Optical emission study of ion composition in an inductively coupled oxygen plasma

The success of ion implantation to precisely modify substrate properties requires control of the incident ion energies to achieve the desired depth of the implanted ions. Oxygen plasmas generally contain both O$^+$ and O$_2^+$ ions, and in plasma immersion ion implantation (PIII) of oxygen, the two will produce different concentration depth profiles due to their different energy/mass ratios. Predicting the overall profile thus requires knowledge of the relative fluxes of the two ion species. Here we combine experiment and modeling to investigate the feasibility of using non-invasive optical emission spectroscopy (OES) to monitor O$^+$ and O$_2^+$ abundances in an oxygen inductively-coupled plasma. Measurements of multiple O, O$_2$, O$^+$, and O$_2^+$ emission line intensities were made as a function of pressure (1-30~mTorr) and power (500-2000~W). While the O$_2^+$ emissions were relatively intense, the O$^+$ emissions were very weak for all conditions examined. Emissions from both ion species were highest at low pressures and at the highest power levels, but the O$^+$ / O$_2^+$ emission ratio varied little with plasma conditions.