Deposition of Giant Magnetoresitance (GMR) Sensors on Large Areas

Magnetic field sensors are broadly applied to detect any type of motion including proximity, rotations or vibrations. Motion sensing is required in numerous applications from industrial robotics, through prosthetics to virtual and augmented reality appliances. When fabricated using thin film technologies (thin film deposition and lithographic patterning), magnetic field sensors relying on giant magnetoresistive (GMR) effect made major impact on our society via realization of high-capacity hard disk drives. This was the key enabler behind the development of cloud storage and social media. Magnetic thin films are used in read heads of hard disk drives or as information bits in magnetic random access memory (RAM) or as electronic compasses in our smartphones for navigation purposes.

The GMR technology puts stringent requirements on a deposition facility. Indeed, GMR stacks typically consisting of multilayers of about 1-nm-thick Co and Cu layers (see Fig. 1) require precision in the film thickness of better than 0.1 nm. Only in this case, it is possible to fabricate high-performance sensors revealing large changes of electrical resistance with magnetic field. To comply with these requirements on the deposition accuracy over large areas, conventionally, thin film based magnetic field sensors are fabricated on planar substrates like Si wafers and are thick and rigid.

The need to realize magnetic field sensing functionality for flexible and wearable electronics stimulated development of technologies to prepare high-performance GMR sensors on polymeric foils of different thickness ranging from 1 μm up to 150 μm. This trend concerned also the realization of mechanically flexible anisotropic magnetoresitance (AMR) sensors, tunneling magnetoresistive (TMR) sensors and giant magnetoimpedance (GMI) sensors. It was demonstrated that metal-based magnetic field sensors have very good adhesion to polymeric support (see Fig. 2) and reveal the same magnetoresistive performance when prepared on rigid Si wafers and on polymeric foils of different thickness (see Fig. 3). Even when prepared on large area polymers, magnetic field sensors can be lithographically patterned at high precision enabling appealing new type of functional devices for large area transparent electronics (see Fig. 4). Lab scale demonstrations highlighted the application potential of flexible magnetoelectronic devices for Internet of Things (IoT), smart home and eMobility.

When prepared on ultrathin polymeric foils (about 1 μm thick), magnetic field sensors can be applied to human skin for the realization of magnetosensitive smart skins. They enable touchless interactivity with our surrounding based on the interaction with magnetic fields (geomagnetic fields or permanent magnets), which is relevant for human-machine interfaces for virtual and augmented reality. Touchless interactivity is useful when objects cannot be physically touched due to safety or security restrictions as was relevant during the COVID-19 pandemic.

When prepared on thicker foils like 100-μm-thick Kapton, it is possible to realize mechanically flexible magnetic field sensors, which can operate in the entire automotive industry relevant temperature range from -40 to 165 °C. Flexibility and thinness allow magnetic field sensors to be inserted in a narrow air gap of electrical machines and drives for real time monitoring of their performance and/or realization of magnetic field-based control. This can improve reliability of devices, increase their dynamic performance and reduce maintenance costs, which is important for further development of eMobility.

The use of polymeric support accommodating GMR stacks enabled a new technology, which allows to fabricate printed magnetic field sensors. In this case, after fabrication over large areas, magnetic multilayers are removed from the support and processed in a functional magnetosensitive paste. This paste can be printed using screen or dispenser printers to realize GMR-based printed magnetoelectric switches for printed interactive electronics like postcards or commercial materials.

These demonstrations and application ideas became mature enough and stimulated technology transfer of flexible and printed magnetic field sensor technologies. These activities require large area high precision deposition tools, which are able to prepare GMR multilayer stacks on large area polymeric foils. With scia Multi 300, the fabrication of high performance GMR stacks on polymeric foils of different thickness over areas of up to 300 mm is successfully validated.

The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is using a scia Multi 300 to produce flexible magnetoelectronics. We like to thank especially Dr. Denys Makarov, Head of Intelligent Materials and Systems, for providing this application note.