In this paper we propose a modified fabrication process to finely define the region surrounding the resonator that undergoes Si etching in CMRs. The unpredictable undercut has been shown to lead to Q instability within microns' variation. High aspect ratio SiO2 trenches were used as barrier to confine the release area. Thanks to the high selectivity of SF6 over SiO2, long Si etch steps can be performed without changing the release area. Our results show an excellent Q stability over etching time (<4%) which allows relaxation of Si etching and fine control of the released area. Moreover, since the geometry of the released area is known and set by design, models including the emptied region can be used as predictive tools.