Ceyer, Sylvia T.

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Reaction Dynamics at Semiconductor Surfaces4. Reaction Dynamics at Semiconductor SurfacesAcademic and Research StaffProf. S.T. CeyerGraduate StudentsJ.D. Beckerle, M.B. Lee, M. McGonigal, J. Simonson, S.L. TangSylvia T. CeyerThe mechanism of the reactions of a gaseous species such as F2 with a semiconductor surfacesuch as Si is complex. The mechanism may involve several processes, including the adsorption ofthe molecule on the semiconductor surface, dissociation of the adsorbed molecule, diffusion of thedissociated species to a reactive site, reaction with the surface, and then desorption of the productmolecule. The purpose of this research is to study the dynamics of the first two processes, theadsorption and dissociation of a molecule incident on a surface. We would like to understand how amolecule falls apart on a surface. For example, does the molecule first adsorb and then diffuse to areactive site and then dissociate or are the dissociation and adsorption processes concerted? Whatfeature of the potential energy surface of the molecule-surface interaction dictates which dynamicsoccur? Variation of the energy of the incident molecule is a convenient probe of these dominantfeatures of the interaction potential because the dynamics and hence the observed dissociationprobability depend strongly on the incident energy relative to the interaction potential.However, studies of the effect of the energy of the incident molecule on the dissociation probabilityare virtually nonexistent because most surface studies are undertaken after adsorption of ambientmolecules. This implies that molecules strike the surface from all directions and with energiescharacterized by a 300 K Maxwell-Boltzmann distribution. We have just begun to design and build anapparatus to investigate the effect of the incident energy on the probability of dissociative adsorption.This apparatus employs molecular-beam techniques to define and control the energy of the incidentmolecule (up to 50 kcal/mole and 10 kcal/mole translational and internal excitations, respectively).The presence and amount of dissociation are detected sensitively (> 0.1% of a monolayer) byhigh-resolution electron-energy-loss spectroscopy, a vibrational spectroscopy. These fundamentalstudies on the effect of a molecule's incident energy on dissociative adsorption are importantbecause of the unknown chemistry these "hot" neutral molecules carry out during the plasma etchingof semiconductors, where translationally and internally excited molecules are produced by numerouscollisions with fast ions and electrons. The long-range goal of this research is to providefar-reaching correlations between the observed dynamics and the potential energy surfaces on whichmolecule-surface reactions occur.RLE P.R. No. 125RLE P.R. No. 125 20

Reaction Dynamics at Semiconductor Surfaces

19 Reaction Dynamics at SemiconductorSurfaces4. Reaction Dynamics at Semiconductor SurfacesAcademic Research StaffProf. S.T. CeyerGraduate StudentsJ. D. Beckerle, M.B.LeeSylvia T. CeyerThe mechanism of the reaction of a gaseous species such as F2 with a semiconductor surface suchas Si is complex. The mechanism may involve several processes, including the adsorption of themolecule on the semiconductor surface, dissociation of the adsorbed molecule, diffusion of thedissociated species to a reactive site, reaction with the surface, and then desorption of the productmolecule. The purpose of this research is to study the dynamics of the first two processes, theadsorption and dissociation of a molecule incident on a surface. We would like to understand how amolecule falls apart on a surface. For example, does the molecule first adsorb and then diffuse to areactive site and then dissociate or are the dissociation and adsorption processes concerted? Whatfeature of the potential energy surface of the molecule-surface interaction dictates which dynamicsoccur? Variation of the energy of the incident molecule is a convenient probe of these dominantfeatures of the interaction potential because the dynamics and hence the observed dissociationprobability depend strongly on the incident energy relative to the interaction potential.However, studies of the effect of the energy of the incident molecule on the dissociation probabilityare virtually nonexistent because most surface studies are undertaken after adsorption of ambientmolecules. This implies that molecules strike the surface from all directions and with energiescharacterized by a 300 K Maxwell-Boltzmann distribution. We have just begun to design and build anapparatus to investigate the effect of the incident energy on the probability of dissociative adsorption.This apparatus employs molecular-beam techniques to define and control the energy of the incidentmolecule (up to 50 kcal/mole and 10 kcal/mole translational and internal excitations, respectively).The presence and amount of dissociation are detected sensitively (20.1% of a monolayer) by high-resolution electron-energy-loss spectroscopy, a vibrational spectroscopy. These fundamental studieson the effect of a molecule's incident energy on dissociative adsorption are important because of theunknown chemistry these "hot" neutral molecules carry out during the plasma etching ofsemiconductors, where translationally and internally excited molecules are produced by numerouscollisions with fast ions and electrons. The long-range goal of this research is to provide far-reachingcorrelations between the observed dynamics and the potential energy surfaces on which molecule-surface reactions occur.RLE P.R. No. 124

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Reaction Dynamics at Semiconductor Surfaces

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