We investigated the etch rate of SiO2 and Si in an electron cyclotron resonance (ECR) etching system as a function of H-2 adding percentage to fluorocarbon gases such as CF4 and C4F8. The selectivity increases considerably in the case of C4F8 plasma as the H-2 percentage increases. Actinometric optical emission spectroscopy and appearance mass spectrometry were employed to elucidate the mechanism of selective SiO2/Si etching in ECR plasma and the following are confirmed. First, the relative behavior of CF2 radical density as a function of H-2 percentage in a CF4 ECR plasma system is very similar to that in a rf capacitively coupled plasma (RFCCP) system but its increasing rate with the addition of H-2 is Smaller by an order of magnitude compared to the case of CF4 RFCCP system. Second, the dominant polymer precursors that are responsible for the selective SiO2 to Si etching is CF among CFx (x=1-3) radicals in C4F8+H-2 ECR plasma and the decreasing rate of the relative concentration of atomic F is higher in CF4 ECR plasma than in C4F8 ECR plasma as the H-2 percentage increases. Therefore, it is suggested that the F abstraction reaction with H in the gas phase process is more responsible for the selective SiO2 etching than the F scavenging reaction in C4F8 ECR plasma when H-2 is added.