수직자화형 STT MRAM의 초고집적 공정을 위한 터널자기저항비 및 수직자기이방성 향상에 관한연구

Abstract

Perpendicular spin-transfer-torque magnetoresistance random-access memory (p-STT-MRAM) as an alternative terra-bit-integration non-volatile memory, has a fast write time of ~10 ns, non-volatile memory operation, and low power consumption. A p-STT-MRAM cell is fabricated with a metal-insulator-semiconductor (MOS) transistor and a perpendicular magnetic tunnel junction (p-MTJ). It shows a digital bit of “0” or “1” depending on the resistance variation of MTJ. MTJ consists of three parts: free layer, pinned layer, and a thin insulator layer. The direction of magnetization of the free layer and pinned layer can be changed individually by the current. When the direction of the magnetization of the layers is the same, the state is called parallel state (Rp) where the resistance is low; for the other state named anti-parallel state (RAP), the resistance is high. The relative resistance change between the two states is defined as the tunneling magnetoresistance (TMR).A higher TMR value means reading operation more accurately. This work aims to improve the TMR ratio. In this dissertation, we used the molybdenum (Mo) seed layer to substitute for Ta in a Ta/CoFeB/MgO structure to achieving high TMR ratio. At high temperature, Ta seed layer could not avoid the Ta diffusion into MgO tunneling barrier and destroyed perpendicular magnetic anisotropy (PMA) characteristics of CoFeB. As a result, TMR ratio dramatically reduced at 400 oC. On the other hand, for the Mo seed films, the annealing temperature and the changing of the thickness have no significant effect on the PMA of the Mo seed samples. Ta is more sensitive for temperature, especially annealing at 400 oC, and Mo is more stable at high temperature. In a summary, we investigated a Mo layer impacted on perpendicular magnetic anisotropy of Ta/CoFeB/MgO structure. Annealing at high temperature, the sample of Ta/Mo/CoFeB/MgO structure steadily achieved a high value of the perpendicular magnetic anisotropy. A bcc (110) texture of Mo layer and bcc (001) orientation of CoFeB was shown to be important relation with improving the perpendicular anisotropy and achieving a high TMR.