Intermittent injection of trisdimethylaminophosphorus (TDMAP) is applied for selective etching of the InP in ultrahigh vacuum by using molecular layer epitaxy equipment. The etching depth is controlled by the number of injection cycles of TDMAP in a self-limiting fashion. The etch rate per injection cycle increases with the injection time of TDMAP and is saturated. The etch rate is well described by a Langmuir-type equation. The etch rate is independent of the injection time at times longer than 0.3 s, and is also independent of the injection pressure of TDMAP. This is almost independent of the evacuation time. The activation energy for etching with TDMAP is determined to be 1.27 eV, which is half of that for the decomposition of phosphorus from InP substrate. Well-controlled digital etching is realized at substrate temperatures as low as 350 degrees C.