Integration of Sputtered Molybdenum Disulfide Layers into a Wafer Thin Film Technology for Memristive Devices

In the last decade, various classes of memristive materials have demonstrated promising properties for emerging information processing technologies such as neuromorphic computing. Among the innovative materials for memristive devices, the class of transition metal dichalcogenides (TMDCs) has attracted much attention, due to their excellent physical properties and scaling capability, which allows scaling to atomically thin layers with defined electronic structure. However, many of the physically relevant properties have so far only been demonstrated on single devices made from TMDC flakes produced by exfoliation which commonly suffer low reproducibility with the fabricated flakes/sheets being rather sensitive against ambient influences like humidity and light. Thus, to exploit their full potential in applications, reliable thin-film processes at wafer-level are requested to tailor the materials precisely for the desired application and integrate them into functional devices and systems. It is therefore beneficial to introduce homogeneous layers on a large scale, enabling to encapsulate targeted areas from the ambience and build vertical devices of controllable size fingerprints.<br/>This contribution therefore presents a technology platform that allows TMDC materials to be integrated into a thin-film process. In detail, a wafer-level 4-inch thin-film technology for fully encapsulated molybdenum disulfide (MoS₂) memristive devices is presented, allowing for a systematic statistical analysis of electrical properties. The developed method, paired with material analyses, further enables us to compare the different material configurations and draw conclusions about the physical switching mechanism and suitability for integration into circuits. MoS₂ layers of 10 nm thickness were deposited in a tri-layer process, sandwiching the material between two 30 nm electrodes using magnetron sputtering. Devices were structured using an optical lithography lift-off process, comprising more than 32,000 devices in six different sizes, areas ranging from 10x10 to 50x50 square micrometers. The sputtered MoS₂ was found to have an amorphous structure. Devices were encapsulated using high quality chemical vapor deposited silicon dioxide (CVD-SiO₂). To understand and tailor the switching mechanism of the memristive devices, the electrode material (active and non-active) was varied and the influence of the encapsulation of the MoS₂ were investigated in detail.<br/>Experimenting with different electrode materials, Cu and Ag showed a strong interaction between electrode and MoS₂, deploying transmission electron microscopy (TEM). Even though these combinations tend to exhibit switching behavior, they have an undesirably high variability due to the material interactions.<br/>Furthermore, comparing the current-voltage characteristics measured on both encapsulated and non-encapsulated devices revealed that in the encapsulated devices, no resistive switching could be observed. In contrast, I-V hysteresis could be found in non-encapsulated devices. To realize the objective of encapsulated, reliable, and reproducible devices, technology was then adapted from tri-layer to bilayer sputtering, introducing a defined oxidation step for the MoS₂ layer before the deposition of top electrode and encapsulation.<br/>The therewith resulting devices were systematically evaluated, studying their current-voltage characteristics, number of resistive states, retention time, on/off ratio, and switching energy. Finally, a model of the switching is proposed, explaining the cause of memristive characteristics in sputtered MoS₂ and their link to external influences.<br/><br/>Funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 434434223 – SFB 1461 and the Carl-Zeiss Foundation via the Project MemWerk as well as support by the Center of Micro- and Nanotechnology (ZMN), a DFG-funded core facility at TU Ilmenau, is gratefully acknowledged.