A free‐standing, extremely thin silicon membrane, prepared by chemical vapor deposition of an epitaxial layer and subsequent anisotropic, selective etching, has been studied. The epitaxial layer consisted of a sandwich structure of undoped and highly boron/germanium‐doped films, serving as an etch stop. The results of double‐ and triple‐crystal x‐ray diffractometry in the Bragg and Laue case, i.e., in reflection and transmission, respectively, are reported using x radiation between 8.05 (CuK&agr;1) and 17.5 keV (MoK&agr;1) photon energy. The thickness of the crystal was determined toz0=(30±1) &mgr;m and its curvature to <7×10−4m−1. The influence of the doped etch stop layer on the x‐ray reflectivity is discussed in detail.