Enhanced Reliability of Top-pinned Perpendicular Magnetic Tunnel Junction by Post-oxidation of Sputtered MgO Barrier
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Enhanced Reliability of Top-pinned Perpendicular Magnetic Tunnel Junction by Post-oxidation of Sputtered MgO Barrier Chikako Yoshida 1, Hideyuki Noshiro 1, Yuichi Yamazaki 1, and Toshihiro Sugii 1 1
Fujitsu Limited, 10-1 Morinosato-Wakamiya, Atsugi, 243-0197, Japan
ABSTRACT We proposed an MgO barrier which is fabricated by combination of rf-sputter deposition of MgO film and subsequent in-situ post oxidation (PO). We found that the perpendicular magnetic anisotropy (PMA) of the CoFeB layer formed on this MgO barrier with PO was improved. We also found that a short error rate reduced drastically and a magnetoresistance (MR) ratio increased about 20% for the magnetic tunnel junction (MTJ) with this MgO barrier with PO. In addition, we showed that this MgO barrier with PO has long endurance life compared with conventional sputtered MgO barriers, and has a potential to operate over 1016 write cycles. Furthermore, we have observed that the PO could suppress the Fe diffusion into the MgO barrier and form Fe-O bonding at MgO/CoFeB interface using electron energy-loss spectroscopy (EELS). The obtained results might be involved to the improvement of PMA and MTJ characteristics. INTRODUCTION CoFeB/MgO based magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) are promising candidates for high density spin-transfer torque (STT) MRAM owing to their high tunneling magnetoresistance (TMR) and large interfacial PMA. As the MTJ shrinks in size, the MgO barrier must be thin enough to keep the junction resistance reasonably small. This may cause reliability problems such as electric shorts and barrier breakdowns [1]. Typically, the MgO film is deposited by rf-sputtering, but the sputtered MgO film contains a large amount of oxygen vacancy defects (assigned as F center) which attributed to the leakage current of MTJs [2]. We consider that in-situ post oxidation (PO) of MgO layer immediately after the sputter deposition can reduce the density of the oxygen defects. In this study, we have investigated the PO effects on the PMA of CoFeB/MgO/CoFeB films, and electric properties and endurance characteristics of top-pinned MTJs with PMA. SAMPLE PREPARATION All stack films studied here were deposited on 300-mm Si/SiN/SiO2 substrates with 2 hours post annealing at 300°C in a high vacuum. Two types of MgO barriers with resistance area product (RA) of ~7 :Pm2 were compared; one is MgO with PO, prepared by sputtering a 0.86nm-thick MgO layer followed by an in-situ 5s natural oxidation, and the other is MgO without PO, which consists of a 0.93-nm-thick sputtered MgO film.
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We fabricated a top-pinned MTJ characterized by the following stacked structure: bottom electrode/CoPt (6)/Ru (3)/Ta (1)/CoFeB (0.8)/ MgO barrier/CoFeB (1.7)/Ta (0.4)/ CoPt (6)/Ru (1)/CoPt (10)/top electrode (thickness i
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