Magnetometer

Summary

A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, one that measures the direction of an ambient magnetic field, in this case, the Earth's magnetic field. Other magnetometers measure the magnetic dipole moment of a magnetic material such as a ferromagnet, for example by recording the effect of this magnetic dipole on the induced current in a coil. The first magnetometer capable of measuring the absolute magnetic intensity at a point in space was invented by Carl Friedrich Gauss in 1833 and notable developments in the 19th century included the Hall effect, which is still widely used. Magnetometers are widely used for measuring the Earth's magnetic field, in geophysical surveys, to detect magnetic anomalies of various types, and to determine the dipole moment of magnetic materials. In an aircraf

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Vertical Hall sensors are capable of measuring surface-parallel components of the magnetic field. They allow therefore relatively easy conception of single-chip multi-axial magnetic sensors compared to solutions using horizontal Hall plates. The modern trend in the field of Hall sensors is to integrate them into electronic circuitry for signal processing. Before this thesis, highly sensitive vertical Hall sensors completely compatible to a technology adequate for co-integration of electronic circuitry were not available. This is the reason why we present in this thesis the developed knowledge that is necessary for the design and manufacture of highly sensitive vertical Hall sensors in CMOS technology. We first present the principles of the technology choice. A wide selection of CMOS processes is presently available, but in order to obtain optimal sensitivity we have to choose "the right" one. The performances can vary easily by 50% from one technology to another. The best choice are CMOS high-voltage technologies since they deliver n-diffusion layers with relatively low doping level and deep junction depth. We present a novel layout of vertical Hall sensors that has six contacts in the active sensor zone and is a development of the known layout that uses only four contacts. The additional contacts improve the sensitivity and reduce the systematic offset considerably. If we re-use layouts that were optimal for not-CMOS compatible technologies, we will see that the results are not satisfying. We have to develop our specific design parameters that are optimal for the chosen technology. The key for high sensitivities is the strong miniaturization of the devices down to technological limits given by the design rules, but sometimes even beyond that. Some of the design rules can be broken with benefit for the obtained sensor devices. In general, minimum contact sizes and contact distances are preferable as well as a minimum sensor thickness. The difficulty is however to find the absolute minimum before device degradation or even failure will occur. Our optimized sensors achieved voltage related sensitivities of about 0.04 V/(VT) and current related-sensitivities up to 400 V/(AT) that are comparable to existing CMOS compatible horizontal Hall plates. Such sensitivities were never reported for a standard CMOS process, without any additional pre- or post-processing steps. The miniaturization of the Hall devices has however also negative aspects e.g. an increase in sensor offset, noise and output non-linearity. Although Hall sensors have many advantages in comparison to other magnetic sensors (simple structure, low fabrication costs, very good linearity, robustness) they have the drawback of a big Abstract offset voltage that is often too big for many applications. This is why an enormous effort was made by researchers in the past to develop offset reduction methods for horizontal Hall plates. We state in this thesis that, in principle, the same techniques are applicable on vertical Hall sensors but for an optimal efficiency certain principles/parameters have to be respected. We reveal the relations of important parameters as the type of layout (number of sensor contacts), the use of single or coupled sensors, the type of spinning-current, bias level etc. In the optimal case we can obtain residual offset values lower than 200 μT for bias voltages up to 2 V. The spinning current method was originally developed for offset compensation, but it has other beneficial influences on the sensor output signal as well. It is capable to remove a large amount of flicker noise if the spinning is executes at adequately high clock frequencies. We show that the efficiency of flicker noise removal is dependent on the bias current through the sensor. We discovered another positive side effect of the spinning-current method: the elimination of the planar Hall effect. While the planar Hall sensitivity for our sensors is initially about 1% of the normal one at B = 2 T, we obtain with the compensation technique values of only 0.02%. After the presentation of general characteristics of the separate sensors, we present two microsystems based on the developed vertical Hall sensors. At first, we present a 2D magnetic microsystem which is well adapted for the construction of contactless angular encoders with a measurement range of 360°. We profit here especially from the very low offset (< 400 μT) obtained with the spinning current method, since offset is in general the main source of angular errors in such systems. Existing angular measurement systems need in general a special (offset) calibration of the sensor unit in order to achieve an accuracy of 2.5‰ full scale. We obtain such a precision directly with our system for a wide temperature range from -30 to 100 °C. With one specific sensor calibration for the compensation of residual offset, sensitivity mismatch and phase mismatch of the two axes, we attain a precision of about 0.5‰ FS over the same temperature range, a performance never before reported with a magnetic sensor system. The microsystem is therefore an excellent candidate for the majority of low-cost angular measurement applications. As a second example we presented the first fully CMOS integrated three-dimensional Hall probe, consisting of a combination of one Hall plate and four vertical Hall devices. The microsystem allows an application as a universal magnetic field probe in the wide field range from 0.1 mT to 20 T. The probe can be easily used for small volumes because of its small size and the fact that the signal processing circuitry already is directly integrated onto the chip. The precision as a 3D magnetometer is limited by the sensitivity mismatch of about 5% FS. The probe is an ideal candidate for low-cost teslameter applications with limited precision.

EPFL2005

Adiabatic Demagnetisation Refrigeration Extension for SQUID Magnetometer

Kjersti Skaaraaen Herberg

A method for decreasing the minimum temperature achievable by a SQUID magnetometer is presented, based on adiabatic demagnetisation refrigeration. The method is found to successfully provide cooling to below 0.26 K for more than an hour. However, performing magnetisation measurements strongly influences the temperature and shortens the duration to a few minutes in the original setup. The cause is most probably re-evaporation of condensed helium, and possible improvements to prevent this effect are suggested.

2014

Integrated fluxgate compass for portable applications

Predrag Drljaca

Modern applications in industry, navigation and medicine demand small and sensitive magnetic field sensors. In this thesis we have developed a new planar micro fluxgate magnetometer. High resolution, low power consumption and orthogonal fields detection makes it suitable for portable compass applications. The developed sensor is a fully integrated device based on CMOS process that provides driving and readout electronics, plus the metallization layers for the excitation and pickup coils. A soft amorphous ferromagnetic material is bonded and structured, using an additional, postprocess implemented on a whole 6 inch wafer to form sensor core on the surface of the chip. The single core principle was selected with the intention to elongate the core, reduce the power consumption and increase sensitivity. We employed a new extraction method, based on the detection of mean value of the induced voltage peaks in the pickup coil, which carry the information on the external field. Several models and tools for the optimization of the core geometry, excitation, as well as pickup coils have been developed. For the first time, the effect of perming has been investigated. Perming is manifested as a change in sensor offset after the magnetic shock. A model was developed, which uses equivalent magnetic pole pieces to account for the contribution of the "frozen" domains not affected by the excitation. Model shows that the effect is predominantly dependent on the level of the core saturation. The electronics has been integrated in a conventional 1 µm CMOS process. Apart from the excitation, a single electronic circuit is used for both axes in order to decrease the power consumption, reduce the chip size, and obtain better matching. The signal extraction principle is accomplished using a box-car circuit. To reduce the influence of the 1/f noise of the amplifiers, we implemented a modulation technique, using an additional modulator. Modulated signal is amplified and finally demodulated using the correlated double sampling demodulator. We have implemented a new digital feedback, for the first time applied on the excitation coils. It consists of comparator, 8-bit U/D counter and current sources to create magnetic field for contra action. The output of the counter represents the digital information on the external field and is transmitted using I2C communication block integrated on chip. The fabricated sensor works from a single 2.5 V power supply and consumes 8.5 mW in open loop mode and 10 mW with a feedback. With the excitation at 350 kHz, the magnetometer exhibits the sensitivity of 2950 V/T and 8 kHz bandwidth. Equivalent noise spectral density at 1 Hz is equal to 12 nT/√Hz. When used as a compass, the sensor has the angular error of ± 1° without and ± 3° using feedback. Measured perming is equal to 1.4 µT, after the shock of 200 mT. Performance of the developed micro-fluxgate is better in terms of noise, angular error, perming, operational voltage, as well as power consumption, than any other fully integrated micro fluxgate presented up to date. In addition, the fabrication is based on the batch and low cost CMOS technology and compatible post-process so we can expect commercialisation in the near future.

EPFL2004