The suitability of reactive ion etching for the fabrication of microelectro mechanical systems (MEMS) has been evaluated by characterizing the change of lateral dimensions versus depth in etching deep structures in silicon. Fluorine, chlorine, and bromine containing gases have provided the basis for this investigation. A conventional planar RIE (reactive ion etching) reactor has been used, in some cases with magnetic field enhancement or an inductive coupled plasma source and low substrate temperature. For RIE based on Cl-2 or Cl-2/HBr plasma a slightly "positive" (top wider than bottom) slope is achieved when etching structures with a depth of several 10 mu m, whereas a "negative" slope is obtained when etching with an SF6/CCl2F2-based plasma. A pattern transfer with vertical walls is obtained for RIE based on SF6 (with O-2 added) when maintaining the substrate at low temperature (approximate to-70 degrees C). Further optimization of plasma chemistries and RIE procedures should result in runouts on the order of 0.1/100 mu m depth in Si as well as in organic materials.