Tunnel Magnetoresistance with Plasma Oxidation Time in Doubly Oxidized Barrier Process

Magnetic tunnel junctions (MTJ) with the tunnel barrier oxidized in two steps with plasma were fabricated to obtain a structurally uniform AlO _x insulator. Plasma oxidation of 10 Å-thick AI layer for 5-20 sec formed the initial oxide barrier on top of which a second oxide layer was deposited by oxidizing 13 Å-thick Al for 120 sec. The doubly oxidized junctions exhibited a magnetoresistance (MR) ratio of 27-31 % depending on the oxidation period of the initial oxide layer while only 24 % was obtained for the junction deposited in one-step oxidation with a similar stack configuration. The junction resistance of the MTJ increased monotonically with oxidation time while minimal deterioration of the MR ratio was observed when oxidation time increased from 5 sec to 20 sec. Transmission electron microscopy of the junctions also confirmed that the AlO_x thickness was thinner for the doubly oxidized junctions compared to the singly oxidized MTJ. X-ray photoelectron spectroscopy of the double junction also strongly suggested that the initial oxide layer prevents over-oxidation of the bottom electrode. Our results suggest that the AlO_x oxidized in two steps produces improved junction performance as well as a wider processing window.