Selective direct laser writing of interdigitated pyrolytic carbon microelectrodes

Laser pyrolysis is an emerging alternative for the fabrication of carbon-based microelectrodes. In the so-called carbon MEMS process (CMEMS), carbon microelectrodes have been fabricated by UV-lithography followed by furnace pyrolysis at >900°C [1]. The high processing temperatures limit the types of films and carrier substrates that may be used and require a significant amount of energy. Laser pyrolysis, on the other hand, allows the patterning and pyrolysis to be done in a single step. Furthermore, it eliminates the need for high-temperature compatible substrates while greatly reducing the overall energy budget. Examples of devices made by laser pyrolysis are carbon micro super-capacitors [2] and biosensors [3], while substrates allowing for laser pyrolysis range from polyimide to potatoes [4].
In this work, we present the fabrication of micro gap interdigitated electrodes (IDEs) by laser pyrolysis for electrochemical sensing. For the fabrication process, we have proposed a novel approach involving the inclusion of a photo absorber (Pro-Jet 800NP, FujiFilm Imaging Colorants Ltd., UK) in otherwise clear SU-8 photoresist (SU-8 2035, Kayaku Advanced Materials, Japan). The absorber allows us to perform selective direct laser writing of carbon IDEs using a low-power, continuous wave, near-infrared, semi-conductor diode laser (Omicron BrixX 808-800HP, Omicron-Laserage Laserprodukte GmbH, Germany) [5]. The laser is mounted in a modified mask-less aligner (Heidelberg μPG 101IR, Heidelberg Instruments Mikrotechnik GmbH, Germany) with a pneumatic autofocus facilitating a focal diameter of 32.3 μm. It operates at a wavelength of 806 nm with adjustable output power from 5-800 mW. We have demonstrated [5], that laser pyrolysis with this laser is only possible in the absorber-modified regions of the SU-8 (Fig.1a-b) and that the porosity of the resulting pyrolytic carbon microelectrodes can be controlled by the scan speed. The lasing was done in either an ambient or an inert nitrogen atmosphere by purging with the desired gas through the pneumatic focus in the write head. We have found, that the nitrogen atmosphere provides better results in terms of electrical conductivity of the resulting carbon lines. Furthermore, we have been able to produce lines with a width as low as 13.5 ± 0.4 μm and achieve a conductivity up to 14.2 ± 3.3 S/cm [5]. We can write adjacent lines with an insulating gap down to ~5 μm (Fig.1d). The lines either sit in the bottom of a protective groove in the SU-8 for added robustness, or are free standing by developing the surrounding SU-8 (Fig.1e-f).
Furthermore, our experiments and finite element models suggest, for the first time, that the laser pyrolysis is a self-limiting process. In essence, the laser-induced pyrolytic carbon acts as a heat sink upon conversion from SU-8 to pyrolytic carbon. Due to the substantially increased thermal conductivity the pyrolytic carbon directs the heat away from the laser spot so quickly, that further laser-annealing of the carbon becomes impossible except at very high laser powers (Fig.2a).
Due to the high resolution and design freedom of the laser pyrolysis process in absorber-modified SU-8, we were able to fabricate carbon micro-IDEs with micron-sized gaps between the electrode fingers (Fig.2b). These electrodes were characterized electrochemically after a 10 min oxygen-plasma treatment at 300 W (Diener Electronic Zepto, Diener Electronic GmbH, Germany). The electrochemical characterization tests were conducted in 10 mM K₄Fe(CN)₆ (in phosphate buffered saline at pH 7.4 using 100 mM KCl as supporting electrolyte) by a PalmSens4 potentiostat (PalmSens BV, The Netherlands) against an Ag/AgCl reference electrode. We have written different IDEs with various design parameters in terms of the number of fingers, finger lengths, and inter-electrode gaps. The gaps are much smaller than what is reported elsewhere for laser-written IDEs [2], which should enable redox-cycling in electrochemical sensing. Preliminary electrochemical characterization of the laser pyrolysed carbon micro-IDEs have shown (Fig.2c), that the laser-written electrodes are promising as interdigitated on-chip working and counter electrodes in electrochemical IDE-based sensing devices.
In conclusion, we introduce a rapid, versatile, and low-energy cost fabrication method for carbon microelectrodes with a large degree of design freedom. The electrodes can be released through developing of the surrounding photoresist and can be used for electrochemical applications, which we are currently exploring.