Selectivity in atomically precise etching: Thermal atomic layer etching of a CoFeB alloy and its protection by MgO

The central unit in an integrated memory device is a magnetic tunnel junction (MTJ) consisting of two ferromagnetic layers, often using complex alloys such as CoFeB, separated by an insulating barrier such as MgO. Atomic level precision is required to fabricate miniaturized devices, and atomic layer etching (ALE) is one of the most promising methods to do this. However, at the nanoscale, it is imperative to maintain the concentration of each element in an alloy during etching, and it is important to not affect the insulating barrier, i.e. the etching process should stop at MgO. Here we use thermal dry etching of CoFeB alloy thin films with sequential doses of chlorine and 2,4-pentanedione (acetylacetone, acacH). Patterned samples are modified with atomic level precision, and the process is completely selective to the removal of CoFeB alloy in ALE regime without changing the alloy composition and without etching MgO that is used as a protecting layer. The etching process was investigated by comparing the film thickness on a patterned surface before and after ALE process using atomic force microscopy (AFM). The viable key features of the reaction mechanism were proposed by detection of key desorbing fragments during a heating ramp via temperature-programmed desorption (TPD) experiments. Ex-situ X-ray photoelectron spectroscopy (XPS) was performed to characterize the surface during ALE process.