Surface flattening processes of metal layer and their effect on transport properties of magnetic tunnel junctions with Al–N barrier

Abstract

In order to form ultrathin insulating layer in magnetic tunnel junctions (MTJs), two surface flatting processes of metal films are investigated. Oxygen-additive sputter-deposition process was applied to the bottom Cu electrode and the Al layer to be nitrided. Dry-etching process was applied for the surface treatment of lower Co–Fe layer. As a result, the surface roughness of stacked ultrathin Al layer to be nitrided is reduced from 3.2Å to 1.7Å, and the tunnel magnetoresistance (TMR) ratio of the MTJs increases from 1% to 26% while maintaining resistance-area product (R×A) less than 5×102Ωμm2 in the Co–Fe∕Al(6A)–N∕Co–Fe MTJs. We conclude that the decrease of the surface roughness of Al layer is one of the key factors to realize high performance MTJs with low R×A and high TMR ratio.