Measurements of nitrogen atom density in N2/Ar sputtering plasma for fabrication of high-mobility amorphous In2O3:Sn films

Amorphous In2O3:Sn (a-ITO) has attracted attention because of the advantages such as smooth surface and high etching rate. We have recently succeeded in sputtering deposition of a-ITO films with high mobility 61 cm$^{\mathrm{2}}$/Vs by introducing N2 into the deposition atmosphere. Here, aiming to clarify effects of N of a-ITO film growth, we measure absolute density of N atom in N2/Ar sputtering plasma by using vacuum UV absorption spectroscopy. ITO films were fabricated by RF magnetron sputtering on glass substrates at 150C with Ar-N2 mixed gas. We observed that the morphology is changed from polycrystalline to amorphous by introducing N2 into the deposition atmosphere. Furthermore the mobility of a-ITO films was found to be greatly dependent on N2 flow rate. The electron Hall mobility increases from 48 to 55 cm$^{\mathrm{2}}$/Vs with increasing N2 flow rate ratio from 3 to 5{\%}, where the absolute density of N atom in the plasma increases from 3.78 to 7.44 (10$^{\mathrm{10}}$ cm$^{\mathrm{-3}})$. Since the N composition ratio in ITO films is almost constant for N2 flow rate ratio of 3--5{\%}, the difference in the adsorption/desorption behavior of N atoms on the growth surface brings about the change in the film properties.