A planar microwave discharge device has been developed, where microwave power is radiated uniformly from an antenna of multiple slots, and overdense argon plasmas with uniformities better than 3 % for 30 cm diameter has been obtained [Y. Yasaka, D. Nozaki, K. Koga, M. Ando, T. Yamamoto, N. Goto, N. Ishii, T. Morimoto, Plasma Sources Sci. Technol. 8 (1999) 530.]. In order to apply this device to material processing, an operation mixing nitrogen gas was preformed and density of nitrogen radical was measured by using a mass spectroscopy. The result was that the density of nitrogen radical had better uniformity when the base argon plasma is uniform. Numerical simulation including gas phase reactions also gave better uniformity of nitrogen radical on uniform base plasma. (c) 2005 Published by Elsevier B.V.