Investigation of selective SiO2-to-Si etching in an inductively coupled high-density plasma using fluorocarbon gases

Abstract

A recent, important development in low-pressure plasma processing is the radio frequency inductively (RFI) coupled high density discharge. Its ability to create high densities of excited and charged species at low pressures (<10−3 Torr) makes it an attractive etching tool. In this work we have examined selective etching of SiO2 over Si using a home-built RFI source. CHF3, C2F4, C3F6 and their mixtures with hydrogen were examined. Without biasing of the substrate strong fluorocarbon deposition occurs over the investigated pressure range from 5 to 20 mTorr. As the pressure increases the ion current density decreases, whereas the fluorocarbon deposition rate increases. Both parameters increase roughly linearly with inductive rf power from 500 up to 1250 W. Etching was achieved by rf biasing. When the pressure is reduced from 20 to 6 mTorr, the oxide and silicon etch rates decrease less than 20% for all gases. The highest oxide etch rate of 830 nm/min at 350 W rf bias power is achieved for C3F6. Adding H2 decreases the etch rates for oxide and silicon for all gases. The drop of the silicon etch rate is considerably higher than for the oxide etch rate resulting in a better selectivity. The best selectivity of 45 is achieved for C2F4 when 30% H2 is added into the discharge. The results obtained with the RFI source are compared to results with a microwave electron cyclotron resonance discharge.