MECHANISMS OF EXCIMER LASER CLEANING OF AIR-EXPOSED SI(100) SURFACES STUDIES BY AUGER-ELECTRON SPECTROSCOPY, ELECTRON ENERGY-LOSS SPECTROSCOPY, REFLECTION HIGH-ENERGY ELECTRON-DIFFRACTION, AND SECONDARY-ION MASS-SPECTROMETRY

The structure and composition of air-exposed Si(100) surfaces irradiated by a KrF (248 nm, 5.0 eV, 22 ns pulses) excimer laser in ultrahigh vacuum (UHV) have been studied using Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and reflection high-energy electron diffraction (RHEED). The near-surface region was compositionally profiled using secondary-ion mass spectrometry (SIMS). Pulsed laser irradiation at energies below the melting threshold (almost-equal-to 500 mJ cm-2) led to significant reductions in surface C and O concentrations but detectable contamination remained even after 200 pulses. However, surfaces exposed to several laser pulses at energies just above the melting threshold were atomically clean (less than 0.01 monolayer of impurities) as judged by AES and EELS and exhibited a 2 x 1 RHEED pattern. The peak in the EELS spectrum associated with surface dangling bond states was slightly lower in intensity and higher in energy than results obtained from thermally cleaned samples, while other surface states associated with the 2 x 1 reconstruction and noninteger RHEED streak intensities were also slightly less intense. SIMS profiles of the irradiated wafers revealed C and O impurities distributed throughout the melt volume. Thus, excimer laser irradiation produces clean Si(100) surfaces by a combination of desorption (accounting for approximately 1/3 to 1/2 of the decrease in impurity concentration) and dissolution leaving a disordered (2 x 1) surface. Annealing irradiated wafers at 600-degrees-C for 1 min in UHV produced sharp 2 x 1 RHEED patterns and EELS spectra identical to those obtained from thermally cleaned (greater-than-or-equal-to 900-degrees-C) surfaces.