CHEMISTRY OF SILICON SURFACES AFTER WET CHEMICAL PREPARATION - A THERMODESORPTION SPECTROSCOPY STUDY

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 surf ace-analytical tools available in an ultrahigh vacuum. For the first t ime thermodesorption spectroscopy can be utilized, without interferenc e from typical artefacts like contamination introduced by the ex situ preparation, to investigate the chemical termination and molecular com position 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 m ultiplexing a mass spectrometer and analyzing thermally desorbed molec ules over a wide range of masses simultaneously, we can separate quant itatively between the major surface-terminating molecules that are inh erently responsible for the different chemical surface properties (hyd rophilic due to -OH groups after wet chemical or UV/ozone-enhanced oxi dation; hydrophobic due to termination with hydrides after etching wit h hydrofluoric acid/ammonium fluoride, HF/NH4F) and minor species char acteristic for details of the preparation process (physisorbed residue s upon omission of a final rinsing step; contributions from residual c arbon-hydride contamination). Furthermore, distinct desorption channel s 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: addit ional slow isotropic attack of bulk silicon; HF/NH4F: anisotropic atta ck by fast removal of higher hydride defect sites on Si(111)] and a de termination of the resulting microroughness of the silicon surface. Sl ow regrowth of an ultrathin oxide on HF-treated surfaces during final rinsing with water can be monitored by a separation between H2O desorp tion at high temperature from surface silanol groups and low-temperatu re desorption of physisorbed water.