Stable encapsulation structures for Permalloy films

This paper presents a study of the effects of annealing on the micromagnetic properties of encapsulated Permalloy films. The stress relaxation during annealing causes observable changes in the micro-magnetic state of the Permalloy. The amount of stress relaxation is highly sensitive to the shape of the Permalloy film and the encapsulation material. After Permalloy films with negative magnetostrictive coefficient are annealed, the easy axis of bar-shape yokes remains in the transversal direction while those of ring-shape yokes and heart-shape yokes rotate so that the easy axis is normal to the film edge. The annealing properties of bar-shape Permalloy films in six different encapsulation structures are compared. Permalloy films encapsulated entirely in hardbaked photoresist are consistently more stable. The uneven external stress is absorbed by the relatively softer hardbaked photoresist buffer layer and is re-distributed evenly to the Permalloy. A better control of the anisotropy of the Permalloy yoke can be obtained by placing a hardbaked photoresist buffer layer between the alumina overcoat and the Permalloy