Influence of conductor steps in passivation stress in magnetic bubble memory devices

Abstract

Stress from the overlay passivation (sputtered SiO2) significantly degrades bubble propagation margins in ion-implanted bubble devices. The present work investigated the stress field from the passivation by computer simulations (boundary element method). In order to reduce the passivation stress, a buffer layer between the passivation and the conductors was investigated. The effects of two factors in the buffer layer have been discussed: (i) Young’s modulus of the buffer layer and (ii) a planarizing characteristic of the buffer layer. The simulation shows that changing Young’s modulus is not effective in reducing the passivation stress. On the other hand, a degree of planarization has a great relation with the passivation stress, i.e., the passivation stress decreases with increasing the planarization by the buffer layer. Therefore, we introduced the thermal-reflow-type polyimide which exhibits excellent planarization. The devices with this structure were fabricated and showed no influence from SiO2 passivation.