Xidex                                                     Vapor Phase Editing of Carbon                                   ...
Limitations of FIB Processing                                             Xidex’s Parallel MPGD ModuleThe ability to edit ...
System for Vapor Phase Editing                                           4 minutes. A summary of the beam energies, curren...
a)                                                          b)                                                   Figure 5 ...
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Xidex application note vapor phase editing of carbon nanotube based nanodevices 110313

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This Application Note describes an editing process for: (1) precise nanometer-scale linear etching operations, including carbon nanotube (CNT) cutting, shortening, cleaning, and other operations involving individual CNTs, and (2) precise micron-scale area etching operations, including cleaning entire areas of unwanted nanotube overgrowth. All of these operations were achieved using the NanoBot® nanomanipulator equipped with Xidex’s Parallel Multi-Precursor Gas Delivery (Parallel MPGD) system.

Xidex manufactures and sells the NanoBot® system, an easy-to-use, highly versatile, user programmable nanomanipulator featuring specialized end-effectors for nanodevice fabrication and testing inside scanning electron microscopes (SEMs) and focused ion beam (FIB) tools.

Our mission is to enhance the R&D productivity of nanoscientists and nanotechnologists in both industry and academia. We offer best-in-class turnkey solutions to customers with well-defined requirements that are stable over time, and highly adaptable solutions to customers who need a system that can easily be augmented with additional plug-and-play nanopositioners and end-effectors as their needs evolve.

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Xidex application note vapor phase editing of carbon nanotube based nanodevices 110313

  1. 1. Xidex Vapor Phase Editing of Carbon Nanotube Based Nanodevices: Using the NanoBot ® System with Gas Delivery Application Note Vladimir Mancevski, Xidex CorporationPrecise Site Selective CNT EditingThis Application Note describes an editing process for: (1) An extra CNTprecise nanometer-scale linear etching operations, including Lateral CNT to be removedcarbon nanotube (CNT) cutting, shortening, cleaning, and emitterother operations involving individual CNTs, and (2) precisemicron-scale area etching operations, including cleaningentire areas of unwanted nanotube overgrowth. All of theseoperations were achieved using the NanoBot ® Si Postnanomanipulator integrated with a gas delivery injectionmodule. Si Post 500 nmApplications that motivated this work include fabrication andrepair of CNT-based scanning probe microscope (SPM) tips Figure 2 – Example of a lateral (horizontal) CNT deviceand CNT-based electron emitters. Figures 1-3 show examples fabricated by Xidex for use as a lateral field emitter.of CNT cutting and area cleaning experiments performed inrelated research. Figure 1 shows SEM images of two sets ofcarbon nanotubes that have been cut using electron beaminduced etching process [1]. The CNTs in image a) wereetched using a line scan, and the CNTs in image c) were cutin a box scan. Image d) illustrates the com petitivecontamination that can accompany the process. Figure 2 500 nmshows an example of a lateral (horizontal) CNT devicefabricated by Xidex for use as a lateral field emitter. Figure 3 Figure 3 – An excess CNT strung from a silicon post (viewed top down) and the surface, before (left) and aftershows the result of using selective CNT etching to remove an (right) it was removed using vapor phase editing.extra CNT extending from a silicon post to the substrate. Nanomanipulator-Based Gas Delivery Nozzles It has been shown that operating a system of gas delivery nozzles with the NanoBot ® unit results in an optimized, localized precursor pressure and flux, while at reduced chamber pressures. One important outcome with the use of this gas delivery system is that the etching time and rate both improve by minimizing the separation distance between the sample surface and the gas nozzle. It has also been observed that, for a smaller sample-nozzle gap, the probe current decreases. This decrease in probe current is due to ionization and competitive positive current flow, which increases with decreased spacing because of enhanced local pressure. The end-result is that the higher local pressure is responsible for an increased and optimal etching rate. These Figure 1 - SEM images of two sets of carbon nanotubes results demonstrate that a smaller nozzle gap leads to a that have been cut using an electron beam induced faster etch rate and a lower sample current. etching (EBIE) process. 1
  2. 2. Limitations of FIB Processing Xidex’s Parallel MPGD ModuleThe ability to edit materials at the nanoscale level is critical Xidex’s Parallel Multi-Precursor Gas Delivery (MPGD) module isfor the ongoing nanotechnology revolution. While standard available as an end-effector for the NanoBot® nanomanipulator, andand emerging lithographic techniques will continue to play a can deliver water vapor locally to a sample inside an SEM. Thiscritical role in nano-fabrication processes, nano-fabrication enables a gas precursor-assisted electron beam-based process thatalso requires highly directed materials editing techniques can be used for precise site-selective carbon nanotube (CNT)which are site-selective. As geometries shrink and wafer editing. The result is a simple, effective, and damage-free way tocost-of-ownership increases, nanoscale re-manufacturing and control fabrication and repair of CNT-based nanodevices.repair techniques will be increasingly important. Currently Details of Xidex’s Parallel MPGD system can be downloaded in theadopted methods for selectively depositing or etching micro- “Products” section at www.Xidex.com.and nanoscale features utilize ion beam deposition and The Parallel MPGD system can be configured with internallyetching, laser ablative etching (using far field and near field mounted reservoirs, or with externally mounted reservoirs, as wasoptics), and mechanical abrasion using a fine micro-tip. Of done in this study (Figure 4). A multi-nozzle fixture is attached to athese techniques, selective focused ion beam (FIB) NanoBot X,Y,Z nanopositioning end-effector, located within the SEMprocessing is probably the most mature technology that has or FIB sample chamber. Up to four different gas precursors can bebeen extended into the nanoscale. While suitable for some accommodated. Examples include Pt, W, Au, TEOS, and O2, inapplications, FIB processing has several drawbacks that addition to water vapor. Key capabilities of this MPGD system are:make it difficult to extend into many other applications. Themost severe drawback when using a Ga+ FIB, is Ga+ (i) Valves mounted on the nanopositioner enable fast on-off controlimplantation into the substrate material, which can of the gas flow within each individual nozzle.deleteriously change the properties (optical, electrical, (ii) Each gas travels through a separate tube and nozzle, therebymechanical, and biological) of the material [2]. Additionally, precluding contamination from residual traces of a previouslycharging inherent to the ion-solid interaction causes proximity used process gas. This separation eliminates unwantedeffects that can lead to “riverbed effects” which erode nearby reactions between incompatible gasses, as may be the casefeatures while the heavy ion beam is scattered, and induce when multiple gasses share the same tubing.sputtering. Consequently, although focused ion beam (iii) Availability of separate delivery tubes for each gas enables fastprocessing is a very effective technique in many nanoscale switching between multiple gasses; i.e., without having to waitapplications [3], an alternative damage-free site-selective to purge the previous gas that was used.nanomaterials editing technique is needed for fabrication and (iv) The gas delivery nozzle assembly can be moved in threerepair of CNT-based devices used in many emerging orthogonal directions, using the X,Y,Z nanopositioning endapplications – enter the Xidex NanoBot system! effector, providing access to larger areas of the sample. This also allows better control of the nozzle-sample gap. Figure 4 – Parallel MPGD System with External Reservoirs 2
  3. 3. System for Vapor Phase Editing 4 minutes. A summary of the beam energies, currents, andA NanoBot system with an externally mounted water reservoir SEM settings investigated is provided in Table 2.was used to for site selective vapor phase editing of CNTs (asper Figure 4). The NanoBot unit and MPGD nozzle assemblywere mounted inside a Hitachi S 4000 SEM. This model is notan environmental SEM, and many other SEM brands andmodels are also compatible with the NanoBot system.Water vapor was delivered to a Gauge 26 metal nozzle with 254μm ID. With the help of the nanomanipulator, the nozzle couldbe placed 1 mm - 10 μm from the sample, depending on theangle of the nozzle with respect to the SEM stage. Relationship Between Nozzle-Sample Distance and Etching Time and RateRelationship Between Nozzle-Sample Distance and The relationship between the nozzle-sample distance and theBackground Operating Pressure etching time and rate was investigated by measuring theImproved etching of carbon nanotubes has been correlated with etching time and the sample current. The etching rate wasthe small distance between the nozzle and the sample, proving computed by knowing the size of the etched CNT. For thisthat the small distance between the nozzle and the sample experiment, the needle-sample distance was changed from 87results in an increase of the localized gas pressure which in turn μm to 328 μm by doubling the gap each iteration, and thenis responsible for the improved etching of carbon nanotubes. returning back to the smallest gap that could be used for that sample, 76 μm, to verify that some bias was not being built up.Because it is impossible to directly measure the localized Figure 6 shows the result of this trial. To be consistent, all CNTprecursor pressure at the sample, the local pressure has been cuts were done on the same multi micron long carbon nanotubecomputed by knowing the chamber pressure and the gas nozzle where each cut was a few microns away from the other. Forgeometry. To estimate the localized pressure/flux from the example in Figure 6, for 164 μm it can be seen that the CNTnozzle, a program initially developed by Kohlmann et al. [4] was was cut in a segment. During these experiments theused. The program inputs the flow rate of precursor gas (for magnification was also kept the same for all trials, at 35 kx forinstance in standard cubic centimeters per second, as imaging and 100 kx during etching. Table 3 lists all thedetermined from the throughput calculations). To find the parameters for this experiment.approximate gas spot area and associated pressures, the gasmolecular weight and temperature are also required. Finally,geometrical factors that determine the area to which the gas isapplied are input. Table 1 lists the parameters, units and somenotes regarding the model parameters. One important conclusion from this experiment is that the etching time and rate improved for a smaller gap between the sample and the nozzle, as shown in Table 3. Further, for a smaller gap the probe current decreased, as per Table 3. The change in the probe current as a function of distance is interpreted as ionization and competitive positive current flow,The next issue to be resolved was to determine the most which increases with a decrease in gap spacing, because of aneffective beam energy for etching, which turned out to be 5 keV. enhanced local pressure. Because the smaller nozzle distanceCNTs can also be cut with modest sample currents of 10−80 shows faster etching rates and lower sample currents, thepA, as shown in Figure 5. Although the etching process is results from Table 3 lead to the conclusion that the localslower at lower currents, one benefit of a low current is that the pressure is responsible for the increased etching rate.etching process is more selective. With currents of less than 10pA a CNT could not be etched in a reasonable time of less than 3
  4. 4. a) b) Figure 5 – Cutting a CNT, before (a) and after (b)Gap(μm) Gap Image CNT Before Cut CNT After Cut87328 Figure 6 – Demonstration of gas delivery system fixed to a nanomanipulator that allows precise positioning of the nozzle gap to the sample, with a range of 50 μm to 1 mm and more. The resulting nozzle proximity results in improved CNT etching capabilities. References NanoBot Sales [1] Images provided by Phip Rack, The University of Tennessee For product inquiries please contact: at Knoxville. Dr. Ray Eby [2] M.C. Peigon, Ch. Cardinaud, and G. Turban, J. Appl. Phys. 70(6), 15 phone: 312-545-6527 September pg. 3314-3323 (1991). e-mail: nanoray@sbcglobal.net [3] S.J. Randolph, J.D. Fowlkes, P.D. Rack, Critical Reviews of Solid For direct contact: State and Materials Sciences, Vol. 31, p. 55-89 (October 2006) Xidex Corporation [4] Kohlmann, K., Thiemann, M. and Bringer, W. E-beam induced X-ray 8906 Wall Street, Suite 703 fax: 512-339-9497 mask repair with optimized gas nozzle geometry. Microelectronic Austin, Texas 78754 e-mail: info@xidex.com phone: 512-339-0608 web: www.xidex.com Engineering 13 (1991), 279. Xidex Corporation © 2011, Xidex Corporation. All right reserved. NanoBot, Xidex, and Xidex manufactures and sells the NanoBot ® system, an easy-to- the Xidex logo are trademarks of Xidex Corporation. Other use, highly versatile, user-programmable nanomanipulator built for trademarks are property of their respective owners. Product use inside scanning electron microscopes (SEMs) and focused ion specifications and descriptions in this document are subject to beam (FIB) tools. The NanoBot system transforms a SEM or FIB change without notice into a workshop for nanodevice fabrication and testing. Xidex 110313 Corporation was founded in 1997 as an Austin -based Texas Corporation by Vladimir Mancevski, President and Chief Technology Officer and Dr. Paul F. McClure, CEO. 4

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