Paper PS-ThM2
Radical Probe System for In-Situ Measurements of Hydrogen, Oxygen and Nitrogen Radical Densities

Thursday, October 24, 2019, 8:20 am, Room B131

The current state-of-the-art methods to identify presence of radical species in vacuum chambers are optical methods, which suffer from the lack of spatial resolution and require expensive optical equipment. Center for Plasma Material Interactions (CPMI) at the University of Illinois developed a catalytic radical probe array to measure concentrations of reactive species in low temperature plasma with high spatial resolution. Radical probes as plasma diagnostic tool can be used to determine radical densities of hydrogen, nitrogen and oxygen in any continuous plasma source in vacuum environment. The basic principle and advantage of a probe array is the capability to distinguish between different gas species due to several sensitive elements acting as recombination catalysts [1]. The catalytic coatings cover an area of several square millimeters on the tip of a sheathed thermocouple. The catalytic probe surface provides efficient recombination of active species with subsequent energy release as a heat. All the probes are exposed to the same background plasma heating/cooling mechanisms, but the temperatures are not the same due to the fact that different catalytic materials have different recombination coefficients, therefore a temperature difference between probes is generated. The system consists of two additional probes, first to obtain the overall heat flux on probe array, and the second is a reference probe with surface chemically active to all gases.

Lifetime of radical probes, specifically catalytic surface degradation, depends highly on vacuum conditions, chamber contamination and the fact that radical species cause surface properties to change. Lifetime of radical probe system is usually three hours. However, probe surface cleaning has been achieved by applying a bias which causes contamination layer on the probe tip to sputter via ion bombardment. Argon gas is used to sputter clean probe tip. Additionally, if probe shows signs of high contamination then a long sputtering process is used to remove all the catalytic material from the probe tip and then magnetron sputtering is used to redeposit new catalytic coating. The array of several probes is capable to distinguish between different gas species with sub centimeter spatial resolution. The probes give accurate results in a broad range of reactive species concentrations from about 10¹² to 10¹⁴ cm^-3.

Reference:

[1] M Mozetic, M Kveder, M Drobnic, A Paulin, and A Zalar. Determination of atomic

hydrogen density with catalytic probes. Vacuum, 45(10-11):1095-1097, 1994.