Residence time effects on high aspect ratio SiO2 hole etching with a 90% Ar addition to C4F8 have been studied using a planar type neutral loop discharge. An adopted wide-type antenna instead of the original narrow one enabled us to improve uniformity of ion currents, electron densities, and plasma generation over a wide range of pressure. Thus, the residence time (tau) dependence of radical and ion densities, and etched features were investigated for various pressures (P). It was found that the conditions which achieved high aspect ratio features followed a straight line passing an origin in the tau-P diagram. This demonstrated that the high aspect ratio feature etching was determined unequivocally by the adequate flow rate, because the gradient of the line resulted in the reciprocal of the flow rate. On the other hand, at short tau, a large amount of radicals generated due to suppressed recombination produced the excess polymer deposition, causing the "etch stop." At longer tau, ions and radical densities decreased due to recombination and thus dominated Ar+ ion sputtered the mask resist, forming the tapered feature.