Etching characteristics of SrBi2Ta2O9 film with Ar/CHF3 plasma

Abstract

Among the ferroelectric thin films that have been widely investigated for ferroelectric random access memory (FRAM) applications, the SrBi2Ta2O9 (SBT) thin film is appropriate as a memory capacitor material due to its excellent fatigue endurance. However, very few studies on the etch properties of SBT thin films have been reported, even though dry etching is an area that demands a great deal of attention in the very-large-scale integration of ferroelectric thin-film capacitors for FRAM applications. In this study, SrBi2Ta2O9 thin films were etched using a magnetically enhanced inductively coupled Ar/CHF3 plasma. Etch properties, such as etch rate, selectivity, and profile, were measured for different gas mixing ratios of CHF3/(Ar+CHF3), while the other process conditions were fixed at rf power of 600 W, dc-bias voltage of −150 V, and chamber pressure of 5 mTorr. The maximum etch rate of SBT thin films was 1650 Å/min under CHF3/(Ar+CHF3) of 0.1. Selectivities of SBT to Pt and photoresist masks were 1.35 and 0.94, respectively. The chemical reaction and compositional change of the etched surfaces were investigated by x-ray photoelectron spectroscopy analysis. The Sr and Ta atoms of SBT films react with fluorine; Sr–F and Ta–F are then removed by physical sputtering by Ar ions. The surface of a SBT film etched with CHF3/(Ar+CHF3)=0.1 was analyzed using secondary ion mass spectrometry. Scanning electron microscopy was used to examine etched profiles of SBT thin films. The wall angle of a SBT film etched using CHF3/(Ar+CHF3)=0.1 was approximately 85°.