# Three‐Dimensional Heterogeneous Bonding for High‐Density and Low‐Noise TMR Sensing Arrays

## TL;DR

A 3D bonding technique doubles TMR sensor junctions without increasing size, reducing noise and enabling high-density magnetic sensing.

## Contribution

A novel 3D heterogeneous bonding method for TMR sensors that doubles junctions and reduces noise without increasing chip area.

## Key findings

- Double-layer TMR devices achieved a void ratio of 0.73% through optimized bonding parameters.
- Angled etching increased magnetoresistance ratio to 172% and reduced magnetic noise to 0.97 nT·Hz−1/2.
- The method enables high-density, low-noise magnetic sensing without altering device dimensions.

## Abstract

Tunneling magnetoresistance (TMR) devices are crucial for high‐density low‐power magnetic sensing owing to the increase in data‐intensive applications. This study presents a three‐dimensional heterogeneous integration bonding technique for TMR sensors. The number of serially connected magnetic tunnel junctions is doubled without enlarging the chip area by vertically bonding two TMR film stacks, and the low‐frequency 1/f noise is effectively suppressed. The sputtering parameters of the Cr/Au bonding layer, argon‐ion activation conditions, and bonding pressure are optimized to achieve a void ratio of only 0.73%. Double‐layer TMR devices are fabricated via backside silicon removal, photolithography, and ion‐beam etching. A 45° etching angle mitigates sidewall redeposition, increases the magnetoresistance ratio from 149% to 172%, and decreases the magnetic noise to 0.97 nT·Hz−1/2 at 1 Hz. The proposed approach enables high integration and low noise without altering the device dimensions and has strong potential for applications in high‐density magnetic‐sensing arrays. The three‐dimensional heterogeneous bonding strategy proposed in this study vertically integrates double‐layer TMR films, doubles the number of tunnel junctions without enlarging the device footprint, and significantly suppresses the 1/f noise through an optimized bonding process. This approach enables high‐density, low‐noise, low parasitic resistance magnetic sensing for next‐generation spintronic devices.

This study demonstrates a three‐dimensional heterogeneous bonding approach to fabricate compact TMR sensing units with double junction numbers and improved magnetic performance. Optimized Au─Au bonding and angled etching improve device integrity, noise characteristics, and magnetoresistance. This method provides a scalable route to dense, low‐noise spintronic sensor arrays suitable for emerging data‐intensive applications.