Planarization, Fabrication, and Characterization of Three-Dimensional Magnetic Field Sensors

Nanomagnetism deals with magnetic phenomena in nanoscale structures, involving processes at the atomic level. Magnetic sensors, which exhibit the surprising giant magnetoresistance (GMR) effect, are some of the first real applications of nanotechnology, and have become very important in the last two decades. In addition, high-performance magnetoresistance (MR) measurement is a critical technique in modern electrical applications, including electronic compasses, aviation navigation, motion tracking, noncontact current sensing, rotation sensing, and vehicle detection. Both GMR and tunneling magnetoresistance (TMR) sensors have been used in the state-of-art electronic compasses. A new planar design layout of a vector magnetometer is proposed in this report. It can sense variations in three-dimensional (3-D) magnetic fields. The planarization of a vector magnetometer is carried out with consideration of materials, magnetic schematics, as well as transducer circuit designs. The optimization of an advanced magnetic material for use in GMR and TMR sensors and its planarization in a 3-D design are crucial practical issues. This paper presents an overview of the planarization of vector magnetometers and the development of its applications. It focuses on recent works, covers an analytic model of magnetoresistive sensors, and methods of thin film fabrication. It also addresses the planar vector magnetometer with a flux-guide, the chopping technique, and techniques for microfabrication of substrates. Planarization in magnetic sensors will become increasingly exploited as nanomagnetism grows in importance.

I. Introduction

Magnetic field sensors are being increasingly exploited in many fields, including geophysics, magneto-cardiography, industrial non-destructive evaluation, magneto-encephalography. They are also used to detect magnetic anomalies according to their sensitivity to the strength of magnetic field [1]. The detection of geomagnetic fields in navigation was the first practical use of magnetism, and has become one of the most significant methods of orientation [2]. The need for greater accuracy and higher spatial resolution has motivated great advances in measurement techniques and materials development. In 1857, the anisotropic magnetoresistance in ferromagnetic materials was discovered by William Thomson [3]. However, his discovery would not be exploited for 100 years. In fact, before the development of film technology, the magnetoresistance effect did not have a commercial application. Materials such as permalloy (NiFe) have a preferred magnetic orientation, so that when a current passes through, the material is magnetized in a direction parallel to the current. When an external magnetic field is applied perpendicular to the direction of the current, the magnetization direction rotates toward the direction of the magnetic field, changing the resistance of the film. In the last few decades, anisotropic magnetoresistance (AMR) sensors have been developed, and now have a wide range of industrial applications, such as in current sensing, electronic compasses, and in measurement of angles, positions and speed [4] –[6]. However, the AMR coefficient in the common thin film permalloy is only about 2.5% [7]. In 1986, a study of the GMR effect revealed antiferromagnetic coupling in a Fe/Cr magnetic multilayer composed of multiple repetitions of ferromagnetic (FM) and non-ferromagnetic (NM) layers [8]. This effect was discovered simultaneously by Baibich [9]. Albert Fert and Peter Grünberg who were the first to use the word “giant” [10] to refer to the huge difference in the resistance between GMR and the previously known AMR. The results opened up a new line of research into the magneto-resistance (MR) effect, including colossal magneto-resistance (CMR) [11], TMR [12], and its applications. Spin polarization is increasingly attracting more researchers [13], and new directions and materials for spintronic devices and spin logic have been presented [14], [15]. A special type of GMR is a spin valve, which Dieny exploited and applied in the read heads of hard disk drives (HDD) [16]. The exchange coupling of an AFM and an FM layer involves the exchange bias effect discovered by Meiklejohn and Bean [17] in 1956. This GMR effect has been exploited in data storage and new magnetic sensors. Recently, high-speed data recording on a spin carrier has been reported [18]. The importance of GMR was acknowledged in 2007, when the Nobel Prize in Physics was awarded jointly to Albert Fert and Peter Grünberg “for the discovery of giant magnetoresistance” [19].