A novel fabrication process for uniform ultrafine SiO2 gaps with perfectly vertical sidewalls is proposed and experimentally demonstrated. The gaps without undulation were successfully fabricated by splitting the SiO2 layer along the guidelines established beforehand, utilizing thermal stress between the SiO2 layer and the InP substrate. High uniformity of the gap width of less than 5% along 200 mu m has been obtained. Minimum gap width of 20 nm has been realized. Moreover, this "split mask" was found to be applicable as a selective regrowth mask and a dry etching mask.