Optimization of a negative oxygen ion beam*

Negative ion beams are of interest to a wide range of applications and many previous studies have investigated, e.g., the properties of negative hydrogen ion beams with beam diameters on the centimeter scale or larger. Less work has been done with other ion species and smaller beam sizes, in the millimeter range or smaller. However, such beam properties are required in Secondary Ion Mass Spectroscopy (SIMS). In this context, we are developing a negative oxygen ion source capable of long-term (days) steady state operation. The source utilizes inductively coupled plasmas using a novel antenna design. It can operate to produce positive or negative ion beams. A filter magnetic field was implemented at the beam exit for deviating electrons. Studies were done with the goal of optimizing the ion beam profile. Preliminary results show a linear relationship between beam current with RF power, and a non-linear relationship with filter magnetic field strength, chamber pressure, and voltage on the beam extraction grids. Changes in chamber size and wall temperature are also found to affect the beam quality, indicating the significance of surface effects in the creation/destruction of negative oxygen ions. Results will be presented in comparison with a traditional solenoid shape antenna.