High-Throughput Printing and Post-Processing Approaches for Realization of Novel Flexible Magnetoresistive Sensors

The research in the field of additive manufacturing is increasingly focusing on realization of electrically conductive interconnects, passive and active components such as resistors, capacitors, inductors, and application-specific electronic devices [1]. The additive manufacturing technologies are offering high design flexibility and high throughput, are material saving and have a great potential of the production cost reduction. The flexible printed magnetoresistive (MR) sensors based on various physical principles are novel components in printed electronics demonstrated in recent years using additive manufacturing methods [2]. However, the large-scale manufacturing of these sensors and their industrial uptake are hampered by the lack of commercially available magneto-resistive inks and scalable post-printing processing methods to enable optimal functionality of the components.<br/>Our work is focused on establishing a fully additive approach for realization of flexible MR-sensors and their arrays. We address the complete process chain from MR-material particles selection/preparation, through the ink formulation, printing and post processing of the structures, their characterization and demonstration of defined use cases. Two types of materials were used to formulate MR-inks: i) commercially available µm-size spherical Bi particles showing non-saturating large magnetoresistance (LMR), and (ii) permalloy (Py) flakes of own development showing anisotropic magnetoresistance (AMR). In the latter case, a method was developed to ensure a production of the particles with 100 nm thickness and in-plane dimensions of a few micrometers with a yield of approx. 2.7 g/day which can be further upscaled. It is based upon large-area PVD deposition of the corresponding materials as thin films onto glass substrates coated with a special sacrificial layer, following lift-off process of the thin films, and their ultrasonic milling. Based on these powder materials, screen-printing compatible inks were formulated with solid fraction as high as 82-90 wt.% (Bi particles) and 21 wt.% (Py flakes). Electrical contacts were realized either using commercially available or IKTS-proprietary Ag inks. Different layouts of LMR and AMR sensors were created and printed on flexible polymer foils (PET, PI).<br/>The core of our approach is a combination of scalable printing methods (screen-, dispenser-, inkjet-printing) with high power diode laser post-processing. The line-shaped laser beam was generated by microoptically optimized high-power diode laser array operating in the near-infrared spectral range using continuous wave mode. In contrast to standard furnace heating this approach enables treatment on the millisecond time scale, preventing temperature-induced damages of sensitive substrates and permitting selective sintering of printed films in air. The laser beam was swept along the printed structures to remove the organic phase and sinter particles of the magnetoresistive material. Depending on the type of material, the dwell time was varied from 3 to 20 ms and the laser intensity from 0.8 to 2.0 kW/cm². In case of Bi-based LMR material, the sensors with a strong linear response within a broad magnetic field range 100 mT - 7 T were achieved. The resulting magnetic field sensors based on Py flakes showed a very promising AMR effect in the range of 0.5% at much lower magnetic fields of 2-3 mT. The applicability of the sensors and their arrays for contactless switching and flexible human-machine interfaces is demonstrated. The relationship between the sensor manufacturing parameters and the resulting structure and properties is analyzed to define pathways to further improvement of their performance.<br/>[1] A.H. Espera, et al. Prog. Addit. Manuf. 4, 245 (2019)<br/>[2] M. Ha, et al. Adv. Mater. 33, 2005521(2021)