Fabrication process development for a high sensitive electrochemical IDA sensor S. Partel a,b,⇑ , M. Mayer a , P. Hudek a , C. Dinçer b , J. Kieninger b , G.A. Urban b , K. Motzek c , L. Matay d a Vorarlberg University of Applied Sciences, 6850 Dornbirn, Austria b Department of Microsystem Engineering (IMTEK), University of Freiburg, 79110 Freiburg, Germany c Fraunhofer Institute for Integrated Systems and Device Technology, 91058 Erlangen, Germany d Institute of Informatics, SAS, Dúbravská cesta 9, 84507 Bratislava, Slovakia article info Article history: Available online 20 April 2012 Keywords: Mask aligner Photoresist calibration DRM Mask aligner lithography simulation Electrochemical sensor IDA Interdigitated electrode array Para-aminophenol PAP abstract We present recent results on a development and fabrication process of the electrochemical sensor with high sensitivity. The electrochemical sensor is based on an enzyme-linked immunosorbent assay (ELISA). The used ELISA provides a redox active species as intermediate product for the electrochemical detection. The increase in sensitivity due to the redox cycling process was evaluated with an interdigitated elec- trode array (IDA) consisting of 300 fingers with 2 lm width and 1 lm gap each. The lithography process was simulated to estimate the impact of the sensor stack and the illumination source during the lithog- raphy step on the sensor’s critical features. Development characteristics of the photoresist were precisely determined using a multi-wavelength dissolution rate monitor (DRM). The final sensor was tested with the ferri/ferro cyanide couple as well as with para-aminophenol (PAP). Both results prove amplification factors of more than 10 measured in a flow cell. The results presented demonstrate that high sensitivity electrochemical immunosensors based on redox cycling at IDAs on Pyrex substrate can be fabricated with conventional but highly optimized UV lithography. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction Interdigitated electrodes (IDEs) are often used for electrochem- ical detection. Different configurations increasing detection sensi- tivity have been already published [1–4]. One method to increase the sensitivity compared to the conventional amperometry is to reduce and oxidize redox-active molecules. This method is called redox cycling and offers the opportunity to detect even single mol- ecules [5]. By changing the charge state of the targeted molecule repeatedly an amplification of the detected signal can be achieved [2,3]. The geometry of the electrodes plays an important role for the efficiency of redox cycling [6–8]. If the gap between collecting and generator electrode is less than 10 lm the collection efficiency is almost one [2]. Decreasing the size of the electrodes and gap the amplification factor increases [6,10]. To reduce the costs per sensor or even make it possible to fabricate with affordable technologies, different designs and fabrication strategies have been published. A planar sensor design has the advantage of being manufacturable with a process structure of moderate complexity. Electrodes down to 4 lm width are fabricated by using standard mask aligner lithography tools [9]. Decreasing the size of the electrodes and gap the cheaper mask aligner lithography approach, compared to e-beam lithography, can still be applied but with more effort in mask design and process optimization. Expected are electrode sizes and gaps in the sub micron range. Tests with mask aligners have already shown that the electrode size of 300 nm can be fabri- cated. To reduce the time for the lithography exposure tests and mask designs, simulations were used as an additional tool. For such simulations exact information about illumination, resist behavior during development and process conditions are essential to achieve accurate simulation results. Investigations to characterize the illumination source in a mask aligner were provided by SUSS Microtec. To describe the behavior of the photoresist (PR) during development, data from a dissolution rate monitor (DRM) can be used. From this data the necessary information for the lithography process simulation can be extracted. The simulations can be per- formed using software which describes the light propagation in the near-field, such as GenISys LayoutLAB. By combining all these information an estimation of the final structures can be made. Based on the simulation results a mask optimization and process optimization can lead to a decrease in feature size. The electrode width and the gap can range from nanometers to micrometers and different designs have been published [1,2,17–19]. There are approaches to increase the sensitivity of the sensor by changing the design from planar structures to trenches where the electrode surface area increases by constant gap size [1]. The approach of this work was to stay at a planar design for mask aligner lithography but with a high amplification factor by reducing the size of the electrodes and gap. 0167-9317/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.03.028 ⇑ Corresponding author at: Vorarlberg University of Applied Sciences, 6850 Dornbirn, Austria. E-mail address: stefan.partel@fhv.at (S. Partel). Microelectronic Engineering 97 (2012) 235–240 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee