Improvements in Gridless Ion Source Performance

Ion Assisted Deposition (IAD) has become well established as a means of improving optical coating performance and reducing cost. Some performance improvements include durability, stability with humidity and temperature, and better stoichiometry. Cost reductions result from low temperature deposition and increased rates for reactive processes. The rate at which a deposition can occur with IAD is limited by the ion beam power; twice the power would allow twice the rate. Beam power is the electron volts (eV) of the ions times the current of the beam. For most materials there is an upper limit on the eV beyond which the coating properties are degraded due to ion “damage.” Investigations of commercially available End-Hall and Cold Cathode sources have shown that the former has a lower than desired eV capability and the latter tends to be too high. Both sources have exhibited operational stability problems due to source component changes with use and power supply instabilities. The eV characteristics of both are affected by gas flow and chamber pressure. The excess pressures needed for voltage control and high power operation can degrade the performance of the deposited films. The concepts and techniques used to overcome these limitations and increase the power and stability by several times are discussed.