The scanning tunneling microscope has been used to desorb hydrogen from hydrogen-terminated silicon (100) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.

Nanoscale patterning of the hydrogen terminated Si(100)-2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope. Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H-passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.

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Norbornadiene (bicyclo[2.2.1]Hepta-2,5-diene) is shown to chemisorb selectivity at room temperature onto clean Si(100)-2×1 surfaces. Combining the chemoselectivity of this process with scanning tunneling microscope nanolithography allows the formation of nanometer-sized regions having a norbonadiene adlayer. This concept could serve as the basis for creating spatially resolved templates to initiate chemical reactions with other chemical species in the presence of hydrogen-passivated Si(100)-2×1 areas.

Articles you may be interested inOxygen adsorption-induced nanostructures and island formation on Cu{100}: Bridging the gap between the formation of surface confined oxygen chemisorption layer and oxide formationThe selective removal of hydrogen from a passivated Si͑100͒ surface with an ultrahigh vacuum scanning tunneling microscope ͑STM͒ allows nanometer-sized ''templates'' of clean Si͑100͒ to be defined on an otherwise unreactive surface. Such depassivated areas have already been shown to react selectively with O 2 and NH 3 in preference to the surrounding H-terminated surface. This selectivity suggests two more sophisticated approaches to fabricating nanostructures with this technique: ͑1͒ selective metallization by thermal chemical vapor deposition, and ͑2͒ formation of ordered organic monolayers by 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 Si͑100͒ surface results in selective deposition of a metal on the STM-patterned area. In order to prevent hydrogen desorption and loss of selectivity, the metal precursor must dissociate its ligands at relatively low temperatures. In the second case, the patterned surface is exposed to an organic molecule expected to react in a site specific manner with unsaturated Si dimer sites. An example of this site selective reaction is the ͓2ϩ2͔ cycloaddition reaction between carbon-carbon double bonds and the Si dimer bond. Such reactions can result in the formation of spatially resolved nanometer-sized regions containing organic monolayers. In this article we describe 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 Si͑100͒ surfaces, as well as the selective patterning of a nucleation promoter, TiCl 4 , commonly used to initiate aluminum film growth. We also discuss the selective chemisorption of norbornadiene ͑bicyclo͓2.1.1͔ hepta-2,5-diene͒ on STM-patterned areas. STM images reveal the formation of a norbornadiene adlayer with indications of local ordering. Both of these methods show promise as techniques for the fabrication of nanostructures on Si͑100͒ surfaces.

Articles you may be interested inCryogenic variable temperature ultrahigh vacuum scanning tunneling microscope for single molecule studies on silicon surfaces Rev. Sci. Instrum. 75, 5280 (2004); 10.1063/1.1818871Nanometerscale modifications of gold surfaces by scanning tunneling microscope Nanoscale patterning of the Si(IOO)-2x 1 monohydride surface has been achieved by using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) to selectively desorb the hydrogen passivation. Hydrogen passivation on silicon represents one of the simplest possible resist systems for nanolithography experiments. After preparing high quality H-passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(IOO)-2X 1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and the electron current for low voltage patterning. The patterned linewidth varies linearly with the applied tip bias achieving a minimum of < loA at -4.5 Y. The dependence of Iinewidth on electron dose is also studied. For positive tip biases up to 10 V no patterning occurs. The restoration of clean Si(lOO)-2X 1 is suggestive of selective area chemical modifications. This possibility has been explored by exposing the patterned surface to oxygen and ammonia. For the oxygen case, initial oxidation of the patterned area is observed. Ammonia dosing, on the other hand, repassivates the surface in a manner different from that of atomic hydrogen. In both cases the pattern resolution is retained and the surrounding H-passivated areas remain unaffected by the dosing.

Nanometer scale patterning of the Si(100)2 × 1:H monohydride surface has been achieved by using an ultrahigh-vacuum (UHV) scanning tunneling microscope (STM) to selectively desorb the hydrogen. After preparing high-quality H-passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field-emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(100)2 × 1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and electron current for low-voltage patterning. The patterned linewidth varies linearly with tip bias, achieving a minimum of less than 10 Å at −4.5 V. The linewidth dependence on electron dose is also studied. For positive tip biases up to 10 V no patterning occurs. The selective chemical reactivity of the patterned surface has been explored by oxygen and ammonia dosing. For the oxygen case, initial oxidation of the patterned area is observed. Ammonia dosing, on the other hand, repassivates the surface in a manner different from that of atomic hydrogen. In both cases the pattern resolution is retained and the surrounding H-passivated areas remain unaffected by the dosing.