Enhancement of reactive species density in nitrogen plasma by mixture of helium and nitridation experiment for silicon

Abstract

In order to increase the electron plasma density and/or temperature in electron cyclotron resonance (ECR) nitrogen plasma, electron acceleration has to be enhanced by reducing the electron collision frequency. One method is to add gas species with a lower electron collision cross-section, compared with the case of nitrogen gas. For this purpose, the helium gas was added into the ECR nitrogen plasma, where the total gas pressure and the absorption power of microwave were kept the same. The emission intensities of nitrogen molecular ion and excited nitrogen molecular were measured as a function of the ratio of helium pressure to nitrogen gas pressure. The densities of both excited nitrogen molecular and nitrogen molecular ion showed maxima to the ratio of helium gas pressure to nitrogen gas pressure. Hence, the densities of such reactive species can be maximized by the addition of helium gas into nitrogen plasma. Based upon the present results, the nitridation experiment for silicon was conducted by changing the ratio of helium gas pressure. In this irradiation experiment, the substrate was negatively biased and the temperature was kept 900 K. The degree of nitridation well corresponded to the change of density of reactive species.