This article describes an in situ trench etching and releasing technique to fabricate high aspect-ratio beams for high performance microelectromechanical systems (MEMS) accelerometers using magnetically enhanced reactive ion etching (MERIE) technique. In the conventional process, lateral encroachment due to the release etching is severe underneath the SiO2 etching mask. In this process, the sidewall of the beams was passivated by inhibiting layers formed during the HBr/SiF4/O-2 trench etching, and the beams were not attacked by the subsequent SF6 release etching. Auger electron spectroscopy showed the etch inhibiting layer to consist mostly of Si and O. This was also confirmed by x-ray photoelectron spectroscopy as SiOx (1 < x < 2) was detected as a predominant species. The in situ process eliminated (1) sidewall deposition step and (2) floor etching step of the conventional process. In addition, the dependency of MERIE etching rates on open ratio and pattern size was studied for high aspect-ratio MEMS structures. The MERIE etching rates of Si substrate were insensitive to the open ratio in the range of from 10% to 50%, in contrast to inductively coupled plasma etching. The capacitance and yield results from MEMS accelerometers made by the in situ process were acceptable to replace the conventional process.