A method for manufacturing a semiconductor device includes providing a substrate having a first surface and a second surface, the second surface is on the opposite side of the substrate facing away from the first surface. The method further includes forming a first portion of an opening by etching a portion of the substrate from the first surface, forming a buffer layer on an inner surface of the first portion, etching a bottom of the buffer layer to expose an area of the underlying substrate, and etching the exposed area of the substrate to form a second portion of the opening. The method also includes performing an isotropic etching on the second portion of the opening to obtain a flask-shaped opening and filling the opening with a filling material. The method also includes partially removing a portion of the second surface and the filling material from the second portion of the opening.