Influence of Free Layer Surface Roughness on Tunnel Magnetoresistance in 300 mm CMOS-compatible MTJ Stacks

The magnetic tunnel junction (MTJ) is a highly versatile device widely used in magnetoresistive random-access memory (MRAM), magnetic sensors and prospective as a read device in racetrack memory. Tuning the MTJ stack to match the desired properties, such as tunnel magnetoresistance (TMR), magnetic anisotropy or coercive field of the free layer, requires careful optimization of the deposition parameters as well as precise thickness control. Here, the deposition of individual layers in a wedged manner across 300 mm wafers is proposed to engineer the thicknesses within the stack more efficiently. Furthermore, this technique provides detailed insights into effects related to surface roughness, magnetic anisotropy and TMR.

I. Introduction

MAGNETIC TUNNEL JUNCTIONS (MTJs) have applications in magnetoresistive random-access memory (MRAM) [1], as magnetic field sensors, as reading and writing devices in racetrack memory [2] and in neuromorphic computing [3]. MTJ stacks usually consist of a complex system of layered metallic and non-metallic thin films (Fig. 1a). Consequently, the deposition parameters have to be carefully chosen to ensure smooth interfaces. In this study, we investigate the impact of different sputter modes on free layer (FL) surface roughness, and consequently, on magnetic anisotropy and tunnel magnetoresistance (TMR).