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

Executive Summary

This research demonstrates a novel Physically Unclonable Function (PUF) utilizing arrays of randomly magnetized micromagnets read out via Nitrogen-Vacancy (NV) diamond magnetometry, offering a robust solution for hardware security and anti-counterfeiting.

Core Value Proposition: The PUF relies on the uncontrollable, random magnetic polarity (0 or 1) of 10⁴ nickel micromagnets fabricated on a silicon substrate, serving as a unique, unclonable identifier.
High-Speed Readout: The NV widefield magnetic imager allows simultaneous measurement of all 10⁴ micromagnets, achieving an effective bit readout rate of 5800 bits/s, with the potential for high Signal-to-Noise Ratio (SNR=10) acquisition in approximately 12 seconds.
High Density: The demonstrated areal density is 10⁴ bits/mm², comparable to high-density magnetic hard drives, enabling compact integration into electronic components (e.g., ASICs).
Covert Sensing Capability: Unlike MOKE microscopy, the NV magnetic imager measures fields in a plane above the sample, allowing the micromagnets to be isolated beneath an opaque, non-magnetic protective layer, increasing security.
Robustness: The magnetic polarities remain stable and unaffected when exposed to typical NV sensing bias fields (up to ±1.3 mT), confirming the PUF identifier is reproducible during measurement.
Feasibility and Optimization: The method is straightforward to manufacture (CMOS-compatible) and easy to characterize. Future optimization focuses on reducing the standoff distance (currently 3.6 µm) and improving micromagnet material (e.g., SmCo instead of Nickel) for better coercivity.

Technical Specifications

Parameter	Value	Unit	Context
PUF Material	Nickel (Ni)	50 nm	Micromagnet thickness
Substrate	Silicon	1 µm	With thermal oxide layer
Micromagnet Dimensions	1 x 4	µm²	Bar shape, constrains moment to ±y axis
Array Size / Area	100 x 100 (10⁴)	Micromagnets / 1 mm²	Demonstrated array density
Bit Areal Density (Demonstrated)	10⁴	bits/mm²	Corresponds to 2¹⁰⁰⁰⁰ unique identifiers
Bit Areal Density (Potential)	4 x 10⁴	bits/mm²	Achievable with 5 µm spacing
NV Layer Thickness (Sample A)	4	µm	Used for main text measurements
NV Layer [N] Concentration (Sample A)	20	ppm	¹⁴N grown in
Magnetic Noise Floor (Sample A)	7	µT after 1 s	In 1x1 µm² pixel area
Mean Standoff Distance (Sample A)	3.6	µm	Effective altitude of NV layer above magnets
Air Gap Only (Sample B, 0.15 µm NV)	1.9	µm	Minimum practical NV-micromagnet separation
Typical Measured Field Strength	~20	µT	B_z field from micromagnet array
Effective Bit Readout Rate	5800	bits/s	Calculated rate after SNR and summing factors
Optical Diffraction Limit (Setup)	1.4	µm	For 700 nm fluorescence, NA=0.25 objective
Bias Field Robustness	±1.3	mT	Maximum bias field tested without polarity change
Operating Temperature	300	K	Ambient (Room Temperature)

Key Methodologies

The PUF implementation involves three main stages: fabrication, NV magnetic readout, and image processing for bit string conversion.

Micromagnet Fabrication:
- Electron-beam lithography was performed on a silicon substrate (with 1 µm thermal oxide) using a 30 keV electron beam and 950A7 PMMA resist.
- A 50 nm layer of Nickel (Ni) was deposited, followed by a lift-off process to define the 1x4 µm² bar micromagnets.
- A 20 nm Al₂O₃ top layer was added to protect the nickel micromagnets from oxidation.
NV Magnetic Readout (Widefield Microscopy):
- A diamond chip containing the NV layer (4 µm thick, 20 ppm ¹⁴N) was placed directly on top of the micromagnet array (NV side down) to minimize the standoff distance.
- The NV layer was illuminated with a 532 nm pump laser beam, causing red fluorescence.
- A probe microwave field was applied to interrogate the NV ground-state magnetic sublevels.
- The resulting NV fluorescence intensity, which is dependent on the local magnetic field, was imaged onto a camera sensor through a 650 nm long-pass filter, producing a magnetic field projection map (B_[111]).
Magnetic Image Processing and Bit String Conversion:
- The measured B_[111] map was converted to a B_z map (magnetic field component along the z-axis) using 2D Fourier transform techniques to simplify analysis and improve spatial separation.
- Image analysis (using scikit-image and opencv2) was used to align the magnetic grid by first applying Canny edge detection and convex hull finding to determine the rotation offset.
- The B_z map was divided into 100x100 cells, one for each micromagnet.
- The magnetic polarity (bit state) was extracted by calculating the difference (ΔB) between the summed B_z field strength in the top half (ΣB_top) and the bottom half (ΣB_bottom) of each cell.
- If ΔB > 0, the bit state is ‘1’ (moment along +y); if ΔB < 0, the bit state is ‘0’ (moment along -y).

Commercial Applications

This NV-readout micromagnet PUF technology is highly relevant for hardware security, anti-counterfeiting, and physical cryptography applications, particularly where high-density, robust, and covert tagging is required.

Hardware Security and Anti-Counterfeiting:
- ASIC Tagging: Embedding the micromagnet array directly onto sensitive Application-Specific Integrated Circuits (ASICs) to provide a unique, unclonable fingerprint for counterfeit protection.
- Supply Chain Integrity: Using the PUF as a magnetic tag for verifying the authenticity of high-value electronic components or modules throughout the supply chain.
- Covert Identification: The ability to read the magnetic state through opaque protective layers allows for covert tagging that is difficult for counterfeiters to access or copy.
Physical Cryptography and Data Storage:
- Physical Cryptographic Key: Utilizing the random bit string as a physical key for cryptographic operations in situations where electronic storage of the key is undesirable or insecure.
- Long-Term Data Stability: Magnetic domains offer favorable long-term stability (potentially billions of years), making this PUF suitable for archival or long-life security applications.
Advanced Magnetic Sensing:
- NV Magnetometry Applications: The core technology (NV widefield magnetic imaging) is applicable to other fields, including imaging magnetic domains in hard drives, studying superconducting and ferromagnetic phase transitions, and current flow mapping in materials like graphene.

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