Dry release for surface micromachining with HF vapor-phase etching

Abstract

A new method for dry etching of silicon dioxide for surface micromachining is presented to obtain very compliant polysilicon microstructures with negligible stiction problem and to greatly simplify the overall releasing procedure as well. By etching the sacrificial silicon dioxide with hydrofluoric acid (HF) vapor instead of conventional aqueous HF solution, the need for subsequent rinsing and an elaborate drying procedure is eliminated. Condensation of water on the etch surface is first identified as the cause that prevented the success of HF vapor release in the past. Use of an anhydrous HF/CH3OH mixture under low pressure solves the problem of water condensation and enables us to take advantage of vapor-phase etching (VPE) for surface micromachining, The mechanism of oxide etching with the HF/CH3OH mixture is explained, and the developed VPE system is described and characterized, Polysilicon cantilevers up to 1200 mu m in length are successfully released with this HF VPE technique, The beams tested are 2 mu m thick with a 2-mu m gap from the substrate, and no antistiction dimples are used, The fabricated structures are observed using both scanning electron microscopy (SEM) and an optical profilometer, The reported VPE technique provides a robust releasing method for polysilicon microstructures and is compatible with integrated circuit (IC) fabrication, even including cluster processors.