Chemistry of silicon surfaces after wet chemical preparation: A thermodesorption spectroscopy study

Abstract

Ultraclean wet chemical preparation in air and a fast new load-locking technique opens up a way to characterize real Si(111) surfaces after processing for microelectronic device fabrication with the proven surface-analytical tools available in an ultrahigh vacuum. For the first time thermodesorption spectroscopy can be utilized, without interference from typical artefacts like contamination introduced by the ex situ preparation, to investigate the chemical termination and molecular composition of silicon surfaces after initial wet chemical key processes in semiconductor technology (chemical and UV/ozone-enhanced cleaning, liquid and gaseous phase etching, rinsing with de-ionized water). By multiplexing a mass spectrometer and analyzing thermally desorbed molecules over a wide range of masses simultaneously, we can separate quantitatively between the major surface-terminating molecules that are inherently responsible for the different chemical surface properties (hydrophilic due to –OH groups after wet chemical or UV/ozone-enhanced oxidation; hydrophobic due to termination with hydrides after etching with hydrofluoric acid/ammonium fluoride, HF/NH4F) and minor species characteristic for details of the preparation process (physisorbed residues upon omission of a final rinsing step; contributions from residual carbon–hydride contamination). Furthermore, distinct desorption channels of hydrogen molecules from mono-, di-, and trihydride surface states allow a characterization of the pH-dependent etching by H2O:HF:NH4F [concentrated HF: selective removal of surface oxides; dilute HF: additional slow isotropic attack of bulk silicon; HF/NH4F: anisotropic attack by fast removal of higher hydride defect sites on Si(111)] and a determination of the resulting microroughness of the silicon surface. Slow regrowth of an ultrathin oxide on HF-treated surfaces during final rinsing with water can be monitored by a separation between H2O desorption at high temperature from surface silanol groups and low-temperature desorption of physisorbed water.