Currently available categories of magnetometers are either electronic or quantum and both have significant limitations. Existing magnetometers that use purely electronic detection (Hall, magneto-resistance, or flux-gate devices) have sensitivities limited by their electronic Quality Factor (“Q-factor”) that depends on the electrical resistance or friction of the electrons traveling through the metal in the circuit; it is difficult to reduce this factor (and increase sensitivity) for room-temperature operations.
In contrast, mechanical resonators made from semiconductor-grade silicon (such as those used in the mPhase magnetometer), exhibit mechanical Q-factors approaching 10,000 or more at room temperature, which is a tremendous improvement over the electronic detection types.
The highest sensitivity magnetometers commercially available today are the second category, quantum magnetometers. These require cooling to cryogenic temperatures. Called SQUIDs (for Superconducting Quantum Interference Devices), these devices only work at the temperature where liquid helium boils, -455°F, making such magnetometers expensive and bulky—ill-suited for remote-sensing applications.
The mPhase magnetometer overcomes the cooling problem and enables a micro-scale, low-cost magnetometer that does not require cooling. The mPhase magnetometer should be up to 100-1,000 times more sensitive than existing commercial devices.
The mPhase magnetometer will be smaller, more sensitive, less costly, and operate at room temperature, thus enabling a new class of sensor systems with appeal to specialized military as well as mass-market commercial applications.
This expected improvement in sensitivity may enable the passive detection (range, bearing and mass) of metallic objects. In military applications, for example, the proposed magnetometer may be able to detect the presence of a solider carrying a rifle at approximately 150 feet from a single sensor, and larger targets such as a car may be detected at 800 feet from a sensor.
Basic sensor designs have been under development at Bell Labs for several years, and the process technology to manufacture these sensors is well understood. In May we announced the first successful production of our prototypes in the Bell Labs clean room.
We expect the new magnetometer designs will achieve substantial improvements in sensitivity by modifying the micro-scale magnetometers to include nano-scale features.
We are currently testing two different designs, both based on Micro-Electro-Mechanical Systems (MEMS) that take advantage of the high Q-factor of the mechanical resonance in single crystals of silicon. Resonance is similar to the fundamental frequency of a tuning fork; when tapped, a tuning fork will vibrate for a length of time inversely proportional to the internal friction of vibration within the metal of the tuning fork.
A comparable tuning fork made from single crystal silicon (with less internal friction than that of the hardest metal) will vibrate almost a thousand times longer. Based on this principle, a device employing a high Q resonator will have enhanced amplitude of vibration at the resonance frequency and will display a greater sensitivity to external perturbations. This mechanical sensitivity can be converted to magnetic field sensitivity by coupling the mechanical motion of a bar or a paddle constructed from silicon to the ambient magnetic field, which is the principle of how the mPhase magnetometer works.
The mPhase and Bell Labs technical team is examining both a static design using an integrated magnetic film as well as a dynamic one through motion of the silicon bar or paddle. These designs are not necessarily mutually exclusive and could potentially be incorporated into a hybrid design depending on the end application. |
