A novel fabrication technique to introduce a deep grating in a vertical coupler filter was studied in order to realize a polarization-insensitive semiconductor optical add and drop multiplexing device with low crosstalk characteristics. The process of fabricating the vertical coupler filter, consisting of InGaAsP quasi-square waveguides buried in InP and an InGaAsP/InP grating between the waveguides, is described. Devices with a grating having a depth of 200 nm were fabricated by a five-step metal organic vapor-phase epitaxy (MOVPE) process accompanied by reactive ion etching (RIE) and wet etching. The deep grating was formed by RIE, and was realized by optimizing surface treatment prior to MOVPE over-growth and growth conditions. The diffraction efficiency of the filter was successfully improved, and an extremely low crosstalk of - 36dB was achieved.