We have studied the in situ HCl etching of Si active areas on patterned wafers. After some in situ HCl etching at 20 Torr of Si(1 0 0), we have locally obtained 2 mu m long areas with misorientation angles around 4.5 degrees towards < 1 10 >. Furthermore, we have evidenced a recess shape transition from convex (T <= 865 degrees C) to concave (T >= 895 degrees C) as the etch temperature increases, with a nearly flat surface with no facets at T = 880 degrees C. The morphology of the etched structures at a given time, temperature and P-HCl/P-H2 ratio will be a function of the slope lengths and the pattern dimensions. Different kinds of surfaces (rounded areas, facets) were obtained in 3.5 mu m x 3.5 mu m Si windows after HCl etching at 850 degrees C during 300 s, depending on the stress within. Thermal oxidations can indeed be used to increase by 65 MPa up to 110 MPa the compressive stress in those Si windows which are bordered by SiO2 shallow trench isolation. An increase of the misorientation angle from 4.5 degrees up to 6 degrees occurred after the above-mentioned HCl etch when switching from conventional to highly strained Si windows. For the shortest etching times studied here (150 s), a selective etching of 3.5 mu m x 3.5 mu m Si windows edges is responsible for the misorientation. The etch is then more uniform. Stress gradients might consequently be one of the main misorientation causes. We have also probed the influence of the shallow trench isolation (STI) thickness on the misorientation. A morphological difference before HCl etching has been shown to be responsible for the transition from sloped to rounded areas. A local loading effect may prevail in this case. (c) 2007 Elsevier B.V. All rights reserved.