We present a simulation model of fluorine plasma etching of silicon. A mechanism for lag effect in the silicon surface etched by an inductively coupled plasma is investigated using molecular dynamics simulation. The results show that the lag effect is popular in etching process and that the etching rate of wide grooves is higher than that of the narrow ones. A probable reason is that the wide groove is produced more easily than the narrow groove. And the escape rate of product in narrow groove is lower than in wide groove. This is because a lot of products huddle together in the groove, which causes the speed of incident ions to decrease, and thus the energy of ions reaching the surface is reduced. The etching rate increases with the decrease of energy under otherwise identical conditions. On the other hand, the incident F particles are more close to the sidewall, which leads to the fact that the incident F particles will be easier to deposit on the surface of the wall. Then the width of the groove becomes narrower and narrower. The subsequent incident particles will be more difficult to reach the bottom of the groove. The lag effect increases not only with the decrease of the width of the groove but also with the enhancement of energy, and it decreases with temperature rising.