Fabrication technology of ruthenium microelectrode pattern for RF MEMS

Ruthenium (Ru), which is one of noble metals, has been intensively considered as an electrode material of DRAM capacitors, since Ru has high oxidation resistance and high electrical conductivity even in the oxidized state. Furthermore, Ru is also expected to be used as an electrode material of acoustic devices, e.g. FBAR, because of its superior acoustic properties such as high Young's modulus and density. For such applications, a precise etching technique for Ru microelectrode patterns is required. Ru etching proceeds by producing volatile oxidized Ru (RuO₄), and is possible using plasma with O₂ or O₂ plus halogenous gas, e.g. Cl₂ and CF₄. However, the plasma etching also attacks resist and/or an underlayer of Si, SiN etc. In this study, a wet etching process with cerium (IV) ammonium nitrate (CAN)-based etchant and a dry etching process with O ₃ gas were investigated as low-damage Ru etching processes. O ₃ gas etching is better for patterning Ru microelectrodes than CAN-based wet etching. For O₃ gas etching, diluted HF pretreatment is effective to reduce side etching and to improve etching uniformity.