Porous Si is the semi-insulating state of Si, with low thermal expansion mismatch with bulk Si. As a result, it is an excellent material for crosstalk isolation in mixed-signal integrated circuits. We study the formation of isolated porous Si regions in p−-type and p+-type Si substrates with emphasis on the cross-sectional profile of the porous regions. Our study reveals that in addition to the primary undercut due to the isotropic nature of the anodization process, there exists a secondary undercut that is similar in shape to the bird’s beak commonly observed at the edge of field oxides in conventional Si complementary-metal-oxide-semiconductor process. The shape and the extent of the secondary undercut are dependent on the type of mask materials used during selective formation of porous Si as well as the substrate resistivity. The combined experimental and simulation studies pointed to two likely origins of secondary undercuts: the weak adhesion of some of the mask materials and current crowding in bulk Si substrates near the edge of the mask openings. Secondary undercuts result in the erosion of the precious Si chip surface area when a porous Si trench is used for rf crosstalk isolation, and should be minimized. © 2003 American Institute of Physics.