BACKGROUND

1. Field of the Invention

This invention relates generally to sensor arrays for measuring magnetic fields in a PUF fingerprint, and more particularly, to a PUF reader device that incorporates the sensor arrays.

2. Description of the Related Art

U.S. Pat. No. 9,553,582, incorporated herein by reference, discloses a PUF (Physical Unclonable Function) that contains magnetic particles, which generate a complex magnetic field near the surface of the PUF part. This magnetic field may be measured along a path and data corresponding to the magnetic field components recorded for later comparison and authentication of the PUF part. U.S. Pat. No. 9,608,828, incorporated herein by reference, discloses the advantages of magnetizing the feed stock prior to the injection molding process to achieve a random orientation of the magnetization directions. In these patents, flakes of an NdFeB alloy are cited as the preferred magnetic particles, however other magnetic materials, alloys, and particle shapes may be employed. These flakes are typically about 35 microns thick with irregular shapes varying in width from 100-500 microns but may vary substantially from these ranges. The NdFeB alloy is not easily magnetized because it has an intrinsic coercivity of around 9,000 Oersted. However, once magnetized, the alloy has a residual induction of about 9,000 gauss, and the random locations and magnetic orientations of the particles and flakes produce sharp peaks in the magnetic field strength of about ±10-35 gauss when measured at a distance of about 0.5 mm from the surface of the PUF.

The magnetic PUF technology can be applied to create PUF tags for authenticating passports, secure ID cards, and other non-rotating objects. For these applications, the complex magnetic field structure near the surface of a magnetic PUF, measured over a non-rotating 2-dimensional region can serve as a magnetic “fingerprint.” A low cost means of authenticating the magnetic PUF fingerprint is needed for non-rotating systems that read stationary or translating PUFs. In a rotating PUF sensor system a single sensor can measure the magnetic profile values at multiple angles around a circular path through the PUF fingerprint. This is possible using a single 3-axis Hall effect sensor because the rotation of the PUF element enables the fingerprint to be sampled at a high spatial frequency using a single magnetometer chip. For non-rotating PUF systems with no moving parts, the sampling of the magnetic fingerprint at multiple locations requires multiple magnetic field sensors, or movement of the PUF with respect to the sensors (similar to a credit-card swipe). A resolution of at least 0.1 gauss is preferred given the expected signal amplitudes generated by PUF samples.

SUMMARY OF THE INVENTION

The problem of counterfeit goods is rapidly escalating and affecting many markets and geographies that include not only retail products purchased by unwary consumers, but also products that are important to public safety, and national security. One recent study projected the annual economic impact of global counterfeiting to be over $1T across a broad class of industry segments at more than 2% of the total global economic output. Beyond an economic impact, counterfeiting is posing a risk to public safety with fake products showing up in pharmaceuticals (e.g., prescription drugs), medical equipment (e.g., surgical drills), food products (e.g., baby formula), cosmetics (e.g., make up), liquor (e.g., bourbon) and automotive equipment (eg air bag control unit). Spare parts used in public transportation systems (e.g., planes and trains) are also subject to counterfeiting where failure can have life threatening consequences.

Counterfeiting is also now a major threat to many systems that protect national security. The rise in the recycling of electronic components has increased the likelihood that components that are defective, substandard or unauthorized copies will make their way into the supply chain and impact critical military systems used for national defense (e.g., aircraft, weapons). Important government identification documents used to protect the borders from illegal access (e.g., passports) are also at risk of counterfeiting and there is evidence that counterfeit goods are also a means of supplying cash to terrorist organizations who threaten individuals and nations.

All these counterfeiting threats call for improved methods to identify and eliminate counterfeit products from making their way into the supply chain and impacting the life of individuals or nations in these negative ways. Unique physical objects that contain high entropy from random stochastic manufacturing processes, making them nearly impossible to copy, offer an improved alternative to authentication methods based on cryptographic algorithms often using integrated circuits that can easily be copied. Such a unique physical object (also referred to as Physical Unclonable Function or PUF) can be based on any random occurring property in physical realm such as electrical, optical or magnetic. When a unique physical object is combined with a high-resolution sensing system capable of extracting all the entropy from the object and a secure storage repository to hold the entropy data sensed from the object (for later us in authenticating the object) it can form the basis of a new authentication system offering improved security.

Such a new authentication system is disclosed in U.S. Pat. No. 9,553,582, one that is based on a unique physical object, where the unique physical object is a PUF (Physical Unclonable Function) that contains magnetic particles that are random in size, shape and orientation, which when magnetized generate a complex and random (in amplitude and direction) magnetic field near the surface of the PUF part. This magnetic field may be measured along a path and the data corresponding to the magnetic field components recorded for later comparison and authentication of the PUF part. U.S. Pat. No. 9,608,828, discloses the advantages of magnetizing the feed stock prior to the injection molding process to achieve a random orientation of the magnetization directions.

In these patents, flakes of an NdFeB alloy are cited as the preferred magnetic particles, however other magnetic materials, alloys, and particle shapes may be employed. These magnetic particles have a diameter greater than 25 μm and have an average diameter from 50-500 μm and an average thickness less than the diameter. The magnetic field from the surface of the PUF is generated by the magnetized NdFeB alloy that once magnetized has a residual induction of about 9,000 gauss, and the random locations and magnetic orientations of the particles and flakes produce sharp peaks in the magnetic field strength of +/−10-35 gauss when measured with a magnetic sensor at a distance of about 0.5 mm from the surface of the PUF. When measured with a magnetic sensor from a distance less than 0.5 mm from the PUF part, the magnetic field strength increases to greater than +/−35 gauss. The non-homogeneous magnetic field on the surface of the PUF part is random in amplitude and direction and may change substantially across the surface of the PUF part at distances less than 500 μm and can make complete peak to peak changes at distances less than 1 mm.

A unique physical object PUF part that is constructed with these characteristics produces a magnetic field across its surface which can be sensed with a reader device such as that disclosed above. This system discloses a magnetometer chip sensor array that provides a two-dimensional sensing system capable of measuring the complex magnetic field structure near the surface of a magnetic PUF over a non-rotating 2-dimensional region.

As shown inFIG. 1, the Z-component (normal to a PUF surface) of a representative magnetic PUF fingerprint does not change significantly over 0.1-0.2 mm of travel along the surface of the PUF. This is because the Hall-effect sensor or sensing element in this example is about 0.5-1.0 mm above the magnetized flakes generating the magnetic field. An economical array of Hall-effect sensors for sampling and validating the magnetic fingerprint would have the sensors nominally spaced approximately at least 0.4 mm apart for the flake sizes discussed in the related art. The average flake length for this approximation is 0.3 mm. The ratio of the minimum separation to particle length is approximately 1.33.

A low cost PUF fingerprint reader can be constructed using multiple discrete 3-axis magnetometer chips. A 1.46 mm×1.46 mm wafer level chip size package, for example, could be placed on a circuit card with a 2 mm center-to-center spacing. This will enable the placement of a 5×5 array of 3-axis Hall-effect sensors within a 10 mm×10 mm window in this example, which would result in the sensing locations spanning an 8 mm×8 mm window for such an arrangement.

The random 3-axis magnetic field sensed from a two-dimensional region from a non-rotating PUF part can serve as the magnetic “fingerprint” for this authentication system.

In some applications it is desirable to manufacture the PUF tags to be as small as possible in order to reduce the cost of the tag, but to do so without decreasing the security provided by the random magnetic field characteristics of the unique physical object. In this case, magnetic particle sizes at the lower end of the range (25 μm minimum diameter and 50-500 μm average diameter) may be used which may produce a magnetic field amplitude at the lower end of the +/−10-35 gauss range when measured with a magnetic sensor 0.5 mm from the surface of the PUF part. In such a system, to avoid any loss of magnetic field data from the reduced dimensions, it is necessary to increase the two-dimensional spatial resolution of the sensor array to the range of the particle diameters (25-500 μm inclusive). Furthermore, since the magnetic field strength decreases inversely with the cube of the distance, it is important to reduce the distance of the sensor array to the surface of the PUF part to less than 0.5 mm in order to amplify the magnetic field strength generated by the small particles at the lower end of the +/−10-35 gauss range.

In yet other applications, it is desirable to manufacture the PUF tag to be a large as possible to maximize security of the system by providing more random data from a larger magnetic “fingerprint.” In this case, magnetic particles could range from 25 μm minimum diameter to 50-500 μm average diameter and generate a magnetic field from +/−10-35 gauss range when measured with a magnetic sensor 0.5 mm from the surface of the PUF part. To measure a PUF part with a non-homogeneous magnetic field such as this with the highest accuracy (to reduce false positive and false negatives necessary for the highest security,) it is necessary to increase the two-dimensional spatial resolution of the sensor array to the range of particle diameters (25-500 μm inclusive). Further, in order to measure large PUF tags (e.g., greater than 1 mm×1 mm), with the highest accuracy, it is necessary to use a sensor array that has a large field of view that can measure the entire surface of the PUF part at one time (without moving the sensor in relation to the PUF part), that has a means to calibrate out any sensor to sensor variation across the sensor array, and that has a means to reduce measurement distortion caused by movement of the sensor in relation to the PUF part while the measurement is being made.

This invention discloses methods that improve the measurement accuracy, resolution and throughput of magnetic sensor arrays when used to measure the magnetic field across a surface of two-dimension PUF part. The multi-axis magnetic field data from these improved measurements can be combined with a verification algorithm such as a Pearson Correlation Function (or similar pattern recognition algorithm) to enable higher reliability identification of genuine and non-genuine PUFs, thus increasing the security of the system in terms of false positive and false negatives for arbitrary small and arbitrary large two-dimensional PUF's.

