Plasma carbonization was used to study the effect of carbon incorporation on the work function of metal nitride gates. The work functions were shifted by as much as 200 mV upon the incorporation of 8% carbon. X-ray photoelectron spectroscopy shows that these shifts in the work function values are mainly driven by the formation of metal-carbon and carbon-carbon bonds in the metal gate. In particular, the work function decreased with increasing the metal-carbon bond concentration, reaching as low as 4.28 eV for TaC with 50% carbon. This correlation of bonding with the work function can be used to tune the work function of metal gates. (C) 2008 The Electrochemical Society.