Removal of copper from silicon surfaces using hexafluoroacetylacetone (hfacH) dissolved in supercritical carbon dioxide

Copper was etched from a silicon surface using the chelator hexafluoroacetylacetone (hfacH) dissolved in supercritical carbon dioxide (scCO ₂) at 40-60°C and 100-250 atm. Copper was deposited on Si(100) using doped HF solutions in the form of 10-90 nm Cu islands, as shown by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) indicated the islands were composed of Cu(I) ₂O due to air exposure before etching was attempted. Oxidation of the Cu(I) was performed using aqueous 30% H ₂O ₂ or a UV-Cl ₂ gas phase, forming shells of Cu(II)O or Cu(II)Cl ₂, respectively, surrounding cores of Cu(I) ₂O. SEM images showed that the Cu(II)O had a flake morphology. The Cu(II) shells were removed selectively to the Cu(I) ₂O cores by processing with pure scCO ₂ and rapidly releasing the system pressure (300 atm/min). Mechanical failure of the Cu(II)O and Cu(II)Cl ₂ when CO ₂ in stress corrosion cracks quickly expanded delaminated these layers, leaving only Cu(I) ₂O on the surface. Etching of both Cu(II) and Cu(I) was achieved when oxidized samples were processed in scCO ₂ containing approximately 120 ppm of hfacH for 2 min. Nucleophilic attack of Cu(II) centers by hfacH formed copper(bis-hexafluoroacetylacetonate), Cu(hfac) ₂ and water, or the monohydrate Cu(hfac) ₂· H ₂O, which was soluble in scCO ₂. The Cu(hfac) ₂·H ₂O byproduct is proposed to oxidize Cu(I) ₂O to Cu(II), allowing attack and etching by hfacH.