Reactive ion etching (RIE) of silicon trenches has been studied as a function of trench width, ranging from 0.4 to 2.5 μm, in a CF3Br/SF6 plasma. The average etch rate inside a Si trench becomes slower as the trench width decreases. This etching phenomenon is called RIE lag. Also, we have measured the ion saturation current under a negative dc bias voltage with a miniature Faraday cup inside a cylindrical hole. The loss of the ion current occurs mostly within a depth equal to the hole diameter, independent of the hole size, and the current loss is directly proportional to both pressure and RF self-bias voltage at the substrate electrode. The lighter the mass of the ions, the more ions are deflected to the sidewall. Theoretical simulation shows that the electric field is perturbed primarily by the shape of the trench, and that the space charge near the trench plays an important role in ion deflection. Since the field perturbation is greater at the edge of the trench, the ion loss becomes more severe in a narrower trench, leading to the RIE lag. The sidewall profile observed in Si trenches shows a consistent picture of the field effect, particularly at low pressure where ion-neutral collisions become insignificant. © 1991, The Electrochemical Society, Inc. All rights reserved.