Selective Removal of Native SiO2 Using XeF2

Thursday, October 15, 2015: 08:40
Phoenix East (Hyatt Regency)
A. Hinckley (University of Arizona), P. Mancheno-Posso, C. S. Lai (LAM Research), and A. J. Muscat (University of Arizona)
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).