High-Performance Flexible Magnetic Tunnel Junctions for Smart Miniaturized Instruments

TL;DR

This paper presents a low-cost process to create flexible magnetic tunnel junctions (MTJs) on silicon, maintaining high magnetic performance and mechanical reliability, suitable for integration into miniaturized wearable and biomedical devices.

Contribution

A novel batch fabrication method transforms rigid MTJs into flexible, high-performance devices on silicon, enabling their use in compact, wearable applications without performance loss.

Findings

Flexible MTJs retain magnetic properties after bending.

MTJs withstand over 1000 mechanical cycles without fatigue.

Integrated MTJs enable sensitive magnetic detection on biomedical devices.

Abstract

Flexible electronics is an emerging field in many applications ranging from in vivo biomedical devices to wearable smart systems. The capability of conforming to curved surfaces opens the door to add electronic components to miniaturized instruments, where size and weight are critical parameters. Given their prevalence on the sensors market, flexible magnetic sensors play a major role in this progress. For many high-performance applications, magnetic tunnel junctions (MTJs) have become the first choice, due to their high sensitivity, low power consumption etc. MTJs are also promising candidates for non-volatile next-generation data storage media and, hence, could become central components of wearable electronic devices. In this work, a generic low-cost regenerative batch fabrication process is utilized to transform rigid MTJs on a 500 {\mu}m silicon wafer substrate into 5 {\mu}m thin, mechanically flexible silicon devices, and ensuring optimal utilization of the whole substrate. This method maintains the outstanding magnetic properties, which are only obtained by deposition of the MTJ on smooth high-quality silicon wafers. The flexible MTJs are highly reliable and resistive to mechanical stress. Bending of the MTJ stacks with a diameter as small as 500 {\mu}m is possible without compromising their performance and an endurance of over 1000 cycles without fatigue has been demonstrated. The flexible MTJs were mounted onto the tip of a cardiac catheter with 2 mm in diameter without compromising their performance. This enables the detection of magnetic fields and the angle which they are applied at with a high sensitivity of 4.93 %/Oe and a low power consumption of 0.15 {\mu}W, while adding only 8 {\mu}g and 15 {\mu}m to the weight and diameter of the catheter, respectively.