Fabrication of metallic nanowires with a scanning tunneling microscope

Abstract

A procedure to pattern thin metal films on a nanometer scale with a scanning tunneling microscope ͑STM͒ operating in air is reported. A 30 nm film of hydrogenated amorphous silicon ͑a-Si:H͒ is deposited on a 10 nm film of TaIr. Applying a negative voltage between the STM tip and the a-Si:H film causes the local oxidation of a-Si:H. The oxide which is formed is used as a mask to wet etch the not-oxidized a-Si:H and subsequently, the remaining pattern is transferred into the metal film by Ar ion milling. Metal wires as narrow as 40 nm have been fabricated. Since a-Si:H can be deposited in very thin layers on almost any substrate, the presented procedure can be applied to structure all kind of thin films on a nanometer scale. © 1995 American Institute of Physics.During the previous 5 years, the scanning tunneling microscope ͑STM͒ has attracted interest as a tool for lithography on a nanometer scale. [1][2][3][4][5][6][7] In scanning tunneling microscopy tip-substrate interactions are very local, making it possible to modify the surface of a substrate to a very high lateral resolution ͑Ͻ20 nm͒. Several methods to fabricate metallic nanowires using this technique, have been demonstrated. Ehrichs et al. 2 and McCord et al. 3 have used an STM induced chemical vapor deposition ͑CVD͒ process in which a metalorganic gas is decomposed between the STM tip and the substrate, depositing metal on the substrate surface. A drawback of this technique is that there is only a limited choice of metals which can be deposited, due to the number of gas precursors which are available. Alternatively, the STM can be used to expose organic resist layers. For example, McCord et al. 3 have used poly͑methylmethacrylate͒ PMMA, Marrian et al. 4 have used SAL 601 and recently Stockman et al. 5 have used a Langmuir-Blodgett film. Since these layers do not always conduct sufficiently, the STM tip can penetrate the resist during lithography which can limit the tip lifetime due to mechanical interactions between tip and layer.In this letter, we present a method to pattern a thin film on a nanometer scale with an STM operating in air, using on a conducting resist layer. The method was inspired by the work of Dagata et al. 1 who demonstrated that a hydrogen terminated silicon ͑111͒ surface can be locally oxidized with the STM. The oxide layer which is formed can act as a mask for etching the silicon, as was demonstrated by Snow et al. 6A thin film could be patterned with this method if a silicon layer which is both stable against oxidation in air and sufficiently conducting could be deposited on the metal film. Hydrogenated amorphous silicon ͑a-Si:H͒ is a material which can fulfill both requirements. It is stable against oxidation in air because of its high hydrogen content ͑Ϸ10 at. %͒.8 Thin films of a-Si:H can be highly doped, to make them electrically conducting for STM operation. In addition, with plasma enhanced chemical vapor deposition ͑PECVD͒ a layer of a-Si:H can be deposited on almost any surface at low temperature, because of the high r...

Laser Application of Polymers

“…Most of these techniques take advantage of the spatial resolution of the electronic emission from a tip to locally expose ultrathin electron resists [367][368][369][370], such as self-assembled monolayers [371,372] or Langmuir-Blodgett films [373], or to directly modify the structure of the superficial layer [374][375][376][377], such as the oxygenation of hydrogenated silicon [378,379]. A few methods based on mechanically engraving a soft layer with the sharp atomic microscope tip have also been proposed [380].…”

Germanium nanowires sheathed with an oxide layer

“…1,2 Current interest focuses on several forms of quasi-1D materials, which include ͑i͒ shell structural nanotubes, such as carbon nanotubes, which can be formed from curved graphitic sheets with pentagonal and hexagonal networks of sp 2 -bonded elemental carbons, 3,4 ͑ii͒ cast-in-tube nanowires, which are fabricated by filling hollow channel templates of porous anodic alumina and carbon tubes, 5,6 ͑iii͒ island-shaped nanowires, which are synthesized on a substrate surface using various techniques such as molecular beam epitaxy, 7 electron-beam lithogaphy, 8 and scanning tunneling microscopy ͑STM͒ pattern writing, 9 and ͑iv͒ free-standing nanowires, which have been grown by laser ablation in vapor phase. 2,[10][11][12] Nevertheless, the formation of free-standing nanowires is difficult due to the constraint of surface problems.…”

SiO2and Si nanoscale patterning with an atomic force microscope

“…Presently there is a large interest in lithographic techniques based on scanning probe microscopy (SPM) (for a review see, e.g., [1][2][3][4] and references therein). In spite of the slow scanning speed compared with electron or ion beam lithography the SPM technique is advantageous because it is free from the proximity effect or radiation-induced crystal damage.…”