Name: Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices (Video) | JoVE Journal
Uploaded: 2017-12-07T14:00:00.000+00:00
Duration: 9 min 14 s
Description: Learn flow-assisted dielectrophoresis for fabricating high-performance silicon nanowire field effect transistors. This scalable method enables efficient self-assembly of solution-processable nanodevices.

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5 minutes

The authors would like to thank ESPRC and BAE systems for financial support, and Prof. Brian A. Korgel and his group for the supply of SFLS grown silicon nanowires used in this work.

Caution: All procedures unless otherwise stated take place in a clean room environment and risk assessments have been done to ensure safety during nanowires and chemicals handling. Nanomaterials may have a number of health implications which are as of yet unknown, and so should be handled with appropriate care21.
NOTE: The process starts with the preparation of the substrates, followed by the first photolithography and metal deposition steps to define the DEP contacts. The nanowires are then assembled via DEP and a further optional photolithographic and metal deposition step can be performed to deposit top contacts onto nanowires. The nanowire transistor devices current-voltage characteristics are then measured using a semiconductor characterization kit.
1. Preparation of Substrates
<ol>
	<li>Cut a doped n-type silicon/silicon dioxide wafer into suitable sizes, e.g., 2.5 cm2.</li>
	<li>During cutting, ensure the top surface of the wafer is not touched or scratched.</li>
	<li>Run a diamond scriber across the surface in one continuous motion to make a cut.</li>
	<li>Split the wafer along the cut.</li>
	<li>Place the samples on a substrate holder and sonicate for 5 min in an ultra-sonic bath at 100% power (450 W), first in deionized water, then acetone, and finally isopropanol (IPA). 
	NOTE: See Table of Materials for CAS numbers and suppliers.</li>
	<li>Dry the substrates with a nitrogen gun to remove any remaining IPA or dust from the surface.</li>
	<li>Plasma ash the samples in oxygen plasma at 100 W for 5 min to remove any remaining organic residues.</li>
</ol>
2. Photolithography Bilayer Process for Contacts
NOTE: A bilayer photolithography process is used to create electrodes. The photolithography process is conducted in a yellow room to prevent decay of photoresist materials.
<ol>
	<li>Heat the sample at 150 &#176;C for 15 min using a hotplate, to remove any remaining water from the surface. 
	NOTE: This is to ensure adhesion of photoresist; however chemical primers such as HMDS may also be used.</li>
	<li>Remove the sample from the hot plate and place it on a spin-coater.</li>
	<li>Using a&#160;pipette, drop approximately 1 mL of photoresist A on the surface until the entire sample is uniformly covered. 
	NOTE: See Table of Materials for the exact photoresist used.</li>
	<li>Spin the sample at 4,000 rpm for 45 s, to produce a film thickness of approximately 250 nm. If electrodes that are thicker than 150 nm are to be deposited, change this recipe.</li>
	<li>Remove the sample from the spin-coater and place it on a hotplate at 150 &#176;C for 5 min.</li>
	<li>Remove the sample from the hotplate and leave the sample to rest for 5 min in a 50% humidity box. This is to ensure rehydration of the photoresist22. 
	NOTE: If the humidity of the lab is greater than 50%, the sample may be left to rest in the air.</li>
	<li>Place the sample back onto the spin-coater and pipette approximately 1 mL of photoresist B on the surface of the substrate.</li>
	<li>Spin the sample at 3,500 rpm for 45 s, giving a film thickness of approximately 500 nm.</li>
	<li>Place the sample on a hotplate at 120 &#176;C for 2 min.</li>
	<li>Remove the sample from the hotplate and leave to rest in a 50% humidity box for 5 min.</li>
	<li>Expose the sample using a mask-aligner and photomask to UV light for 6.7 s for a total of 180 mJ of exposure. 
	NOTE: The exact exposure dose might need to be adjusted depending on a particular model of mask aligner.</li>
	<li>Remove the sample from the mask-aligner and develop by immersing it in photoresist developer&#160;for 30 s. 
	NOTE: See Table of Materials for the exact developer.</li>
	<li>Remove the sample from the developer, immerse the sample into deionized water, and rinse it to stop the development process.</li>
	<li>Check the photolithography using an optical microscope. A polarizer can be used to check the bilayer undercut which should appear as faint lines around the channel. The time may be adjusted if too much or two little undercut is achieved.</li>
</ol>
3. Deposition of Metal Contacts
NOTE: Electron beam (E-beam) deposition is used to deposit electrodes onto the prepared photoresist. This process can also use thermal evaporators or other types of metal thin film deposition techniques.
<ol>
	<li>Place the samples in the E-beam chamber; pump it down until a high vacuum is reached. In this case, a vacuum of around 1 x 10-6 mTorr is reached.</li>
	<li>Deposit 2 - 6 nm of titanium which acts as an adhesion layer followed by 30 nm of gold for the DEP contacts.</li>
	<li>Remove the samples from the E-beam chamber.</li>
	<li>Perform the lift-off procedure by removing most of the photoresist and excess metal. This is done by placing the samples into a beaker of photoresist remover for 15 min.</li>
	<li>Remove the samples from the beaker of photoresist remover A and place into another clean beaker of photoresist remover&#160;for a further 15 min. This is to prevent any large metal particles from settling on the sample.</li>
	<li>Complete lift-off by sonicating the beaker for 5 min at 50% power.</li>
	<li>Remove the samples from the bath one-by-one, ensuring to rinse off any material with IPA to prevent undesired metal particles from settling between electrodes. 
	NOTE: The electrodes are now prepared for the DEP alignment of nanowires.</li>
</ol>
4. DEP of Nanowires
<ol>
	<li>Prepare a solution of silicon or other nanowires in anisole of approximately 1 &#181;g/mL concentration. In this experiment, the solution is briefly sonicated for 15 s at the lowest power setting possible to remove any flocculation. Other solvents can be used such as toluene and N,N-dimethylformamide (DMF)1.</li>
	<li>Check the solution by drop casting a 10 &#181;L of the nanowire formulation onto a sacrificial substrate.</li>
	<li>Inspect the substrate with deposited nanowires using a polarized optical microscope (POM). The silicon nanowires are birefringent and hence can be easily seen in POM. If there are no nanowire clumps visible, and most of nanowires are well dispersed on the substrate, then the next stage can begin, otherwise the solution is re-sonicated, and the nanowire concentration might need to be adjusted. It may take several attempts to achieve the correct nanowire dispersion.</li>
	<li>Place the prepared sample with electrodes onto 30&#176; (vs. horizon) inclined platform with the device channel aligned horizontally. The dispersion flow direction needs to be perpendicular to the electrodes edges to allow more efficient nanowire alignment.</li>
	<li>Contact the electrodes using micro-probes connected to a frequency generator1.</li>
	<li>Set the desired frequency and voltage are on the frequency generator. In this experiment, use a DEP signal voltage of 10 V peak-to-peak and a 1 MHz sinewave. 
	NOTE: Increasing the frequency up to 20 MHz can help to collect nanowires with high conductivity and low trap density1,2. See reference1 for a detailed discussion. DEP signal frequency range indicated here was obtained by conducting SFLS Si nanowires impedance spectroscopy and collection time analysis, as described in reference1. Other types of nanowires with higher or lower charge carrier mobility, doped nanowires, or nanowires obtained by other growth methods can have different DEP signal frequency range resulting in the collection of high quality nanowires.</li>
	<li>Switch-on the frequency generator and drop approximately 10 &#181;L of nanowire solution using a micropipette onto the device area. 
	NOTE: Placing the sample at an angle (30&#176;) helps to create a gravity-assisted slow flow of the liquid. Alternatively, a capillary action using a glass slide can be used6.</li>
	<li>Apply the DEP signal for 30 s and then switch-off the frequency generator.</li>
	<li>Remove the sample and rinse very gently with IPA.</li>
	<li>Dry-off the sample very gently using a nitrogen gun. A polarized optical microscope may be used to inspect the sample and adjust parameters 
	NOTE: The DEP signal voltage, frequency, and the nanowire dispersion density can be adjusted to achieve a reproducible desired density of nanowires, from a few nanowires to a few hundred per device1,2.</li>
</ol>
5. Deposition of a Secondary Metal Layer
<ol>
	<li>To achieve improved current injection in NW FETs, apply a second metallic contact on top of the nanowire. 
	NOTE: This contact deposition process follows the exact same steps as sections 2 and 3, in both photolithography and metal deposition, except that only a gold layer is deposited.</li>
</ol>
6. I-V Characterization of Nanowire Devices
NOTE: The samples are now complete and can be used in subsequent experiments or their I-V characteristics can be measured to establish nanowire FET electrical properties. The fabricated devices are back-gated FETs, where doped silicon wafer serves as the common gate, and SiO2 layer serves as the gate dielectric.
<ol>
	<li>To establish electrical contact with the gate, remove a small area of the silicon oxide at the edge of the sample using a diamond scriber.</li>
	<li>Use a nitrogen gun to remove any undesired silicon dioxide particles.</li>
	<li>Place three microprobes (source, drain, and gate) on the gold source-drain electrode contacts, with the gate probe on the area with removed SiO2.</li>
	<li>Use a semiconductor characterization system to take I-V measurements.</li>
	<li>Measure transfer and output scans of NW FETs as these give information on the performance of the device and the electrical properties of the nanowires1,17,23. Note that transfer measurements involve stepping source-drain voltage and sweeping gate voltage. The output characteristics are measured by sweeping source-drain voltage and stepping gate voltage.</li>
</ol>

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<li>Doh, I., Cho, Y. H. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0924424705000464">A continuous cell separation chip using hydrodynamic dielectrophoresis (DEP) process.</a> Sensrs. and Actrs, A: Phys. 121 (1), 59-65 (2005).</li>
<li>Freer, E. M., Grachev, O., Duan, X., Martin, S., Stumbo, D. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High-yield+self-limiting+single-nanowire+assembly+with+dielectrophoresis%5BTitle%5D)+AND+%22Nat.+nanotech%22%5BJournal%5D)">High-yield self-limiting single-nanowire assembly with dielectrophoresis.</a> Nat. nanotech. 5 (7), 525-530 (2010).</li>
<li>Monica, A. H., Papadakis, S. J., Osiander, R., Paranjape, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Wafer-level+assembly+of+carbon+nanotube+networks+using+dielectrophoresis%5BTitle%5D)+AND+%22Nanotech%22%5BJournal%5D)">Wafer-level assembly of carbon nanotube networks using dielectrophoresis.</a> Nanotech. 19, 85303(2008).</li>
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<li>Hanrath, T., Korgel, B. A. <a target="_blank" href="http://onlinelibrary.wiley.com/doi/10.1002/adma.200390101/abstract">Supercritical fluid-liquid-solid (SFLS) synthesis of Si and Ge nanowires seeded by colloidal metal nanocrystals.</a> Adv. Mat. 15 (5), 437-440 (2003).</li>
<li>Heitsch, A. T., Akhavan, V. A., Korgel, B. A. <a target="_blank" href="http://pubs.acs.org/doi/abs/10.1021/cm2007704">Rapid SFLS synthesis of Si nanowires using trisilane with in situ alkyl-amine passivation.</a> Chem. of Mat. 23 (11), 2697-2699 (2011).</li>
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The authors confirm that there are no conflicts of interest.

The successful fabrication and performance of the devices depend on several key factors. These include nanowire density and distribution in the formulation, the choice of solvent, the frequency of DEP, and the control of the number of nanowires present on the device electrodes1.
One of the critical steps in achieving repeatable working devices is the preparation of a nanowire formulation without clusters or clumps. The formulation can be sonicated before DEP to reduce t...

Controllable and reproducible assembly of nanoparticles in pre-defined substrate locations is one of the main challenges in solution-processed electronic and photonic devices utilizing semiconducting or conducting nanoparticles. For high performance devices, it is also highly beneficial to be able to select nanoparticles with preferential sizes, and particular electronic properties, including, for example, high conductivity and low density of surface trap states. Despite significant progress in nanomaterials growth, including nanowire and nanotube materials, some variations of nanoparticle properties are always present, and a selection step can significantly improve nanoparticle-based device performance1,2.
The purpose of the flow-assisted DEP method demonstrated in this work is to address the above challenges by showing controllable semiconducting nanowires assembly onto metallic contacts for high performance nanowire field effect transistors. DEP solves several problems of nanowire device fabrication in a single step including positioning of nanowires, alignment/orientation of nanowires, and selection of nanowires with desired properties via DEP signal frequency selection1. DEP has been used for numerous other devices ranging from gas sensors3, transistors1, and RF switches4,5, to the positioning of bacteria for analysis7.
DEP is the manipulation of polarizable particles via the application of a non-uniform electric field resulting in nanowires self-assembly across the electrodes8. The method was originally developed for the manipulation of bacteria9,10 but has since been expanded to the manipulation of nanowires and nanomaterials.
DEP solution processing of nanoparticles enables semiconductor device fabrication that significantly differs from traditional top-down techniques based on multiple photomasking, ion implantation, high temperature14, annealing, and etching steps. Since DEP manipulates nanoparticles that have already been synthesized, it is a low-temperature, bottom-up fabrication technique11. This approach allows large-scale nanowire devices to be assembled on almost any substrate including temperature-sensitive, flexible plastic substrates6,12,13.
In this work, high performance p-type silicon nanowire field effect transistors are fabricated using flow-assisted DEP, and the FET current-voltage characterization is conducted. The silicon nanowires used in this work are grown via the Super Fluid Liquid Solid (SFLS) method15,16. The nanowires are intentionally&#160;doped, and are approximately 10 - 50 &#181;m in length and 30 - 40 nm in diameter. The SFLS growth method is very attractive since it can offer industry scalable amounts of nanowire materials15. The proposed nanowire assembly methodology is directly applicable to other semiconductor nanowire materials such as InAs13, SnO23, and GaN18. The technique can also be expanded to align conductive nanowires19 and to position nanoparticles across electrode gaps20.

<table><tbody><tr><td>Silicon/silicon dioxide wafer, CZ method growth, 100mm diameter,&amp;nbsp; 300 nm oxide thermal growth,&amp;nbsp; n-doped phosphorus</td><td>Si-Mat (Silicon materials)</td><td>-</td><td>http://si-mat.com/</td></tr><tr><td>Acetone (200ml)</td><td>Sigma Aldrich</td><td>W332615</td><td>-</td></tr><tr><td>Isopropanol (200ml)</td><td>Sigma Aldrich</td><td>W292907</td><td>-</td></tr><tr><td>Deionised water (150ml)</td><td>On site supply</td><td>-</td><td>-</td></tr><tr><td>Photoresist (A) SF6 PMGI under etch photoresit (approx 1 ml per sample)</td><td>Microchem&amp;nbsp;</td><td>-</td><td>http://microchem.com/pdf/PMGI-Resists-data-sheetV-rhcedit-102206.pdf</td></tr><tr><td>Photoresist (B) S1805 photoresit) (approx 1 ml per sample)</td><td>Microchem&amp;nbsp;</td><td>-</td><td>http://www.microchem.com/PDFs_Dow/S1800.pdf</td></tr><tr><td>Photoresist developer Microposit&amp;nbsp; MF319&amp;nbsp; (100ml)</td><td>Microchem&amp;nbsp;</td><td>-</td><td>http://microchem.com/products/images/uploads/MF_319_Data_Sheet.pdf</td></tr><tr><td>Photoresist remover Microposit remover 1165 (300ml (2 baths 150 each))</td><td>Microchem&amp;nbsp;</td><td>-</td><td>http://micromaterialstech.com/wp-content/dow_electronic_materials/datasheets/1165_Remover.pdf</td></tr></tbody></table>

flow-assisted dielectrophoresis: a low cost method for the fabrication of high performance solution-processable nanowire devices

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

Bilayer photolithography results in clean sharply defined electrodes. In the example (Figure 1A), inter-digitated finger structure was used with a channel length of 10 µm. These structures allow a large area to assemble the maximum number of nanowires when the DEP force is applied. Figure 1B shows a schematic of a bottom-gate nanowire FET device.
Incorrect nanowire disp...

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

flow-assisted-dielectrophoresis-low-cost-method-for-fabrication-high

Research

JoVE Journal

Engineering

Dielectrophoresis (DEP) is an effective method to manipulate cells.  Printed circuit boards (PCB) can provide inexpensive, reusable and effective electrodes for contact-free cell manipulation within microfluidic devices.  By combining PDMS-based microfluidic channels with coverslips on PCBs, we demonstrate bead and cell manipulation and separation within multichannel microfluidic devices.

Separating Beads and Cells in Multi-channel Microfluidic Devices Using Dielectrophoresis and Laminar Flow

We describe the device fabrication and measurement protocol for carbon nanotube based high frequency biosensors. The high frequency sensing technique mitigates the fundamental ionic (Debye) screening effect and allows nanotube biosensor to be operated in high ionic strength solutions where conventional electronic biosensors fail. Our technology provides a unique platform for point-of-care (POC) electronic biosensors operating in physiologically relevant conditions.

Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

Contactless dielectrophoresis (cDEP) achieves sorting and enrichment of particles via their intrinsic dielectric properties. Fluidic electrode channels replace metallic electrodes traditional to DEP, suiting cDEP to non-damaging sterile characterization and sorting of biological particles. We demonstrate how to prepare a cDEP microdevice and conduct cell characterization and sorting experiments.

Label-free Isolation and Enrichment of Cells Through Contactless Dielectrophoresis

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

This protocol demonstrates how to achieve femto molar detection sensitivity of proteins in 10 µL of whole blood within 30 min. This can be achieved by using electrospun nanofibrous mats integrated in a lab-on-a-disc, which offers high surface area as well as effective mixing and washing for enhanced signal-to-noise ratio.

Fully Automated Centrifugal Microfluidic Device for Ultrasensitive Protein Detection from Whole Blood

We propose a simple self-assembly technique of silica colloidal nanoparticles to create a nanofluidic junction between two microchannels in polydimethylsiloxane (PDMS). Using this technique, a nanoporous bead membrane with a pore size down to ~45 nm was built inside a microchannel and applied to electrokinetic preconcentration of DNA samples.

Creating Sub-50 Nm Nanofluidic Junctions in PDMS Microfluidic Chip via Self-Assembly Process of Colloidal Particles

This protocol describes a solution-based fabrication strategy for high-performance, flexible, transparent electrodes with fully-embedded, thick metal mesh. Flexible transparent electrodes fabricated by this process demonstrate among the highest reported performances, including ultra-low sheet resistance, high optical transmittance, mechanical stability under bending, strong substrate adhesion, surface smoothness, and environmental stability.

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

The article aims to introduce a standard and reliable fabrication procedure for the development of future low dimensional nanoelectronics.

A Standard and Reliable Method to Fabricate Two-Dimensional Nanoelectronics

In this work, we describe a protocol to fabricate iron nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

This protocol describes a straightforward process that utilizes convenient plastic micro-molds for simple microembossing operations to fabricate microchannels on nanofibrillated cellulose paper, achieving a minimum width of 200 µm.

Microembossing: A Convenient Process for Fabricating Microchannels on Nanocellulose Paper-Based Microfluidics

Learn flow-assisted dielectrophoresis for fabricating high-performance silicon nanowire field effect transistors. This scalable method enables efficient self-assembly of solution-processable nanodevices.

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices (Video) | JoVE Journal

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

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