Influence on selective SiO2/Si etching of carbon atoms produced by CH4 addition to a C4F8 permanent magnet electron cyclotron resonance etching plasma

Abstract

Changes in the densities of fluorocarbon radicals and fluorine atoms in a size-scalable large-area compact permanent magnet electron cyclotron resonance etching plasma source employing C4F8 gas with CH4 addition have been investigated. Measurements using infrared laser absorption spectroscopy and actinometric optical emission spectroscopy show that, for a pure C4F8 plasma, the dominant species is CF2 radicals with a density of the order of 1013 cm−3, followed by fluorine atoms, CF3 and CF2 radicals which have a density an order of magnitude lower at 1012 cm−3. The densities of the different fluorocarbon radical species were found to display different dependencies on increasing CH4 gas addition. Hollow cathode absorption spectroscopy was used to estimate the carbon atom density for the first time, to the best of our knowledge, in an etching plasma. The carbon atom density in the plasma increases linearly with CH4 gas addition between 20 and 80%. Analysis of actual SiO2/Si etching revealed that the etch selectivity and carbon atom to fluorine atom ratio follows a similar trend indicating a direct correlation between the carbon atom density in the plasma and the etch selectivity.

Etching Mechanism Based on Hydrogen Fluoride Interactions with Hydrogenated SiN Films Using HF/H2 and CF4/H2 Plasmas

“…To establish a relationship between the changes in ER and the effect of H 2 addition, the number density of fluorine atomic concentrations obtained from optical actinometry method is illustrated in Figure . The obtained densities were on the order of magnitude of 10 12 cm –3 , which is consistent with the previously published results . As mentioned above, the primary uncertainty in accuracy comes from the integration of the excitation cross section at different energy levels for F and Ar.…”

Etching mechanisms of Si and SiO2in fluorocarbon ICP plasmas: analysis of the plasma by mass spectrometry, Langmuir probe and optical emission spectroscopy

“…Then, HF loss mechanisms are expected to be similar in both mixtures and the higher HF concentration obtained in methane discharges may be explained by creation mechanisms. HF is created by two main reactions either in gas phase or more probably at the reactor wall because of the low pressure condition used here [12,14]:…”

“…Relative atomic hydrogen concentration increases linearly with additive gas and its value is the same for a given H 2 or CH 4 percentage (not shown here). The higher HF concentration in methane discharges may come from a third mechanism [12] involving CH x radicals detected only in methane mixtures (figure 9(c)):…”

“…Nevertheless, higher oxide etch rates are obtained. New chemistries have been thus studied [9][10][11][12][13][14] to improve selectivity without decreasing the oxide etch rate too much. In parallel, a few papers have been devoted to the understanding of the etching mechanisms in order to anticipate the etching process.…”

Cross sections for electron collision with difluoroacetylene

“…Hence plasma etching with various fluorocarbon gases is often applied in such cases. To achieve good etch selectivity in Si while maintaining a reasonable etch rate of SiO 2 in silicon devices various fluorocarbon gases such as CHF 3 , C 3 F 8 , CH 2 F 2 and c-C 4 F 8 with low F/C ratio have been used [10,11]. The F/C ratio for the fluorocarbon molecules CF 4 , CHF 3 , c-C 4 F 8 , C 5 F 8 and C 4 F 6 decreases gradually in this order, taking the values 4, 3, 2, 1.6 and 1.5 respectively.…”