Description
Using optically trapped microparticles, the sheath of a plasma can be probed in a non-invasive way. To investigate the effects of secondary electron emission from the surface of the electrode on the sheath of a capacitively coupled radio frequency plasma, the force on single optically trapped microparticles was measured as each particle traversed the plasma bulk and sheath toward the electrode. Measurements were carried out for electrodes made of stainless steel, aluminum, titanium, magnesia and glass-ceramic, under varying argon gas pressures and rf voltages.
We observe significant changes in the electric field force on the microparticle depending on the electrode material. We attribute the changes in the location of the sheath edge and the forces on the particle to the emission of secondary electrons from the electrode surface, and resulting changes of the particle’s charge.
Langmuir-probe measurements confirm that the electrode material significantly affects plasma density and electron temperature, supporting the findings obtained with the non-invasive microparticle method.