Selective Removal of Native SiO2 Using XeF2

Selective removal of native silicon dioxide (NOx) relative to the underlying Si substrate and to other films present in a device structure is important in forming low resistance electrical contacts. Xenon difluoride (XeF₂) was chosen as the F-source because of the weak molecular bonds and the absence of precursors that could promote the uncontrolled formation of water on the surface. Etching of SiO₂ with XeF₂ has been shown to require the etching byproducts of elemental Si in close proximity.¹ Both NOx and thermal SiO₂ (TOx) films were etched with XeF₂, titanium tetrachloride (TiCl₄), and water vapor at different temperatures, pressures and process sequences. Samples were analyzed in situ with x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The O 1s XPS spectrum in Fig. 1 after static XeF₂ exposure at room temperature and 1 Torr showed a decrease in the O coverage to near the detection limit. The surface was visibly roughened by this process because the Si substrate was etched spontaneously by F atoms produced by the reaction of XeF₂. Flowing XeF₂ at a pressure below 10 mTorr, NOx was fluorinated without removing it and without roughening the underlying substrate. The fluorinated NOx was desorbed by heating above 677 °C. The TPD spectra in Fig. 2 show that F disrupted the oxide causing it to desorb beginning at about 190 °C. Si desorbed at this temperature likely as SiF₄ based on the SiF₃ daughter species observed. Control experiments showed that the onset of native oxide desorption was about 770 °C without XeF₂. Atomic force microscopy (AFM) images showed that the surface roughness was unchanged by low pressure XeF₂ and heating to high temperature.

In order to understand the reaction kinetics and reduce the high temperature anneal required for NOx removal, XeF₂ was dosed at 120 °C and TiCl₄ was used as a co-reactant because of the facile reaction that it undergoes with silanol groups. Three treatments of XeF₂ at 120 °C removed all oxide as shown by the O 1s XPS spectra in Fig. 3. More controlled NOx removal was achieved via the introduction of TiCl₄ prior to XeF₂. The Ti 2p XPS spectra in Fig. 4 shows that TiO₂ was formed after TiCl₄ pulses at 250 °C, and the TiO₂ was removed by XeF₂ under continuous flow conditions. In addition, peak area ratios between O 1s and Si 2p states decreased by 20% after XeF₂ exposure. Based on an initial thickness measured via ellipsometry of 15.8±0.2 Å, the amount of NOx etched was about 3 Å or approximately one monolayer. The surface was fluorinated after XeF₂ exposure and unreactive to further TiCl₄ pulses. Two process sequences were demonstrated for the controlled removal of NOx, one involving XeF₂ and high temperature annealing and the other involving sequential doses of TiCl₄ and XeF₂.

1. Veyan, J.F.; Halls, M.D.; Rangan, S.; Aureau, D.; Yan, X.M.; Chabal, Y.J. J. Appl. Phys. 108, 114914 (2010).