Cl surfaces induced by surface melting: etching and cleaning

Pulsed laser driven modifications of one Cl monolayer (ML) chemisorbed on Si(100) are studied by time of flight mass spectrometry (TOF) of the desorbed molecules, Auger electron spectroscopy @ES) of the modified surface, and Cl depth profile analysis by secondary ion mass spectrometry (SIMS). The experimental conditions are that of surface melting. Evidence of Cl diffusion to the bulk during surface melting, and of strong segregation of Cl in Si during surface recrystallization, are presented. The following branching ratios for 1 Cl ML initially adsorbed on clean Si are measured independently: (a) (from TOF and depth profiler measurements) 0.58 f 0.07 ML desorb reactively in the form of Sic1 and SiCl,. The Sic1 to SiCl, ratio is 1.3 + 0.3; (b) (f rom AES measurements) 0.37 f 0.13 ML are found after the laser pulse in a thin surface layer of 7 atomic planes; (c) (from SIMS measurements) 0.10 + 0.10 ML are found in deeper layers, the solubility of Cl in solid Si being lower than 1019 cmm3, the sensitivity of our SIMS for Cl; (d) 0.12 f 0.07 ML desorb unreactively in the form of Cl and Cl,. By order of increasing importance, unreactive desorption, stoichiometry and diffusion to the bulk during the laser pulse have the practical effect of limiting the maximum etch rate to 0.40 + 0.03 Si ML per laser pulse under the conditions of surface melting. Although the laser pulse induces diffusion of Cl towards the bulk during melting, the strong segregation of Cl during recrystallization limits Si contamination by Cl at undetectable levels to our SIMS. Cl in Si is a very good case for laser cleaning, as is shown by comparison with C and 0.