Reactions of Bare Silicon Cluster Ions: Prototypical Deposition and Etching Versus Cluster Size
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REACTIONS OF BARE SILICON CLUSTER IONS: PROTOTYPICAL DEPOSITION AND ETCHING VERSUS CLUSTER SIZE
M. L. MANDICH, W. D. REENTS, JR. AND V. E. BONDYBEY AT&T Bell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974 ABSTRACT Prototypical silicon deposition and etching reactions have been observed in gas phase reactions of size selected bare silicon cluster ions at room temperature. Laser evaporation of bulk silicon just outside the ion cell of a Fourier transform mass spectrometer produces abundant positive and negative silicon cluster ions. These cluster ions are trapped inside the cell for subsequent study of their bimolecular reactivity with various neutral reagents. Deposition type reactions occur with silanes, e.g. CH 3 SiH3 , and increase the number of silicon atoms in the cluster. Etching reactions occur in reactions with certain halogen or oxygen containing reagents. For example, NO 2 and XeF 2 react to destroy the silicon clusters by sequential loss of a silicon atom. Overall the reactivity of small silicon cluster ions correlates with chemistry which occurs at two distinct types of dangling bonds in the clusters: either a lone pair of electrons or a single unpaired electron.
INTRODUCTION Deposition and etching reactions are essential to the manufacture of silicon devices. Yet, large gaps exist in the understanding of the mechanisms by which these reactions occur. This can be attributed in large part to the highly complex chemical behavior of silicon surfaces as well as the experimental difficulties that are currently inherent in studies of surface reactivity. Our approach is to study an analogous system: the microscopic surfaces on silicon clusters. The particular type of clusters that we have examined are either positively or negatively charged aggregates of 2-9 atoms isolated in the gas phase. This offers one distinct advantage: the bimolecular reactivity of these clusters can be unambiguously monitored. We have investigated the reactions of both positively and negatively charged silicon clusters and found a rich and varied chemistry that changes with cluster size and reagent composition. 1 -51 Although these clusters have geometries and electronic structures that differ significantly from bulk silicon surfaces, many of their reactions are remarkably similar to those that are used in chemical processing. For example, both deposition and etching reactions that add and remove silicon atoms from the clusters have been observed. These two types of reactions appear to involve two distinct types of dangling bonds on the silicon clusters which are similar in electronic character to dangling bonds found on various silicon surfaces. Thus the reactions of small silicon clusters may provide a model for understanding some of the details of the reactions of specific sites on bulk silicon surfaces.
EXPERIMENTAL METHOD Gas phase silicon cluster ions are created by laser evaporation of a stationary bulk silicon target located just outside the trapped ion cell of-a modified Nicolet FT/MS-1000 Fourier transform ion cyclo
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