In this work, we report on the atomic layer deposition (ALD) of HfNx thin films by employing CpHf(NMe2)(3) as the Hf(IV) precursor and Ar-H-2 plasma in combination with external RF substrate biasing as the co-reactant. Following up on our previous results based on an H-2 plasma and external RF substrate biasing, here we address the effect of ions with a larger mass and higher energy impinging on HfNx film surface during growth. We show that an increase in the average ion energy up to 304 eV leads to a very low electrical resistivity of 4.1 x 10(-4) omega cm. This resistivity value is achieved for films as thin as 35 nm, and it is an order of magnitude lower than the resistivity reported in literature for HfNx films grown by either CVD or ALD, while being comparable to the resistivity of PVD-grown HfNx films. From the extensive thin film characterization, we conclude that the impinging ions during the film growth lead to the very low electrical resistivity of HfNx films by suppressing the oxygen incorporation and in-grain nano-porosity in the films.