Résumé:
The compatibility of some Hall sensors with CMOS electronics allows the co-integration of sensors and electronics on the same chip to obtain a low cost microsystem. In addition to a careful sensor design, the relationships between the system components, i.e. sensors and electronics, allow us to improve substantially the behavior of the microsystem. The first objective of this thesis is to create a toolbox to construct very accurate CMOS Hall microsystems; each so-called tool solves a limitation of Hall sensors. We also built for angular measurements a 2-dimensional Hall microsystem, i.e. a microsystem with two orthogonal axes of sensitivity. This microsystem is based on vertical Hall sensors placed orthogonally.
CMOS Hall sensors suffer from several non-idealities, such as low sensitivity, sensitivity drift, offset and non-linearities. What’s more, a 2-dimensional sensor suffers from sensitivity mismatch between the X- and Y-sensors and non-orthogonality between the measurement axes. Sensitivity drift and offset are the most challenging problems for which new tools are necessary.
The first tool is calibration coils for the self-calibration of Hall microsystems. These coils are in-situ using the metallization layers of the technology. We obtain a coil efficiency of 290mT/A and 230mT/A for respectively miniaturized Hall plates and miniaturized CMOS vertical Hall sensors. Note that miniaturized sensors are obligatory for efficient coils. Using external laboratory voltage and current references, we obtain an outstanding thermal drift of less than 30ppm/°C. We also propose a calibration scheme, called geometrical reference, which is independent of the electric references; the sensitivity is in that case a function of only the sensor and the coil geometry.
The second tool is an efficient reduction of the Hall sensor offset using the spinning current technique. We analyze the sources of residual offset of our microsystem to find the major sources of residual offset: the electronic circuit and the sensor itself. We develop a feedback scheme for the spinning current method to reduce drastically the residual offset by one order of magnitude to a 1.2mV standard deviation from the output. Miniaturized sensors are degenerated, because they suffer from non-linearities; that’s why a four phases spinning current is required. We also multiplex the electronics for the X and Y axis of the angular sensor in order to guarantee the matching of the amplification chain and to reduce the electronics surface.
Our main industrial challenge, based on accurate Hall microsystems, is an absolute 360° angular position sensor without any dead angles reaching a 0.3% (1.1°) and 0.1% (0.36°) accuracy respectively without and with calibrations. The measurement principle, we choose at the beginning of this work, consists in measuring the magnetic field from a permanent magnet placed in front of the sensor on the rotating shaft. The magnet generates a rotating magnetic field at the surface of the CMOS chip. Measuring this field along two orthogonal axes allow us to calculate the angle over 360° with and without calibration using the arctangent function. This principle is very robust in respect to mechanical tolerances, to the variations of the sensor sensitivity and to the strength of the permanent magnet.
We develop for the first time a CMOS angular sensor based on vertical Hall sensors. We reach an outstanding accuracy of 0.5° (1.4‰) with only an easy offset compensation. In this thesis work, the sources of non-linearities are also studied and explained, allowing their calibration. With additional sensitivity mismatch and non-orthogonallity compensations, an accuracy of 0.17° (0.5‰) is obtained. These results are compared with those of a sensor using an integrated ferromagnetic disk and Hall plates. With an offset compensation, the accuracy is degraded to 2° (5.6‰) due to the tolerances during the post processing. However an accuracy of 0.4° (1.1‰) can be obtained with an additional calibration of the gain mismatch. This sensor is better adapted to low field measurements. The full range is limited by the saturation of the ferromagnetic elements and the field is amplified by the concentrators. The sensor based on vertical Hall sensors, is better adapted to high precision, especially without complex calibrations, with a larger magnetic field. Measurements demonstrate that coils, our first tool, is useless for angular measurement because the ratio of the X and Y signals is calculated, getting ride of the drifts if they are matched. The accurate matching of these drifts is typically less than 10ppm/°C even when low cost plastic packaging is used. The low offset microsystem, our second tool, is required to reach high accuracy and low thermal drift without calibration.
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