Effects of CF 4 Addition on the Growth of Amorphous CN x Films by Plasma Decomposition of CH 4 -N 2 Gas Mixtures
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ABSTRACT Hydrogenated amorphous carbon nitride (a-CN,:H) films are deposited at 50-300'C by radiofrequency plasma decomposition of CH4 -CF4 -N 2 gas mixtures. Effects of fluorine radicals on the deposition rate, composition, and on chemical bonding are investigated. When CF 4 gas is added as partial replacement of CH 4 gas, the deposition rate at 200'C steeply increases with increasing flow rate ratio RCF = [CF 4 ] / ([CH 4 ]+[CF 4 ]) up to -0.3, and then it decreases gradually. It is found that the nitrogen content, x, increases from 0.22 to 0.46 when RCF increases from 0 to 0.75. The inclusion of fluorine atoms into the films is less than 5 at%. The infrared absorption spectra shows strong enhancement of the absorption bands related to various forms of C-N bonding by the CF 4 addition. It is found that the addition of CF 4 enhances the formation of C-N bonds in PECVD a-CNx:H films. INTRODUCTION After the theoretical prediction by Liu and Cohen", 2, carbon nitride have continuously attracted a great deal of attention as a novel candidate for superhard materials. The possible application of carbon nitride is now not limited to hard coatings but extended to field electron emitters 3, photosensitive devices 4 and optical wave guides5 . In order to synthesize carbon nitride with high quality, a number of techniques, such as ion implantation 6,7 , reactive ion beam deposition 8,9 , plasma-en15 16 5 4 3 hanced chemical vapor deposition (PECVD) ,4,1-13, reactive sputtering ,1, and laser ablation , have been attempted by many researchers. Nevertheless, the synthesis of stoichiometric carbon nitride, C3N4 , is still one of the most significant subject in this field. The PECVD method is widely used to deposit hydrogenated amorphous carbon nitride (aCN,:H) films. However, when the a-CN,:H film is deposited using CH 4-N 2 plasma, nitrogen atoms appears hard to incorporate into the film, in comparison with those deposited with reactive sputtering or laser ablation using a graphite target and N 2 gas. In those cases, hydrogen radicals in the plasma are considered to hinder the growth of a-CNx:H films due to their high etching activity to nitrogen atom. In order to reduce the concentration of such hydrogen radicals effectively, we propose supply of fluorine radicals into the CH4 -N2 plasma. When fluorine radicals exist together with hydrogen radicals, the equilibrium concentration of hydrogen radicals should be reduced through the formation of stable HF molecules. In this article, we focus on the growth of a-CN,:H films by PECVD using CH4 -N 2 gas mixtures. It is investigated that effects of the addition of CF 4 gas as a source of fluorine radicals. We demonstrate that the addition of CF 4 enhances the growth and nitrogen incorporation of PECVD a-CNx:H films. EXPERIMENT Sample films were deposited by radio-frequency (rf) PECVD using CH 4 -CF 4 -N2 gas mixtures. The rf power of 10 W was inductively coupled to the feed gas. The reactor tube, which was made 505
Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
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