Printable Magnetoelectronics

Printed electronics are about to revolutionize the field of conventional electronics offering low-cost, large area, high-volume, high-throughput production in roll-to-roll or sheet-feed processing techniques. Electronic components fabricated by printing are light weight and small, thin and flexible, inexpensive and disposable [Moo12]. Already at the very beginning of the development of this technological platform various application directions for printed electronics aiming for transparent, bendable/flexible functional devices have been proposed (Figure 1): displays [Sir98], including organic light-emitting diodes (OLEDs), liquid crystal display (LCDs), various types of sensors [Som04], radio frequency identification (RFID) tags [Bau03] and organic solar cells [Yel11].

In order to realize the vision of printable electronics, it is necessary to replicate all components of conventional rigid electronics in a printable form. The field of modern electronics is very general including interconnects, optoelectronics and magnetoelectronics. Although, cost-efficient versatile electronic building blocks, such as transistors, diodes and resistors resulting in printable optoelectronics and printable communication modules are already available as printed counterparts of conventional semiconductor elements, printable magnetoelectronics and contactless magnetic field driven switches operating at ambient conditions had been reported only very recently [Kar12, Mak13]. For this purpose, magnetic sensors with a high sensitivity operating at room temperature have to be developed as inks, pastes or paints. The fabrication of printable magnetoelectronics is challenging, mainly due to the lack of appropriate sensing compounds at ambient conditions. In order to be in line with the concept of printable electronics, magnetic sensors should satisfy the following basic requirements: low-cost, high-volume production on large areas of standard printing materials, disposability and processability.

 

Figure 1: The family of printable electronic devices includes solar cells, printable displays and communication modules. Recently, a new member was added to the family - printable magnetoresistive sensor elements. Image is taken from [Mak13].

Only very recently, magneto-sensitive inks revealing a giant magnetoresistive (GMR) effect at room temperature were developed [Kar12, DE patent]. Fabrication in brief: GMR stacks consisting of Co/Cu bilayers are grown on top of a polymer buffer layer prepared on a rigid wafer. After deposition, the samples are rinsed in acetone to release the GMR stacks from the substrates. The metallic film is filtered out, dried and then ball milled. The resulting powder is filtered through a grid that defines the maximum lateral size of a GMR flake to about 150 µm. A magneto-sensitive ink is prepared by mixing the GMR powder with a binder solution. To demonstrate the functionality of a printable GMR sensor element on a proof-of-concept level, the sensor is integrated in an electronic circuit fabricated directly on the paper of a postcard (Figure 2).

Conductive silver paste was used to paint the interconnects. For demonstration, we chose a hybrid circuit consisting of painted elements (interconnects and sensor) combined with conventional solid state discrete transistors, capacitors, resistors and a light emitting diode (LED). When the postcard is closed, the LED is switched off (Figure 2). Opening the postcard switches the LED on. The ON/OFF state of the LED is triggered by a small permanent magnet that modifies the resistance of the printable magnetic sensor. This change of resistance, in turn, alters the open/close state of the transistors in the amplification cascades (Figure 2) regulating the current flow through the LED. Note that all these discrete elements (resistors, transistors, diodes) used for electronic circuitry as well as the permanent magnets are available for printable electronics. Therefore, the fabrication of a fully printable circuit with an integrated magnetic sensor is possible.

Figure 2: (top) The printable magnetic sensor is integrated in a hybrid electronic circuit (amplification cascade with an LED) fabricated on the paper of a postcard. The LED ON/OFF state is triggered by a permanent magnet that modifies the resistance of the sensor. This change of resistance, in turn, alters the open/close state of the transistors in the amplification cascades regulating the current flow through the LED. (bottom) A magnified view of the hybrid electronic circuit with the printed magnetic sensor. The amplification circuit contains the current source Ibias to bias the sensor, three amplification cascades with three transistors T1-T3, resistors R1-R7 and capacitors C1-C3. The last amplification cascade supplies LED. The image is adopted from [Kar12, Mak13].

In this project we focus on several issues which have to be solved on the way towards the commercialization of this technology. Among them the most crucial are a high-volume production of magneto-sensitive ink and a demonstration of large-scale printing of the ink. There are strategies that will help us to overcome the aforementioned aspects. To increase the produced amount of the ink it is instructive to choose different support materials. The best would be to switch from the Si-based substrates to large-area polymer sheets. We already demonstrated that magnetic sensors with good GMR response can be prepared even on regular transparency. Regarding the second issue: At the moment, we applied a regular painting process to bring the magneto-sensitive ink to the surface. Potentially, other methods, such as roll-to-roll, flexography, spray coating and screen printing can be applied. However, the size of the GMR flakes in the ink as well as the viscosity of the ink has to be optimized separately for each of these printing processes. In this respect, more investigation is required to understand the influence of the size of GMR flakes on the resulting GMR response of the magneto-sensitive ink. Furthermore, different binder solutions have to be tested to adjust the viscosity of the ink.

References:
[Moo12] P. F. Moonen et al., Adv. Mater. 24, 5526 (2012).
[Sir98] H. Sirringhaus et al., Science 280, 1741 (1998).
[Som04] T. Someya et al., Proc. Nat. Acad. Sci. 101, 9966 (2004).
[Bau03] P. F. Baude et al., Appl. Phys. Lett. 82, 3964 (2003).
[Yel11] A. Yella et al., Science 334, 629 (2011).
[Kar12] D. Karnaushenko et al., Adv. Mater. 24, 4518 (2012).
[DE patent] https://register.dpma.de/DPMAregister/pat/register?AKZ=1020110779078
[Mak13] D. Makarov et al., ChemPhysChem 14, 1771 (2013).




This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration
under grant agreement no 306277.