magnetic noise.
Isotopically Enriched ¹²C Diamond: Essential for achieving the best possible SNR and coherence times necessary for high-speed magnetic sensing.
Custom Thin Film SCD: Specifically engineered SCD wafers with NV layers ranging from 0.1 µm to 5 µm, tailored to balance magnetic sensitivity and spatial resolution requirements for high-density PUF arrays.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing MPCVD growth parameters for quantum sensing applications. We provide expert consultation on material selection, doping profiles, and surface preparation necessary to achieve the sub-micron standoff distances and low noise floors required for next-generation magnetic PUF and hardware security projects.

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

View Original Abstract

A physically unclonable function (PUF) is an embedded hardware security measure that provides protection against counterfeiting. Here, we present our work on using an array of randomly magnetized micrometer-sized ferromagnetic bars (micromagnets) as a PUF. We employ a 4μm thick surface layer of nitrogen-vacancy (NV) centers in diamond to image the magnetic field from each micromagnet in the array, after which we extract the magnetic polarity of each micromagnet using image analysis techniques. After evaluating the randomness of the micromagnet array PUF and the sensitivity of the NV readout, we conclude by discussing the possible future enhancements for improved security and magnetic readout.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0005335

References

2013 - Physically Unclonable Functions: Constructions, Properties and Applications