Oxygen Atom Reactions with Well-Characterized Surface Adlayers on Si(100)
M. Litorja, L.M. Struck, and S.A. Buntin
Objective: To quantitatively measure reaction probabilities and mechanistic details for radicals reacting with well-characterized surfaces of industrial relevance. Specifically, use a novel radical beam source to characterize the interactions of ground and electronically excited O atoms with adsorbate-covered Si(100).
Problem: For microelectronic device fabrication, a more quantitative understanding of the surface chemistry of semiconductors is necessary as the critical dimension of device features continues to shrink and the cost of empirical process optimization continues to rise. Industry-based strategic planning for the continued development of microelectronic processing clearly highlights an increased reliance on modeling and simulation for process optimization. However, it is recognized that there is a significant deficiency in our current knowledge of radical/surface interactions; there is a critical need to elucidate mechanisms and quantify probabilities for radical/surface reactions to meet roadmap goals and next-generation demands. In device fabrication, oxygen plasmas are often used to remove photoresist, and plasma-enhanced processes involving Si/O/H-based precursors are used for silicon dioxide deposition. For these applications, it is known that O atoms are key in the surface chemistry, but reaction rates and mechanisms are not known. With the drive towards lower pressure and temperature plasmas, it is necessary to determine the influence of electronic excitation on the surface reactivity of O atoms. Dramatic reactivity differences between (1D) and (3P) O atoms have been documented for reactions with small molecules in the gas phase (1D metastable, excited state is 1.97 eV above the 3P ground state). The effects of electronically excited O atoms in surface reactions, however, have not been systematically addressed.
Approach: We have developed a laser-photolysis-based method of producing a relatively "clean" flux of atomic radicals. This source is uniquely suited for probing O atom reactions since it is capable of generating very well-defined incident O atom fluxes; that is, exclusively ground-state (3P) O atoms are produced from the 193 nm photolysis of SO2, while a 50/50 mix of (3P) and (1D) O atoms is produced by the 157 nm photolysis of O2. Well-characterized, fully saturated surface adlayers of deuterium (D), acetylene (C2H2) or ethylene (C2H4) on Si(100) are prepared in an ultrahigh vacuum environment. These adlayers are then subjected to varying exposures from the O atom beam source and the evolution of the surface adlayer composition (e.g., oxygen and carbon relative to silicon) is followed by Auger electron spectroscopy.
Results and Future Plans: Studies
thus far have considered O atom surface oxidation using only the 157 nm photolysis
of O2. For the monodeuteride-terminated Si(100) surface, the data
in Figure 1 shows no difference in the oxidation rate for surface temperatures
of 290 K and 580 K, indicating that the oxidation process is not activated by
the surface energy. While O atoms exposure dependencies have not yet been fully
determined for C2H2 and C2H4 adlayers,
the results indicate that there is no significant difference in oxidation rate
for C2H2-, C2H4-, and D-Si(100).
In addition, the surface carbon does not appear to be diminished by the O atom
oxidation. These results suggest that the oxidation occurs at sites other than
those associated with the Si-dimer bond, perhaps by insertion in the Si backbonds.
Further characterization of these systems is planned, including an evaluation
of the dependence of the initial deuterium coverage.
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Last Updated
March 5, 2002
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