Testing new chemistries for mask repair with focused ion beam gas assisted etchingCritical dimension ͑CD͒ control during silicon gate etching has been investigated with state-of-the-art chemistries. In particular, we have compared the etched profile of both isolated and dense gates obtained after the main etch step of a gate etch process using HBr/Cl 2 /O 2 and HBr/Cl 2 /O 2 /CF 4 gas mixtures, and study the influence of the CF 4 /O 2 ratio in this mixture. We demonstrate that the gate etch profile is mainly driven by the passivation layer deposited on the gate and mask sidewalls during the etching. Due to aspect ratio dependant etching effect the passivation layer formation is thinner in dense than in isolated structures resulting in significant profile microloading. However, CF 4 addition to HBr/Cl 2 /O 2 strongly minimizes the difference in passivation layer thickness between dense and isolated lines thus potentially improving the critical dimension control. These results will be discussed in terms of chemical composition of the passivation layer and deposition mechanisms, based on previous studies by x-ray photoelectron spectroscopy and mass spectrometry studies. Finally, we will discuss the influence of the soft-landing and overetch steps on the final profile of the gates, and show that aspect ratio dependant etch rate during the main etch step of the process can become an additional source of CD microloading.