Flexible MgO-Based Magnetic Tunnel Junctions on Silicon Substrate

Flexible electronic devices are emerging in many areas, providing novel features and creating new applications [1]. Due to their ubiquitous utilization, flexible magnetic sensors [2] play a critical part in this development. In particular, magnetic tunnel junctions (MTJs) are of great interest, because of advantages like low power consumption or high sensitivity. We report the development of flexible MTJs on a silicon substrate fabricated by a low-cost batch process [3]. Thereby, conventionally fabricated MTJ devices are transformed into flexible ones by thinning down the silicon wafer from 500 μm to 5 μm. This process leads to thin, bendable silicon devices, while maintaining their original performance. The fabrication process steps are shown in figure 1a to e. The MTJs were fabricated by standard multilayer magnetic stack deposition with an MgO tunnel barrier of 1.6 nm thickness. After backside etching of the wafer the thickness of the substrate was reduced to 5 mm as shown in the inset of figure 1f. The resulting flexible MTJ films are extremely bendable with a diameter down to 500 mm (figure 1f). The magnetic properties and magneto-transport properties of the flexible MTJs were the same as those of the rigid ones. The TMR ratio was tested under different bending conditions (figure 2a) and no degradation was found as shown in figure 2b. The reliability of the flexible MTJs was evaluated by exposing them to periodic strain cycles. To this end, an MTJ sample was mounted to an elastomeric support, which was repetitively stressed moving it from a flat state to a tensile state and visa-versa. Figure 2c shows that even after 1000 cycles, the TMR ratio showed no relevant differences (figure 2c). The presented method enables fabricating silicon devices without comprising any of their performance characteristics like cost/yield advantage or integration density. Entire devices can be fabricated prior to thinning, benefiting from standard batch processes without introducing any constraint in design, thermal budgets or fabrication methods. Therefore, these flexible silicon-based MTJs are a crucial contribution on the way to integrated state-of-the-art flexible magnetoelectronics. The maturity of silicon fabrication techniques makes the flexible MTJ sensors on silicon semiconductor substrates the natural choice for very large integration of high-performance electronic applications on flexible substrates.