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Aligned implantation via integrated scanning probe
RESEARCH NEWS
Nanoscale writing in fountain-pen mode FABRICATION & PROCESSING
Ink molecules diffuse to the substrate from the liquid-air interface at the probe tip's annular aperture. (© 2005 Wiley-VCH.)
Dip-pen nanolithography uses controlled transfer of molecular ink from an atomic force microscope (AFM) tip to a substrate for direct-write patterning of sub-15 nm features. But replenishing ink limits throughput and necessitates realignment during complex patterning. Attempts to overcome this by continuous feeding of ink through an aperture at the apex of a hollow AFM tip have been limited to feature sizes of >100 nm by the tip’s outer diameter and by ink flooding the substrate through surface tension. Now, a Northwestern University team has demonstrated sub-100 nm molecular
patterning in fountain-pen mode [Kim et al., Small (2005) 1 (6), 632]. Their nanofountain probe (NFP) is fabricated on a Si chip for mounting on commercial AFMs. It integrates an on-chip reservoir and microfluidic channels embedded in cantilevers that, through capillary action, drive a 1 mM solution of molecular ink to a volcano-like dispensing tip. This has an annular aperture that controls the position of the liquid-air interface around its core. Molecules then diffuse from the interface to the substrate. The tip shows controlled transport of molecular ink to Au substrates without flooding. The smallest linewidth achieved is 40 nm (about a third of the tip radius). Standard micromachining enables scalability to massively parallel arrays of NFPs and reservoirs for high-throughput patterning over large areas with multiple inks. “The technology will likely lead to many highimpact applications in nanosensors, biotechnology, and pharmaceuticals, including nanolithography, combinatorial nanochemistry, biosensors, and nanodevices,” says team leader Horacio D. Espinosa.
Aligned ion implantation has been demonstrated by forming reproducible patterns of dots in a layer of resist on Si. To achieve single-atom devices, experiments Cantilever with hole and imaging tip are ongoing to (© 2005 American Chemical Society.) reduce hole diameter to 10 nm. The use of highly charged Bi60+ ions enhances resist-developing, as well as the creation of secondary electrons above the surface and electron/hole pairs inside the solid. This allows the detection of single implanted ions, though not yet with scanning probe alignment in place. Mark Telford
Mark Telford
Mark Telford
FABRICATION & PROCESSING
10
July/August 2005
CHARACTERIZATION Coworkers at the Universites of Bath and Bristol in the UK, and the Institut Laue-Langevin in France, have found topological and chemical order in glass persisting to distances beyond the nearest-neighbor length scale [Salmon et al., Nature (2005) 435, 75]. Using neutron diffraction, the researchers probed two binary network-forming glasses with contrasting atomic bonding: the ionic halide ZnCl2 and the covalent chalcogenide GeSe2. The diffraction patterns are composed of overlapping individual patterns that describe the relative positions of pairings of like or unlike atoms (e.g. Zn-Zn, Cl-Cl, and Zn-Cl). They used isotopic substitution, where an element is replaced by an alternative isotope that scatters neutrons differently. By measuring diffraction for different isotopic compositions, patterns for individual atom pairings can be separated. For both ZnCl2 and GeSe2 structures, they found: a strong preference for Zn or Ge to have four Cl or Se nearest neighbors arranged locally as a tetrahedron, generating a chemical short-range ordering that propagates to long range (about 60 Å, or 30 nearest-neighbor distances); plus intermediate-range order (about 6 Å) associated with the way the tetrahedra interlink. “Our results give information that will provide a severe test of models of glasses,” says Bath’s Philip S. Salmon. “They may also lead to the preparation of new materials by rational design, including optically active glasses containing rare-earth ions for fiber lasers and amplifiers, and glasses for nuclear waste storage.” The researchers aim to discover if the results also hold for oxide glasses.
Aligned implantation via integrated scanning probe As part of their work on quantum computing, which relies on placing single dopant ions in Si, a team from the Lawrence Berkeley National Laboratory, the University of California, Berkeley, and the University of Kassel, Germany has integrated an ion beam with a scanning force microscope [Persaud et al., Nano Lett. (2005), doi: 10.1021/nl0506103]. Previously, direct-write doping was performed by ion implantation using focused ion beam (FIB) systems. But, to avoid ions reaching the sample during alignment, an electron beam is needed for imaging. Instead, passing an ion beam through an aperture in the cantilever of a scanning probe tip allows highresolution imaging and noninvasive alignment of the ion beam for implantation of targeted device features to <10 nm accuracy. Feature size is limited by the cantilever hole diameter. For cantilevers that are microns thick, holes drilled by FIB are limited to an aspect ratio of ~5:1. But they can be narrowed by depositing a Pt film thick enough to stop ions (a few hundred nanometers) then redrilling to give a 300 nm wide hole.
Long-range order in glass
Nanoscale writing in fountain-pen mode FABRICATION & PROCESSING
Ink molecules diffuse to the substrate from the liquid-air interface at the probe tip's annular aperture. (© 2005 Wiley-VCH.)
Dip-pen nanolithography uses controlled transfer of molecular ink from an atomic force microscope (AFM) tip to a substrate for direct-write patterning of sub-15 nm features. But replenishing ink limits throughput and necessitates realignment during complex patterning. Attempts to overcome this by continuous feeding of ink through an aperture at the apex of a hollow AFM tip have been limited to feature sizes of >100 nm by the tip’s outer diameter and by ink flooding the substrate through surface tension. Now, a Northwestern University team has demonstrated sub-100 nm molecular
patterning in fountain-pen mode [Kim et al., Small (2005) 1 (6), 632]. Their nanofountain probe (NFP) is fabricated on a Si chip for mounting on commercial AFMs. It integrates an on-chip reservoir and microfluidic channels embedded in cantilevers that, through capillary action, drive a 1 mM solution of molecular ink to a volcano-like dispensing tip. This has an annular aperture that controls the position of the liquid-air interface around its core. Molecules then diffuse from the interface to the substrate. The tip shows controlled transport of molecular ink to Au substrates without flooding. The smallest linewidth achieved is 40 nm (about a third of the tip radius). Standard micromachining enables scalability to massively parallel arrays of NFPs and reservoirs for high-throughput patterning over large areas with multiple inks. “The technology will likely lead to many highimpact applications in nanosensors, biotechnology, and pharmaceuticals, including nanolithography, combinatorial nanochemistry, biosensors, and nanodevices,” says team leader Horacio D. Espinosa.
Aligned ion implantation has been demonstrated by forming reproducible patterns of dots in a layer of resist on Si. To achieve single-atom devices, experiments Cantilever with hole and imaging tip are ongoing to (© 2005 American Chemical Society.) reduce hole diameter to 10 nm. The use of highly charged Bi60+ ions enhances resist-developing, as well as the creation of secondary electrons above the surface and electron/hole pairs inside the solid. This allows the detection of single implanted ions, though not yet with scanning probe alignment in place. Mark Telford
Mark Telford
Mark Telford
FABRICATION & PROCESSING
10
July/August 2005
CHARACTERIZATION Coworkers at the Universites of Bath and Bristol in the UK, and the Institut Laue-Langevin in France, have found topological and chemical order in glass persisting to distances beyond the nearest-neighbor length scale [Salmon et al., Nature (2005) 435, 75]. Using neutron diffraction, the researchers probed two binary network-forming glasses with contrasting atomic bonding: the ionic halide ZnCl2 and the covalent chalcogenide GeSe2. The diffraction patterns are composed of overlapping individual patterns that describe the relative positions of pairings of like or unlike atoms (e.g. Zn-Zn, Cl-Cl, and Zn-Cl). They used isotopic substitution, where an element is replaced by an alternative isotope that scatters neutrons differently. By measuring diffraction for different isotopic compositions, patterns for individual atom pairings can be separated. For both ZnCl2 and GeSe2 structures, they found: a strong preference for Zn or Ge to have four Cl or Se nearest neighbors arranged locally as a tetrahedron, generating a chemical short-range ordering that propagates to long range (about 60 Å, or 30 nearest-neighbor distances); plus intermediate-range order (about 6 Å) associated with the way the tetrahedra interlink. “Our results give information that will provide a severe test of models of glasses,” says Bath’s Philip S. Salmon. “They may also lead to the preparation of new materials by rational design, including optically active glasses containing rare-earth ions for fiber lasers and amplifiers, and glasses for nuclear waste storage.” The researchers aim to discover if the results also hold for oxide glasses.
Aligned implantation via integrated scanning probe As part of their work on quantum computing, which relies on placing single dopant ions in Si, a team from the Lawrence Berkeley National Laboratory, the University of California, Berkeley, and the University of Kassel, Germany has integrated an ion beam with a scanning force microscope [Persaud et al., Nano Lett. (2005), doi: 10.1021/nl0506103]. Previously, direct-write doping was performed by ion implantation using focused ion beam (FIB) systems. But, to avoid ions reaching the sample during alignment, an electron beam is needed for imaging. Instead, passing an ion beam through an aperture in the cantilever of a scanning probe tip allows highresolution imaging and noninvasive alignment of the ion beam for implantation of targeted device features to <10 nm accuracy. Feature size is limited by the cantilever hole diameter. For cantilevers that are microns thick, holes drilled by FIB are limited to an aspect ratio of ~5:1. But they can be narrowed by depositing a Pt film thick enough to stop ions (a few hundred nanometers) then redrilling to give a 300 nm wide hole.
Long-range order in glass