which a given thickness of the silicon substrate 21 remains from the silicon oxide film 22 this is to physically protect the silicon deposition layer 23 since the thickness of the silicon deposition layer 23 is relatively thinly formed than the thickness of the silicon substrate 21 referring now to fig4 c the second nitride film 24 b formed on the top of the silicon deposition layer 23 is patterned to expose a given region of the silicon deposition layer 23 then a second etch process is performed to form a plurality of apertures 25 only one is shown in the drawing at this time the second etch process is performed with a wet etch the aperture 25 has a reverse trapezoid shape by a recipe of the second etch process the silicon oxide film 22 is exposed at the bottom of the aperture 25 at this time etching is performed at the bottom of the silicon substrate 21 where the first nitride film 24 a is not formed so that the remaining silicon substrate is removed and the bottom of the silicon oxide film 22 is exposed thus the silicon substrate 21 is divided centering on the aperture 25 a lower base of the aperture 25 is 10 u02dc 100 nm in size and the top of the aperture 25 is 1 u03bcm u02dc 100 u03bcm in size at this time the depth of the aperture 25 functioning as a waveguide is 1 u03bcm u02dc 10 u03bcm referring.