Multiple frequency characterizations of a CMOS planar fluxgate magnetic sensor

A single-axis CMOS micro-fluxgate magnetic sensor, including device design, manufacturing process and harmonic frequency characterizations, is presented. The device is designed in a planar coil structure with a ferromagnetic core featuring low saturation magnetic induction of 10 μT. The chip is 2.08 mm × 1.29 mm and fabricated by TSMC CMOS 0.35 μm 2P4M process. The miniature magnetic core is prepared by wet etching process, and precisely allocated to the designated position of the sensor chip by electrostatic adhesion 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 3^rd harmonic frequencies are well described by even-function behavior while the characteristics at the 2^nd and 4^th 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 available at even-order harmonic frequencies, and vice versa. Finally, the micro-fluxgate demonstrates a nearly linear B-V relationship around the zero point region if the 4^th order harmonic frequency is employed, which is superior to other harmonic extractions and feasible for low magnetic field measurement.