Effect of helium ion beam treatment on the etching rate of silicon nitride

We investigated the effect of the helium ion implantation on the etching rate of silicon nitride in hydrofluoric acid. 30 keV helium ions were implanted into a 500-nm-thick silicon nitride film on silicon. Ion fluences from 1015 to 1017 cm−2 were used. Etching was performed in a hydrofluoric acid solution. All samples were investigated with a scanning electron microscope and atomic force microscope. It was found that helium ion implantation can increase the etching rate by a factor of three. This results in the formation of a well in the implanted area after etching. The maximum depth of the well is about 180 nm and is limited by the penetration depth of 30 keV helium ions. Two possible reasons for enhanced etching are suggested: enhancement by ion-induced defects and electrostatic interaction of ions of the etchant with ion-induced space charge of silicon nitride. The recombination of ion-induced defects is also discussed.