We examined the etch characteristics of SiO2 contact holes which have the sizes of 0.3-1.2 mu m in a C4F8 + H-2 electron cyclotron resonance plasma etching system. We varied the chamber pressure (3-7.5 mTorr), microwave power (300-800 W), gas mixing ratio, bias voltage (100-300 V) and bias frequency (100 - 800 kHz) as the experimental parameters to investigate the reactive ion etching (RIE) lag. The RIE lag improved, as the bias voltage increased and this RIE lag improvement was thought to be due to the increase of ion energy incident on the substrate. The RIE lag also improved as the chamber pressure decreased and this improvement is due to the increase of ion current density as the chamber pressure decreases. When we increased the bias frequency from 100 to 800 kHz maintaining the same bias voltage, the RIE lag improved considerably. We calculated energy distribution of ions striking the rf biased substrate using Monte Carlo particle-in-cell method to understand this phenomenon. The ion energy distributions are bimodal when the bias frequencies are below 1 MHz, but as the bias frequency increases from 100 kHz to 1 MHz, the low energy part decreases and the high energy part increases, and it is suggested that this change of ion energy distribution is responsible for the RIE lag improvement with the bias frequency increase.