The effect of gas composition on the properties of silicon oxynitride thin film prepared by low-pressure inductively coupled Ar/N₂ plasma

Silicon nitride (Si₃N₄) has been widely used in semiconductor devices and applications, including the passivation layer in organic light-emitting diode (OLED). However, it becomes more important to adjust the refractive index value of silicon nitride layers. For example, the refractive index difference between the passivation layer and emission layer is one of the problems degrading OLED performance. Recently, silicon oxynitride (SiO_xN_y) thin films possessing different refractive indexes have been fabricated by varying their nitrogen and oxygen content. In this work, we control the refractive index of SiO_xN_y thin film by changing the N₂ and Ar flow ratio in 13.56 MHz radio-frequency inductively coupled plasma (RF-ICP). To study the effects of gas composition in the plasma on the properties of thin films, nitrogen atom (N) and nitrogen molecular ion (N₂⁺) density are investigated using actinometry and simulation via the synthetic spectrum method, respectively. We observed that when the gas ratio defined as [N₂/(N₂+Ar)] increases from 0.1 to 0.9, the density of N₂⁺ increases from 4.76×10¹⁵ m ^{− 3} to 1.71×10¹⁶ m ^{− 3} and the refractive index of the thin film increases from 1.54 to 1.57. The results indicate that N₂⁺ plays the important role in changing the properties of thin film during the plasma process.