Researchers Publish Proof of Concept Detailing Inkjet Additive Manufacturing Process for Memory Devices

Every day, at work, at home, and out in the world, our lives involve the use of low-performance electronic functions that are implemented in computer chips for items like credit cards. These memory devices have been gradually becoming more flexible, which is a good thing. But historically, not many people have worried too much about their integration and ease of fabrication in low-performance applications. But this attitude could soon be changing along with increasing availability of technology and research efforts. Recent additive manufacturing work by a group of researchers from both INRS-EMT in Canada and Germany’s Munich University of Applied Sciences (MUAS) is delving into the ways that 3D printing technology can benefit memory storage.

We all know that additive manufacturing can cut costs and overall production time, but it can also streamline process flows. In the case of memory devices, the use of additive manufacturing can get rid of material removal steps and complex lithography, without causing the feature size to shrink – this isn’t critically important for all memory devices, but certainly for the ones needed for higher-level, more demanding applications.

The joint research group published a paper in the journal Applied Physics Letters, titled “Fully inkjet printed flexible resistive memory,” describing their proof of concept using inkjet printing, which has been used with digital memory technology before, and resistive memory (ReRAM).

The paper’s abstract reads, “Resistively switching memory cells (ReRAM) are strong contenders for next-generation non-volatile random access memories. In this paper, we present ReRAM cells on flexible substrates consisting of Ag/spin-on-glass/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate). The complete cell is fabricated using a standard inkjet printer without additional process steps. Investigations on the spin-on-glass insulating layer showed that low sintering temperatures are sufficient for good switching behavior, providing compatibility with various foils. The cells feature low switching voltages, low write currents, and a high ratio between high and low resistance state of 10⁴. Combined with excellent switching characteristics under bending conditions, these results pave the way for low-power and low-cost memory devices for future applications in flexible electronics.”

The principle behind the group’s ReRAM concept, which can help other researchers begin to streamline the mass production of printable electronics, is not difficult.

Bernard Huber, an INRS-EMT doctoral student who works in the Laboratory for Microsystems Technology at MUAS, explained, “In any kind of memory, the basic memory unit must be switchable between two states that represent one bit, or ‘0’ or ‘1.’ For ReRAM devices, these two states are defined by the resistance of the memory cell.”

The group uses a conductive-bridge random access memory, or CB-RAM, in their concept, and that colored inks are not used in their 3D printing process. The CB-RAM concept is not new, and Christina Schindler from MUAS and Andreas Ruediger of INRS-EMT, the leaders of the research group, have prior experience with more conventional CB-RAM cells.

Huber explained that “0” for their CB-RAM is “a high-resistance state represented by the high resistance of an insulating spin-on glass, which separates a conducting polymer electrode from a silver electrode. The ‘1’ is a low-resistance state, which is given by a metallic filament that grows into the spin-on glass and provides a reversible short-circuit between the two electrodes.”

He continued, “we use functional inks to deposit a capacitor structure—conductor-insulator-conductor—with materials that have already been deployed in cleanroom processes. This process is identical to that of an office inkjet printer, with an additional option of fine-tuning the droplet size and heating the target material.”

The paper explains that the main element of one of the group’s ReRAM cells is an insulating layer, which is a matrix for metallic filament growth and ion migration. Each individual memory cell is comprised of a roughly 100 μm x 100 μm crosspoint structure, and a FujiDimatix DMP 2831 inkjet printer was used to print all three layers for the memory cells, using a cartridge with a 10 pl drop volume.

The joint research group’s innovative work and proof of concept is pretty significant for the world of memory devices.

“We not only demonstrated that a complete additive (printing) process was possible but also that the performance parameters are comparable to cleanroom-fabricated devices. The biggest technological appeal is the mechanical flexibility of our memory tiles, and the fact that all materials required for processing are commercially available,” said Schindler. “From our proof of concept, we’re paving a road toward optimization. Our biggest surprise was how little device performance depends on the fabrication process.”

This means that 3D printing processes can be used to fabricate extremely low-cost flexible electronics, and as Ruediger explained, another field that could potentially benefit from their work is print-on-demand electronics.

“At present, the main source of versatile electronics is field-programmable gate arrays that provide a reconfigurable circuitry that can be adopted for different purposes with predefined limitations,” Rueidger said.

There is a lot of potential in the print-on-demand field, and we’ll be keeping an eye on future developments as research in this area progresses.

Schindler said, “Just imagine supermarkets printing their own smart tags or public transport providers customizing multifunctional tickets on demand. ‘Wearables’ that explicitly require flexible electronics may also benefit. The costs for such a printer, after optimization of the process steps, could drop to within the range of current inkjet printers.”

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[Sources/Images: Phys.org, Applied Physics Letters]