Design considerations and optimization for 3-axis anisotropic magneto-resistive sensors

MEMS electronic compass devices are becoming an attractive solution for improving navigation assistance tools provided on mobile devices. These devices use the earth's magnetic field in order to obtain the orientation of the device. An anisotropic magneto resistive sensor consists of permalloy thin film that has been deposited under a strong magnetic field. This results in an easy axis in the material along which it can be magnetized. When a field is applied along the sensing axis, the magnetic field inside the sensor is rotated which in turn causes a change in resistance. By measuring this resistance change, the magnitude of the magnetic field is determined. This paper presents a formalized procedure recommended for the development of new AMR sensors. The optimization of concentrators, used for transforming fields normal to the die (z-axis) into an in-plane field measured using a planar sensor is also presented. Finally the paper also reports and discusses simulation results obtained using permalloy guard rings in order to minimize applied in-plane fields in the z-axis sensor.