Focused ion beam production of nanoelectrode arrays

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Materials Science and Engineering C 28 (2008) 777 – 780 www.elsevier.com/locate/msec

Short communication

Focused ion beam production of nanoelectrode arrays A. Errachid a,⁎, C.A. Mills a , M. Pla-Roca a , M.J. Lopez a , G. Villanueva b , J. Bausells b , E. Crespo c,1 , F. Teixidor c , J. Samitier a a

Nanobioengineering Research Laboratory (IBEC) and Electronic Department of University of Barcelona (CIBER), C/ Martí I Franqués 1, 08028 Barcelona, Spain b Centro Nacional de Microelectronica (IMB-CSIC), Campus UAB, 08193 Bellaterra, Spain c Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) Campus UAB, Spain Available online 18 October 2007

Abstract We present a method for the production of nanoelectrodes using focussed ion beam techniques (FIB). The electrodes utilise nanometric holes milled in a silicon nitride based pasivation layer, followed by wet etching of a silicon oxide based pasivation layer, to expose an underlying gold electrode. After functionalisation using a surface assembled monolayer and an electrochemically grown polypyrrole, these gold nanoelectrodes have been tested, via cyclic voltammetry, in the detection of [Fe(CN)6]4−/3− ions. The nanoelectrodes will be used to investigate the electrical properties of nanometric biological specimen. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanoelectrodes; Focused ion beam; Cyclic voltammetry; Polypyrrole

1. Introduction To investigate the electrical properties of individual nanometric biological specimen, such as single biomolecules, electrodes with dimensions on an equivalent scale need to be developed. These nanoelectrodes present technological development challenges due to their small size. However, once fabricated, the electrodes offer the ability to examine the fundamental electronic processes occurring between individual nanometric species and their surroundings. After production, the electrodes must be carefully managed to avoid electrical breakdown of the substrate or electrode damage, due to the high field present at the electrode/substrate interface [1]. Therefore, the electrode design must be carefully considered, to optimise electrode shape and proximity, and fabrication must be performed with care. The drive towards the production of nanoelectrode architectures stems from a number of advantages. Primarily, a decrease

⁎ Corresponding author. Tel.: +34 93 403 71 79; fax: +34 93 403 71 81. E-mail address: [email protected] (A. Errachid). 1 Eulalia Crespo is enrolled in the U.A.B. PhD program. 0928-4931/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2007.10.077

in electrode dimensions means that they occupy a smaller area than analogous microelectrodes. The small size of the electrodes can also enhance selectivity and sensitivity. The improvement of these latter properties as a function of electrode size stems from the fact that only a small number of analyte molecules can be detected at a nanoelectrode's surface. Using a suitable sensing layer, significant selection and sensitivity improvements can be achieved. A common sensing layer for biosensors is polypyrrole, which has been developed as an interface medium between inorganic electrodes and biological and chemical sensing layers [2,3]. Normally, such nanoelectrodes are produced in distinct planar architectures, based on two-electrode configurations, for applications such as single molecule biosensors [1], and interdigitated electrodes, where a thin film sensing element is used [4]. Such electrodes have been fabricated using techniques such as electron beam lithography (EBL) [5], mechanical break contacts [6] and electrochemical deposition [7]. Each of these techniques has their own drawbacks, ranging from the requirement for a chemical resist layer [8] to lack of reproducibility in electrode production. Here, we describe the production of nanogap electrodes in a vertical architecture using a novel technique based on the combination of focussed ion beam (FIB) milling with wet

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Fig. 1. Device architecture: 1. Silicon substrate, 2. SiO2 layer (800 nm thick), 3. Au/Cr layer (50/30 nm thick), 4. SiO2 passivation (400 nm thick), 5. Si3N4 layer (400 nm thick), 6. Epoxy encapsulation, 7. Connections to Au layer, 8. FIB milled area, 9. SiO2 etched area.

etching. FIB milling can be utilised as a direct process for the removal of material from a required substrate [9]. It therefore precludes the need for a resist (and its associated chemistry), such as that required in EBL. The electrodes are fabricated for integration into nanoelectrode arrays and have been used to produce cyclic voltammetric analysis of iron species in solution. 2. Experimental 2.1. Nanoelectrode fabrication The microelectronics fabrication process for the planar nanoelectrodes consists of 2 photolithographic steps. Thermal oxidation is used to grow an oxide layer (0.8 μm thick) on ptype silicon (525 μm thick). A photoresist layer is then applied and electrode shapes defined using standard UV photolithography. A chromium/gold layer (30 nm/50 nm thick) is thermally deposited and patterned by lift-off. SiO2 (400 nm thick) and Si3N4 (400 nm thick) PECVD layers are then deposited which act as passivation layers. Contact pads for electrical connection are opened by reactive ion etching (RIE). After dicing the device, FIB is used, at beam currents of 30 pA, to directly remove Si3N4 to a depth just entering the underlying SiO2 layer. The SiO2 is then wet etched using Sioetch to expose the underlying Au. The device architecture is given in Fig. 1. Fig. 2 shows arrays of nanoelectrodes with diameters from 300 to 140 nm. Fig. 3 shows a single nanoelectrode of 40 nm diameter [10]. Finally, the nanoelectrodes are mounted, wirebonded and encapsulated using epoxy resin.

Fig. 3. SEM image of a single nanoelectrode with a 40 nm diameter fabricated by FIB/etching.

2.2. Monolayer functionalisation The nanoelectrodes were functionalised, and used to detect [Fe(CN)6]4−/3− ions using cyclic voltammetry (CV). The electrode surface was cleaned and immediately immersed in a solution of 16–mercaptohexadecanoic acid (MHDA, 1 mM) and the phospholipid dipalmitoyl–sn–glycero–3–phospho– ethanolamine–N–(biotinyl) (biotinyl–PE, 0.1 mM) in ethanol (21 h, r. t.), forming a mixed monolayer on the electrode surface [11,12]. 2.3. Electrochemical polymerization of PPy[3,3′–Co(1,2–C2B9H11)2] The poly(pyrrole) was prepared with the dopant ion carboltbis ion (PPy[3,3′–Co(1,2–C2B9H11)2]) films galvanostatically grown over the surface of pristine gold nanoelectrodes, in a single compartment cell with a standard three electrode system, and at a constant current, using an EG&G PAR273A potentiostat–galvanostat. Electropolymerisation was carried out from a solution of pyrrole (0.1 M), Cs[3,3′–Co(1,2–C2B9H11)2] (0.035 M) and acetonitrile (MeCN, 1 wt.%) in water [13–16]. Typical operating conditions were 100 mA applied current for 10 s. The extent of film growth was determined by measuring the electrical charge through the electrodes during polymerisation

Fig. 2. SEM images of nanoelectrode arrays with diameters of (a) 300 nm, (b) 200 nm, and (c) 140 nm. The FIB milled and subsequently etched holes penetrate through the silicon based upper layers to the lower gold electrode.

A. Errachid et al. / Materials Science and Engineering C 28 (2008) 777–780

Fig. 4. Variation of the potential of a nano-ISE during the electropolymerisation growth of PPy[3,3′–Co(1,2–C2B9H11)2].

(Fig. 4). Polymer film growth showed that there was a suitable electrical connection to form the required bond between the PPy [3,3′–Co(1,2–C2B9H11)2] and the Au nanoelectrode. 2.4. Electrochemical characterisation The nanoelectrodes were characterised using a classical 3electrode electrochemical cell (Voltalab 400, Radiometer Analytical) containing a buffer solution of phosphate buffered saline (PBS, pH = 7) with NaCl (140 mM), KCl (2,7 mM), Na2HPO4 (0.1 mM), KH2PO4 (1.8 mM), and the redox couple Fe(CN)63−/Fe(CN)64− (5 mM) (Fig. 5). 3. Results Fig. 6 shows the pristine Au nanoelectrode response in the presence of [Fe(CN)6]4−/3− . The appearance of current upon applying potential suggests that the etching step was successful and that no problems occur due to capillary effects. The shape of the cyclic voltammograms of current vs. potential for oxidation of ferrocene show similar forms to those described previously in the literature [17,18]. Due to the diffusion of solutions at the surfaces of nano-electrodes, the total diffusion limited current is composed of the planar flux and radial flux diffusion compo-

Fig. 5. Electrochemical cyclic voltammetry cell: 1. Gold nanoelectrodes (WE), 2. Platinum counter electrode, 3. Saturated calomel reference electrode (SCE).

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Fig. 6. Cyclic voltammetry using gold nanoelectrodes in PBS solution in the presence of 5 mM [Fe(CN)6]4−/3−, before (black line) and after (red line) thiol SAM functionalisation.

nents. This leads to a non-uniform current density, with higher currents at the edges of the electrodes, which leads to sigmoidal voltammograms. After functionalisation with the thiol SAM, the reduction in the current in the cyclic voltammagram, from 7 to 3 mA cm− 2 at − 400 mV, shows that the Au surface has been functionalised, although optimisation of the functionalisation is required to decrease the current to a nominal value. To achieve this, the immersion time of the nanoelectrodes in the MHDA/biotinyl– PE solution needs to be increased in order to obtain complete coverage. Cyclic voltammograms recorded after electrodeposition of the polypyrrole (Fig. 7) indicated that the polymer was conductive and electroactive in sodium nitrate (NaNO3) solution (0.1 M) at a scan rate of 50 mV s− 1. However, when compared to pristine gold nanoelectrodes (Fig. 6), the polypyrrole coated electrodes produced higher oxidation and reduction currents (Fig. 7). This can be explained as being due to the nitrate ions present in the conducting polymer during the oxidation/reduction process.

Fig. 7. Cyclic voltammetry of PPy[3,3′–Co(1,2–C2B9H11)2] onto a gold nanoelectrode. Initial scan in supporting NaNO3 electrolyte solution (0.1 M) at a scan rate of 50 mV s− 1.

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4. Conclusions A novel nanoelectrode array architecture has been presented using a rapid production method combining FIB and wet etching techniques; the first time this method has been used for the production of chemical sensors, and will lead to the production of nano-biosensor arrays. We have demonstrated that such nanoelectrode arrays can be simply functionalised via SAM or electrochemical deposition of polymeric sensing layers, and can be used for cyclic voltammetric analysis of ions in solution. Further development of the electrode system could integrate counter and pseudo-reference (Ag/AgCl) nanoelectrodes, which can be scaled down to nanometer dimensions required for single molecule biosensors. Future studies will focus on using the nanoelectrodes for the detection and examination of other redox couples in solution with a rapid, robust response. Acknowledgement The authors would like to acknowledge the support for this work through the European 6th Framework projects “SpotNOSED” and “CellPROM”, and the Spanish ministry for Science and Education through the projects (TEC2005-07996C02-02/MIC, HF2004-0055 and MAT2006-05339) and from DURSI under grant 2005/SGR/00709. C. Mills acknowledges support from the ‘Ramon y Cajal’ programme. References [1] G. Maruccio, P. Visconti, S. D'Amico, P. Calogiuri, E. D'Amone, R. Cingolani, R. Rinaldi, Microelectron. Eng. 67-68 (2003) 838. [2] S. Hamilton, M.J. Hepher, J. Sommerville, Sens. Actuators, B, Chem. 107 (2005) 424.

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