The etching processes and mechanisms for sidewall formation were investigated for the in situ trench etching and releasing process, using a magnetically enhanced reactive ion etcher (MERIE), and for the Bosch process, using an inductively coupled plasma (ICP) etcher. For the Bosch process, almost vertical sidewall profiles were obtained up to etching depth of similar to100 mum. The average height of the scallops on the sidewall was similar to70 nm and the average erosion underneath the mask was similar to0.4 mum, when trenches 50 mum deep were formed. This limited the fabrication capability of small feature sized microelectromechanical system structures of less than similar to1 mum. For the in situ processes using MERIE, the reactive ion etching (RIE) lag was significant when the etching depth reached similar to50 mum. The RIE lag was significantly reduced when the ICP was used at pressure below 50 mTorr. Scallops and undercuts were not observed when the in situ process was used. Anger electron spectroscopy and x-ray photoelectron spectroscopy analysis revealed that the sidewall passivation layer from the in situ process consisted of SiOx (1