Harmonic frequency characterisations of a CMOS micro fluxgate sensor for low magnetic field detection

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 10μT. The chip is measured 2.08 mm × 1.29 mm and fabricated by TSMC CMOS 0.35μm 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.