Oxygen Radical Density Measurement in O 2 –N 2 Gas Mixture Plasma by Means of a Thin Platinum Wire NOBUYUKI HARAKI, SAIENSU NAKANO, SHIGERU ONO, and SHINRIKI TEII Musashi Institute of Technology, Japan SUMMARY The oxygen radical density was measured in a weakly ionized plasma in an O 2 –N 2 gas mixture by using a simple platinum thin-wire sensor. The increased temperatures of the platinum wire caused by the energy released in the recombination of two oxygen radicals on the platinum surface were measured by varying the nitrogen gas mixture ratio and were compared with the oxygen radical densities theoretically calculated under the same plasma conditions. The relation between the wire temperature and the oxygen radical densities was cross-checked by quantitative meas- urement of oxygen radical densities with a quadrupole mass spectrometer. All of these results made it possible to deter- mine the oxygen radical density experimentally from the platinum wire temperature alone. The influence of the ambient gas temperature on the radical density measure- ment is also discussed. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(4): 14–20, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20018 Key words: platinum; oxygen radical; recombina- tion; plasma; catalyst. 1. Introduction Many researchers have made efforts to measure the radical density in weakly ionized plasmas because of the important role of radicals in various plasma applications. However, the conventional optical technique and the con- ventional mass analysis technique for radical density meas- urement require large-size instruments with high equipment cost. Oxygen plasmas have been used for the formation of oxide films, surface modification, and the like, and in many cases the oxygen radicals play important roles. We discov- ered red heating of a thin platinum wire immersed in a plasma that included oxygen radicals, which is attributed to oxygen radical recombination on the platinum surface. Thus, the platinum wire temperature depends on the oxygen radical density, and the oxygen radical density should be determinable from the platinum wire temperature. The purpose of this research is the development of a simple oxygen radical measurement method using this heating of a thin platinum wire in an oxygen-containing plasma. We used a low-pressure glow discharge plasma in the experiment. The oxygen radical density was determined from the temperature rise of a platinum wire exposed to the plasma, and was also compared with oxygen radical density measurements using a quadrupole mass spectrometer. This paper describes the oxygen radical density measurement method using a thin platinum wire and also describes the effect of the gas temperature on this measurement method in order to adapt it to a wide radical density range. 2. Experimental Apparatus and Method 2.1 Experimental apparatus Figure 1 shows a schematic diagram of the experi- mental apparatus. The entire apparatus consists of a dis- charge tube which generates the plasma, a quadrupole mass spectrometer used to analyze the constituent species of the plasma, a spectroscopic measurement system which ana- lyzes light emissions from the plasma, and an oxygen radical sensor using a platinum wire. The plasma was generated by a DC low-pressure discharge. A cylindrical Pyrex discharge tube with an inner diameter of 24 mm was used in the experiment. The anode and cathode of the discharge tube were stainless-steel cy- lindrical electrodes 500 mm apart. An oxygen–nitrogen gas mixture adjusted to suitable proportions was introduced from above the discharge tube and evacuation was per- formed from the lower part of the discharge tube. Two ports © 2004 Wiley Periodicals, Inc. Electrical Engineering in Japan, Vol. 149, No. 4, 2004 Translated from Denki Gakkai Ronbunshi, Vol. 123-A, No. 11, November 2003, pp. 1075–1080 14