María-Dolores Cubells-Beltrán

Magnetic Field Sensors Based on Giant Magnetoresistance (GMR) Technology: Applications in Electrical Current Sensing.

The 2007 Nobel Prize in Physics can be understood as a global recognition to the rapid development of the Giant Magnetoresistance (GMR), from both the physics and engineering points of view. Behind the utilization of GMR structures as read heads for massive storage magnetic hard disks, important applications as solid state magnetic sensors have emerged. Low cost, compatibility with standard CMOS technologies and high sensitivity are common advantages of these sensors. This way, they have been successfully applied in a lot different environments. In this work, we are trying to collect the Spanish contributions to the progress of the research related to the GMR based sensors covering, among other subjects, the applications, the sensor design, the modelling and the electronic interfaces, focusing on electrical current sensing applications.

Full Wheatstone Bridge Spin-Valve Based Sensors for IC Currents Monitoring

Full Wheatstone bridge spin-valve-based electrical current sensors at the IC level are presented. Prototypes with different geometrical parameters have been designed, fabricated and fully characterized. DC characterization has been carried out, for measurement of insertion losses, linearity, voltage offset and sensitivity. Current ranges from 10 muA to 100 mA can be covered with these sensors with excellent linearity and sensitivities above 1 mV/(VmiddotmA) . AC characteristics have also been analyzed and bandwidths exceeding 100 kHz are demonstrated. Moreover, the temperature coefficients have been extracted in the range of -20degC to +60degC. In order to highlight the design properties, dependence of the sensors performance with external magnetic perturbations and self-heating have also been measured and quantified. The associated errors are in the range of 1%-2% of the full scale.

Integration of GMR Sensors with Different Technologies

Less than thirty years after the giant magnetoresistance (GMR) effect was described, GMR sensors are the preferred choice in many applications demanding the measurement of low magnetic fields in small volumes. This rapid deployment from theoretical basis to market and state-of-the-art applications can be explained by the combination of excellent inherent properties with the feasibility of fabrication, allowing the real integration with many other standard technologies. In this paper, we present a review focusing on how this capability of integration has allowed the improvement of the inherent capabilities and, therefore, the range of application of GMR sensors. After briefly describing the phenomenological basis, we deal on the benefits of low temperature deposition techniques regarding the integration of GMR sensors with flexible (plastic) substrates and pre-processed CMOS chips. In this way, the limit of detection can be improved by means of bettering the sensitivity or reducing the noise. We also report on novel fields of application of GMR sensors by the recapitulation of a number of cases of success of their integration with different heterogeneous complementary elements. We finally describe three fully functional systems, two of them in the bio-technology world, as the proof of how the integrability has been instrumental in the meteoric development of GMR sensors and their applications.

A DC behavioral electrical model for quasi-linear spin-valve devices including thermal effects for circuit simulation

An advanced model for quasi-linear spin-valve (SV) structures is presented for circuit simulation purposes. The model takes into account electrical and thermal effects in a coupled way in order to allow a coherent representation of the sensor physics for design purposes of electronics applications based on these sensor devices. The model was implemented in Verilog-A and used in a commercial circuit simulator. For testing the model, different SV structures have been specifically fabricated and measured. The characterization included DC measurements as well as steady-state and transient thermal analysis. From the experimental data, the parameters of the model have been extracted. The model reproduces correctly the experimental measurements obtained for devices with diverse sizes in different electrical and thermal operation regimes.

Current-Based Measurement Technique for High Sensitivity Detection of Resistive Bridges With External Balancing Through Control Voltages

We present a novel approach based on differential measurements of dc currents with very high sensitivity suitable for the detection of very small variations of resistors in Wheatstone full-bridge configurations. External control voltages allow for the compensation of the bridge unbalancing avoiding the need of changing its elements so making the solution suitable for integrated sensor systems. The proposed current-based measurement technique has been implemented through three different circuits, in transimpedance configuration and without the use of any further amplification stage, employing only two active blocks that allow for a very high integration level. The main characteristics of these solutions, developed both in current-mode (CM) and voltage-mode (VM) approaches, have been preliminary evaluated through PSPICE simulations so validating the new approach. The results have demonstrated the capability of the presented circuits to reveal, with linear responses, relative variations of the bridge resistors lower than 10-6 % with respect to their nominal values, providing detection sensitivities up to 150 V/% (i.e., 500 μA/%) with the CM solutions and 1750 V/% in the case of the VM scheme. Moreover, experimental measurements have also been conducted by implementing the new technique on breadboard with discrete commercial components confirming the performances of the proposed approach. In this regard, a detection resolution of resistive variations as low as 0.00008%, corresponding to about 0.001Ω, has been achieved in the measurement of only one resistor of the bridge with an improvement of the detection sensitivity of a factor 500 with respect to the standard basic resistive bridge.

GMR Based Sensors for IC Current Monitoring

The Giant MagnetoResistance (GMR) effect is a magnetic coupling mechanism that can be obtained in multilayer structures of few nanometers thick. In these devices, and at room temperature, the resistance is a function of the external magnetic field, at optimal levels for being used as sensors. Since the GMR effect was reported, scientists and engineers have dedicated their effort to this topic. This way, after two decades, a a very good knowledge of the GMR underlying physics together with notable designs of GMR based devices are nowadays available. They were initially used in the read heads of hard drives, but the constant evolution that this technology has experienced has open new fields of application, mainly related to the measurement of small magnetic fields using miniaturized devices, such as biotechnology and microelectronics.

Giant Magnetoresistance (GMR) Magnetometers

Since its discovering in 1988, the Giant Magnetoresistance (GMR) effect has been widely studied both from the theoretical and the applications points of view. Its rapid development was initially promoted by their extensive use in the read heads of the massive data magnetic storage systems, in the digital world. Since then, novel proposals as basic solid state magnetic sensors have been continuously appearing. Due to their high sensitivity, small size and compatibility with standard CMOS technologies, they have become the preferred choice in scenarios traditionally occupied by Hall sensors. In this chapter, we analyze the main properties of GMR sensors regarding their use as magnetometers. We will deal about the physical basis, the fabrication processes and the parameters constraining their response. We will also mention about some significant application, including developments at the system level.

Sub-mA current measurement by means of GMR sensors and state of the art lock-in amplifiers

Electric current measurement at the range of μA in integrated circuit has been traditionally carried out by micro-electronically engineered systems, such as current mirrors or charging capacitors. However, off-line, i.e., non-intrusive methods provide advantages related to size and power consumption. In this sense, giant magnetoresistance (GMR) magnetic sensors are optimal due to their sensitivity and CMOS compatibility. In this work, we make use of specifically designed CMOS GMR-based current sensors in combination with a custom electronic interface based on a low-voltage low-power lock-in amplifier, demonstrating the capability of this combination for current measurement in the range of μA.