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Micro/Nano Lithography

Nanoscale electrical contacts made using a focused ion beam

Focused-ion-beam-assisted deposition is used to make a new kind of electrical contact to wires just tens of nanometers in diameter.
8 June 2006, SPIE Newsroom. DOI: 10.1117/2.1200604.0199

Metallic nanowires (NWs) have attracted considerable attention in recent years due to their unique physical properties and their potential application as interconnects in future generations of electronics. At low voltage, these metallic NWs behave as almost perfect one-dimensional (1D) conductors. These fascinating electrical properties make a single NW an ideal experimental system for investigating electron transport in a low-dimensional system.

Nowadays the fabrication of these materials is relatively well controlled, but sometimes their characterization is inadequate because of the difficulties in accessing them, especially to extract electrical properties. Measuring the electrical characteristics of a single NW, having a diameter of nanometers and macroscopic length, requires novel and technically challenging experimental tools.

Metallic Au nanowires have been electrochemically synthesized into 20μm-thick, ion-track-etched, polycarbonate membranes with a pore diameter of the order of 100nm1,2 (see Figure 1). Prior to contact deposition, the polycarbonate membrane template was dissolved in dichloromethane (Cl2CH2) and the wires were collected on holey carbon grids. Figure 2 shows scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the nanowires. The high-resolution TEM (HRTEM) image of a nanowire in Figure 2(c) shows a dense grain structure, with an estimated grain size of about 5nm.

Figure 1. This schematic illustration shows the experimental setup for electrochemical template synthesis of Au nanowires.

Figure 2. (a) This scanning electron microscope (SEM) image shows 100nm-diameter Au nanowires form completely filled pores after dissolving the membrane in dichloromethane and collecting the wires on holey carbon grids. The wires have a length of 20±3μm, with hemispherical caps that form when the pores are filled up on the top of membrane surface. (b) This bright-field TEM image shows 100nm Au nanowires. (c) An HRTEM image of a single Au nanowire shows that it has even edges and exhibits a bamboo structure, with grains elongated in the growth direction. The grain sizes were estimated to be 5nm.

To establish good, ohmic, four-point contacts to NWs, a solution containing NWs was dispersed on top of a lithographically pre-patterned Au electrode on an oxidized silicon wafer, as shown in Figure 3(a). The NWs were then connected to the pads using focused-ion-beam (FIB)-assisted Pt deposition, as seen in Figure 3(b) and 3(c). The Pt deposition in the FIB system involves decomposition of trimethyl-methylcyclopentadienyl-platinum (C9H16-Pt) gas molecules, which results in a local deposition of Pt material on the sample in the area where the ion beam is scanned in a predefined pattern.

Figure 3. Focused-ion-beam (FIB)/SEM image shows connection of four contacts to a Au nanowire. (a) The left-hand panel shows the four-point structure. (b) The middle panel illustrates the Au nanowire located on the sample before contacting to the electrodes. (c) The right-hand panel shows how this Au nanowire is connected to the electrodes.

The electron-transport properties of NWs can be studied in detail from current-voltage (I–V) characteristics. The I–V curves may be either linear (ohmic) or non-linear, due to size or electrostatic and inelastic scattering of the electrons at the wire surfaces. Electron scattering can also occur due to intrinsic defects and impurities or, at high voltages, as a result of phonon emission.

The resistance, R, of a metallic NW can be described by R=(ρL)/A, where ρ the resistivity of the material, A is the cross sectional area of the NW, and L is its length. The resistivity of the NW depends on the material.

The I–V characteristics of single Au nanowires showed ohmic behavior, with a resistivity of 2.8±0.2 × 10-4Ωcm (see Figure 4). This value of the resistivity is two orders of magnitude larger than that for bulk gold, 2.2 × 10-6Ωcm.3 Possible explanations for this could be the reduced grain size and the nanowire wall roughness, both leading to an increase of the electron scattering at the grain boundaries or at the nanowire boundary. However, the former mechanism seems to be a reason for the large variation in measured resistivity values in this work.

Figure 4. The FIB-assisted deposition allowed this room-temperature, four-point current-voltage measurement of an Au nanowire.

The nanofabrication methods described above allow a large number of nanostructures, such as quantum dots, photonic crystals, fluorescent polymer nanoparticles, metal and ceramic nanoparticles, to be modified in a controlled and precise manner.

Sima Valizadeh and Jan Åke Schweitz 
Engineering Sciences, Universitet Uppsala