Contains reports on two research projects with publication lists in each section.Joint Services Electronics Program Contract DAAL03-89-C-0001MIT Energy Laboratory Synthetic Fuels CenterNational Science Foundation Grant CHE 85-08734Petroleum Research Fund Contract 19014-AC

Cain, Gerald R.

Ceyer, Sylvia T.

Daley, Sean P.

Johnson, Andrew D.

Laughlin, Kenneth B.

Li, Yulin

Maynard, Kevin J.

Pullman, David P.

Schulberg, Michelle T.

DSpace@MIT

Chapter 4. Chemical Reaction Dynamics at SurfacesChapter 4. Chemical Reaction Dynamics at SurfacesAcademic and Research StaffProfessor Sylvia T. Ceyer, Dr. Kenneth B. Laughlin, Dr. Yulin Li, Dr. Kevin J. Maynard, Dr. David P.PullmanGraduate StudentsSean P. Daley, Andrew D. Johnson, Michelle T. Schulberg, Julius J. YangUndergraduate StudentGerald R. Cain4.1 Dynamics of the Reactionof Fluorine with Si(100)SponsorJoint Services Electronics ProgramContract DAAL03-89-C-0001Project StaffProfessor Sylvia T. Ceyer, Dr. Kenneth B. Laughlin,Dr. Yulin Li, Dr. David P. Pullman, Michelle T.Schulberg, Gerald R. CainIt has long been believed that molecular fluorine,F2, would not react with or etch Si. To overcomethis problem, scientists used a plasma environ-ment. Because the plasma dissociates the mole-cular F2 or other fluorine containing molecules intoF atoms, it was believed that these species areresponsible for reacting with the Si surface to formthe volatile product SiF 4. While atomic fluorine, F,certainly does react with Si, we have shown that,contrary to popular belief, molecular F2 also reactswith Si(100) with very close to unity probability(~ 0.95).With support from the Joint Services ElectronicsProgram, we used our molecular beam-ultrahighvacuum surface scattering apparatus to direct awell-characterized, monoenergetic beam of F2molecules at a Si(100) surface. Not only have weshown that fluorine reacts via the well-knownmechanism in which both F atoms of the incidentF2 molecule adhere to the surface (known as dis-sociative chemisorption), but also that some of theincident F2 reacts so that only one of the F atomssticks to the surface while the other atom is scat-tered from the surface. This latter mechanism iscalled an abstraction reaction and, while its exist-ence has been predicted theoretically, we were thefirst group to demonstrate it experimentally.In essence, the dangling bonds on the Si surfacestrip off one of the F atoms as the F2 molecule fliesby it. We were able to observe this mechanism inthe angular and time-of-flight distribution meas-urements only as the direct consequence of thehigh resolution, the high signal to noise, and thecollisionless environment of the detector in ourapparatus. This mechanism could not have beenobserved with any other existing apparatus eithercustom-built or commercially available. Asexpected, the surface temperature has no effect onthe abstraction process, but the incident energy ofthe F2 molecule does determine the branching ratiobetween F atom abstraction and dissociativechemisorption.Presently, we are continuing our angular and time-of-flight (energy) distribution measurements of thescattered F and F2 signal to more accurately quan-tify this ratio as a function of the kinetic energy ofthe incident F2 molecule. These accurate measure-ments are necessary because the F atomabstraction mechanism has implications for modelsof the plasma environment during etching reac-tions. The abstraction of F2 by the Si surface pro-vides an additional source of F atoms, in additionto the known source of F atoms from the gasphase decomposition of F2 in the plasma. There-fore, this source of reactive species must now beincluded in a quantitative model of the plasmaetching environment.PublicationsCeyer, S.T. "New Mechanisms for Chemistry atSurfaces." Sci 249:133 (1990).Ceyer, S.T., D.J. Gladstone, M. McGonigal, andM.T. Schulberg. "Molecular Beams: Probes ofthe Dynamics of Reactions on Surfaces." InPhysical Methods of Chemistry. 2nd ed. Eds.B.W. Rossiter, J.F. Hamilton and R.C. Baetzold.New York: Wiley, 1991. Forthcoming.Gladstone, D.J., M.T. Schulberg, K.B. Laughlin, M.McGonigal, and S.T. Ceyer. "Design of a PowerChapter 4. Chemical Reaction Dynamics at SurfacesSupply for Resistive Heating of SemiconductorCrystals." In preparation.4.2 Dynamics of the Reactionof Fluorine with FluorinatedSi(100)SponsorJoint Services Electronics ProgramContract DAAL03-89-C-0001Project StaffProfessor Sylvia T. Ceyer, Dr. Kenneth B. Laughlin,Dr. Yulin Li, Dr. David P. Pullman, Michelle T.Schulberg, Gerald R. CainWhile the reaction probability of F2 with a cleanSi(100) surface is near unity, we have shown thatthe probability for reaction decays to zero as thefluorine coverage increases to one monolayer.That is, F2 is not unreactive with Si but it isunreactive with a fluorinated surface of Si. Thislack of reactivity with the fluorinated Si surface isthe source of the misconception that F2 does notreact with Si. The lack of reactivity with thefluorinated surface precludes the build up of a suf-ficient layer of fluorine to produce the volatile etchproduct, SiF 4. However, we have shown that ifthe kinetic energy of the incident F2 molecule isincreased above a threshold value of 6 kcal/ mol( - 0.25 eV), the reaction probability of F2 with afluorinated Si surface increases linearly with thenormal component of kinetic energy. Theenhancement in the reaction probability allowsenough fluorine to be deposited to form the vola-tile etch product, SiF4. Therefore, we haveobserved efficient etching of Si(100) at 300K foran incident F2 normal kinetic energy of 15kcal/mol (0.5 eV). This energy is two orders ofmagnitude below the energies used in plasmas.This result establishes that Si can be etched withlow energies using molecular beam techniqueswithout the use of plasmas. The low energiesafforded by molecular beam techniques preventthe introduction of radiation damage or defectsinto the Si lattice, which is a typical result ofplasma etching.PublicationSchulberg, M.T. The Reaction of MolecularFluorine with Silicon (100): Adsorption,Desorption, and Scattering Dynamics. Ph.D.diss., Dept. of Chem., MIT, 1990.4.2.1 New Mechanisms for SurfaceProcessesSponsorsMIT Energy LaboratorySynthetic Fuels CenterNational Science FoundationGrant CHE 85-08734Petroleum Research FundContract 19014-AC5Project StaffProfessor Sylvia T. Ceyer, Dr. Kevin J. Maynard,Andrew D. Johnson, Sean P. DaleyWith partial support from the Joint Services Elec-tronics Program (JSEP), we have found that thekinetic energy of an inert gas atom incident onCH4 physisorbed on Ni(111) activates the disso-ciative chemisorption of CH4 just as the transla-tional energy of the CH4 molecule incident on thesurface. This process occurs because the impactof the inert gas atom pounds the molecularlyadsorbed CH4 into the distorted shape of the tran-sition state that leads to dissociation. The productsof the dissociative chemisorption event after colli-sion induced activation, identified by high resol-ution electron energy loss spectroscopy as anadsorbed methyl radical and an adsorbed hydrogenatom, are identical to those observed after transla-tional activation. This observation represents thediscovery of a new mechanism for dissociativechemisorption: collision induced dissociation ofadsorbates. These results for translational and col-lision induced activation as well as the transla-tional activation of F2 on fluorinated Si(100)signal the demise of the long-standing and perva-sive notion in surface science that the surface isthe all-important and sole source of energy in acti-vating the dissociation of a molecule at oradsorbed on a surface.With the goal of uncovering the detailed mech-anism and dynamics of this new process, theabsolute cross section for collision induced disso-ciation of CH4 physisorbed on Ni(111) is meas-ured over a wide range of kinetic energies andangles of incidence of a Ne, Ar and Kr atom beam.Unlike the translational activation of CH4, whichexhibits strict normal energy scaling, the collisioninduced dissociation cross section displays acomplex dependence on the energy of theimpinging inert gas atoms characteristic of neithernormal nor total energy scaling. A two-step,dynamic model for the mechanism for collisioninduced dissociation provides excellent agreementwith the energy and angular dependence of thecross section for dissociation. The model showsthat the origin of the breakdown in normal energyscaling in the inert gas kinetic energy is the range36 RLE Progress Report Number 133of impact parameters which contribute to the dis-sociation cross section. By properly summing overthe impact parameter, the model calculations allowthe previous translational activation results to bemapped onto the cross sections for collisioninduced dissociation. In this way, translationalactivation and collision induced activation areshown to be completely consistent. They aresimply different ways to provide the energy todeform the CH 4 molecule but, once deformed, themechanism for the dissociation is the same.In competition with collision induced dissociation,another process, collision induced desorption,occurs. Previously, the desorption of adsorbatesby the impact of an inert, neutral species has beenpredicted, but ours was the first experimentalobservation of this process. Specifically, the abso-lute cross section for collision induced desorptionof CH 4 physisorbed on Ni(111) is measured as afunction of the kinetic energy and incident angleof an Ar beam. The mechanism for desorption isshown to involve a direct and impulsive,bimolecular collision between Ar and CH 4. Mole-cular dynamics simulations show that the compli-cated energy and incident angle dependence ofthe desorption cross section are the consequenceof the competition between the decrease in theenergy transferred in the normal direction and theincrease in the collision cross section as the inci-dent angle increases. The results of detailed tra-jectory calculations also assess the minor roles ofmultiple and mirror collisions, normal energyaccommodation and neighboring molecules.The impact of the observations of collisioninduced chemistry and desorption for under-standing surface chemistry in a high pressure envi-ronment is potentially large because, in a highpressure environment, an adsorbate-coveredsurface is continually bombarded by a large flux ofhigh energy molecules. Therefore, having shownthat collision induced processes occur, we believethat no mechanism for surface reactions underhigh pressure conditions including those thatoccur in plasma environments, can now be consid-ered complete without an assessment of theimportance of collision induced processes as amajor reaction step. In fact, there are many unex-plained observations in the literature of effects ofinert gases on the rates of high pressure, heteroge-neous catalytic reactions. It is now important toreinvestigate these reactions in light of the know-ledge that these collision induced processes occur.Collision induced chemistry and desorption areadditional reasons why surface chemistry at highpressures is often very different from the chemistryin UHV environments.Knowledge of these microscopic origins for thepressure gap has allowed us to develop a schemeChapter 4. Chemical Reaction Dynamics at Surfacesto bypass the high pressure requirement by simplyraising the energy of the incident molecule or col-lisionally inducing dissociation. We have used theformer trick to synthesize and identify spectro-scopically by high resolution electron energy lossspectroscopy, an adsorbed CH3 radical for the firsttime under low pressure, ultrahigh vacuum condi-tions. More recently, higher resolution and highersensitivity spectra of CH 3, CH 2D and CD 3 haveconfirmed a Fermi resonance between the over-tone of the asymmetric deformation and the lowfrequency or "soft" C-H symmetric stretch. Thesespectra have also allowed a symmetry analysis tobe carried out that establishes that the CH3 speciesis adsorbed with C3v symmetry in a threefoldhollow site with the hydrogens either eclipsed overor staggered between the surrounding Ni atoms.Because of our unique ability to produce a CH3species, we have been able to probe its stability.Above 150 K, CH 3 begins to dissociate toadsorbed CH. An unambiguous identification ofthe spectrum as that of CH is only possiblebecause of the high resolution (32cm- 1 FWHM)and high sensitivity (5 x 106 counts/sec for theelastically scattered electron beam) of our spectro-meter. This assignment is also supported by thespectra measured after the thermal decompositionof the mixed isotope CH 2 D. This work rectifies aprevious assignment of a spectrum in the literatureto a CH species.Our ability to bypass the high pressure requirementhas allowed us to carry out a high pressurereaction at low pressure: the synthesis of C6 H6from CH 4. In addition, because this reaction iscarried out at low pressure, we have been able toidentify the adsorbed intermediates by high resol-ution electron energy loss spectroscopy and todetermine the mechanism of this reaction. Thesynthesis is effected by exposing a monolayer ofCH4 physisorbed on Ni(111) at 47 K to a beam ofKr atoms. The collision of the incident Kr with thephysisorbed CH 4 distorts the CH 4 from itstetrahedral configuration, thereby lowering thebarrier to dissociation into an adsorbed methylradical and an adsorbed hydrogen atom. As thesurface temperature is raised to 230 K, all theadsorbed CH 3 dissociates to CH and the CHrecombines to form adsorbed C2 H2. Some of theC2H2 trimerizes to adsorbed C6H6 and at 410 K and425 K, respectively, the atomically adsorbedhydrogen desorbs as H2 and some of thechemisorbed C6H6 desorbs. The gas phasebenzene is detected mass spectrometrically in athermal desorption experiment. These data alsoprovide mechanistic information useful to the pos-sible extrapolation of this synthesis from molecularbeam-UHV environments to more practical condi-tions.Chapter 4. Chemical Reaction Dynamics at SurfacesPublicationsBeckerle, J.D., A.D. Johnson, and S.T. Ceyer."Collision Induced Desorption of PhysisorbedCH4 from Ni(111): Experiments and Simu-lations." J. Chem. Phys. 93:4047 (1990).Ceyer, S.T. "New Mechanisms for Chemistry atSurfaces." Sci. 249:133 (1990).Ceyer, S.T. "The Activation and Reactions of CH4on Ni(111)." Paper presented at the GordonConference on Organometallic Chemistry, June1990.Ceyer, S.T. "Collision Induced Absorption of Hinto Ni( 111)." Paper presented at the AmericanChemical Society Meeting, Washington, D.C.,August 1990.Ceyer, S.T. "Adsorbate Synthesis with MolecularBeams." Paper presented at the Chemistry atSurfaces Symposium, University of California,Irvine, October 1990.38 RLE Progress Report Number 133

Chemical Reaction Dynamics at Surfaces

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Chemical Reaction Dynamics at Surfaces

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