Low electrical resistivity in thin and ultrathin copper layers grown by high power impulse magnetron sputtering

Abstract

Ultrathin copper (Cu) layers are in continuous demand in several areas, such as within microelectronics and space, as well as in instrumentation technology requiring an electrical resistivity as low as possible. However, the performance of modern copper connections is limited by the size-dependent value of the film resistivity, which is known to increase when the layer thickness is reduced to a few tens of nanometer. In this work, the authors have successfully deposited Cu thin films from 20 to 800 nm exhibiting reduced electrical resistivity by using a high power impulse magnetron sputtering (HiPIMS) process. The electrical and microstructural properties of such films were compared to samples deposited by conventional direct current magnetron sputtering (DCMS) within the same thickness range. For films as thin as 30 nm, the electrical resistivity was reduced by ∼30% when deposited by HiPIMS compared to DCMS, being only three times larger than the copper bulk value. The HiPIMS Cu films exhibit larger grain size and reduced grain boundary density, which reduce the scattering of charge carriers and thereby the resistivity of the thin film. These larger grains are likely due to the highly ionized precursor flux of the HiPIMS discharge, which in the present work is controlled by an external substrate bias.