Removal of Copper from Silicon Surfaces Using Hexafluoroacetylacetone (hfacH) Dissolved in Supercritical Carbon Dioxide

Abstract

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) 2 O due to air exposure before etching was attempted. Oxidation of the Cu(I) was performed using aqueous 30% H 2 O 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) 2 O. SEM images showed that the Cu(II)O had a flake morphology. The Cu(II) shells were removed selectively to the Cu(I) 2 O 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) 2 O 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 2 O, which was soluble in scCO 2 . The Cu(hfac) 2 ·H 2 O byproduct is proposed to oxidize Cu(I) 2 O to Cu(II), allowing attack and etching by hfacH.