A Self-Aligned Two-Step Reactive Ion Etching Process for Nanopatterning Magnetic Tunnel Junctions on 300 mm Wafers

We demonstrated a self-aligned two-step reactive ion etching (RIE) process to pattern high density magnetic tunnel junction (MTJ) arrays. We did the RIE for the top electrode (TE) and stop in the middle of the tunnel barrier. A nitride conformal film was coated on the device pillars as a dielectric spacer. The conformal spacer protects the tunnel barrier from shorting by redeposition and provides a mask for the bottom electrode (BE) RIE. We used this process and completed perpendicular MTJ devices with our process flow. We tested the devices by measuring magnetic field switching and spin transfer torque switching. We get tunneling magnetoresistance (TMR) up to 100%, switching current as low as 60 μA at 100 ns, switching current density J_c0 as low as 2.5 × 10⁶ A/cm² and endurance above 10⁹ for devices as small as 50 nm in diameter. The results are compared with devices from a TE RIE only process, and we find minimum damage was made by the BE RIE. We also discuss the size dependence of MTJ parameters such as TMR and free layer coercive field and offset field, which is very related to the RIE process.