Postdeposition relaxation of internal stress in sputter-grown thin films caused by ion bombardment

Abstract

A hitherto unreported postdeposition method of relaxing ultrahigh internal stresses prevailing in sputter-grown thin films is the subject of the present research. A significant reduction of stress has been confirmed by x-ray diffraction and independent substrate deflection measurements for HfN films treated with Si+ ions of various energies (450 keV, 500 keV, and 1.1 MeV). The particular sequence of the performed experiments has allowed us to deduce the most likely scenario of the resulting stress relaxation. We argue that the observed reduction of internal stress had been caused by structural changes, namely the transport of interstitial defects occurring within the thermal spikes induced by ion bombardment and an increase in vacancy concentration. Simple theoretical considerations proved that the existence of an amorphous silicon interlayer formed right under the HfN film during the bombardment cannot be the cause of the observed stress relaxation. The employed Auger electron spectroscopy, transmission electron microscopy, and x-ray diffraction technique confirmed that the ion bombardment did not seriously affect either the composition, dislocation structure, or texture of nitride films. The relative softening of HfN after ion bombardment was found to be caused by amorphization of the substrate directly under the film. The penetration of incident particles deeper than predicted by a numerical simulation was attributed to a distinct crystallographic texture determined experimentally for HfN films. It was confirmed that modification with Si+ ions does not affect the resistivity of HfN films.