The Effect of Oxygen on the Work Function of Tungsten Gate Electrodes in MOS Devices

The effect of oxygen on the work function of tungsten electrodes has been studied. As expected, it was found that the presence of the metastable A15 $(\beta)$ phase is correlated with oxygen concentration. Tungsten films with minimal oxygen content contain only the stable bcc $(\alpha)$ phase. However, after a forming gas anneal was performed on MOS capacitors made from these films, it was found that the $\beta$ phase converts completely to a strained $\alpha$ phase. Despite this uniform transformation, it was found that the work function of the tungsten gate electrodes varies from 4.54–4.91 eV, depending on the partial pressure of oxygen that the tungsten films were grown in. This is a novel work function tuning technique, and it is hypothesized that the observed variation is due solely to the incorporation of oxygen from the growth environment into the W layer at the $ \hbox{SiO}_{2}/\hbox{W}$ interface.