Process-induced damage and its recovery for a CoFeB–MgO magnetic tunnel junction with perpendicular magnetic easy axis

Abstract

We investigate the effect of process-induced damage (PID) caused by reactive ion etching using methanol (Me–OH) gas on the magnetic properties of the CoFeB free layer in a magnetic tunnel junction with a perpendicular easy axis (p-MTJ), and on the tunnel magnetoresistance (TMR) ratio of CoFeB–MgO p-MTJs. The dot pattern of the MTJ stack with size varied from 65 to 430 nm etched by the Me–OH plasma showed a smaller coercivity (Hc) than that fabricated by Ar ion milling. The increase in Hc was observed in the dot pattern of large size (430 nm) upon increasing He/H2 plasma treatment temperature after the Me–OH etching. A possible origin of the increase in Hc is the increase in nucleation field after He/H2 treatment. This suggests that Hc of the large dot pattern has the potential to be an index for detecting PID during the MTJ fabrication process. The TMR ratio of CoFeB–MgO p-MTJ deteriorated after the Me–OH plasma etching. This PID was considered to be due to oxidation from the pattern edge of the CoFeB free layer of the MTJ. The recovery process by the He/H2 plasma treatment was examined just after the Me–OH etching to reduce the oxidized part. The median TMR ratio of 102%, which is 5% higher than that of the sample without the He/H2 treatment, was observed after applying this reductive treatment at 180 °C. In addition, the recovery process had scalability with MTJ size, as the effect was observed more clearly in the MTJ smaller than 97 nm.