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Microelectrodes fabrication using laser scissor
Materials Letters 61 (2007) 3829 – 3832 www.elsevier.com/locate/matlet
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 2006 Available 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 we fabricated the gold microelectrodes having a gap spacing of less than 3 μm using this system. A 500 Å thickness of gold was coated on standard glass 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 of gold electrodes on the above slides. Microstructures up to the resolution limit (0.25 μm) of optical microscope can be fabricated using this cold ablation by laser. © 2007 Published by Elsevier B.V. Keywords: Optical Tweezer; Microelectrodes; Laser
1. Introduction The micro- and nano-fabrication technologies provide easy fabrication of micro- and nano-electrode arrays, nanostructure, microfluidics and peripherals for the development of “laboratory-on-a-chip” technology. These devices boost the low cost research in areas of nanobiotechnology, biotechnology, targeted drug delivery, Bio-MEMS, Biomolecular electronics etc. Microelectrodes are commonly used in electrochemistry based biosensors due to their small physical size, which have advantage over larger electrodes in terms of sensitivity, decreased influence of the solution resistance, low charging currents, greater signal-to-noise, and the ability to perform measurements in extremely small microenvironments or sub-microliter sample volumes [1–3]. A number of techniques have been proposed to prepare microelectrodes and microelectrode arrays [4–6]. Microfabrication using conventional lithography requires high cost masks and other maskless lithography techniques are tedious and time consuming. Here we fabricated the gold microelectrodes by simple technique without any mask using laser scissor. Laser scissor is a laser-based micropreparation technique that allows one to selectively cut out and specifically obtain any type of desired pattern. It is quick and convenient and
above all can be performed without mechanical contact; thus, it is possible to work without danger of contamination or infection. Optical/laser microdissection system or laser scissor is commonly used for isolation of single chromosome, nucleus, cell, tissue etc. The extremely high photon density in the narrow focus of the pulsed laser allows the system to cut or ablate biological structures and thin films. The physical principle of laser cutting is a locally restricted ablative photodecomposition process without heating, so it is possible to ablate desired pattern without damaging adjacent regions [7,8]. Optical micromanipulation with noncontact laser microbeams has been demonstrated to be an alternative approach to the more common procedures using mechanical microtools. This simple, precise technique significantly advances laser technology for microdissection and isolation [9–11]. This technique is similar to the direct writing by laser; in this gold is cold ablated directly by 337 nm laser microbeam. The laser cuts around the target specimen, which yields a clear-cut gap between the selected and nonselected specimens. In addition, unwanted material within a larger selected area can be selectively destroyed with a few laser shots. 2. Experimental 2.1. Gold deposition on glass surface
⁎ Corresponding author. Tel.: +91 172 2656285; fax: +91 172 2657267. E-mail address: [email protected] (L.M. Bharadwaj). 0167-577X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.matlet.2006.12.076
Gold coating on glass surface was carried out at room temperature by thermal evaporation using HIND HIVAC
3830
S. Kumar et al. / Materials Letters 61 (2007) 3829–3832
Fig. 1. Effect of different energy of laser on gold ablation.
VACUUM COATING UNIT. The pressure during evaporation was kept at 2 × 10− 5 mbar. Thickness of deposited gold layer was determined by surface texture analysis system DEKTAK 3030 ST (Sloan, Veeco) and was found to be 500 Å.
more gold ablation (Fig. 1). The power and energy per pulse duration was calculated 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 μJ Peak power per pulse ¼ Energy=time ¼ 300 μJ=4 ns ¼ 75 kW Peak power per pulse at objective ¼ Energy available at objective=time ¼ 40 μJ=4 ns ¼ 10 kW Average power ¼ Ppeak pulsewidth frequency
2.2. Cleaning of gold coated slide The gold surface was thoroughly cleaned sequentially in methanol, acetone and isopropanol-2 for 10 min each. The substrate was then rinsed three times with deionized water. Finally the substrate was rinsed with ethanol and was dried. 2.3. Fabrication of microelectrodes using dissection system Microelectrodes were fabricated using Optical Tweezer cum microdissection combi system (PALM system, Germany). 2.4. Characterization of microelectrodes Characterization of microelectrodes was carried out using Scanning Electron Microscope (JEOL JSM 6100) and SIGNATONE Probe Station, CA, USA having Hewlett Packard 4155A Semiconductor Parameter Analyzer (internal resistance of ≥ 1013 Ω and current resolution of 10 fA) and fitted with Bausch and Lomb Micro zoom II High Performance Microscope. 3. Results and discussion We used Optical Tweezer cum microdissector system (PALM Combi) for fabrication of microelectrodes. Gold-coated slide was observed under bright field microscope and gold pads were designed using the software. The gold was ablated by a pulsed laser beam (337 nm, nitrogen) that has pulse duration of 4 ns and a frequency of 33 Hz. The maximum energy of laser beam was 300 μJ and out of 300 μJ only 40 μJ was available at objective. Gold ablation depends upon the energy applied. We have optimized the energy for ablation of gold. From Fig. 1, it is clear that 20 μJ energy at objective is insufficient to ablate the gold. 40 μJ energy gives
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 ablate gold with a minimum width of 1.64 μm and maximum of 11.7 μm by laser having an energy of 40 μJ, power per pulse of 10 kW and average power of 1.32 mW. We started gold ablation with low energy and increased energy and hence peak power, average power in steps till we get some ablation. The peak power of 5 and 5.3 respectively did not produce any spot on gold slide indicating that this much power is insufficient to ablate the gold and can not be used to fabricate micro-/ nano-electrodes of desired spacing. Further increase in peak power beyond this point i.e. 5.5 kW produces ablation and results in the formation of spot on the gold slide and acts as threshold peak power for the formation of micro-/nano-electrodes using laser scissor.
Table 1 Effect of laser energy on gold ablation on gold coated glass surface Serial no. Energy available Peak power Average power Width of spot at objective (μJ) per pulse (kW) (mW) (in μm) on gold slide 1 2 3 4 5 6 7 8 9 10 11 12
20 21.2 22 24 26 28 30 32 34 36 38 40
5 5.3 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
0.66 0.69 0.72 0.79 0.85 0.92 0.99 1.05 1.12 1.18 1.25 1.32
– – 1.641 3.359 4.698 5.970 8.047 8.672 8.359 9.305 10.643 11.719
S. Kumar et al. / Materials Letters 61 (2007) 3829–3832
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Fig. 2. Microelectrodes visualized under optical microscope at 40×.
Fig. 3. I–V characteristics of microelectrodes.
Here we used energy of 26 μJ and average power of 0.85 mW for fabrication of gold microelectrodes having a gap of around 3 μm. The peak power per pulse is very high (around 10 kW at objective) and is
more than sufficient to remove the gold from the glass surface as depicted in Fig. 1. By increasing exposure time of laser and by applying it repeatedly, desired structures were achieved (Fig. 2). These
Fig. 4. SEM images of microelectrodes after probing with signatone-probing station.
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S. Kumar et al. / Materials Letters 61 (2007) 3829–3832
programmed using software to remove selected area at desired locations after specific gap/position repeatedly) and cost effective, the microelectrode array can even be used as disposable electrode. As the density and resolution required for microelectrode applications increases in the future, these capabilities will likely prove to be valuable. Electrodes of very small gap spacing can be used for immobilization of DNA between these and hence studying its electrical properties. Acknowledgements
Fig. 5. SEM images of microelectrodes at high resolution.
fabricated microelectrodes were then washed with acetone followed by distilled water and dried. The microelectrodes were characterized using signatone-probing station having Hewlett Packard 4155A Semiconductor Parameter Analyzer (internal resistance of ≥1013 Ω and current resolution of 10 fA). The current voltage characteristics showed open circuit and high resistance indicating the complete removal of gold between 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 microelectrodes. This technique is very simple, precise, reliable and do not take much time to fabricate micro-/nano-electrodes of gap spacing of up to the resolution limit of the optical microscope. This technique can be used for fabrication of micro-/nanostructures of any design and pattern to meet research purpose and application demands. Complex, multi-material patterns could be made by using this technique for direct writing on the substrate. Since the reproduction is easy (as it can be
The authors greatly acknowledge the financial support provided by Department of Information Technology and Department of Science & Technology Government of India. Sandeep Kumar and Rajesh Kumar thanks Council of Scientific and Industrial Research for their research fellowship. Authors are thankful to the Director Central Scientific Instruments Organization (CSIO), Chandigarh, for providing necessary facilities. Special thanks are due to late Dr. Rakesh Kumar for the stimulating discussions on the fabrication method. References [1] J. Heinze, Angew. Chem., Int. Ed. Engl. 32 (1993) 1268. [2] R.M. Wightman, D.O. Wipf, Electroanalytical, vol. 15, 1988, p. 267, Chapter 3. [3] S. Pons, M. Fleischman, Anal. Chem. 59 (1987) 1391A. [4] G. Buß, M.J. Schöning, H. Lüth, J.W. Schultze, Electrochim. Acta 44 (1999) 3899. [5] B.A. Grzybowski, R. Haag, N. Bowden, G.M. Whitesides, Anal. Chem. 70 (1998) 4645. [6] G. Sreenivas, S.S. Ang, I. Fritsch, W.D. Brown, G.A. Gerhardt, D.J. Woodward, Anal. Chem. 68 (1996) 1858. [7] N.L. Simone, R.F. Bonner, J.W. Gillespie, M.R. Emmert-Buck, L.A. Liotta, Trends Genet. 14 (1998) 272. [8] R. Srinivasan, Science 234 (1986) 559. [9] K. Schütze, I. Becker, K.F. Becker, et al., Genet. Anal. 14 (1) (1997) 1. [10] A. Clement-Sengewald, K. Schütze, A. Ashkin, G.A. Palma, G. Kerlen, G. Brem, J. Assist. Reprod. Genet. 13 (1996) 259. [11] M. Schindler, Nat. Biotechnol. 16 (8) (1998) 719.
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 2006 Available 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 we fabricated the gold microelectrodes having a gap spacing of less than 3 μm using this system. A 500 Å thickness of gold was coated on standard glass 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 of gold electrodes on the above slides. Microstructures up to the resolution limit (0.25 μm) of optical microscope can be fabricated using this cold ablation by laser. © 2007 Published by Elsevier B.V. Keywords: Optical Tweezer; Microelectrodes; Laser
1. Introduction The micro- and nano-fabrication technologies provide easy fabrication of micro- and nano-electrode arrays, nanostructure, microfluidics and peripherals for the development of “laboratory-on-a-chip” technology. These devices boost the low cost research in areas of nanobiotechnology, biotechnology, targeted drug delivery, Bio-MEMS, Biomolecular electronics etc. Microelectrodes are commonly used in electrochemistry based biosensors due to their small physical size, which have advantage over larger electrodes in terms of sensitivity, decreased influence of the solution resistance, low charging currents, greater signal-to-noise, and the ability to perform measurements in extremely small microenvironments or sub-microliter sample volumes [1–3]. A number of techniques have been proposed to prepare microelectrodes and microelectrode arrays [4–6]. Microfabrication using conventional lithography requires high cost masks and other maskless lithography techniques are tedious and time consuming. Here we fabricated the gold microelectrodes by simple technique without any mask using laser scissor. Laser scissor is a laser-based micropreparation technique that allows one to selectively cut out and specifically obtain any type of desired pattern. It is quick and convenient and
above all can be performed without mechanical contact; thus, it is possible to work without danger of contamination or infection. Optical/laser microdissection system or laser scissor is commonly used for isolation of single chromosome, nucleus, cell, tissue etc. The extremely high photon density in the narrow focus of the pulsed laser allows the system to cut or ablate biological structures and thin films. The physical principle of laser cutting is a locally restricted ablative photodecomposition process without heating, so it is possible to ablate desired pattern without damaging adjacent regions [7,8]. Optical micromanipulation with noncontact laser microbeams has been demonstrated to be an alternative approach to the more common procedures using mechanical microtools. This simple, precise technique significantly advances laser technology for microdissection and isolation [9–11]. This technique is similar to the direct writing by laser; in this gold is cold ablated directly by 337 nm laser microbeam. The laser cuts around the target specimen, which yields a clear-cut gap between the selected and nonselected specimens. In addition, unwanted material within a larger selected area can be selectively destroyed with a few laser shots. 2. Experimental 2.1. Gold deposition on glass surface
⁎ Corresponding author. Tel.: +91 172 2656285; fax: +91 172 2657267. E-mail address: [email protected] (L.M. Bharadwaj). 0167-577X/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.matlet.2006.12.076
Gold coating on glass surface was carried out at room temperature by thermal evaporation using HIND HIVAC
3830
S. Kumar et al. / Materials Letters 61 (2007) 3829–3832
Fig. 1. Effect of different energy of laser on gold ablation.
VACUUM COATING UNIT. The pressure during evaporation was kept at 2 × 10− 5 mbar. Thickness of deposited gold layer was determined by surface texture analysis system DEKTAK 3030 ST (Sloan, Veeco) and was found to be 500 Å.
more gold ablation (Fig. 1). The power and energy per pulse duration was calculated 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 μJ Peak power per pulse ¼ Energy=time ¼ 300 μJ=4 ns ¼ 75 kW Peak power per pulse at objective ¼ Energy available at objective=time ¼ 40 μJ=4 ns ¼ 10 kW Average power ¼ Ppeak pulsewidth frequency
2.2. Cleaning of gold coated slide The gold surface was thoroughly cleaned sequentially in methanol, acetone and isopropanol-2 for 10 min each. The substrate was then rinsed three times with deionized water. Finally the substrate was rinsed with ethanol and was dried. 2.3. Fabrication of microelectrodes using dissection system Microelectrodes were fabricated using Optical Tweezer cum microdissection combi system (PALM system, Germany). 2.4. Characterization of microelectrodes Characterization of microelectrodes was carried out using Scanning Electron Microscope (JEOL JSM 6100) and SIGNATONE Probe Station, CA, USA having Hewlett Packard 4155A Semiconductor Parameter Analyzer (internal resistance of ≥ 1013 Ω and current resolution of 10 fA) and fitted with Bausch and Lomb Micro zoom II High Performance Microscope. 3. Results and discussion We used Optical Tweezer cum microdissector system (PALM Combi) for fabrication of microelectrodes. Gold-coated slide was observed under bright field microscope and gold pads were designed using the software. The gold was ablated by a pulsed laser beam (337 nm, nitrogen) that has pulse duration of 4 ns and a frequency of 33 Hz. The maximum energy of laser beam was 300 μJ and out of 300 μJ only 40 μJ was available at objective. Gold ablation depends upon the energy applied. We have optimized the energy for ablation of gold. From Fig. 1, it is clear that 20 μJ energy at objective is insufficient to ablate the gold. 40 μJ energy gives
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 ablate gold with a minimum width of 1.64 μm and maximum of 11.7 μm by laser having an energy of 40 μJ, power per pulse of 10 kW and average power of 1.32 mW. We started gold ablation with low energy and increased energy and hence peak power, average power in steps till we get some ablation. The peak power of 5 and 5.3 respectively did not produce any spot on gold slide indicating that this much power is insufficient to ablate the gold and can not be used to fabricate micro-/ nano-electrodes of desired spacing. Further increase in peak power beyond this point i.e. 5.5 kW produces ablation and results in the formation of spot on the gold slide and acts as threshold peak power for the formation of micro-/nano-electrodes using laser scissor.
Table 1 Effect of laser energy on gold ablation on gold coated glass surface Serial no. Energy available Peak power Average power Width of spot at objective (μJ) per pulse (kW) (mW) (in μm) on gold slide 1 2 3 4 5 6 7 8 9 10 11 12
20 21.2 22 24 26 28 30 32 34 36 38 40
5 5.3 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
0.66 0.69 0.72 0.79 0.85 0.92 0.99 1.05 1.12 1.18 1.25 1.32
– – 1.641 3.359 4.698 5.970 8.047 8.672 8.359 9.305 10.643 11.719
S. Kumar et al. / Materials Letters 61 (2007) 3829–3832
3831
Fig. 2. Microelectrodes visualized under optical microscope at 40×.
Fig. 3. I–V characteristics of microelectrodes.
Here we used energy of 26 μJ and average power of 0.85 mW for fabrication of gold microelectrodes having a gap of around 3 μm. The peak power per pulse is very high (around 10 kW at objective) and is
more than sufficient to remove the gold from the glass surface as depicted in Fig. 1. By increasing exposure time of laser and by applying it repeatedly, desired structures were achieved (Fig. 2). These
Fig. 4. SEM images of microelectrodes after probing with signatone-probing station.
3832
S. Kumar et al. / Materials Letters 61 (2007) 3829–3832
programmed using software to remove selected area at desired locations after specific gap/position repeatedly) and cost effective, the microelectrode array can even be used as disposable electrode. As the density and resolution required for microelectrode applications increases in the future, these capabilities will likely prove to be valuable. Electrodes of very small gap spacing can be used for immobilization of DNA between these and hence studying its electrical properties. Acknowledgements
Fig. 5. SEM images of microelectrodes at high resolution.
fabricated microelectrodes were then washed with acetone followed by distilled water and dried. The microelectrodes were characterized using signatone-probing station having Hewlett Packard 4155A Semiconductor Parameter Analyzer (internal resistance of ≥1013 Ω and current resolution of 10 fA). The current voltage characteristics showed open circuit and high resistance indicating the complete removal of gold between 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 microelectrodes. This technique is very simple, precise, reliable and do not take much time to fabricate micro-/nano-electrodes of gap spacing of up to the resolution limit of the optical microscope. This technique can be used for fabrication of micro-/nanostructures of any design and pattern to meet research purpose and application demands. Complex, multi-material patterns could be made by using this technique for direct writing on the substrate. Since the reproduction is easy (as it can be
The authors greatly acknowledge the financial support provided by Department of Information Technology and Department of Science & Technology Government of India. Sandeep Kumar and Rajesh Kumar thanks Council of Scientific and Industrial Research for their research fellowship. Authors are thankful to the Director Central Scientific Instruments Organization (CSIO), Chandigarh, for providing necessary facilities. Special thanks are due to late Dr. Rakesh Kumar for the stimulating discussions on the fabrication method. References [1] J. Heinze, Angew. Chem., Int. Ed. Engl. 32 (1993) 1268. [2] R.M. Wightman, D.O. Wipf, Electroanalytical, vol. 15, 1988, p. 267, Chapter 3. [3] S. Pons, M. Fleischman, Anal. Chem. 59 (1987) 1391A. [4] G. Buß, M.J. Schöning, H. Lüth, J.W. Schultze, Electrochim. Acta 44 (1999) 3899. [5] B.A. Grzybowski, R. Haag, N. Bowden, G.M. Whitesides, Anal. Chem. 70 (1998) 4645. [6] G. Sreenivas, S.S. Ang, I. Fritsch, W.D. Brown, G.A. Gerhardt, D.J. Woodward, Anal. Chem. 68 (1996) 1858. [7] N.L. Simone, R.F. Bonner, J.W. Gillespie, M.R. Emmert-Buck, L.A. Liotta, Trends Genet. 14 (1998) 272. [8] R. Srinivasan, Science 234 (1986) 559. [9] K. Schütze, I. Becker, K.F. Becker, et al., Genet. Anal. 14 (1) (1997) 1. [10] A. Clement-Sengewald, K. Schütze, A. Ashkin, G.A. Palma, G. Kerlen, G. Brem, J. Assist. Reprod. Genet. 13 (1996) 259. [11] M. Schindler, Nat. Biotechnol. 16 (8) (1998) 719.