Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (a) CMOS Technology <s> This paper presents a CMOS two-dimensional (2-D) vector magnetic sensor system integrating two planar microfluxgate sensors and the complete electronics for sensor excitation and signal readout. The system is based on an industrial 0.8-/spl mu/m double-poly, double-metal CMOS technology with ferromagnetic NiFeMo cores added in a simple postprocessing sequence. The fluxgate sensors are embedded in a /spl Sigma//spl Delta/ analog-to-digital converter for a stable and precise digital detection of weak magnetic fields. A cascaded /spl Sigma//spl Delta/ modulator topology is utilized to obtain second-order noise shaping and to suppress pattern noise. Within the range of /spl plusmn/50 /spl mu/T, the system nonlinearity is less than 1.5 /spl mu/T. The angular resolution as a 2D vector sensor is less than 4/spl deg/ for a measured magnetic induction of 50 /spl mu/T. This makes the 2-D microfluxgate magnetometer suitable for use as fully integrated electronic compass. <s> BIB001 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (a) CMOS Technology <s> An electronic compass made of a new planar 2D micro-fluxgate sensor is presented. The magnetometer is integrated in a standard CMOS process, and uses a post-processed cross-shaped ferromagnetic amorphous core. This core is diagonally placed above a single square excitation coil common to both measurement axes. The silicon chip includes the driving and readout electronics, the excitation and pick-up coils made of the two CMOS process metallization layers. The ferromagnetic core is integrated from commercially available amorphous metal ribbons, by using a post-process compatible with standard IC technologies. Such a ferromagnetic core presents outstanding magnetic characteristics compared to usual electroplated cores. The micro-fluxgate works with a pulse-shaped driving current which allows the sensor output to be independent of the driving frequency. The current pulse width ratio of 1/8 reduces drastically the power consumption. For magnetic fields within "60 mT, the micro-fluxgate sensor exhibits a magnetic sensitivity of 3760 VrT at 125 kHz with an amplification gain of 26.4 dB. The angle error on the Earth’s magnetic field is "1.58. The chip has a power consumption of 12.5 mW for a 17 mA-peak driving current; its area is of 5.3 mm2. <s> BIB002 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (a) CMOS Technology <s> Abstract A CMOS micro fluxgate sensor, including device design, manufacturing process and harmonic frequency characterizations, is described in this paper. The single-axis device is designed in a planar coil structure with a ferromagnetic core featuring low saturation magnetic induction of 10 μ T . The chip is measured 2.08 mm × 1.29 mm and fabricated by TSMC CMOS 0.35 μ m 2P4M process. The miniature magnetic core is formed by wet etching process and precisely allocated to the designated position of the sensor chip by using an aligner. To characterize the micro-fluxgate sensor, the output voltage of the induction coil and external magnetic field is especially measured at the excitation, second, third and forth harmonic frequencies respectively. Experimental results reveal that the B-V diagram at excitation and the 3rd harmonic frequencies are well described by evenfunction behavior while the characteristics at the 2nd and 4th harmonic frequencies are expressed by odd-function operation. Besides, it is also observed that the maximum output voltage amplitudes at odd-order harmonic frequencies tend to occur when the maximum field-to-voltage transfer coefficients (i.e., dV/dB) become prominent at even-order harmonic frequencies, and vice versa. To measure the low magnetic field, it is found that the micro fluxgate demonstrates a nearly linear B-V relationship around the zero-magnetism region while the 4th order harmonic frequency is employed. The maximum responsivity can be further enhanced up to 174.62 mV/T. The frequency response of the device is measured as 90 Hz, i.e., a rapid response time of 0.01 s. <s> BIB003 </s> Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review <s> (a) CMOS Technology <s> Abstract This paper presents a dual-core (Vacquier-type) micro-fluxgate magnetic sensor fabricated on a silicon chip based on standard CMOS technology. The silicon chip is 2.5 mm × 2.5 mm in dimension, and the micro-fluxgate sensor occupies the area of 2.5 mm × 1.8 mm. The sensor consists of magnetic cores, planar pick-up coils, bottom excitation coils (CMOS Al interconnections) and upper excitation coils (wire-bonding Al wires). The micro-solenoid excitation coils consisting of aluminum bonding wires and CMOS metallic layers can generate out-of-phase excitation magnetic fields strong enough to saturate the two magnetic cores. It was found that the sensor's sensitivity can be optimized and the field noise spectral density can be minimized by adjusting the excitation current as well as the excitation frequency. The maximum responsivity of 8.1 V/T occurs at the second harmonic when the excitation frequency is 110 kHz; however, the minimum magnetic field noise is found to be 2.6 nT/√Hz at 1 Hz under 20-kHz excitation. In comparison with other miniature planar fluxgates with similar dimensions reported to date, our device has a relatively low field noise spectral density. <s> BIB004
For these microfluxgates, planar technologies are applied (specifically CMOS) to implement the necessary circuitries and coils. The chips currently available on the market are equipped with two integrated orthogonal microfluxgate sensors with their corresponding A/D conversion circuitry. The application of coils in microfluxgates is perhaps the most original thing about them. They are created in the shape of various metal overlapping layers separated from each other by dielectric material (silicon oxide) BIB001 . They measure magnetic fields in a similar way as other fluxgate magnetometers: saturating ferromagnetic cores (actually, nickel-iron-molybdenum cores) and measuring the lag produced by external fields. These kind of devices have the following advantages compared with conventional fluxgates:  They need little space.  They provide linear responses for overhead magnetic inductions (there are certainly "conventional" fluxgates that offer a better response, but not in such a reduced area of 50 μT).  They measure the chip plan (360°) without ambiguity.  Their production is easy and very cheap using planar technology.  They have the whole control circuitry integrated. However, in spite of these advantages, one must keep in mind that these sensors are specifically calibrated to measure the earth field. At least for the moment, there are no commercial microfluxgate sensors able to function in other magnetic ranges. Their maximum linear output range is of ±100 μT. Technological development concerning microfluxgates has brought on the market different chip models. For example, the chip developed by Chiesi et al. BIB002 . This microfluxgate carries a circuitry set up with the standard CMOS process and a ferromagnetic core produced thereafter. As for the Chiesi chip, the developed micro-sensor consists essentially of a parallel fluxgate magnetometer of two axes. The uniqueness of the setting lies in the geometric layout of its elements, which allows the use of one single coil to saturate the two cores, optimizing the length of the latter. The trick consists in locating the two cores crosswise and perpendicularly to each other, above the square coil. This way the secondary coils are located at the ends of both cores. For a given exciting flow, with this arrangement the core magnetization increases up to a 30% compared with the classic setting. The magnetic cross shaped core is realised by means of photolithography and chemical attack of amorphous tapes and subsequently attached to the previously produced chip with CMOS technology BIB004 BIB003 . Both secondary coils, located under the core ends, are series connected along the measuring axes with the objective of obtaining a differential configuration. This cancels the contribution of the exciting field to the total induced signal. The device is powered by a 5 V supply with a frequency of 125 kHz. The exciting flow is of 17 mApeak, giving a consumption of 12.5 mW. The output signal shows a linear response in a range between -100 and 100 μT. The resulting sensitivity of the sensor equals 3700 V/T with an error of less than 0.4% and with an accuracy level of 1.5° in the obtained angle for a horizontal field of 9 μT 1.5°.