07) 3829–3832www.elsevier.com/locate/matlet

Materials Letters 61 (20

Microelectrodes fabrication using laser scissor

Sandeep Kumar, Rajesh Kumar, Awdhesh Kumar Shukla, Lalit M. Bharadwaj ⁎

Biomolecular Electronics and Nanotechnology Division (BEND), Central Scientific Instruments Organisation (CSIO), Sector 30-C, Chandigarh, India

Received 6 October 2006; accepted 19 December 2006Available online 8 January 2007

Abstract

Today laser microdissection system or laser scissor is frequently used for isolation of single chromosome, nucleus, cell, tissue etc. Here wefabricated the gold microelectrodes having a gap spacing of less than 3 μm using this system. A 500 Å thickness of gold was coated on standardglass slide. A UV-laser (λ 337 nm) of 4 ns pulse duration having an energy of 22 μJ is sufficient to cold ablate the gold, was used for fabrication ofgold electrodes on the above slides. Microstructures up to the resolution limit (0.25 μm) of optical microscope can be fabricated using this coldablation by laser.© 2007 Published by Elsevier B.V.

Keywords: Optical Tweezer; Microelectrodes; Laser

1. Introduction

The micro- and nano-fabrication technologies provide easyfabrication of micro- and nano-electrode arrays, nanostructure,microfluidics and peripherals for the development of “labora-tory-on-a-chip” technology. These devices boost the low costresearch in areas of nanobiotechnology, biotechnology, targeteddrug delivery, Bio-MEMS, Biomolecular electronics etc.Microelectrodes are commonly used in electrochemistry basedbiosensors due to their small physical size, which have ad-vantage over larger electrodes in terms of sensitivity, decreasedinfluence of the solution resistance, low charging currents,greater signal-to-noise, and the ability to perform measurementsin extremely small microenvironments or sub-microliter samplevolumes [1–3]. A number of techniques have been proposed toprepare microelectrodes and microelectrode arrays [4–6].Microfabrication using conventional lithography requires highcost masks and other maskless lithography techniques aretedious and time consuming. Here we fabricated the goldmicroelectrodes by simple technique without any mask usinglaser scissor. Laser scissor is a laser-based micropreparationtechnique that allows one to selectively cut out and specificallyobtain any type of desired pattern. It is quick and convenient and

⁎ Corresponding author. Tel.: +91 172 2656285; fax: +91 172 2657267.E-mail address: lalitmbharadwaj@hotmail.com (L.M. Bharadwaj).

0167-577X/$ - see front matter © 2007 Published by Elsevier B.V.doi:10.1016/j.matlet.2006.12.076

above all can be performed without mechanical contact; thus, itis possible to work without danger of contamination or infection.Optical/laser microdissection system or laser scissor is com-monly used for isolation of single chromosome, nucleus, cell,tissue etc. The extremely high photon density in the narrowfocus of the pulsed laser allows the system to cut or ablatebiological structures and thin films. The physical principle oflaser cutting is a locally restricted ablative photodecompositionprocess without heating, so it is possible to ablate desired patternwithout damaging adjacent regions [7,8]. Optical micromanip-ulation with noncontact laser microbeams has been demonstrat-ed to be an alternative approach to the more common proceduresusing mechanical microtools. This simple, precise techniquesignificantly advances laser technology for microdissection andisolation [9–11]. This technique is similar to the direct writing bylaser; in this gold is cold ablated directly by 337 nm lasermicrobeam. The laser cuts around the target specimen, whichyields a clear-cut gap between the selected and nonselectedspecimens. In addition, unwanted material within a largerselected area can be selectively destroyed with a few laser shots.

2. Experimental

2.1. Gold deposition on glass surface

Gold coating on glass surface was carried out at roomtemperature by thermal evaporation using HIND HIVAC

Fig. 1. Effect of different energy of laser on gold ablation.

Table 1Effect of laser energy on gold ablation on gold coated glass surface

Serial no. Energy availableat objective (μJ)

Peak powerper pulse (kW)

Average power(mW)

Width of spot(in μm) ongold slide

1 20 5 0.66 –2 21.2 5.3 0.69 –3 22 5.5 0.72 1.6414 24 6 0.79 3.3595 26 6.5 0.85 4.6986 28 7 0.92 5.9707 30 7.5 0.99 8.0478 32 8 1.05 8.6729 34 8.5 1.12 8.35910 36 9 1.18 9.30511 38 9.5 1.25 10.64312 40 10 1.32 11.719

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VACUUM COATING UNIT. The pressure during evapora-tion was kept at 2×10−5 mbar. Thickness of deposited goldlayer was determined by surface texture analysis systemDEKTAK 3030 ST (Sloan, Veeco) and was found to be500 Å.

2.2. Cleaning of gold coated slide

The gold surface was thoroughly cleaned sequentially inmethanol, acetone and isopropanol-2 for 10 min each. Thesubstrate was then rinsed three times with deionized water.Finally the substrate was rinsed with ethanol and wasdried.

2.3. Fabrication of microelectrodes using dissection system

Microelectrodes were fabricated using Optical Tweezer cummicrodissection combi system (PALM system, Germany).

2.4. Characterization of microelectrodes

Characterization of microelectrodes was carried out usingScanning Electron Microscope (JEOL JSM 6100) andSIGNATONE Probe Station, CA, USA having HewlettPackard 4155A Semiconductor Parameter Analyzer (internalresistance of ≥1013 Ω and current resolution of 10 fA) andfitted with Bausch and Lomb Micro zoom II High PerformanceMicroscope.

3. Results and discussion

We used Optical Tweezer cummicrodissector system (PALMCombi)for fabrication of microelectrodes. Gold-coated slide was observed underbright field microscope and gold pads were designed using the software.The gold was ablated by a pulsed laser beam (337 nm, nitrogen) that haspulse duration of 4 ns and a frequency of 33 Hz. The maximum energy oflaser beam was 300 μJ and out of 300 μJ only 40 μJ was available atobjective. Gold ablation depends upon the energy applied. We haveoptimized the energy for ablation of gold. FromFig. 1, it is clear that 20μJenergy at objective is insufficient to ablate the gold. 40 μJ energy gives

more gold ablation (Fig. 1). The power and energy per pulse duration wascalculated as follows;

Maximum energy of UV laser ¼ 300 μJ; Pulse width ¼ 4 ns;Frequency ¼ 33 Hz

Energy available at Objective after various losses ¼ 40 μJPeak power per pulse ¼ Energy=time ¼ 300 μJ=4 ns ¼ 75 kWPeak power per pulse at objective

¼Energy available at objective=time ¼ 40 μJ=4 ns ¼ 10 kWAverage power ¼ Ppeak � pulsewidth� frequency

From Table 1, it was observed that laser having energy of 22 μJ,power per pulse of 5.5 kW and average power of 0.72 mW can ablategold with a minimum width of 1.64 μm and maximum of 11.7 μm bylaser having an energy of 40 μJ, power per pulse of 10 kWand averagepower of 1.32 mW. We started gold ablation with low energy andincreased energy and hence peak power, average power in steps till weget some ablation. The peak power of 5 and 5.3 respectively did notproduce any spot on gold slide indicating that this much power isinsufficient to ablate the gold and can not be used to fabricate micro-/nano-electrodes of desired spacing. Further increase in peak powerbeyond this point i.e. 5.5 kW produces ablation and results in theformation of spot on the gold slide and acts as threshold peak power forthe formation of micro-/nano-electrodes using laser scissor.

Fig. 3. I–V characteristics of microelectrodes.

Fig. 2. Microelectrodes visualized under optical microscope at 40×.

3831S. Kumar et al. / Materials Letters 61 (2007) 3829–3832

Here we used energy of 26 μJ and average power of 0.85 mW forfabrication of gold microelectrodes having a gap of around 3 μm. Thepeak power per pulse is very high (around 10 kW at objective) and is

Fig. 4. SEM images of microelectrodes after

more than sufficient to remove the gold from the glass surface asdepicted in Fig. 1. By increasing exposure time of laser and by applyingit repeatedly, desired structures were achieved (Fig. 2). These

probing with signatone-probing station.

Fig. 5. SEM images of microelectrodes at high resolution.

3832 S. Kumar et al. / Materials Letters 61 (2007) 3829–3832

fabricated microelectrodes were then washed with acetone followed bydistilled water and dried. The microelectrodes were characterized usingsignatone-probing station having Hewlett Packard 4155A Semicon-ductor Parameter Analyzer (internal resistance of≥1013Ω and currentresolution of 10 fA). The current voltage characteristics showed opencircuit and high resistance indicating the complete removal of goldbetween electrodes (Fig. 3). The scanning electron microscope (SEM)images of microelectrodes taken after probing are shown in Fig. 4.Fig. 5 shows the SEM image of microelectrode at high resolution.

4. Conclusion

We have demonstrated the technique to fabricate microelec-trodes. This technique is very simple, precise, reliable and donot take much time to fabricate micro-/nano-electrodes of gapspacing of up to the resolution limit of the optical microscope.This technique can be used for fabrication of micro-/nano-structures of any design and pattern to meet research purposeand application demands. Complex, multi-material patternscould be made by using this technique for direct writing on thesubstrate. Since the reproduction is easy (as it can be

programmed using software to remove selected area at desiredlocations after specific gap/position repeatedly) and costeffective, the microelectrode array can even be used asdisposable electrode. As the density and resolution requiredfor microelectrode applications increases in the future, thesecapabilities will likely prove to be valuable. Electrodes of verysmall gap spacing can be used for immobilization of DNAbetween these and hence studying its electrical properties.

Acknowledgements

The authors greatly acknowledge the financial supportprovided by Department of Information Technology andDepartment of Science & Technology Government of India.Sandeep Kumar and Rajesh Kumar thanks Council of Scientificand Industrial Research for their research fellowship. Authorsare thankful to the Director Central Scientific InstrumentsOrganization (CSIO), Chandigarh, for providing necessaryfacilities. Special thanks are due to late Dr. Rakesh Kumar forthe stimulating discussions on the fabrication method.

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