Abstract: A method for separating a joined substrate type wafer, which wafer is composed of a pair of semiconductor substrates joined through an insulation film, utilizes dielectrics through simple processing steps. Trenches for separating a semiconductor substrate with dielectrics are dug from the surface of the substrate and a dielectrics film is deposited on the surface of the substrate including the trenches. Then poly-crystalline silicon under layer is grown by CVD method to a thickness of about 0.5 .mu.m. Thereafter, a poly-crystalline silicon filler layer, which is deep enough to fill the trenches, is grown over the underlying poly-crystalline silicon under layer, followed by selectively removing the two poly-crystalline silicon layers from the surface of the substrate excluding the regions inside the trenches. An alternative embodiment contemplates depositing a second dielectrics film interposed between the poly-crystalline silicon under layer and the poly-crystalline silicon filler layer.