MECHANISM OF SELECTIVE SIO2 SI ETCHING WITH FLUOROCARBON GASES (CF4, C4F8) AND HYDROGEN MIXTURE IN ELECTRON-CYCLOTRON-RESONANCE PLASMA-ETCHING SYSTEM/

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 pl asmas. We investigated the etch rate of SiO2 and Si in an electron cyc lotron resonance (ECR) plasma etching system as functions of gas press ure, gap distance between the ECR layer and the substrate, and hydroge n percentage in fluorocarbon gases such as CF4 and C4F8. The selectivi ty increases considerably in the case of the C4F8 plasma as the gas pr essure, hydrogen percentage, and the gap distance increase. Actinometr ic optical emission spectroscopy (OES) and appearance mass spectrometr y (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 v arious fluorocarbon radicals was investigated for CF4 and C4F8 ECR pla smas. The following results were obtained. First, the CF2 radical beha vior investigated by OES coincides well with that by AMS. Second, the behavior of CF2 radical density as a function of hydrogen percentage i n the CF4 ECR plasma is very similar to that in the rf capacitively co upled plasma (RFCCP), but its increasing rate with H-2 addition is sma ller by an order of magnitude compared to using the CF4 in RFCCP syste m. Third, the dominant polymer precursor which is important for the se lective SiO2 to Si etching is CF in the C4F8+H-2 ECR plasma, and as th e hydrogen percentage increases, the decreasing rate of the relative c oncentration of atomic F is higher in the CF4 ECR plasma than in the C 4F8 ECR plasma. Therefore, it is suggested that the F abstraction reac tion 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 pla sma in the case with hydrogen addition. Fourth, the densities of all t he fluorocarbon radicals increase contrary to the decrease of atomic F when the pressure increases from 2.5 to 10 mTorr when the H-2 additio n is used. Last, the relative concentration of the F atom decreases as the distance from the ECR layer increases axially, but the fluorocarb on radicals do not show considerable changes. (C) 1996 American Vacuum Society.