Atomic layer etching of molybdenum with O2 and NbCl5

Molybdenum is a material of significant interest in the development of future interconnects for microelectronics. This is due to a slower increase in resistivity with diminishing critical dimension as compared to copper, and the potential for integration to silicon and dielectric layers without diffusion barriers. These future interconnect technologies require accurate patterning and manufacturing processes. With thermal atomic layer etching, isotropic and selective etching is possible with the highest degree of control over the etching process. This paper describes the development of an atomic layer etching process for etching metallic molybdenum using O2 and NbCl5. Thermodynamic calculations reveal that in addition to etching the oxidized surface, NbCl5 reacts with the molybdenum metal in a self-limiting fashion, which enables etching during both of the reactant pulses. The oxidation of the metal surface is diffusion-limited, and the etch per cycle is highly temperature dependent, ranging from 0.2 to 3.9 Å at 250–400 °C. No negative effects on film crystallinity or resistivity were observed, and roughness of partially etched films was only slightly increased. Some chlorine residues were seen on the surface with in vacuo x-ray photoelectron spectroscopy, as well as trace amounts of Nb. No metallic residues could be seen on a fully etched sample.