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Dry Cleaning Technology for Removal of Silicon Native Oxide Employing Hot NH3/NF3 Exposure

A new dry cleaning technology for removal of silicon (Si) native oxides from contact holes employing a hot ammonium (NH3)/nitrogen trifluoride (NF3) mixture has been studied. The NH3/NF3 mixture heated at a high temperature in a quartz tube enabled etching of the silicon dioxide (SiO2) film placed in the downstream region. The mechanism of the etching reaction which was investigated using in-situ infrared spectroscopy and X-ray photoelectron spectroscopy analysis was revealed as follows: NF3 alone in the NH3/NF3 mixture was decomposed above 600°C, probably producing NFx (x=1, 2) and fluorine atoms. These active species reacted with NH3, thereby generating ammonium hydrogen fluoride (NH4F ·HF) and/or ammonium fluoride (NH4F) in the gas phase which are considered to be SiO2 etchants. The reaction of these molecules with SiO2 generated an ammonium hexafluorosilicate ((NH4)2SiF6) product on the Si surface which was liberated at a temperature above 70°C, leaving the hydrogen terminated surface. The application of this method to actual contact holes demonstrated successful removal of the Si native oxide grown on the Si surface at the hole bottom.