Laser Based 3D-Magnetic Field Sensor Formation
Position determination using a magnetic field is excellent sensor technology. Sensors based on the Giant Magneto Resistance Effect (GMR) and the Tunneling Resistance Effect (TMR) are particularly suitable. They are used to determine angles of rotation and positions, among other things for steering wheels, the speed of rotating wheels, or the precise control of electronically commutated motors. GMR and TMR sensors have particular strengths in mobile devices when determining their position in the earth's magnetic field (in cell phones, smartwatches, wearables, mobile robots, or VR glasses). Monolithic sensors manufactured on a wafer achieve the highest accuracy and sensitivity.
The individual sensors are based on layer stacks of ferromagnetic materials, separated by non-magnetic layers. The initial orientation of the magnetic fields is permanently generated by the formation or pinning process. When using sensors, the orientations of the magnetic fields in the magnetic layers are changed by external magnetic fields and thus the conductivity of the individual sensor. The combination of several individual sensors to form a Wheatstone Bridge increases the sensitivity. If several bridges are combined, a 3D sensor is obtained.
Up to now, 3D sensors were mainly manufactured in complex process chains with great effort and finally assembled by packaging. In the paper, innovative laser-based technology is presented, which allows significant simplification of sensor production by selective pinning and at the same time enables better quality sensors.
The individual sensors are heated by individual laser pulses and permanently oriented by a strong magnetic field applied at the same time. Due to the selective formation, it is theoretically possible to program any number of different orientations on a wafer.
The selective process has numerous advantages for the sensor properties: very small individual sensors can be addressed precisely by the laser spot. The evaluation electronics can be integrated on the same wafer and is not damaged by the formatting process. The resulting devices have a higher integration density. The individual sensors are much more homogeneous - the 3D sensors are therefore more sensitive and precise.
The production process is becoming more cost-effective and reliable: Smaller sensors and smaller permissible distances during formatting allow more sensors per wafer. Different single sensor geometries and arrangements on the wafer can be formatted per recipe without changing hardware or even support wafers. The throughput is high and error-prone production steps (mounting / un mounting, packaging) are eliminated.
In the paper, the presented technology will be introduced in detail and compared with the previously used technologies. The implementation in a system suitable for production will be presented. Best practice examples and the achievable sensor properties will be discussed.
The new technology has proven its suitability for production in the first industrial installations. The adaptation to a different wafer and sensor concepts is very simple thanks to the laser-magnet concept and has been demonstrated in Technology demonstrations.