Patterning of Cu, Co, Fe, and Ag for magnetic nanostructures

Wet and dry etching of thin metallic multilayer structures is necessary for the development of sensitive magnetic field sensors and memory devices based on spin–valve giant magnetoresistance elements. While it is well established that Cu, Co, and Fe are soluble in HNO₃ and H₃PO₄ at room temperature, little effort has been made to investigate selective wet and dry etch chemistries. For example, we find Ag is not etched in H₂SO₄, HCl, or H₃PO₄ under conditions where etch rates for the other metals are in the range of 2000–60 000 Å/min. Electron cyclotron resonance (ECR) SF₆/Ar plasmas provide etch selectivities of ⩾5:1 for Ag over Cu, Co, and Fe, while lower selectivities are obtained with CH₄/H₂/Ar. Cl₂-based plasma chemistries leave significant metal–chlorine surface residues, which can be removed in situ by low ion energy H₂ or Ar plasma treatments that eliminate corrosion problems. Cu etch rates in excess of 3000 Å/min at 25 °C can be obtained in ECR Cl₂/Ar discharges because the high ion flux prevents formation of a CuCl_X-rich selvedge layer, which normally only is volatile for etch temperatures ⩾220 °C in conventional reactive ion etch systems. Photoresist masks suffer severe reticulation under ECR conditions, at least for microwave powers >400 W, and SiO₂ or SiN_X thin films offer much better etch resistance.