Experimental Verification of a New Mechanism for Dissociative Chemisorption: Atom Abstraction

Abstract

Abstraction of a F atom from incident F2 by Si(100)-(2 x 1) is demonstrated by detection of the scattered, complementary F atom. He atom diffraction measurements are consistent with abstraction occurring at dangling bond sites. The low probability for single atom adsorption (P~= 0.10~0.01) relative to the total adsorption probability (P, = 0.96~0.02) in the zero coverage limit indicates that the second F atom can also be trapped by the dangling bonds. Both the single and two atom adsorption probabilities decay to zero when the dangling bonds are saturated at 1 ML coverage. Atom abstraction represents a mechanism distinct from the classic one for dissociative chemisorption. PACS numbers: 82.65.Pa, 34.50.Bw One important function of a surface in heterogeneous catalysis or chemical vapor deposition is to cleave a bond of an incident molecule, thus forming two adsorbed reactive fragments. The dissociation proceeds by the formation of two bonds to the surface because the formation of two bonds is energetically necessary for bond cleavage. This is the classic view of dissociative chemisorption [1], and indeed this mechanism is operative in many moleculesurface systems. In contrast, one can envision bond cleavage by formation of a single surface-atom bond if the energy released by this bond formation is greater than the bond energy of the incident molecule. Generically, this mechanism is known as abstraction and is a welldocumented reaction mechanism between two molecules in the gas phase. In this Letter, we document the dissociative chemisorption of a molecule on a surface by an abstraction mechanism.Specifically, we demonstrate that a Si (100)-(2 X 1) surface abstracts one F atom from an incident F2 molecule by detecting the complementary F atom scattered back into the gas phase with a triply differentially pumped, rotatable, quadrupole mass spectrometer in a molecular-beam -surface-scattering UHV apparatus. In addition, we couple these experiments with He atom diffraction from the resulting fluorinated surface to demonstrate that the Si surface dangling bonds are responsible for the abstraction and are the sites for F adsorption. Because of the experimental complexities in the detection of reactive radicals such as F atoms, this mechanism has gone undetected in numerous previous studies of the interaction of fluorine and fluorine containing molecules with Si. The apparatus has been described in detail [2]. The triply differentially pumped F2 beam is formed by expansion of a mixture of 1% F2 in Kr. The velocities of the incident and scattered beams are determined by cross correlation time-of-flight (TOF) methods. The beam is directed at an n-type Si(100) crystal that can be heated resistively and cooled to 120 K. The crystal is etched [3] prior to insertion into the vacuum chamber where it is repetitively sputtered and annealed (1120 K for 30 min) until Auger spectroscopy reveals carbon as the only contaminant, at a 1% or less level. He atom diffraction measurements show the surface to exhibit the welldocumented...

Ab Initio-Derived Dynamics for F2Reactions with Partially Fluorinated Si(100) Surfaces: Translational Activation as a Possible Etching Tool

“…Although it is initially less probable for the Θ F ) 0.5 ML surface as compared to the clean surface, the amount of abstraction stays relative constant regardless of incident energy, causing abstraction to be more likely on the Θ F ) 0.5 ML surface than on the clean surface at energies above 1.0 eV. This is in qualitative agreement with the experimental results of Li et al, 12 who reported a maximum in the number of atom abstraction events at a nonzero coverage. The dissociative chemisorption pathway appears to have changed at least.…”

“…43 Figure 3b shows the average exit energies for F atoms ejected during atom abstraction as a function of increasing incident F 2 COM translational energy. Immediately obvious from this graph is the hyperthermal character of the ejected F atoms, a qualitative feature which has also been observed experimentally by Li et al 12 Even the lowest F 2 incident energy, 0.078 eV, results in an average exit energy for ejected F atoms of 0.4 eV, as compared to the 0.03 eV expected for the average energy of an atom at 298 K, the surface temperature. Clearly the ejected F atoms have not equilibrated with the surface.…”

“…The fluorine molecule was given an initial COM translational energy, and in some cases vibrational excitation, chosen from the list of initial conditions in Table 1. The COM velocity of the F 2 was directed normal to the surface, as in the experiments of Ceyer [11][12][13] and Kummel. 14,15 For cases without vibrational excitation, the atoms in the F 2 molecule were initially separated by 1.435 Å, the minimum energy in the Stillinger-Weber F-F potential.…”

Overview of atomic layer etching in the semiconductor industry

“…This reaction is dissociative, as one chlorine atom at a time from the dimer adsorbs onto a silicon dangling bond. 70 In the simplest two-dimensional model, each silicon surface atom bonds to one chlorine atom. X-ray photoelectron spectroscopy (XPS) data show the SiCl bonding on the surface, with some SiCl 3 groups that may be related to damage generated by the removal step that produces addition silicon reaction sites.…”

Adsorption and abstraction reactions of HCl on a single Si(100) dangling bond

“…When a polyatomic molecule impinges upon a surface, it may be reflected (unreactive scattering), bind to the surface as an intact molecule (molecular adsorption), fragment into adsorbed components (dissociative adsorption), [1][2][3] or fragment into adsorbed and reflected components (abstraction). [4][5][6][7][8] The last two processes are usually exothermic; chemical bond formation between surface atoms and radicals derived from the parent molecule drives the reactions toward a lower energy state. However, a few previous studies have reported that endothermic chemisorption can occur under certain conditions.…”