Approaches to nanofabrication on Si(100) surfaces: Selective area chemicalvapor deposition of metals and selective chemisorption of organic molecules

The selective removal of hydrogen from a passivated Si(100) surface with an ultrahigh vacuum scanning tunneling microscope (STM) allows nanometer-size d ''templates'' of clean Si(100) to be defined on an otherwise unreactive s urface. Such depassivated areas have already been shown to react selectivel y with O-2 and NH3 in preference to the surrounding H-terminated surface. T his selectivity suggests two more sophisticated approaches to fabricating n anostructures with this technique: (1) selective metallization by thermal c hemical vapor deposition, and (2) formation of ordered organic monolayers b y reaction with specific organic molecules. In the first case, an intrinsic difference in the reaction rate of a metal precursor with the clean and H- terminated Si(100) surface results in selective deposition of a metal on th e STM-patterned area. In order to prevent hydrogen desorption and loss of s electivity, the metal precursor must dissociate its ligands at relatively l ow temperatures. Tn the second case, the patterned surface is exposed to an organic molecule expected to react in a site specific manner with unsatura ted Si dimer sites. An example of this site selective reaction is the [2+2] cycloaddition reaction between carbon-carbon double bonds and the Si dimer bond. Such reactions can result in the formation of spatially resolved nan ometer-sized regions containing organic monolayers. In this article we desc ribe progress toward the fabrication of nanostructures utilizing these two techniques. First, we discuss the use of a new amidoalane precursor for the selective chemical vapor deposition of aluminum on STM-patterned Si(100) S urfaces, as well as the selective patterning of a nucleation promoter, TiCl 4, commonly used to initiate aluminum film growth. We also discuss the sele ctive chemisorption of norbornadiene (bicyclo[2.1.1] hepta-2,5-diene) on ST M-patterned areas. STM images reveal the formation of a norbornadiene adlay er with indications of local ordering. Both of these methods show promise a s techniques for the fabrication of nanostructures on Si(100) surfaces. (C) 1998 American Vacuum Society. [S0734-211X(98)03906-7].