Angular dependences of Si3N4 etch rates and SiO2-to-Si3N4 etch selectivity in C4F6/Ar/O2/CH2F2 plasmas

Summary form only given. Si₃N₄ films are usually used as a protecting layer of a poly silicon gate during the etching of a contact hole. An increase in the etch rates of Si₃N₄ at its curved surface lowers the etch selectivity of SiO₂ with respect to Si₃N₄, resulting in device failure. Therefore, it is important to control the angular dependence of Si₃N₄ etch rates and SiO₂-to-Si₃N₄ etch selectivity. In this work, the angular dependences of the Si₃N₄ etch rate and SiO₂-to-Si₃N₄ etch selectivity in a C₄F₆/Ar/O₂/CH₂F₂ plasma were investigated using a specially-designed Faraday cage system. The Faraday cage allowed one to control the ion-incident angles. Etching was conducted by varying the ratio of CH₂F₂ flow rate to study a role of steady-state fluorocarbon films formed on the surface of the Si₃N₄ substrate during etching. In the absence of CH₂F₂, the etch rate of Si₃N₄ decreased with increasing ion-incident angle. When the CH₂F₂ gas was added, the etch rate had a maximum around 30-50° as the ion-incident angle was changed. The normalized etch yield (defined as the etch yield at a specific angle normalized to one obtained on a horizontal surface) of Si₃N₄ showed a maximum at angles between 45° and 70° at all ratios of CH₂F₂ flow rate. As the ratio of CH₂F₂ flow rate was increased, the maximum value of the normalized etch yield was increased. The etch selectivity of SiO₂ with respect to Si₃N₄ increased with ratio of CH₂F₂ flow rate, but it went through a minimum as the ion-inciden- angle was varied. These results were explained by characteristics of the fluorocarbon films (such as F/C ratio, rates of etching and deposition) formed on the Si₃N₄ substrate during the etching process.