A method for distinguishing a specific region in a sample to be observed by a microscope is disclosed. The method includes the steps of (a) forming a first concavity on a first side of the specific region by a focus ion beam (FIB) technique, (b) forming a second concavity on a second side of the specific region opposite to the first side by the focus ion beam technique, and (c) filling the first concavity and the second concavity with a first metallic packing and a second metallic packing respectively for defining the specific region to be observed.