Paper NM-WeM10
Fabrication of Large-area Magnetic Ring Arrays using Ion Beam Proximity Lithography

Wednesday, October 17, 2007, 11:00 am, Room 615

Arrays of 500nm/300nm outer/inner diameter permalloy rings on a 700nm pitch were fabricated to study transitions between the micromagnetic configurations within these structures. The existence of these states, which are both very stable and confine the magnetic flux within the material, is the basis for next-generation, high-density magnetic memory devices. To form these rings, oxidized silicon substrates were first coated with a 5nm thick Ta layer, a 10nm permalloy (Ni₈₀Fe₂₀) layer deposited in a magnetic field to induce anisotropy in-plane, and a 5nm thick Ta capping layer. Circular openings were defined in a 200nm thick resist layer, deposited on the magnetic stack, using ion beam proximity lithography (IBP) and a conformal coating of SiO₂ was applied to the surface by reactive sputter deposition. A directional reactive ion etch removed the oxide coating at the base of the opening and on top of the resist but preserved the oxide wall coating. After removing the resist in an oxygen ashing step, a ring-shaped hard mask remained that was transferred into the underlying permalloy using argon ion milling. Our fabrication approach has two distinct advantages over direct-write processes (i.e., electron beam): the patterns can be formed with much higher throughput using the parallel IBP approach (the entire mask pattern was replicated with a single exposure lasting less than 20 sec.) and the pattern of dots is significantly easier to control than the more complex ring structure. Hysteresis loops were measured using a vibrating sample magnetometer and show evidence of the switching sequence observed in micromagnetic simulations of these structures using the OOMMF code. However, the size and shape variation in the patterning process mask used for printing the dots must first be reduced to better understand the switching behavior. The results do suggest that the field required for onion-to-vortex transition and field required for vortex-to-onion transition to be 85Oe and 330Oe, respectively. We are currently reducing the size variation in the rings by fabricating improved IBP masks with more uniform openings to achieve smaller size distributions, which is expected to substantially reduce the switching field distribution. Currently we have fabricated large area masks that contain patterns with a standard deviation in size of less than 3nm.