Polysilicon Etchback Plasma Process Using HBr, Cl_2, and SF₆ Gas Mixtures for Deen-Trench Isolation

Etchback of polysilicon used for refilling deep trenches isolating bipolar devices has been investigated in a magnetically enhanced reactive ion etcher using HBr, Cl₂, and SF₆ gas combinations. Etch rates, uniformities, and loading effects have been studied as a function of gas mixtures. Polysilicon etchback processes with uniformity better than 2%, polysilicon etch rate larger than 5000 A/min, and loading factor less than 1.3 were obtained with a gas mixture consisting of less than 9 seem SF₆ flow in a predominant HBr and Cl₂ mixture. It was found that the etch roughness and surface profile of the resulting trench polysilicon was a strong function of Cl₂ percentage in the mixture. The controllable trench polysilicon etchback profile with smooth surface and curvature was obtained by using 60 seem HBr and 50 seem Cl₂ gas mixtures with 6 seem SF₆ gas flow.