Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-06-18
2001-04-10
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
Reexamination Certificate
active
06215318
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a micromechanical magnetic field sensor.
BACKGROUND INFORMATION
From the related art, various micromechanical magnetic field sensors are known, which convert an interaction between an electric current and a magnetic field into a force, based on the Lorentz force acting on electric charges. This force acting on the magnetic field sensor structure causes a deflection, which can be detected by various methods.
To illustrate the related art, first three rocker-shaped sensor structures will be described.
The first known magnetic field sensor structure is a resonant SiO
2
-torsion rocker, as is known from B. Eyre and K. S. J. Pister, Micromechanical Resonant Magnetic Sensor in Standard CMOS, Transducers 97, 1997 Int. Conf. Solid-State Sensors and Actuators, Chicago, Jun. 16-19, 1997.
In this design approach, an aluminum loop is generated on a freely suspended SiO
2
-rocker structure. As a result an electromagnetic excitation (alternating current interacting with the magnetic field) having the mechanical resonant frequency of the vibrator, a mechanical torsional vibration ensues. The amplitude is detected using a piezoresistive Wheatstone bridge circuit. Due to the high attenuation of the structure (SiO
2
—Al), only low qualities (i.e., high attenuation values) are achieved even under vacuum, namely qualities in the range of Q=10. Due to the thin substrates (particularly SiO
2
), the rocker structure bends considerably under the action of a force.
The second known magnetic field sensor structure is a resonant monocrystalline torsion rocker as is known from Z. Kadar, A. Bossche and J. Mollinger, Integrated Resonant Magnetic Field Sensor, Sensors and Actuators A, 41-42, (1994), pp. 66-69.
This magnetic field sensor is composed of a monocrystalline rocker structure suspended on torsion bars, printed circuit traces of aluminum being deposited on the structure. As before, an alternating current having the resonant frequency of the mechanical torsional vibrator is sent through the printed circuit trace. A vibrational amplitude (angle of torsion) arises which is read out capacitively via separate electrodes. The counter-electrode is formed by a patterned, conductive layer, which is deposited on a glass cap having depressions (cavities). The resonant method and the low “structural attenuation” of the vibrator supposedly make it possible to measure magnet fields in the nT range due to a possible vacuum enclosure and the resulting high qualities. In addition, a large dynamic range is indicated by the authors. Feedback printed circuit traces and a synchronous detector (carrier-frequency method) are used for the capacitive readout. The sensor design approach implemented in the laboratory has rest capacities in the range of 0.5 pF. The corresponding manufacturing process is hardly suited for inexpensive series production due to the complicated (and expensive) process steps. This publication does not reveal how the described crossover of printed circuit traces is implemented.
The third known magnetic field sensor structure is a non-resonant design approach, in which a direct current flows through one half of a silicon rocker structure (no separately applied printed circuit traces), the direct current interacting with the magnetic field and producing a Lorentz force. This force is converted into a torsional moment, which then twists the rocker structure. The changes in capacitance of the rocker areas toward the lower-lying counter-electrodes resulting from this torsion are read out using a capacitive method of measurement.
Furthermore, there are magnetic field sensors, which are based on electromagnetic material effects.
For example, the Hall effect is utilized. The Fall effect occurs in current-carrying conductors in the presence of an external magnetic field. The electrons are deflected perpendicularly to their moving direction depending on the current-carrying material. This deflection produces a potential difference between the two sides of the conductor, the potential difference representing the measured Hall effect. The following disadvantages can be enumerated regarding Hall-effect sensors: highly limited resolution (usual resolution limits lie in the mT range), a highly limited dynamic response, a great offset and a high temperature dependence of the measured effect.
Other sensors, which are based on electromagnetic material effects, include magnetoresistive sensors, in which the electrical resistance increases in the presence of a magnetic field (see, for example, M. J. Caruso, Applications of Magnetoresistive Sensors in Navigation Systems, 1997 Society of Automotive Engineers, Inc., Publ. # 970602, pp 15-21).
Fluxgate sensors, in which the sensor and the electronic unit are usually provided on one chip, some of them having a high resolution of typically 9 mV/&mgr;T, were disclosed, for example, by R. Gottfried—Gottfried, W. Budde, R. Jähne, H. Kück, B. Sauer, S. Ulbricht and U. Wende, A Miniaturized Magnetic Field Sensor System Consisting Of A Planar Fluxgate Sensor And A CMOS Readout Circuitry, Sensors and Actuators A54 (1996), pp. 443-447.
With regard to the above known approaches it turned out to be disadvantageous that the magnetic field sensors have either a low resolution, i.e., sensitivity, or a high-temperature-dependent offset, or that their production is very costly.
SUMMARY OF THE INVENTION
The idea, on which the present invention is based, is that a motion of the sensor structure caused by the Lorentz force is detected by electrodes, preferably comb-like ones. In this process, the Lorentz force is utilized in that a current impressed upon a (freely suspended) electric conductor and an externally applied magnetic field cause the freely suspended structure to move laterally.
The use of fixed electrode combs (fixed comb structure) and movable electrode combs (movable comb structure), which are preferably manufactured by known silicon surface micromachining, makes it possible to produce a motion-dependent change in capacitance.
The combs can be arranged parallel or vertical with respect to the direction of motion (i.e., parallel or vertical with respect to the conductor direction, respectively). The combs have a uniform electrical potential, which considerably simplifies the electro-capacitive readout of the motion. A design approach of this kind is not known since in all known design approaches, the electrical potential of the movable electrode is locus-dependent.
The potential can be tapped by an additional, preferably soft spring, and be used, for example, for an electrical control.
In the capacitive detection of the sensor motion by a capacitive measuring method, the change in distance between fixed and movable electrode combs is converted into an electrical signal, i.e., the area overlap between fixed and movable electrode combs produces a change in capacitance. By the capacitive readout of the amplitude signal, a low temperature sensitivity and a low temperature hysteresis is expected.
The sensor can be used for static and dynamic, particularly resonant operating modes, i.e., a deflection of the sensor structure by a direct current or an alternating current in the magnetic field. In one case, a static deflection results; in an other case, a vibratory movement. In the dynamic operation in combination with a capacitance differential measuring method, transverse accelerations are negligible, depending on the mechanical resonance frequency in each case.
Furthermore, there is the possibility of a vacuum enclosure for the dynamic operation to increase the vibration quality, and consequently, to improve the sensitivity compared to static systems. Compared to static methods of measurement, the operation under a vacuum atmosphere allows a considerably higher sensitivity (high vibration amplitudes combined with high quality by resonance amplification). To detect alternating magnetic fields, a direct current can be sent through the structure instead of an alternating current.
The micromechanical magnetic field sensor a
Emmerich Harald
Kaienburg Joerg
Schellin Ralf
Schoefthaler Martin
Kenyon & Kenyon
Le Roux E. P.
Metjahic Safet
Robert & Bosch GmbH
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