Optimization of a lithographic and ion beam etching process for nanostructuring magnetoresistive thin film stacks

The patterning of multilayer thin-film stacks to create spin valves, with d imensions similar to 100 nm, for magnetic-random-access memories presents n ovel fabrication challenges since the materials commonly used (e.g., Co, Cu , and Ni) do not form volatile compounds, and hence cannot be reactive-ion etched. The consequent necessity of using ion-beam etching (''ion milling'' ) demands a solution to the twin problems of faceting and redepostion of sp uttered material. In addition, antireflection layers are not used during li thography because of the necessity of avoiding high-temperature curing, whi ch would harm the spin valve characteristics. By using a thin SiOx phase-sh ifting layer under the resist, we obtained adequate resist profiles; and by using a 15-nm-thick W hard mask, no measurable redeposition was observed a fter ion milling of cobalt. Improved etch selectivity of W relative to Co i s achieved by using neon as the ion-milling gas rather than argon. A simple model for the enhanced ion-milling selectivity is presented. (C) 2000 Amer ican Vacuum Society. [S0734-211X(00)16906-9].