Sensors and Actuators B 40 (1997) 79-84. B. CHEMICAL. Characterization of microelectrode arrays by means of electrochemical and surface analysis methods '.
B
Sensors and Actuators
B 40 (1997)
79-84
CHEMICAL
Characterization of microelectrode arrays by means of electrochemical and surface analysis methods ’ M. Wittkampf a,*, K. Cammann a, M. Amrein b, R. Reichelt b a Institutfir
Chemo- und Biosensorik, Lehrstuhlfiir Analytische Chemie, WestjXische Wilhelms-Universiit, MendelstraJe 7, D-48149 Miinster, Germany b Institutfiir Medizinische Physik und Biophysik, WestjXsche Wilhelms-Universittit, Robert-Koch-Straje 31, D-48149 Miinster, Germany
Received27 February 1996;revised 1 August 1996;accepted5 August 1996
Abstract Microelectrode arrays have been fabricated using silicon thin-film technology. The arrays consist of 100 platinummicroelectrodesconnected in parallel to a single output line. In addition, a platinum counter electrode as well as a silver/silver chloride pseudo-reference electrode are integrated on the microchip. We present results of surface analysis of the microelectrode arrays by time-of-flight secondary ion mass spectrometry (TOF-SIMS), scanning electron microscopy (SEM) and scanning tunnelling microscopy (STM). The highly surface-sensitive
analysis method SIMS reveals traces of Al and Ag impurities on the microelectrode surfaces.SEM micrographs show the embedding of the electrodesinto insulating silicon oxide and silicon nitride layers. Furthermore, they reveal anunexpectedirregular, corrugated surfacestructure of the platinum electrodes, which are shown to be built up by small platinum grains. The microelectrode array is electrochemicallycharacterized by means of cyclic and linear sweep voltammetry. The methods applied have successfully beenused for the investigation and characterization of the surface structure and the composition of thin-film electrodes. Keywords:
Microelectrodearrays; Siliconthin-film technology;ModifiedCMOS process; Surfaceanalysis;Electrochemicalcharacterization
1. Introduction
Miniaturization of electrodes down to the lower micrometre range results in microelectrodes showing advantageous properties. The diffusion of electroactive substances towards microelectrodes differs from that towards conventionally used macroelectrodes, where only planar diffusion perpendicular to the surface occurs. When a current is flowing at microelectrodes, hemispherically shaped diffusion layers are formed within seconds, which are large in comparison to the electrode radius, Due to the radial component of diffusion, more electroactive particles per surface area reach the electrode within a certain period of time. Sigmoidal cyclic voltammograms instead of peak-shaped ones are obtained, because of a steady-state mass flux to the electrode surface. In addition, the current density increases and, as a result, an improved signal-to-noise ratio is obtained [ 11. The problem of the very low current of a single microelectrode was overcome by the development of an array structure. * Correspondingauthor. Tel: +49 251 980 2004. Fax: +49 2.51980 1911. ’ Part of this publication has beenpresentedat EUROSENSORS VIII, Toulouse,France,25-28 Sept.,1994. 0925-4005/97/$17.000 PIISO925-4005
1997 Elsevier (97)00020-8
ScienceS.A. All rights reserved
In contrast to conventional macroelectrodes, the improved diffusion characteristics of microelectrode arrays allowmeasurements in both stationary and stirred solutions. To obtain optimum properties, well-defined diffusion characteristics have to be established, depending on the electrode radius, their spacing and geometrical arrangement. Thus, &e advantages of a single microelectrode can be preserved even while obtaining a significantly increased current [2], The fabrication of microelectrode arrays (r< 10 pm) is possible using modified complementary metal oxide semiconductor (CMOS) technology. In addition to the common investigations regarding electrochemical properties, problems resulting from the fabrication process and the small dimensions of the electrodes made a comprehensive surface analysis appropriate. To get a close look at the electrodes and their embedding in the insulating layers, mass spectrometry, scanning electron and scanning tunnelling methods were applied. 2. Experimental
2.1. Materials and methods All electrochemical measurements were carried out with a fully computer-controlled system (potentiostat, AUTOLAB
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PSTATlO; software, General Purpose Electrochemical System 3.2, Eco-Chemie B.V., Utrecht, Netherlands). An Ag/ AgC113 M KC1 reference electrode (Metrohm, Filderstadt, Germany) was used as an external reference. The on-chip platinum counter electrode was used for all measurements. Time-of-flight secondary ion mass spectrometry (TOFSIMS) investigations were performed with a device built inhouse (Miinster TOF-SIMS III, Physikalisches Institut, Westfslische Wilhelms-Universitgt, Miinster, Germany) as described in Ref. [ 31. The microelectrode arrays were examined in a high-resolution field emission scanning electron microscope (FESEM; ‘in-lens’ type, model S-5000, Hitachi Ltd, Tokyo, Japan) in high vacuum (p = 5 X lo-’ torr) and in a scanning tunnelling microscope (STM; model STMM-1, Omicron, Taunusstein, Germany) at ambient conditions. Low-magnification imaging of a large surface area of the arrays by FESEM was performed at low accelerationvoltages (e.g., 3 kV) to reduce electrical charge-ups of the insulating material around the microelectrodes [4]. No electrical charge-ups were observed at low magnification at 25 kV after coating the surface of the array with a layer of approximately 10 to 15 nm amorphous carbon. The acceleration voltage used and the lateral scale aregiven at the bottom of the FESEM micrographs. For high-resolution investigations, the bare platinum microelectrodes were imaged in the FESEM operated at 25 kV using secondary electrons (SE). For tunnelling microscopic imaging, the STM was operated at a gap voltage of 1.5 V and a tunnelling current of 50 pA using the constant current mode (for further details of the field emission scanning electron microscopy, see Refs. [5,6] and for the scanning tunnelling microscopy, see Refs. [7,8] ) . All reagents were purchased from Sigma (Deisenhofen, Germany) and of the highest available purity grade. They were used as received.
2.2. Electrode conjiguration The chip under investigation is designed to be used as a transducer for chemical and biochemical sensors. The most suitable layout has been developed by testing different geometrical arrangements of the three-electrode arrangement including various diameters and spacings of the array electrodes [ 91. The transducer chosen for these detailed investigations consisted of two Ag/AgCl pseudo-references, a platinum counter electrode and an array of 100 platinum microelectrodes with a diameter of 2 p,m and an electrode spacing of 20 km in a quadratic arrangement (Fig. 1). The chip was glued onto an epoxy substrate and then bonded with 25 pm aluminium wires. Finally, the bond wires were encapsulated with epoxy resin (Master Bond Polymer Adhesive EP21, HTK GmbH, Hamburg, Germany). For electrochemical measurements,the integrated pseudo-reference electrodeswere potentiostatically chloridized in 0.1 M KC1
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AglAgCl reference
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pseudoelectrode
-bondpads
substrate
contact
i%-counter
alternative AglAgCl reference
electrode
pseudo-
electrode
Fig, 1. Schematic layout of the transducer with the microelectrode array. Al-adhesion
Pt-microelectrode
layer
/
lysilicon L
silicon
line
wafer
Fig. 2. Schematic cross section of a single microelectrode.
solution at a potential of + 150 mV versus Ag/AgC1/3 M KC1 for 60 s. In Fig. 2, a schematiccrosssection of a single electrodeis shown to illustrate the different layers depositedin the modified CMOS process.First, the silicon wafer wascovered with an insulating layer of silicon dioxide. Afterwards, polysilicon wasdepositedfor the electrodeoonnections.As an insulation from the electrolyte solution, silicon nitride and silicon dioxide were deposited. Then, the electrode areaswere etched selectively for these passivatiqg layers until reaching the polysilicon. For the electrodessilver and platinum, respectively, were sputter-coatedwith a layer thicknessof 100 nm and patternedin a lift-off process,In order to increaseadhesion between the polysilicon and the platinum, a layer of aluminium a few nanometresthick wasdepositedby sputter coating [lo].
3. Results and discussion
3.1. Electrochemical
investigations
The diffusion characteristicsof the microelectrode array were examinedby electrochemicalinvestigation in a solution of a reversible redox system.The cyclic voltammogram in a 5 r&l solution of ruthenium(III) -hexamine (Fig. 3 (a) ) showeda sigmoidaltransition to ,thediffusion limiting current without any peaks. This is in a:greementwith the diffusion characteristicsof singlemicroelectrodes [ 11,
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M. Wittkampf
-150. -0.4
-0.3
-0.2 -0.1 0.0
E M vs AgIAgCV3M
0.1
KCI
-0.5
0.0
0.5
1.0
KCI
Fig. 4. Silver contamination of the sputter-coated platinum. Cyclic voltammogram (in 0.1 M KC104, air saturated, 298 K, scan rate 100 mV s-l),
-150. -1.2
-1 .a
E M vs AgIAgCU3M
’ IW t
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-3000
81
-1.0
-0.8
-0.8
E M vs AglAgCWO.
-0.4
-0.2
0.0
7 M KCI
Fig. 3. Electrochemical response. (a) Cyclic voltammogram in 5 mM [Ru(NH~),]~+ (in 0.1 M KCI,nitrogen saturated, 298 K,‘scan rate 100 mV s-l). (b) Linear sweep voltammograms (in 0.1 M KCl, air saturated, 298 K, scan rate 100mV s-‘) before(i) andafter (ii) treatment with
dichromate-sulfuric acid.
When equipped with an internal electrolyte and a gaspermeable membrane the transducer could be used as a Clarktype oxygen sensor, Therefore, measurements in air-saturated solutions were carried out. The oxygen transient obtained (Fig. 3 (b) (i) ) shows areduction plateau between - 850 and - 1050 mV versus the internal pseudo-reference electrode ( Ag/AgCl/O. 1 M KCI) . Furthermore, the transition into the plateau exhibits two different slopes in the range - 300 to - 500 mV and - 500 to - 700 mV versus Ag/AgCl/O. 1 M KCl. To improve the catalytic properties, the array was then treated with dichromate-sulfuric acid for 5 min, followed by an electrochemical reduction at - 160 mV versus Ag/AgCl/ 3 M KC1 in 0.1 M KCl, until the current decreased to less than - 500 pA. Thus, the plateau was shifted into the range - 850 to - 1150 mV versus Ag/AgCl/O.l M KC1 (Fig. 3 (b) (ii) ) with only one slope in the transition region, The diffusion limiting current was only slightly increased. The suitable potential for amperometric oxygen measurements with this transducer is causedby the position of the
reduction plateau, - 900 mV versus Ag/AgCl/O. 1 M KCl. This is some hundreds of millivolts higher than the potentials used in commercial Clark oxygen sensors ( - 600 to - 800 mV versus Ag/ AgCl/O. 1 M KCl). This high cathodic potential causes the well-known problems of silver migration to the working electrode [ 111. Because of the increasing active electrode area, the sensor signal increased, while the lifetime of the pseudo-reference electrode decreased. It was observed that the array electrodes of one lot were contaminated with silver even prior to any measurements,
probably caused by the sputtering process. Scanning the potential between - 1 and + 1 V showed two symmetrical and very sharp peaks around the zero potential of the external reference electrode (Fig. 4). This indicates reversible reduction and oxidation of silver incorporated or adsorbed on the platinum surface [ 111. The same peaks were detected after uncontaminated chip electrodes had been used as Clark-type oxygen sensors for several hours. 3.2. Scanning secondary ion massspectrometry
Traces of silver on unused platinum microelectrodes were also detected by TOF-SIMS. The top monolayer of the substrate under investigation was removed and analysed in a time-of-flight mass spectrometer. By scanning the array surface, a lateral distribution of fragments of the same massto-charge ratio was obtained. This highly sensitive method was used to obtain detailed information on the composition of the electrode surfaces. SiMS proved that the microelectrode surfaces of a new, untreated chip were contaminated with aluminium, in addition to silver (Fig. 5, left column), This may be caused by its use as an adhesive layer underneath the platinum. By a cleaning step with dichromate-sulfuric acid and washing with deionized water, the aluminium was completely removed, in contrast to silver. Due to this procedure, however, traces of chromium were deposited on the electrodes (Fig. 5, right column). Obviously, the platinum of the microelectrodes could clearly be seen in the SIMS images both before and after this treatment (not depicted in the Figures). Organic contaminants such as polymeric layers or silicon oils that might have reduced the catalytic properties of the electrode surfaces were not detected. 3.3. Imaging by FESEM and ST&l
The micrograph in Fig. 6 shows the regular arrangement of the circular microelectrodes having a diameter of approximately 2.5 pm. The sample was tilted in the microscope by an angle of 30”. The tilt axis corresponds to the horizontal direction of the micrograph. Therefore, the scale in the vertical direction is reduced by about 10%. The surface of the insulating material around the microelectrode areas shows a
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Fig. 7. FESEM micrograph showing embedding and grainy stnrcture of a single microelectrode.
after before treatment with dichromate-sulfuric acid Fig. 5. SIMS images of the array surfacefor typical m/z, ratios (field of view, 60 pmX60 pm; raster, 128 X 128 pixel), before (left column) and after (right column) treatment with dichromate-sulfuric acid.
Fig. 6. FCESEMmicrograph ofthe regular arrangement of themicroelectroder (tilted view, 30”).
grainy structure with a grain diameter of about 130 nm. This can be seen in detail in Fig. 7 near (up to 200 nm distance) the outermost cone-shaped edge. At larger distances from this edge, however, some negative electrical charge-ups are indicated as bright, irregular features that are not correlated with
Fig. 8. WSEM mkrographofaplatinumelectroderevealing as well as ‘nano-crystallite’ aggregate structure.
thelargegrains
the real surface structure of the insulating material. The platinum microelectrode area exhibits the typical appearance of a polycrystalline material with grain sizes of approximately 100 to 700 nm. The grains are separated by grain boundaries which appear dark in the SEM micrographs. There are a few small bright round blobs with a size of 20 to 30 nm (see also Fig. 8), which are always located at the grain boundaries. Many bright blobs of similar size and perhaps of the same origin are Gtuated near the inner cone-shaped edge. The large grains are very likely due to the structure of the polysilicon lines. They are tilted with respect to each other and exhibit different mean heights in spite of the constant thickness of the sputter-coated platinum layer, A highly magnified section of the same platinum microelectrode is shown in Fig. 8. The microelectrode revealed a granular structure consisting of preponderant three-cornered bright spots having a size of 10 to’ 30 nm. They may be caused by pyramidal ‘nano-crystallites’ of platinum. It is interesting to note that there were similar granular structures visible within the ditch-like grain boundaries of the large grains. Hence, the effective surface area of a microelectrode exposed to the electrolyte was, in comparison to the geometrical area,
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Fig. 9. STM topograph of the electrode surface (height scale compared to the lowest point).
expected to be significantly larger. This is due to the roughness caused by the ‘nano-crystallites’, the large grains, their boundaries and the cavity between the platinum microelectrode and the insulating layers. Electrolyte solution may penetrate the platinum layer at the grain boundaries and/or leak through the cavity between the electrode and the insulator. This could cause a short circuit or poor performance characteristics of the arrays, respectively. An STM topograph (Fig. 9) of a platinum microelectrode also clearly revealed the large grains as well as the ‘nanocrystallites’ having a mean diameter of 30 to 50 nm. The size of crystallites was larger in the STM topograph than in the SEM micrograph, due to the, radius of the tunnelling tip, Furthermore, the topograph indicated that the large grains had different mean heights (differences of 10 to 20 nm) . As already mentioned, this may be due to the structure of the polysilicon lines. The height of the ‘nano-crystallites’ measured by STM amounted to a few nanometres only. However, this may be an underestimation of the true height, because of the radius of the tunnelling tip. Fig. 10 shows an SEM micrograph view of a microelectrode that has been tilted by 40” with respect to the electron
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beam. In this view cavities between the platinum microelectrode and the attached insulating layer of silicon nitride and also between this layer and the cone-shaped top layer of silicon dioxide can be seen. The extension of these cavities below the insulating layers cannot be determined from the micrograph. These cavities are caused by the very selective but isotropic plasma-etching process of the insulating layers (see Section 2.2). The photoresist is then underetched, resulting in these cavities. Furthermore, at this scale the large grains seemed not to have completely plane surfaces, which is indicated by their topographic contrast. This result is in close agreement with the STM data (see Fig. 9). 4. Conchsions The electrochemical and surface analysis methods CV, TOF-SIMS, FESEM and STM applied in this study have proven to be very well suited for electrode characterization and quality controlling of the modified CMOS process. They allowed the detection of different kinds of surface contaminations as well as the actual morphology and topology of electrode surfaces and their embedding in the insulating layers. Thus, the data obtained provided valuable information for the development of the next generation of thin-filmmicroelectrodes, which are currently under investigation in our laboratory.
Acknowledgeruents The chip investigated was developed in cooperation with and produced by M. Rospert and W. Mokwa from the Fraunhofer-Institut fiir Mikroelektronische Schaltungen und Systeme in Duisburg (Germany). We thank B, Hagenhoff and A. Benninghoven, Physikalisches Institut, IJniversitZit Miinster (Germany) for the TOF-SIMS investigations. This work was supported by the Bundesministerium fiir Forschung und Technologie (BMFT, Grant No. 414-401313 MV 0357) and the Ministerium fiir Wissenschaft und Forschung des Landes Nordrhein-Westfalen.
References
Fig. 10. FESEM micrograph showing cavities caused by the etching process between the electrode surface and the different insulating layers (tilted view, 40”).
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Biographies Michael Wittkumpf graduated in analytical chemistry at the Westftilische-Wilhelms Universit;it in Miinster, Germany, in 1993. His current work as a Ph.D. student at the Institute for Chemical- and Biochemical Sensor Research (ICB) in Miinsteerconcentrates on the development of Clark-type oxygen sensors based on microelectrode arrays. Karl Cammann, born in 1939, received his Ph.D. a# the U~versity of Mu&h in 1975. Since 1979 he has been pro-
~Actautors
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fessor of analytical chemistry at the Universities of Ulm and Munich (Germany) and guest professor at the University of Delaware (USA). In 1987 he received the Ocb-van-der-Grinten Environments Prize for Technology Transfer from the M~is~ of Science and Education, Germany. Now he is full professor at the University of MCinster and head of the Xnstitute for ChBmical- and Biochemical Sensor Research (ICB) . He is mainly engaged in the development of chemical sensors and biosensors and in the development of spectroscopic and c~omato~aphic methods. Matthias Amrein received his Ph.D. in biology at the ETH Zi.irich,Switzerland, in 1989.Afterapost-doctoralfellowship at the ETH Ziirich from 1989 to 1991, he is working at the Institut fii Medizinische Physik ,und Biophysik at the Universitit Melter, Germany. His crtrrent interest is focused on scanning tunnelling and scanning force microscopy of biological samples. ~~~~~~~iche~~ studied physics at the Technische Universitilt Dresden, Germany, where he received his Ph.D, in 1973. In 1973 he joined the Central Institute for Molecular Biology in Berlin, Germany, for post-doctoral work. He moved in 1981 to the Biocenter of the Universittit Basel, SwitzerIand. In 1990 he joined ,the Westf&lische-Wilhelms Universit~t in Monster, Ge~any~ asprofessor for biophysics and became head of the ~ep~~~:nt for Electron Microscopy and Analygis in the. Institute for Medical Physics and Biophysics. His research interests include the investigation of biological, medical and polymeric materials with high-resolution scanning electron, scanning tunnelling and scanning force microscopy.
