APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 23
4 JUNE 2001
Positioning of nanometer-sized particles on flat surfaces by direct deposition from the gas phase Thomas J. Krinkea) and Heinz Fissan Process and Aerosol Measurement Technology, Gerhard-Mercator-University Duisburg, D-47048 Duisburg, Germany
Knut Deppert,b) Martin H. Magnusson, and Lars Samuelson Solid State Physics, Lund University, Box 118, S-221 00 Lund, Sweden
共Received 31 October 2000; accepted for publication 16 April 2001兲 Arrangements of nanometer-sized particles were obtained on plane oxidized silicon substrates by direct deposition from the gas phase. The particles were attracted onto charge patterns created by contact charging. Monodisperse, singly charged indium aerosol particles with a diameter of 30 nm were used as a test case to illustrate this process. Due to the surface treatment, the deposition is highly selective. We were able to create lines of particles with widths as narrow as 100 nm and several millimeters in length. The resolution of the pattern depends mainly on the surface treatment and the tool geometry. Our approach opens the possibility of creating patterns composed of nanometer-sized particles on a flat substrate surface by the simple transfer of charge patterns, without a lithographical process. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1377625兴
Nanometer-sized metal and semiconductor particles are potential components in future photonic or quantum electronic devices.1 This will require a direct positioning of nanoparticles. Many different approaches to create nanostructures using particles or clusters as building blocks have been presented in the literature. Deposition from a suspension using capillary forces gives two- and three-dimensional arrays of crystal-like structures of particles.2 Patterning of an insulating substrate with charges applied by an electron beam attracts particles from the liquid phase with a resolution in the micrometer range.3 Particles from the gas phase have been deposited between micrometer-sized electrodes.4 Individual particles can be arranged one by one to create device structures.5 Nanoscale chains of metal clusters can be fabricated with a resolution better than 200 nm by nucleation at the boundary of lines of photoresist.6 In this letter, we describe a method for nanoparticle positioning, based on aerosol technology without lithographic processing. Charged aerosol particles are deposited on a silicon-oxide surface patterned with lines of induced surface charges, applied to the substrate by contact charging. Here, electrons cross the interface between the insulator and a metal brought into contact with it. After the metal is removed, charge remains in the insulator for several hours. The sign and amount of charge transferred depend roughly linearly on the work function of the metal.7,8 As test aerosol we used monodisperse indium particles with a diameter of 30 nm, fabricated in ultrapure nitrogen by an aerosol generator described in detail elsewhere.9 With a volume flow of 1680 cm3/min, a particle concentration of about 5⫻105 cm⫺3 at ambient pressure and room temperature was achieved. Every particle carries either one positive or one negative charge. For deposition, the aerosol flows into a兲
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an electrostatic precipitator 共ESP兲, which is a vertical, cylindrical chamber designed to remove charged particles from the gas flow by means of a homogeneous electric field.10 The field is applied between the top plate, where the aerosol enters the ESP, and a horizontally positioned electrode on which the substrate is placed. With no electric field applied to the electrode, only drag forces and Brownian motion act on the particles. Under these circumstances, the number of particles deposited on the substrate surface is negligible. This has been confirmed by scanning electron microscope 共SEM兲 investigations, showing that after ‘‘deposition’’ without an electric field no particles are found on the substrate. When a homogeneous electric field of appropriate polarity is applied, the charged particles are driven towards the substrate surface. In combination with the drag forces parallel to the substrate, this leads to a circular deposition spot on the substrate, which, under the given conditions has a diameter of about 15 mm and within which the particles are distributed in a homogeneous but random fashion.11 Simple deposition in an electric field cannot produce patterns of particles, due to the Brownian motion of the particles in the gas phase. Coulomb forces, however, are able to affect the particle motion. In order to attract particles to a desired position, we have developed a method to transfer patterns of surface charges onto the surface of an insulating substrate. As substrates, we used silicon共111兲 with a 0.5-m-thick thermally grown wet oxide and a plane surface. Lines of negative charge were created by contact charging the surface with a stainless-steel needle that was allowed to glide over the surface of the substrate without applying pressure. Both needle and substrate were grounded. A second method was to transfer the patterns with nanoimprint stamps.12 In this case, a nickel plate, patterned with 1 m by 0.2 m lines, and with dots 0.2 m in diameter, was pressed against the substrate. Prior to the charging process, the substrate was transferred into an ultrapure nitrogen atmosphere. A heat treatment 共30
0003-6951/2001/78(23)/3708/3/$18.00 3708 © 2001 American Institute of Physics Downloaded 04 Jul 2002 to 134.91.65.75. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
Appl. Phys. Lett., Vol. 78, No. 23, 4 June 2001
FIG. 1. SEM image of positively charged, 30 nm indium particles deposited on lines of charge transferred by a sliding contact with a stainless-steel needle.
min at 200 °C on a heating plate兲 was carried out in order to remove the contamination layer 共mainly water兲 from the substrate surface. The substrate was then placed in the ESP for 30 min, without the application of an additional electric field. Very narrow lines of particles with widths of less than 100 nm and several millimeters in length could be realized in the first case. The result is demonstrated in Fig. 1. The broadest line has a width of about 250 nm; the narrowest is less than 100 nm wide. The lines differ in coverage; this is probably caused by the nonoptimal tool used. Figure 2 shows one result of a deposition using a nanoimprint stamp. The procedure was the same as in the first example except for the charging process. The particle arrangement corresponds to the patterns on the stamp. In order to prove that this effect can be attributed to the attractive Coulomb forces between the negative charges on the surface and the positively charged particles, the first experiment was repeated using negatively charged particles. Without applying a voltage to the electrode of the ESP, no deposition at all could be observed. However, when a positive electric field of 300 kV/m was applied, particles were attracted to the substrate. As can be seen in Fig. 3, particlefree zones showed up among the otherwise homogeneously distributed particles. The width of these zones was approximately 10 m, with a narrow transition region. The patterns of particle-free zones correspond to the patterns applied with
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FIG. 3. SEM image of a particle-free zone after 30 min of deposition of negatively charged particles on a charged substrate in the presence of a homogeneous electric field.
the steel needle. This indicates a negative charging of the contact area. If the water layer on the substrate is not removed prior to deposition, the linewidth was found to be substantially increased, probably due to charge spreading through the surface layer.13 To investigate this, the charging with the steel needle was done in ambient air and without initial heat treatment of the substrate, after which positively charged particles were deposited. An additional electric field of ⫺300 kV/m was applied in order to make the edge of the influence region visible, but this did not affect the linewidth. The result is shown in Fig. 4. The particles were deposited in an approximately 10-m-wide line lying at the center of a 60m-wide particle-free zone. On the rest of the substrate, the particle distribution was found to be undisturbed. The particle density within the line was higher by a factor of approximately 5–10 compared to the mean density on the rest of the sample. The lines were situated where the steel needle had been made to glide over the substrate. For the fabrication of electronic nanostructures, it is of great importance that the charging process neither destroys nor contaminates the substrate surface. To investigate this,
FIG. 4. SEM image of a line of particles at the center of a particle-free zone, surrounded by a homogeneous distribution of particles, after deposition of FIG. 2. SEM image of positively charged, 30 nm indium particles deposited positively charged particles in the presence of surface charges and a homoon charge patterns transferred by static contact with a patterned nickel geneous electric field. stamp. Downloaded 04 Jul 2002 to 134.91.65.75. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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Appl. Phys. Lett., Vol. 78, No. 23, 4 June 2001
we analyzed the charged areas with atomic-force microscopy 共AFM兲 and with energy-dispersive x-ray spectroscopy 共EDX兲. The AFM investigations showed, within the resolution of 1 nm, no destruction of the substrate surface as long as the contact pressure between needle and tip was kept very low. Actual scratches made with the needle did not attract particles. The EDX investigations gave no indication of material transfer when the steel needle was used. With softer materials, e.g., copper and aluminum, the surface was contaminated but no particles were attracted, and no patterns formed. The amount of charge transferred to the substrate was determined by measuring the current flow while sliding the needle at constant velocity over the substrate surface. The charge density could be estimated to be approximately 4000 elementary charges per square micrometer. Due to the roughness of the tip, the true contact area was probably smaller than our estimate, making the above charge density a lower limit. Computer simulations based on a trajectory model were made, which support our conclusions.14 The experiments have demonstrated that contact electrification of silicon-oxide substrates, in combination with particle deposition from the gas phase of an aerosol, can be developed into a powerful tool for creating nanostructures. We expect that the size range of the building blocks may reach from several hundreds of nanometers down to molecular dimensions. The resolution achievable in these experiments was better than 100 nm, seeming to depend mainly on the geometry of the tool used for the contact charging. Another important aspect is the flexibility of the process, which opens the opportunity for simultaneous creation of structures with resolutions from the millimeter range down to the 100
共or even lower兲 nanometer range. This makes the connection from the macroscopic to the nanoscale possible in one process step. We were also able to show that nanoimprint methods can be used for the charging process, possibly opening the way for efficient parallel patterning. This work was supported by the ESF program NANO and the EU–TMR project CLUPOS. The authors would like to thank Mariusz Graczyk, Jan-Olle Malm, Frank Jordan, and Klaus Tu¨ber for their help during the investigations.
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