An etch process using HCl:H3PO4 (5:1) was developed for the fabrication of highly precise alignment grooves in (100) InP wafers. We found that with In0.53Ga0.47As masks the undercut is negligible compared to titanium masks. The shape of cross-sectional etch profiles was strongly dependent on the orientation of the grooves. We present a model with a complete set of parameters for the present etch system for predicting the cross-sectional profiles of the resulting mesa structures with high accuracy using a graphical construction procedure.