A mixture of fluorocarbon gases having high C/F ratio is commonly used to obtain the high selective etching of SiO2 to Si in high-density plasmas. We investigated the etch rate of SiO2 and Si in an electron cyclotron resonance (ECR) plasma etching system as functions of gas pressure, gap distance between the ECR layer and the substrate, and hydrogen percentage in fluorocarbon gases such as CF4 and C4F8. The selectivity increases considerably in the case of the C4F8 plasma as the gas pressure, hydrogen percentage, and the gap distance increase. Actinometric optical emission spectroscopy (OES) and appearance mass spectrometry (AMS) were employed to study the mechanism of selective SiO2 etching. Ar was used as the actinometer, and the behavior of the F atom and various fluorocarbon radicals was investigated for CF4 and C4F8 ECR plasmas. The following results were obtained. First, the CF2 radical behavior investigated by OES coincides well with that by AMS. Second, the behavior of CF2 radical density as a function of hydrogen percentage in the CF4 ECR plasma is very similar to that in the rf capacitively coupled plasma (RFCCP), but its increasing rate with H-2 addition is smaller by an order of magnitude compared to using the CF4 in RFCCP system. Third, the dominant polymer precursor which is important for the selective SiO2 to Si etching is CF in the C4F8+H-2 ECR plasma, and as the hydrogen percentage increases, the decreasing rate of the relative concentration of atomic F is higher in the CF4 ECR plasma than in the C4F8 ECR plasma. Therefore, it is suggested that the F abstraction reaction in gas phase process such as CF2+H-->CF+HF is more dominant than the F scavenging reaction such as CF2+F+H-->CF2+HF in the C4F8 ECR plasma in the case with hydrogen addition. Fourth, the densities of all the fluorocarbon radicals increase contrary to the decrease of atomic F when the pressure increases from 2.5 to 10 mTorr when the H-2 addition is used. Last, the relative concentration of the F atom decreases as the distance from the ECR layer increases axially, but the fluorocarbon radicals do not show considerable changes.