MEMS magnetic field sensor with integrated signal conversion

Goal of the project is to investigate and design an integrated sensor that detects magnetic fields based on the force they exert on electric conductors. This harmonic Lorentz force is being used to excite resonant oscillations of a micromechanical cantilever carrying a current-conducting lead. The oscillations shall be sampled contactlessly with micro-electrodes and converted into an electrical signal for further processing by means on an application- specific integrated circuit. With appropriately formed mechanical resonators and properly placed micro-electrodes it will be possible to capture orthogonal components of the magnetic field independently of one another. Preliminary experiments have shown that this magneto-mechanical transducer principle can yield very high sensitivities. The expected peak sensitivity will be sufficient to measure the earth magnetic field with good accuracy. The integrated circuit, which will be designed, will generate both the alternating current needed to stimulate the oscillations and the signals required for the capacitive readout. Furthermore, it will process the capacitive signals and extract the amplitude and phase information. By proper control of the alternating current, the measurement circuit will be able to keep the amplitude of the oscillations constant over a wide range of flux density. This allows one to adapt the sensitivity of the transducer according to the magnetic field, which in turn permits a very large measurement range of about seven orders of magnitude. The integrated circuit will be assembled together with the micromechanical part to form a hybrid component. The proposed sensor concept promises compact and easy-to-use magnetic field sensors exhibiting high sensitivity. They are able to indicate both, magnitude and direction of the flux density vector. Within the framework of the project, the difficult task of merging such different technologies as silicon micromachining, resonant magneto-mechanical transduction, capacitive sensing, and signal conditioning with application specific integrated circuits will be solved using scientific research.

A novel construction of micromachined magnetometer detecting both static and alternating magnetic fields was investigated. These devices consist of a U-shaped single-crystal silicon cantilever which bears a thin conductor carrying an alternating current. The cantilever and thin film metal electrodes that are somewhat separated form a vibrating plate capacitance.Movements of the cantilever may be read out conveniently by electronic circuits. A static magnetic field evokes a force acting on the conductor that alternates according to the frequency of the current. It is named after H.A. Lorentz, one of pioneers of electromagnetism and oriented perpendicular to the directions of both the field and the current. Knowing the amount of current, the deflections of the cantilever are a measure of the component of the magnetic flux density that points perpendicular to the current. By changing the drive current, the operating range of the magnetometer can be adapted to cover more than six orders of magnitude. The highest vibration amplitudes are expected, of course, in the vicinity of resonance frequencies of the micromachined structure. Which resonances may be excited depends on the configuration of the field, the current, and most important, on the shape of the resonating structure. In the experiments, emphasis is laid on the investigation of the first symmetric, the first antisymmetric and the second symmetric vibration mode. The resonant enhancement of vibration amplitudes is expressed by the quality factor, which can easily exceed a value of 10000 in vacuum. At standard pressure, however, values of the order of several hundred are typical indicating the importance of gas friction. Developing valid physical models of gas friction is a rather complex task that was accomplished on the basis of a comprehensive experimental study for pressures from 10-7 to 10 times the normal pressure. Significant improvements of the state of the art were achieved in the pressure regime where the mean collision-free path length of the gas particles measures 0.02 to 10 times the characteristic distance of the mechanical arrangement. Magnetic field induced deflections of the cantilever were studied with a laser-Doppler vibrometer in addition to the capacitive readout and resolutions of up to 0.3 of the earth magnetic field were achieved. A hitherto unknown interference effect induced by the readout capacitances in conjunction with the electrical resistance of the current lead on the cantilever was discovered. This complex interaction was studied in great detail since this effect may seriously limit the achievable resolution in specific cases.