Increased boron content for wider process tolerance in perpendicular MTJs

Abstract

Perpendicular CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) prepared from (Co25Fe75)1−xBx alloys are found to have better annealing stability when made with 30at% boron relative to a more typical 20at% boron. A comparison of film-level properties shows that perpendicular magnetic anisotropy (PMA) increases significantly for 30at%B, while the range of electrode thicknesses that maintain a perpendicular easy axis also increases. Because capping layer interdiffusion has been previously suggested to play a role in the breakdown of PMA with annealing temperature, we have isolated its effect by studying the annealing process of thin Ta/CoFeB(2nm)/Ta trilayers. Through analysis of the decrease in Curie temperature during annealing, we can infer that higher boron content indeed suppresses growth of the intermixed CoFeB-Ta dead layer. For device structures and processing conditions where interdiffusion is a limiting factor, increasing boron content is shown to result in substantially improved tunneling magnetoresistance (TMR).