We investigate the reactive ion etching (RIE) lag or aspect ratio dependent etching in etching of a thin Si3N4 layer in the regime of relatively low ion bombardment energy. The surface reaction is studied through the transportation of ions and radicals in high aspect ratio holes. The radical transportation is studied in the regime that the radical concentration is low and the ion flux is saturated. The key parameter for the radical transportation is the high substrate temperature, where radicals can reach the hole bottom, easily resulting in high surface coverage. On the other hand, in a regime of saturated surface coverage with abundant radicals, the ion flux is the reaction limiting parameter for the RIE lag. The comparison of RIE lag with the surface coverage shows that the transportation of depositive radicals is more sensitive to the temperature. Finally, in the range where the neutral and ion fluxes are balanced, we achieved a result of aspect ratio independent etching in a structure with an aspect ratio higher than 13:1.