It has been found that in the etching of SiO2 using CHF3 in an inductively coupled plasma reactor of the planarized coil design, a thin steady state fluorocarbon film can play an important role in determining the rate of etching. This etching is encountered as the amount of bias power used in the SiO2 etching process is increased, and a transition from fluorocarbon film growth on the SiO2 to an oxide etching rate which is consistent with reactive sputtering theory is made. The observed presence of an intermediate region where etching occurs, although a steady state fluorocarbon film suppresses the etch rate from that expected for a reactive sputtering process, has been referred to as the fluorocarbon suppression regime. This work demonstrates the role of the steady state fluorocarbon film present on silicon dioxide during etching within the fluorocarbon suppression regime. X-ray photoelectron spectroscopy studies of the surfaces of partially etched SiO2 have shown a thinning of this film with increasing rf bias power, as well as a decrease in the fluorine content of the surface film as a function of increasing rf bias power. We have found that slight variations in the film thickness, on the order of 1 nm, can result in large variations, approximately 400 nm/min, in the silicon dioxide etch rate. The presence of the film within the suppression regime appears to be due to the overwhelming polymerization ability of high density plasmas, coupled with the inability of the oxide to react sufficiently with the total fluorocarbon particle flux in order to completely remove this film. For this reason these types of reactors exhibit a regime where oxide etching occurs in the presence of a surface film. The film appears to be directly responsible for the observed suppression of the oxide etch rate from that expected for a reactive sputtering process by dissipating the bombarding ion energy, and thereby suppressing the energy flux arriving at the oxide surface.

The slowdown of the oxide etch rate with width of submicrometer structures is known as reactive ion etching (RIE) lag and has been explained by ion shadowing and differential charging of the sidewalls, among other effects [R. A. Gottscho and co-workers, J. Vac. Sci. Technol. B 10, 2133 (1992)]. Here we show for an inductively coupled high density plasma reactor working in the pressure regime from 6 to 20 mTorr that inverse RIE lag is primarily observed, i.e., the etch rates increase as the width of the microstructures decrease. Inverse RIE lag, which was first discussed by Vitkavage et al. [Tegal Plasma Proceedings Symposium, San Francisco, 1991 (unpublished)], may be explained by considering the neutral flux distribution at the structure bottom. The neutral flux has a stronger dependence on the aspect ratio than the ion flux due to its isotropic velocity distribution. The neutral flux distribution has been modeled and is consistent with etching profiles observed at high pressure.

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