Three-dimensional integration technology of magnetic tunnel junctions for magnetoresistive random access memory application

Abstract

Three-dimensional integration processes (based on direct wafer bonding and back-surface silicon removal) for magnetic tunnel junctions with perpendicular magnetization (p-MTJs) were developed. Perfect wafer bonding, namely, bonding without interfacial voids, and damageless silicon removal were successfully demonstrated by using very flat tantalum cap layers. Moreover, p-MTJ nanopillars subjected to these processes exhibited no degradation in magnetoresistance or spin-transfer-torque (STT) switching. Magnetoresistive random access memory (MRAM) technology incorporating these processes (direct wafer bonding and back-surface silicon removal) will make it possible to integrate epitaxial MTJs (with a single-crystal tunnel barrier) and ferromagnetic electrode layers (based on new materials).

Surface activated room-temperature bonding in Ar gas ambient for MEMS encapsulation

“…[1][2][3] SAB has been successfully applied to various semiconductor wafers, [4][5][6][7][8] oxide wafers, 9,10) and thin metal films on them. 11,12) In addition, in the case of wafers without serious bowing and with atomically smooth surfaces, SAB is achieved spontaneously by the attractive force between the surfaces of two wafers without external applied force. 13,14) Room-temperature bonding is quite advantageous in MEMS applications.…”