Nano Magnetic tunneling Junction (MTJ) sensor for on-chip current sensing

Hardware Trojans (HTs) pose a significant threat to the security and integrity of electronic devices across various applications. By covertly modifying or inserting malicious circuitry during design or manufacturing, these Trojans can compromise device functionality, leak sensitive information, or create exploitable vulnerabilities. Detecting and mitigating HTs is essential for ensuring the reliability and confidentiality of electronic systems.

Conventional built-in current sensors (BICS), which utilize invasive components such as shunt resistors or power MOSFETs and are commonly employed for current and power monitoring, can be used to detect HT-induced anomalies. However, in circuits with currents ranging from several milliamperes (mA) to hundreds of mA and operating at mid to high frequencies, these sensors face significant limitations. They may exhibit restricted bandwidth, cause high power dissipation, and alter the overall characteristics of the circuit under test, potentially masking the presence of Trojans or introducing unintended side effects.

In contrast, non-invasive techniques utilizing Tunneling Magnetoresistance (TMR) sensors offer promising alternatives. TMR sensors can detect the Oersted field induced by power line currents without direct electrical contact, providing high sensitivity within a compact form factor. Additionally, scaling TMR sensors’ dimensions to the nanoscale increases the potential to distribute sensors across several critical signal nodes in ICs, enhancing resolution and accurately capturing hidden HTs. However, applying TMR sensors to IC power line current sensing still presents some key challenges, including limited sensing range, hysteresis response, and deviations caused by Joule heating.

This thesis aims to develop compact nano TMR sensors based on Magnetic Tunnel Junctions (MTJs) for IC power line current sensing. At the nanoscale, MTJ devices suffer from weakened magnetic pinning, leading to undesirable hysteresis behavior. We introduce a step-bottom-pinned MTJ design that preserves pinning strength by halting etching above the IrMn layer, achieving a hysteresis-free linear response over a large dynamic range for nanoscale TMR sensors. Additionally, we demonstrate the first integration of a fully nanoscale TMR sensor within a power line current sensing scheme, delivering a ±20 mA sensing range and hysteresis-free linearity that outperforms previously reported micron-sized devices. To address signal deviations caused by current-induced Joule heating, we developed a full Wheatstone bridge TMR sensor featuring a monolithic fabrication process and tilted nano-MTJs with synthetic antiferromagnetic (SAF) top electrodes. By utilizing exchange coupling and spin-flop transitions, the device achieves a hysteresis-free linear magnetoresistance (MR) response while effectively compensating for thermal fluctuations.

By developing nanoscale MTJ sensors and leveraging their non-invasive nature, this research provides a high-performance solution for on-chip current monitoring, enabling effective detection and mitigation of hardware Trojans in electronic devices.