Effective work function of TiN films: Profound surface effect and controllable aging process

Titanium nitride (TiN), with its tunable work function, serves as an electrode metal in the scaling fin-type field-effect transistor and plays the key role for low threshold operation. Measuring the effective work function of thin TiN films is desirable for rapid evaluation before device fabrication. In this work, Kelvin probe force microscopy is applied to study the impact of various factors on the surface potential of TiN films with an uncertainty below 30 mV. By scraping and gauging the potential evolvement of TiN in different circumstances, it is revealed that the surface effect is the major obstacle to determine the work function of the as-deposited TiN. For thick films, the potential drops over 530 mV for a fresh TiN surface relative to that of an aged one. For TiN films thinner than 5 nm, the potential changes by 290 mV due to surface oxidation. This enables a quantitative assessment on the effective work function as well as the surface charge density of TiN films.