A miniature microwave plasma source used for localized surface treatment and processing

Summary form only given. The goal of this project is materials processing on spatially localized areas by applying a small discharge to only the region being processed. A small diameter stream of plasma (less than 2 mm in diameter) is created by focusing microwave energy inside a discharge tube. The discharge then flows out the end of the tube onto the surface being processed delivering ions and reactive radicals. The diameter of the plasma stream from the tube to the material being processed is controlled by an aperture mounted at the end of the tube. The spot size of the localized plasma stream ranges from 2 mm down to 10´s micrometers depending on the aperture size. The discharge is created by using 2.45 GHz microwave energy that is coupled into the discharge using a small re-entrant cavity that has a hollow inner conductor and a small capacitive gap at the end of the cavity. A processing gas mixture is fed through a 2 mm inner diameter (i.d.) quartz tube which is located inside the hollow inner conductor of the cavity. This tube is exposed to a high electric field at the small gap of the cavity thus generating the surface wave plasma. The length of the surface wave discharge in the tube can be extended by increasing the microwave power to the discharge so that the plasma reaches the aperture. At the end of the tube the plasma stream impinges on a substrate that sits on a CAD-guided XYZ stage. A radio frequency (RF) bias is applied between the aperture and the substrate holder to increase the ion energy. The operating pressures range from 0.5 Torr to 10 Torr. The microwave powers utilized ranges from a few Watts to 10´s Watts. Experimental results for etching localized regions on the surface of an ultrananocrystalline diamond (UNCD) thin film with an argon/oxygen discharge will be presented. The surface profile produced on the UNCD after the etching process will also be quantified. This paper also reports on measurements of the plasma density of argon discharg- s operated in the miniature plasma source using a double Langmuir probe placed in the processing area. Additionally, simulations of the plasma excitation using the reentrant cavity will be presented.