Novel fabrication process for sub-micron interdigitated electrode arrays for highly sensitive electrochemical detection

Sensors and Actuators B 205 (2014) 193–198

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Novel fabrication process for sub-micron interdigitated electrode arrays for highly sensitive electrochemical detection S. Partel a,b,∗ , S. Kasemann a , P. Choleva a , C. Dincer b,c , J. Kieninger b , G.A. Urban b,c a b c

Vorarlberg University of Applied Sciences, 6850 Dornbirn, Austria Department of Microsystem Engineering (IMTEK), University of Freiburg, 79110 Freiburg, Germany Freiburg Materials Research Center (FMF), University of Freiburg, 79104 Freiburg, Germany

a r t i c l e

i n f o

Article history: Received 8 July 2014 Received in revised form 11 August 2014 Accepted 21 August 2014 Available online 29 August 2014 Keywords: Interdigitated electrode array p-Aminophenol electrochemical biosensor

a b s t r a c t Interdigitated electrode arrays (IDAs) are often used for electrochemical detection using redox cycling for signal amplification. Its geometry, especially the gap width, is an important factor defining its electrochemical performance (sensitivity and collection efficiency). Minimal defect density on a wafer is a must, as a single shortcut prevents the function of the sensor chip. Therefore, the fabrication of interdigitated electrode arrays is a fundamental step in the integration of electrochemical sensors. This paper presents a novel fabrication approach that allows electrode spacing in the sub-micrometer region by using standard equipment such as UV Mask Aligner, physical vapor deposition and diffusion furnace. The fabrication procedure is a combination of dry etching, thermal oxidation and wet etching. This approach has potential to realize electrode distances down to 140 nm or even less. The proposed method enables the fabrication of sub-micron IDAs with the use of a conventional Mask Aligner and standard silicon technology. Moreover, it allows the electrode spacing adjustment without changing the mask design. The presented results demonstrate high sensitivity electrochemical sensors with amplification factors more than 12 without the need of e-beam lithography. Hence, this fabrication method offers a low-cost alternative to sub-micron e-beam written IDAs. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical biosensors are most commonly used if miniaturization and cost efficiency is playing the major role. The necessary micro- and nano-electrodes can be fabricated by microelectronic techniques with low-cost. There are many different electrode geometries evaluated and newer geometries like coplanar electrodes designs awaken interest in the electrochemical biosensor community [1–6]. However, the planar interdigitated electrode array (IDA) configuration is still a geometry of interest [7–9] especially if the gaps between these electrodes are moving toward the nano-scale [9–11] and the aspect ratio between the metal layer width and gap increases [12]. This approach can amplify the current flow between the electrodes which was demonstrated in 1986 by Bard et al. and is known as redox cycling [13–16]. The amplification is achieved by two electrodes in close vicinity, one is polarized to a potential allowing a reduction process and the other electrode to a potential enabling the

∗ Corresponding author at: Vorarlberg University of Applied Sciences, 6850 Dornbirn, Austria. Tel.: +43 55727927204. E-mail address: [email protected] (S. Partel). http://dx.doi.org/10.1016/j.snb.2014.08.065 0925-4005/© 2014 Elsevier B.V. All rights reserved.

complementary oxidation process. By decreasing the gap (diffusion length) between the electrodes the cycling efficiency can be increased [17,15,18] compared to the conventional amperometry. To fabricate such sub-micron or even nano-electrodes for fundamental research, fabrication techniques like e-beam, ion beam milling [19,20] or stepper lithography are necessary [11,12,21], but certainly not everyone has access to such technologies. Furthermore, they are very expensive and the fabrication process is time consuming. To overcome the drawbacks alternative fabrication approaches as well as UV lithography optimization for sub-micron gap distances are reported previously [10,22]. In this paper we are presenting a novel method for planar IDA fabrication, which enables gap widths down to 140 nm with conventional photolithography tools. The small features are achieved by a new process sequence which involves dry etching, oxidation and wet etching as well as an optimized UV lithography process with structures down to 500 nm. An additional advantage of the presented method is the possibility to set the electrode gap width without changing the photomask, because the actual gap width depends on the oxidation degree of the structure rather than the photolithography process. With this concept a variety of gap widths, all in the sub-micron range, can be easily produced without the need of a high resolution patterning technique. The potential

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of this novel fabrication method is demonstrated by presenting adjustable electrode gap widths down to 140 nm with gold and 300 nm with platinum electrodes. 2. Materials and methods 2.1. Reagents and materials Lithography was carried out on a SUSS MA6 Mask Aligner (Süss Microtech AG, Germany). Photoresist MEGAPOSIT SPR955-CM 0.7 from MicroResist Technology GmbH, Germany was used. Adhesion promoter AZ Ti-Prime from Microchemical GmbH, Germany was applied. The dry etching of the amorphous silicon was performed on a deep RIE ICP system Adixon AMS100 (Alcatel Vacuum technology, France). Thermal diffusion was carried out on an ATV POE 604(ATV technologie GmbH, Germany). Etching solutions and process solutions were purchased from Microchemical GmbH, Germany. The electrode materials platinum (Pt) and gold (Au) were deposited by high vacuum evaporation (Univex 500, Oerlikon Leybold Vacuum GmbH, Germany). Optical microscopy images were obtained using Zeiss MAT. Scanning electron microscopy images (SEM) and measurements were taken with FEI XL30 ESEM FEG and Jeol 7100F equipment. Thickness measurements were performed with a Dektak 8 (Veeco, USA). Characterization of the sputtered Six Ny layer was done by spectroscopic ellipsometry on a VASE system (J.A. Woollam Co., Inc., USA). Simulation software LayoutLAB (GenISys, Germany) was used to optimize the structures and calibrate the photoresist. The input parameters for the calibration were collected by a homemade dissolution rate monitor (DRM).

Fig. 1. Fabrication process of the electrode pattern. (1) Silicon substrate with sputtered Six Ny and a-Si layer. (2) Photoresist which defines the electrode pattern. (3) Dry etching of the sputtered Si layer and removing of the photoresist by O2 plasma. (4) Thermal oxidation of the a-Si layer. (5) Wet etching of the thermal oxidized layer by buffered oxide etch (BOE) 7:1. (6) Evaporation of metal (Ti/Au respectively Ti/Pt). (7) Lift-off of the a-Si layer by KOH 44% etching with ultrasonic assistance. (8) Sputtering of SiO2 layer over complete wafer. (9) Lithography, which defines the active electrode area. (10) Dry etching of SiO2 layer and photoresist removal by oxygen plasma.

2.2. Novel fabrication process of the IDA The following illustration shows the process chain of the novel IDA fabrication. A standard silicon substrate is covered with a sputtered Six Ny film with a thickness of 500 nm and a second layer of silicon with a thickness of at least 500 nm. The wafer is then spin-coated with adhesion promoter (AZ Ti-Prime), followed by a positive tone photoresist (MEGAPOSIT SPR955-CM 0.7) with a final thickness of 500 nm. A softbake step of 60 s on a 100 ◦ C hot plate is performed. The exposure takes place on a SUSS Mask Aligner MA6 with a band pass filter (i-line filter; 365 nm) and an illumination intensity of 125 mJ/cm2 . The post exposure bake is carried out on a hot plate of 115 ◦ C for 120 s. The wafer is than developed in AZ 726 MIF for 45 s in single puddle mode on an EVG 101 and then rinsed with DI-water and spin-dried. The next step is dry etching of the sputtered silicon layer with subsequent photoresist removal by O2 plasma (Fig. 1(2)). The wafer is then placed in a diffusion furnace to oxidize the a-Si (Fig. 1(3)). After oxidation the just grown oxide is removed by wet etching of with BOE 7:1 or any other etchant for SiO2 and shrinks the structure. In this case the oxide was etched in BOE (etch rate ∼ 100 nm/min) for 4 min, ensuring that no oxide is remaining (Fig 3b). The final electrode material platinum (Pt) or gold (Au) with a thickness of 90 nm is deposited by high vacuum evaporation. A 10 nm titanium (Ti) layer underneath the electrode material acts as an adhesion promoter to the Six Ny layer. The evaporation rate for Ti was 0.3 nm/s and for Pt or Au 0.1 nm/s. The final step for IDA fabrication is the lift-off, which is performed in 44% KOH solution at a temperature of 45 ◦ C for 4 min to dissolve the remaining a-Si underneath the evaporated metal layer. Ultrasound supports the lift-off of a-Si and was applied for additional 2 min. Cleaning of the substrate was done with DI-rinse with subsequent nitrogen blow dry. To define the contact pads, the IDA, counter electrode and reference electrode area, the wafer was sputter coated with 200 nm SiO2 and a second photolithography step is

performed. The photoresist AZ1518 is used and the SiO2 layer is etched using reactive ion etching (RIE). This process step can also be done by wet etching with respect to a ∼1 ␮m under etch. After etching, the photoresist was removed by oxygen plasma. 2.3. Biosensor fabrication For the first test of the novel fabrication method a 6 in. quartz mask with feature sizes down to 200 nm was used. The high resolution test patterns demonstrate the potential of the novel method. Whereas, electrochemical performance was evaluated with 5 in. quartz mask featuring IDA patterns of 1 ␮m gap and a 2 ␮m electrode width. The schematic design of a large area IDA is shown in Fig. 2a. Each sensor chip contains two individual IDAs along with a reference electrode and a counter electrode (see Fig. 2b). On the reference electrode silver was electrodeposited after thin-film fabrication and subsequently partially converted electrochemically into silver chloride. The working electrodes have 300 finger pairs with a finger length of 100 ␮m. 2.4. Electrochemical characterization setup The final sensors were characterized using 0.1 M phosphate buffered saline (PBS) solutions with 0.1 M NaCl containing 1 mM ferrocenemethanol (FcMeOH) as a test system as well as 1 mM p-aminophenol (PAP) as a biological redox couple. The electrochemical characterization was done using IDA chips with 150 finger pairs resulting in an active electrode area of 3 × 10−4 cm2 . The resistance between the interdigitated electrodes was tested to be higher than 20 M in its dry-state before any experiment. All measurements were done under diffusion limited conditions (no flow). The two pairs of the IDA were polarized using a bipotentiostat (Autolab PGSTAT128N, Deutsche METROHM GmbH & Co. KG, Filderstadt,

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Fig. 2. (a) Schematic design of the IDA and (b) the final IDA chip.

Germany). One pair of fingers as working electrode was polarized to a potential enabling the oxidation process, the other pair was connected to the second working electrode and polarized to a potential enabling the reduction process. As reference electrode the on-chip silver/silver chloride electrode was used, the platinum counter electrode was on-chip as well. Cyclic voltammograms (CVs) have been recorded polarizing one pair of fingers only (normal mode). CVs comprising redox cycling have been obtained by scanning the working electrode while keeping the second working electrode at a fixed potential. For the amperometric measurements the applied potentials have been −200 mV for the reduction and +500 mV for oxidation process in case of PAP, −100 and +300 mV in case of FcMeOH respectively. Prior to any experiment a cyclic voltammogram in 0.1 M PBS was recorded to ensure the clean-state (residue free surface) of the platinum electrode.

3. Results and discussion 3.1. Fabrication process of the IDA A standard silicon substrate is covered with a sputtered Six Ny film with a thickness of 500 nm and a second layer of silicon with a thickness of at least 500 nm, whereas the Six Ny layer underneath the a-Si acts as a dry etch stop. The thickness of the Six Ny film is uncritical for the fabrication process and can be varied. The thickness of the a-Si layer depends on the desired structure and the initial structure size. Furthermore, for a successful lift-off a minimum thickness of ∼1:3 is a rule of thumb (deposited film to masking feature). For example if the initial structure size is 1 ␮m and a reduction to about 600 nm is desired a thickness of 400 nm should be oxidized. The oxide layer grows almost homogeneously over the complete microstructure. As a result the structure shrinks not only on the sidewalls but also in height for about 200 nm. This means for a 100 nm metal layer the deposited silicon layer should be around 500 nm, considering the 200 nm shrinkage. In general, for single crystal silicon about 46% Si would be consumed during the oxide formation. With the same approach the sputtered Si structure should shrink about 46% and the formation of SiO2 should be about 54% according to the initial thickness. Therefore, by thermally oxidizing the wafer with 200 nm SiO2 , the 1 ␮m structure in a-Si transforms into 816 nm a-Si surrounded by a 200 nm SiO2 layer (Fig. 3a). This process step enables the variable adjustment of the final a-Si structure. For example oxidizing 400 nm of SiO2 shrinks the 1 ␮m a-Si feature down to 632 nm. However, these are ideal values for oxidizing single crystal Si without considering topography. We observed for our structures a higher oxidation rate on sputtered Si,

which results in higher shrinkage of the final structures (shrinking ration of ∼55%). Another observation was the slightly uneven formation of the thermal oxide at the Si to Six Ny interface. The silicon oxidizes at the interface more and a small undercut is formed. This circumstance has a positive effect on the lift-off process by reducing the probability of metal sidewall coverage. Fig. 3a shows the cross section of a 200 nm oxidized a-Si layer (Feature size 520 nm before oxidation). The just thermal grown oxide is removed by wet etching (in our case with BOE 7:1). This process step is uncritical because the applied surrounding material (a-Si and Six Ny ) has a significant etch resistance to the etchant. The final a-Si structures after wet etching are shown in Fig. 3b. The a-Si layer exhibits a thickness of 520 nm before etching and after the two process steps the feature size shrinks down to ∼300 nm (reduction of about 220 nm). The micrograph also visualizes the feature size before the shrinking process due to the fact that the dry etching slightly etches the Six Ny layer underneath the a-Si. The produced structures no longer exhibit a 1:1 duty ratio, which is a minor drawback compared to the gap influence on the cycling efficiency [11,18,21]. In conventional lift-off process a photoresist is

Fig. 3. (a) Scanning electron microscope image of a cross section of 200 nm oxidized a-Si electrodes. Half pitch of the structure was 520 nm before oxidation. (b) Cross section of a-Si structure after etching the 200 nm SiO2 in BOE. (c) The same structure after deposition of Au. Underneath the 500 nm Six Ny layer is recognizable.

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Fig. 4. (a) Scanning electron microscope image of a cross section of an electrode structure made of Au. The gap widths are in the range of 200 nm. (b) Cross section of a Pt electrode with a gap width of 400 nm (initial 600 nm line and space structure). (c) Cross section of Au electrode with a gap width of 140 nm (initial 520 nm line and space). (d) Cross section of Pt electrode with 300 nm gap width.

used for lifting off the evaporated metal layer. In this approach the a-Si layer acts as a lift-off material. The lift-off process was tested for two metals, Au and Pt. Both metals were coated with 10 nm Ti as adhesion layer and a 90 nm thick Au electrodes and a 90nm thick Pt layer for the Pt electrodes. Fig. 3c shows the Au covered wafer before lift-off and illustrates the 500 nm thick Six Ny layer underneath. The lift-off process was performed in KOH. The wet etch step is uncritical like the BOE etch step, because the Six Ny has a very ˚ low etch rate (<1 A/min) compared to Si (>1 ␮m/min) [23]. For both metals (Au and Pt) the lift-off process was successfully performed and gap widths down to 140 nm are already achieved. The achieved resolution is shown in Fig. 4. The micrographs in Fig. 4a and c show the cross section of the Au electrodes with a gap width of 200 nm (Fig. 4a) and 140 nm (Fig. 4c). The cross section of the Pt electrodes is shown in Fig. 4b and c, whereas Fig. 4b shows the line and space structure with a spacing of 400 nm and Fig. 4c, the 300 nm gap width. The combination of SiO2 diffusion (in our case thermal diffusion) and wet etching allows adjusting the final gap width (remaining features in a-Si) between the electrodes. The thicker the thermally grown SiO2 the smaller is the gap between the electrodes obtained after wet etching. This linkage enables changing the gap distance nearly independently of the lithography mask features. This means that a single mask (single interdigitated electrode design) allows the fabrication of various electrode designs by featuring different gap widths with their appropriate electrode widths. For sub-micron Mask Aligner lithography the electrode gap deviation depends on the quality of the photolithography, in general line edge roughness and photo resist profile, but the contact between mask and wafer has the biggest influence. If there is a small proximity gap (>100 nm) the feature width and spacing will change significantly [24]. Due

to this fact the lithography is the critical process step to realize electrode widths in the sub-micron region. 3.2. Electrochemical characterization For both redox systems, FcMeOH and PAP, amplification by redox cycling using the IDAs fabricated with the presented technology was demonstrated. The results from the electrochemical characterization are summarized in Fig. 5. In Fig. 5a cyclic voltammograms of the redox systems are shown. The major difference in the observed curves are an increased current of approximately a factor two comparing PAP to FcMeOH. This is mainly attributed to the two-electron process of the PAP redox system in comparison to the one-electron process of the FcMeOH redox system (see Eqs. (1) and (2)). From these curves the potentials to be used in the amperometric measurements have been deduced. p-aminophenol + 2e− + 2H+ ⇔ p-quinone −

FcMeOH + e ⇔ FcMeOH

+

(1) (2)

Fig. 5b illustrates the onset and stable amplification of redox cycling using the PAP and FcMeOH redox system. For these measurements, the first working electrode (generator) was in scanning mode (potential changed as indicated by the abscissa). The second working electrode (collector) was fixed at a potential of −0.2 V where the reduction of p-quinone (PQI) and at a potential of −0.1 V where the reduction of FcMeOH+ occurs. Compared to conventional cyclic voltammogram (in this paper referred as normal mode) the amplified signal raised by a factor more than 10. The collection factor (the ratio between the absolute values of the collector and the generator currents) was 96% in case of PAP and 98% in case of FcMeOH.

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Fig. 5. (a) Cyclic voltammograms in 0.1 M phosphate buffered saline (PBS), 1 mM Ferrocenemethanol (FcMeOH) and 1 mM p-aminophenol (PAP) at the platinum working electrode with a scan rate of 100 mV/s, potential scale is versus the on-chip silver/silver chloride reference electrode. (b) CVs in 0.1 M PBS with 1 mM PAP (black curve) and 1 mM FcMeOH (red curve) with redox-cycling, applying −0.2 V for PAP and −0.1 V for FcMeOH at the second working electrode. (c) Amperometry in 0.1 M PBS with 1 mM PAP. (d) Amperometry in 0.1 M PBS with 1 mM FcMeOH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The amperometric measurements in 0.1 M PBS with 1 mM PAP are shown in Fig. 5c and in 0.1 M PBS with 1 mM FcMeOH in Fig. 5d. The sensitivity was increased by a factor of 12.8 in case of PAP and 13.3 in case of FcMeOH. This slight difference in signal amplification factors between these redox couples might originate in different diffusion properties. The diffusion coefficient of FcMeOH in 0.1 M PBS is 4.4 × 10−6 cm2 /s (measured using a platinum rotating disk electrode), while the diffusion coefficient of PAP is 4.55 × 10−6 cm2 /s [25]. Aoki et al. derived an equation for the diffusion-controlled currents during redox cycling for IDAs under steady-state conditions [26]. The amplified current is predicted by this equation to 11.2 mA cm−2 in case of PAP and 5.4 mA cm−2 in case of FcMeOH. The results obtained from the measurements were 10.7 mA cm−2 in case of PAP and 4.65 mA cm−2 in case of FcMeOH, which is in good agreement with the theoretical prediction, especially taking into account the uncertainty depending on the value chosen for the diffusion constant. The results of the electrochemical characterization prove that the IDA chip, fabricated with the described novel process sequence, shows comparable characteristics like the ones with state of art production methods [3,16,27,28].

lithography equipment like e-beam writing or stepper technology. Standard microtechnology equipment such as sputtering, Mask Aligner lithography, dry/wet etching, diffusion furnace and evaporation is sufficient to fabricate IDA sensor chips with sub-micron electrode gap widths. Only two lithography steps, one to transfer the IDA geometry and the other to define the passivation layer (openings for the active electrode areas and contact pads) are necessary. The alignment between these two layers is uncritical and can be in the micrometer range. Another major benefit of the presented method is the adjustable gap widths, which is defined during processing almost independent of the lithography step by the diffusion grade of the a-Si layer. The subsequent wet etching process removes the formed SiO2 and reduces the sputtered Si feature size. The accurate control of the shrinking process is achieved by the thermal SiO2 diffusion process. This allows generating various gaps without changing the mask. We expect that with this method electrode gap widths in the nano-meter range (<100 nm) can be obtained. The electrochemical amplification by redox cycling using the IDAs fabricated with this new technology was tested and evaluated with different redox couples. The presented results prove high sensitivity of the electrochemical sensors with amplification factors of approximately 13 with collection factor higher than 96%.

4. Conclusion We have developed a novel method to fabricate interdigitated electrode arrays with electrode gap widths in the sub-micron region (down to 140 nm gap range) with conventional Mask Aligner lithography. This enables almost every institute or research center to fabricate nano-electrode arrays without using expensive

Acknowledgment We would like to thank the Research Centre for Microtechnology at Vorarlberg University of Applied Sciences for providing the facilities to fabricate the biochip.

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Biographies Stefan Partel is a researcher at Vorarlberg University of Applied Sciences located in Dornbirn (Austria) and a PhD student at University of Freiburg (IMTEK). He received his MSc. (Micro-Nanotechnology) in 2007. His main areas of expertise include micro and nano-fabrication techniques, especially photo lithography, simulation of photoresists and development of new processes and their optimization. His current interest is pushing the limits of Mask Aligner lithography for electrochemical devices. Stephan Kasemann received his PhD degree in experimental nuclear physics from the University of Cologne (Germany) in 2001. After working as an R&D scientist in a leading PVD coating equipment manufacturing company, he joined the Research Centre for Microtechnology at Vorarlberg University of Applied Sciences in 2009. Beside others, he is responsible for PVD coating processes for coating of microtechnological samples. Pavlina Choleva received her PhD degree from the University of Cologne (Germany) in 2004. Since November 2006 Pavlina Choleva has been with Vorarlberg University of Applied Sciences in the Research Centre for Microtechnology, where she is responsible for the research and development of micro-electrodeposition of metals, like copper, nickel and iron and also for the preparation of dielectric thin film layers by thermal dry and/or wet oxidation of Silicon substrates. Dr.-Ing. Jochen Kieninger studied Physics and Microsystems engineering at the University of Freiburg. In 2003, he received his diploma as “Dipl.-Ing. Mikrosystemtechnik”. Afterward he worked as scientific staff member and since 2008 as group leader in the Laboratory for Sensors. From 2009 to 2010, he worked as scientific staff member in the School of Soft Matter Research at the Freiburg Institute for Advanced Studies (FRIAS) with Prof. Andreas Manz. Since 2010, he was head of the SENSORTECH group in the Laboratory for Sensors and completed his PhD in 2011 with the academic title “Dr.-Ing.”. In 2012, he was nominated as Lecturer (Akademischer Rat). His main areas of expertise include electrochemical sensors, cell culture monitoring, microtechnology (dry-film resists, electrodeposition, hybrid technology) and sensor applications in medicine. Can Dincer studied Microsystems engineering at the University of Freiburg. In 2009, he received his diploma as “Dipl.-Ing. Mikrosystemtechnik”. Afterward he worked as scientific staff member at the Laboratory for Sensors. Since 2013, Mr. Dincer is head of the Nanosensors group. His main areas of expertise include immunosensors, scanning electrochemical microscope (SECM), microtechnology (dry-film resists, electrodeposition, hybrid technology) and sensor applications in medicine. Prof. Gerald Urban studied technical physics at the TU Vienna and was employed in the neurosurgical department of the AKH Vienna afterwards. Subsequently he worked on his PhD thesis at the Department of Electrical Engineering at the TU Vienna. In 1985, he founded, together with colleagues, the company OSC in Cleveland, Ohio and Vienna. In 1986 and 1987, he was a post-doc at the neurophysiological department in Münster, Germany. From 1990 till 2002, he was scientific director of the Ludwig Boltzmann Institute of Biomedical Microengineering in Vienna. He finished his habilitation in 1994 and became a full professor at the University of Freiburg in 1997. From 1999 to 2002, he was dean of the Faculty for Applied Science at the University of Freiburg.