Micron 68 (2015) 27–35

Contents lists available at ScienceDirect

Micron journal homepage: www.elsevier.com/locate/micron

Advanced fabrication process for combined atomic force-scanning electrochemical microscopy (AFM-SECM) probes Alexander Eifert, Boris Mizaikoff, Christine Kranz ∗ Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Ulm, Germany

a r t i c l e

i n f o

Article history: Received 22 July 2014 Received in revised form 22 August 2014 Accepted 22 August 2014 Available online 16 September 2014 Keywords: FIB milling Atomic force-scanning electrochemical microscopy AFM-SECM AFM tip-integrated electrode Steady-state current simulation

a b s t r a c t An advanced software-controlled focused ion beam (FIB) patterning process for the fabrication of combined atomic force-scanning electrochemical microscopy (AFM-SECM) probes is reported. FIB milling is a standard process in scanning probe microscopy (SPM) for specialized SPM probe fabrication. For AFM-SECM, milling of bifunctional probes usually requires several milling steps. Milling such complex multi-layer/multi-material structures using a single milling routine leads to significantly reduced fabrication times and costs. Based on an advanced patterning routine, a semi-automated FIB milling routine for fabricating combined AFM-SECM probes with high reproducibility is presented with future potential for processing at a wafer level. The fabricated bifunctional probes were electrochemically characterized using cyclic voltammetry, and their performance for AFM-SECM imaging experiments was tested. Different insulation materials (Parylene-C and Six Ny ) have been evaluated with respect to facilitating the overall milling process, the influence on the electrochemical behavior and the long-term stability of the obtained probes. Furthermore, the influence of material composition and layer sequence to the overall shape and properties of the combined probes were evaluated. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The combination of complementary scanning probe techniques like AFM and SECM is leading to improved analytical tools by combining the advantages of both individual techniques (Kranz, 2014 and references herein). In conventional SECM, an ultra microelectrode (UME) is positioned close to the sample surface recording current-distance approach curves. These current–distance approach curves can be described theoretically, and are utilized do determine the distance of the UME by fitting the experimental data to the theoretical current distance curves (Bard et al., 1990; Kwak and Bard, 1989; Mirkin et al., 1992). The UME is then scanned in constant height in the x–y plane across the sample surface. However this so-called constant height imaging mode (Fan, 2012) may lead to falsified electrochemical information or artifacts, if the topography of the sample is unknown, the sample is tilted, and/or the surface roughness is on the same order of magnitude as the electroactive radius of the UME. To overcome these problems, shear-force based positioning, which is derived from near field scanning optical microscopy (NSOM) (Betzig et al., 1992;

∗ Corresponding author. Tel.: +49 731 50 22749; fax: +49 731 50 22763. E-mail address: [email protected] (C. Kranz). http://dx.doi.org/10.1016/j.micron.2014.08.008 0968-4328/© 2014 Elsevier Ltd. All rights reserved.

Shiku and Dunn, 1999; Toledo-Crow et al., 1992) was introduced for UME positioning (Hengstenberg et al., 2000; James et al., 1998; Ludwig et al., 1995). Shear-force based SECM allows discriminating between the topographical information and the electrochemical signal, as the electrode is now scanned in constant distance across the sample surface. However, given the radius of the UME, topographical features can usually not be resolved with a lateral resolution comparable to AFM. In contrast, modifying an AFM probe with an electroactive area leads to an analytical tool for recording topographical information with desired lateral resolution, while allowing simultaneous electrochemical data acquisition. Several research groups contributed to the combination of AFM and SECM following different strategies for implementing electrodes into AFM cantilevers. So far, two main approaches haven been reported in literature. The first approach, which is based on etching and insulating a conductive wire results in a fast and easy to fabricate procedure for “hand-made” AFM-SECM probes (Abbou et al., 2002; Macpherson and Unwin, 2000; Rodriguez et al., 2011) resulting in small electrode dimensions at low costs for the overall fabrication process. Alternatively, combined probes with defined geometry can be fabricated with high reproducibility even at a wafer level using state-of-the-art microfabrication techniques. In principle, such microfabricated bifunctional probes are again associated to two fundamentally different designs. (I) Probes with a conductive

28

A. Eifert et al. / Micron 68 (2015) 27–35

tip apex (Avdic et al., 2011; Derylo et al., 2011; Dobson et al., 2005; Fasching et al., 2006, 2005; Gullo et al., 2006; Hirata et al., 2004; Wain et al., 2011) and (II) AFM-SECM probes with recessed electrodes (Davoodi et al., 2005; Eifert et al., 2012; Kranz et al., 2001; Kueng et al., 2003; Moon et al., 2007; Shin et al., 2007; Sklyar et al., 2005; Wiedemair et al., 2010), which ensure a defined electrode-tosample distance, enable the integration of electrochemical sensing schemes, and facilitate recording of complementary data in a single scan independent of the nature of the sample. Here, “recessed” refers to the fact that the electroactive area is located at a certain distance beneath the insulating AFM tip, as depicted in Fig. 2B. For the fabrication of AFM-SECM probes with integrated electrodes recessed from the insulating AFM tip, typically FIB milling processes are used taking advantage of a state-of-the-art prototyping and microfabrication tool. Alternatively, a FIB-less wafer level batch fabrication process was developed using lithographic microfabrication methods (Shin et al., 2007), albeit providing for somewhat larger electrode dimensions. FIB fabrication is based on a high-energy Ga+ beam, which is usually focused sideways onto the cantilever substrate for material removal with nanometer precision during several consecutive milling steps. Real-time monitoring of the structuring process is typically possible with dual beam FIB/SEM systems. FIB-based patterning and structuring can be enhanced using gas-assisted processes, which selectively enhance or limits the milling rate of specific materials (Efremow, 1985; Stark et al., 1995) or reduce the damaged surface layer thickness (Adams et al., 2003). For example, Stark et al. demonstrated the influence of gas-assisted FIB processing using water vapor to significantly enhance the milling yield of carbon-based materials (frequently termed ‘selective carbon etch’, SCE), while simultaneously inhibiting milling of metals like alumina. Also, gas-assisted milling techniques promote the formation of volatile products leading to reduced re-deposition during milling of small features (Xu et al., 2009). Here, we present a novel semi-automated FIB milling fabrication process, which enables milling AFM-SECM probes from top with an offline-generated computer-assisted drawing (CAD)-file, in combination with a dedicated software (FEI NanoBuilder) designed for patterning complex structures. Using such a semi-automated fabrication process with embedded pattern alignment leads to significantly reduced fabrication time and costs in comparison to conventional FIB processes involving several consecutive milling steps (Eifert et al., 2012; Kranz et al., 2001). Furthermore, milling approaches from top, as presented here, will in future allow fabrication of sophisticated SPM probes at a wafer level enabling mass production. The fabricated probes were electrochemically characterized and then applied during AFM-SECM imaging experiments. Different electrode materials like gold and boron-doped diamond (BDD), as well as different insulation materials including ParyleneC, and Six Ny and intrinsic diamond have been tested in respect to their milling behavior, which plays a significant role in the overall shape and performance of the obtained combined probes. The effect of the electrode surface orientation on the steady-state current due to the milling behavior of the mixed material layers, and the influence on the long-term stability was investigated. In addition, the obtained experimental data were compared to theoretical results obtained from simulations via ComSol Multiphysics.

2. Material and methods 2.1. Reagents and solutions Potassium chloride p.a., and hexamineruthenium(III)chloride were purchased from Sigma–Aldrich (Steinheim, Germany). Solutions for electrochemical measurements contained 10 mmol/l

[Ru(NH3 )6 ]Cl3 and 0.1 mol/l KCl as supporting electrolyte. All solutions were prepared with Elga high purity water (Elga Labwater; VWS Deutschland GmbH, Celle, Germany). Parylene-C for cantilever insulation was purchased from Special Coating System Inc. (Indianapolis, IN, USA). 2.2. Equipment FIB milling was performed using a DualBeam Helios NanoLab 600 (FEI Company, Eindhoven, NL) equipped with a 5-axis piezo driven stage, a gas injection system and a custom-made AFM-chip holder. For electrochemical characterization and electrochemical data acquisition during combined measurements, a CHI potentiostat 832A (CH Instruments Inc., Austin, TX, USA) in combination with a three-electrode cell was used. The three-electrode cell consists of a chlorinated silver wire utilized as quasi reference electrode (Ag/AgCl), a platinum wire as counter electrode and the embedded recessed electrode of the AFM probe serving as the working electrode. AFM and AFM-SECM measurements were performed with an atomic force microscope 5500 (Agilent Technologies, Santa Clara, CA, USA) equipped with a 90 × 90 ␮m multipurpose scanner (Agilent Technologies, Santa Clara, CA, USA) inside a faraday cage. Insulation of the metal-coated cantilevers was performed with a Parylene-C coater (PDS 2010 Labcoater 2; Specialty Coating System Inc., Indianapolis, IN, USA) as described elsewhere (Heintz et al., 2001). 2.3. Modification of commercial AFM probes Standard silicon nitride AFM probes (length: 180 ␮m, nominal spring constant: 0.06 N/m) were coated with a conductive gold layer and an additional insulation layer either Six Ny (Kranz et al., 2001) or Parylene-C with a thickness of approximately 900–1000 nm (Heintz et al., 2001). BDD-coated probes were fabricated by coating silicon AFM probes (NCL probes, NanoWorld) with a nanocrystalline layer of boron-doped diamond (Smirnov et al., 2011), and consecutively with an additional insulating diamond layer of 550–600 nm. 2.4. FIB milling from top (top-mill) The coated silicon nitride AFM probes were further modified using advanced patterning routines with and without gas-assisted pattering. For Parylene-C insulated probes, an intermediate step using gas-assisted milling was performed ensuring an improved electrode surface in respect to the surrounding surface orientation. Water vapor as precursor gas leads to an enhanced formation of volatile COx compounds (Stark et al., 1995) during pattering of carbonaceous materials resulting in an enhanced milling yield. Water vapor is generated by heating MgSO4 × 7H2 O, which is injected into the chamber by a needle-type gas injection (GIS) system close to the sample surface. The water vapor adsorbs onto the surface and interacts with the carbon-based material during the ion-beam bombardment. Hence, more volatile carbon species are generated during sputtering, thereby resulting in an increased milling yield and less re-deposition of milled material. A layer-based CAD program (‘layout editor’, Juspertor UG, Unterhaching, DE) was selected for pattern generation, which is able to accommodate the pyramidal structure of the AFM tip and the challenge of milling different materials. For the presented fabrication process, FIB milling was performed with an accelerating voltage of 30 kV and a beam current of 48 pA. The milling sequence involved a series of square-shaped polygons with a square-shaped hole in the center, which is needed for subsequent tip reconstruction. The outer dimensions of the polygon were resized for every step adapting to the pyramidal geometry of the tip

A. Eifert et al. / Micron 68 (2015) 27–35

29

2.5. Electrochemical characterization

Fig. 1. A: Exemplary overlay of nine polygons taking the pyramidal shape of the tip into account via a steady increase in polygon size; B: cross-sectional schematic view of part 1 of the combined Parylene-C AFM-SECM probe fabrication.

(Fig. 1A). For correct pattern positioning, the embedded alignment routines of the NanoBuilder were used. The alignment routine is based on a comparison of a series of images created by scanning the area around the tip base and a predefined template image, which was generated prior to patterning. For Six Ny -insulated probes, the patterning sequence contained 15–20 steps in dependence of the size of the anticipated frame electrode with an individual step height of 90–120 nm. To ensure the correct step height, the milling yield of each involved material was individually determined by analyzing the depth of a milled structure of the particular material relative to the settings required to achieving a trench of 1 ␮m depth. For Parylene-C-coated probes, a two-step process was applied benefiting from an enhanced milling yield during gas-assisted patterning, which resulted in the exposure of a larger electrode area (Section 4). For contact mode AFM-SECM measurements, the tip apex should consist of a relatively robust wear-resistant material such as diamond, Si, or Six Ny . For polymer-insulated probes such as Parylene-C, which represents a soft material, the polymer material has to be removed from top (Fig. 1B, Fig. 3D) such that the tip apex is re-shaped from the initial cantilever material (here Six Ny ). In a first step, the Parylene-C layer was removed from the top of the pyramid exposing the gold layer at an area of approx. 500 nm in diameter (part 1) (Fig. 3D). For that purpose, simple square-shaped patterns were milled taking for each next step the increasing dimensions of the pattern resulting from the pyramidal shape of the tip into account (Fig. 1A). After milling another 15–20 steps (part 2) depending on the polymer thickness, SCE-assisted milling was applied (part 3) using the square shaped polygons with a center hole. SCE significantly enhanced the Parylene-C milling yield by a factor of 6.2 compared to milling without the gas-assisted process (Table 1). This significant enhancement leads to a change in the resulting electrode (Fig. 3G) geometry caused by the different milling yields of the materials involved in the multi layer/multimaterial structure (Section 4). For the SCE-based milling step, carbonaceous materials such as organic polymers not only lead to enhanced milling yields, but also to less re-deposition. Additionally, the water vapor does not significantly interact with the gold layer, and coincidentally decreases the milling yield of Six Ny by a factor of four (Stark et al., 1995), which protects the original AFM tip by causing less damage during patterning for later tip reconstruction. Irregularities at the outer edges of the pyramidal structure were removed via a final polishing mill using again a square-shaped polygon with a center hole of 2.0–2.5 ␮m in diameter (Fig. 3H).

Electrochemical characterization was performed before and after the milling procedure to characterize the electrochemical performance of the combined probes. Cyclic voltammograms were recorded in a three-electrode setup with the combined probe as working electrode in an aqueous solution containing 10 mmol/l [Ru(NH3 )6 ]Cl3 , in 0.1 mol/l KCl as supporting electrolyte at a scan rate of 100 mV/s. Solutions containing hexamineruthenium(III)chloride were sparged for at least 15 min with Argon prior to use. After electrochemical characterization of the insulation quality, the probes were rinsed with Elga high purity water (18.2 M) and dried at 60 ◦ C for 30 min. After exposing the electroactive area using the top-mill process, the combined AFM-SECM probes were again characterized by cyclic voltammetry. 2.6. Combined AFM-SECM measurements For simultaneous data acquisition during imaging experiments, the current output of the potentiostat was directly fed into an auxiliary signal input channel of the AFM. The topographical data acquisition was performed using contact mode with a scan speed 0.3 line/s and 512 data points per line. The SECM measurements were performed in a three-electrode setup using an electrochemical AFM cell equipped with an Ag/AgCl quasi reference electrode. The integrated frame electrode served as working electrode, and a Pt wire as auxiliary electrode. As a model sample for imaging, a FIBstructured Pt-coated SiO2 glass substrate with defined conductive and insulating areas was used. 3. Simulations For simulating the mass transport towards such combined AFM-SECM probes and the resulting current response, a three dimensional virtual replica of the combined probe was generated (Fig. 2) according to the electrode geometry obtained by the different materials and milling yields. For the numerical calculations, only diffusion towards the electrode was considered in solution. Any interactions of the reduced species of [Ru(NH3 )6 ]Cl3 were considered negligible. For the numerical calculation of the diffusion profile, and consequentially, the resulting current a diffusion coefficient of D = 7.5e−6 cm2 /s (Wiedemair et al., 2010) was used. The concentration of the redox mediator was set to zero at the electrode surface and to the concentration of the bulk solution at the outer solvent box boundaries (Fig. 2). For current simulations, the concentration of the oxidized redox species can be described in a Cartesian coordinate system as c(x,y,z). Based on Fick’s 2nd law of diffusion, the time dependent concentration change is expressed by ∂c/∂t = D × (∂2 c/∂x2 + ∂2 c/∂y2 + ∂2 c/∂z2 ) with c: concentration of the redox mediator and t: time. Only diffusion is considered for mass transport, therefore  the steady state current can be described by it,∞ = n × F × D × (∂c/∂x + ∂c/∂y + ∂c/∂z)dA with n = 1, F = 9.64853e4 C/mol and A = the exposed electrode area. 4. Results and discussion 4.1. Top-milled probes

Table 1 Volume-per-dose parameters of the involved materials. Material

Volume-per-dose (VPD)

Six Ny (insulator) Six Ny (probes) Au Parylene-C Parylene-C (gas-assisted)

0.275 ␮m /nC 0.162 ␮m3 /nC 1.477 ␮m3 /nC 0.296 ␮m3 /nC 1.841 ␮m3 /nC 3

Besides providing the patterning routines, the FEI NanoBuilder software module also includes alignment algorithms for extended patterning sequences, thereby ensuring drift correction and a high precision in pattern positioning. Also, the support of milling at multiple sites for mass fabrication at the wafer level can be envisaged. To achieve maximum precision, the volume-per-dose

30

A. Eifert et al. / Micron 68 (2015) 27–35

Fig. 2. 3D/2D models for steady state current simulations; 3D: model of a fabricated tip with solvent box; 2D: exemplary scheme of an AFM-SECM probe with an inward tilted electrode.

(VPD) parameters for all involved materials have been evaluated (Table 1). 4.2. Parylene-C Parylene-C, a xylylene derivative, is an excellent insulation material used in semiconductor industry and for biomedical devices. It also has been frequently used as insulation material for micro-sized electrodes due to its biocompatibility and pinhole-free coating. It also reveals certain advantages as insulating material for the fabrication process of AFM-SECM probes. For example, Parylene-C represents an insulator material, which is deposited in a cold chemical vapor deposition process, and therefore avoids bending of the modified cantilever due to different thermal expansion coefficients. Furthermore, Parylene-C as insulation layer forms a pinhole-free insulation layer, and the force constant of the cantilever is not significantly changed for layers up to 900 nm. The top-milling process of Parylene-C-coated AFM-SECM probes (Fig. 3) led to combined probes with a variable size of the exposed electrode depending on the number of applied milling steps. With increasing step number of the milling process, the exposed frame size of the electrode was increasing. As described in the experimental section, the Parylene-C coating was removed from the top part of the insulated cantilever (Fig. 3A–D). To avoid undesirable artifacts, gas-assisted milling was applied only during the last step

of the frame electrode exposure (Fig. 3G). For gas-assisted milling steps, the patterning parameters such as dwell time, beam overlap etc. play a significant role, and are key features for a successful patterning sequence. Using trigonometric functions, the exposed (effective) electrode thickness and surface tilt can be calculated. The projected effective electrode thickness Dp = TAu /(R × tan ˛) (Fig. 4) was calculated by comparing the different VPD parameters representing the milling yield of the involved materials using the VPD ratio R = VPD2 /VPD1 (VPD1 = VPD of insulating material; VPD2 = VPD of the conductive layer). Responsible for the tilt effect of the exposed electrode was the VPD ratio and the thickness of the sputtered gold layer (TAu = 100 nm) determined in vertical orientation relative to the cantilever surface. The tilt of the electrode surface is given by  = arctan(Tp /Dp ) = arctan((Tp × R × tan ˛)/TAu ) with Tp = (TAu /R) − TAu for R < 1. A projected effective electrode thickness of 95 nm with an electrode angle of 13◦ was calculated, which was in good agreement with the obtained experimental data showing a thickness of 95.1 nm and a tilt angle of 19◦ . This electrode geometry, which is tilted towards the outer edge, is also beneficial for the diffusion of the redox mediator towards the electrode surface. Given the outward tilt of the electrode, the diffusion is enhanced and a higher steady-state current was observed compared to a flat embedded ring- or frame-shaped electrode with an enhanced mass transfer towards the outer edge of the electrode (Lee et al., 2001).

Fig. 3. Sequence of SEM images representing the top-mill patterning process of a Parylene-C-coated AFM-SECM probe; A–D: part 1; E–H: part 2 + 3.

A. Eifert et al. / Micron 68 (2015) 27–35

Fig. 4. Schematic cross-section view of a gold frame electrode insulated with Parylene-C using selective carbon etch; Dp = projected electrode thickness, Tp = projected thickness of the gold layer, ˛ = pyramid angle (52◦ ),  = electrode angle.

31

Fig. 6. Schematic cross-section of a milled gold frame electrode insulated with Six Ny , Dp = projected electrode thickness, Tp = projected thickness of the gold layer, ˛ = pyramid angle (52◦ ),  = electrode angle.

4.3. Silicon nitride Six Ny is a commonly used coating material in microfabrication with excellent electrical insulation properties even if deposited as thin layer (ε = 7;  = 1012  cm−1 (Piccirillo and Gobbi, 1990)). However, as shown below for a top-mill fabrication process, this material revealed some adverse effects. By applying the developed milling routine to Six Ny -coated probes, a comparatively even insulator surface was generated surrounding the gold frame electrode (Fig. 5). However, as gold has a 5.37 times higher milling yield (Table 1) compared to the Six Ny insulation layer, the ratio of milling at the electroactive layer surrounded by Six Ny led to an exposure of only a small ring/frame electrode with a projected thickness of 15–25 nm, as schematically indicated in Fig. 6. The projected thickness of the exposed electrode (Dp ) can be calculated again using Tp = TAu − TAu /R for R > 1 for the inverse VPD ratio. Utilizing the corresponding equation for the electrode angle led to an effective thickness of the exposed frame electrode of 15 nm with a tilt of −79.9◦ relative to the cantilever surface. As the effective thickness of the electrode was only around 15 nm and the electrode had an inward tilt, Six Ny as insulation material is apparently not suitable, if a top-mill process is applied. By decreasing the milling yield ratio (R), which can be achieved using an insulation material with a higher milling yield as shown earlier, the dimensions of the electrode are increased. Hence, for all further experiments discussed herein, Parylene-C insulation was applied for AFM-SECM probes. Evaluating the SEM images shown in Figs. 3 and 5 reveal that the AFM imaging functionality is

maintained by the re-shaped insulating tip apex. An AFM tiprecessed electrode is exposed, which is surrounded by a flat insulation layer. However, it has to be noted that the effective thickness of the exposed electrode layer depends significantly on the milling properties of the insulating material. 4.4. Electrochemical characterization In order to compare the steady-state currents of Six Ny and Parylene-C-coated probes, all probes were fabricated targeting a frame length of the exposed electrode of 1 ␮m. As shown in Fig. 7, both types of probes show a similar sigmoidal-shaped CV in bulk solution with a cathodic faraday current of 2.3 × 10−10 A (Six Ny ) and 1.5 × 10−9 A (Parylene-C) with only minor capacitive contributions. Comparing the steady-state currents in respect to the insulation layer revealed that the Six Ny -coated probes showed a steadystate current of 2.3 × 10−10 A, which is one order of magnitude smaller than the steady-state current recorded at Parylene-Ccoated probes. This can be attributed to the significant difference in tilt angle of the electrode surface for both types of probes (Six Ny : −79,9◦ ; Parylene-C: +13,0◦ ). As the mass transport towards ring- or frame-shaped electrodes is highest at the outer edge of the miniaturized electrode (Lee et al., 2001) contributing significantly to the steady-state current, a decreased steady-state current for Six Ny coated and an increased current for Parylene-C-coated probes was anticipated as described above.

Fig. 5. Sequence of SEM images representing the top-mill patterning process of a Six Ny -insulated AFM-SECM probe.

32

A. Eifert et al. / Micron 68 (2015) 27–35

Fig. 7. Cyclic voltammograms of top-milled AFM-SECM probes; A: Six Ny -insulated; B: Parylene-C-insulated; recorded in 10 mM [Ru(NH3 )6 ]Cl3 with 0.1 M KCl as supporting electrolyte; scan rate 20 mV/s (Six Ny ), 100 mV/s (Parylene-C).

4.5. Numerical simulations As derived from the electrochemical characterization, the current is strongly correlated to the electrode angle  (Figs. 4 and 6). Therefore, simulations were performed to evaluate the influence of the electrodes angle with respect to the top-mill fabricated probes using Six Ny and Parylene-C as insulation material. Table 2 shows the calculated influence for different simulated electrode angles (−80◦ , 0◦ , +13◦ ). Hereby, the calculation for 0◦ tilt was used as reference point to evaluate the impact on the diffusion-limited current generated in bulk solution. As summarized in Table 2, the simulations confirmed the influence of the tilt angle on the diffusion behavior, and hence, the resulting steady-state currents. As shown in Table 2 and Fig. 7, simulation and experimental data for Six Ny and Parylene-C probes are in good agreement confirming the direct influence of the electrode tilt. 4.6. AFM-SECM imaging Finally, the fabricated probes were evaluated in terms of their functionality for combined AFM-SECM imaging. As a test sample, a FIB-milled structure revealing defined conductive and insulating features in form of the letter “A” was used. Combined measurements with the fabricated probes revealed that so far only electrodes with a VPD ratio R < 1 are suitable for long term measurements. Experiments have shown that the electrochemical signal of probes with a VPD ratio R > 1 experience a rapid degradation of the electrochemical signal, which may be attributed to contamination of the significantly smaller electrode due to a cumulative deposition of contaminations during AFM-SECM data acquisition (Fig. 5H, 8A) inside the crevasse formed during the top-mill process. As previously indicated, the fabrication of Six Ny -coated combined probes did not yield suitable probes to date. In contrast, Parylene-C-coated probes with a VPD ratio R < 1 showed excellent stability, allowing several hours of imaging without significant decrease of the recorded electrochemical or topographical response. The images shown in Fig. 9 were recorded

in contact mode AFM and feedback mode SECM. In SECM feedback mode, a redox mediator cycle is utilized to obtain for example information on the surface conductivity. During the SECM experiment, a redox mediator is continuously converted at the electrode, and the determined current depends on the diffusive flux of the mediator species towards the electrode surface. If the electrode is scanned across a conductive surface, the converted mediator species can be regenerated at the sample surface to its initial oxidation state. This regeneration-cycle leads to an enhanced mass transfer towards the electrode compared to the flux in bulk solution, and thus, to a higher current. If on the other hand a non-conductive surface is present, the diffusive flux is partially blocked during imaging , and therefore the detected current is decreased compared to the signal in bulk solution (Kwak and Bard, 1989). As anticipated, the electrochemical response of the Parylene-C-coated bifunctional AFM-SECM probe corresponds well to the recorded AFM image. The elevated areas of the micro structured sample in the topographical image correspond to the platinum-coated glass features. These areas also show a higher cathodic current due to the reoxidation of the redox active species at the conductive surface (i.e., positive feedback effect). In contrast, areas where the platinum was removed and the SiO2 glass substrate is exposed, a decreased cathodic current is measured due to hindered diffusion of the redox mediator towards the tip-integrated electrode. As the re-shaped AFM tip has a curvature in the range of commercially available probes, high-resolution topographical imaging can be achieved. The electrochemical resolution obtained with an electrode frame length of 1.2 ␮m is approximately 300 nm (Wiedemair et al., 2008). An enhanced electrochemical resolution can be achieved by reducing the electrode size, i.e., by exposing a smaller frame electrode during the fabrication process.

Table 2 Calculated current ratio for combined AFM-SECM probes fabricated using a top-mill process. Electrode tilt

Current ratio

−80◦ 0◦ +13◦

0.683 1 1.166

Fig. 8. Exemplary diffusion profile (simulated) for [Ru(NH3 )6 )Cl3 as redox mediator at an electrode with A: a negatively tilted geometry (−80◦ ) and B: with a positively tilted geometry (+13◦ ).

A. Eifert et al. / Micron 68 (2015) 27–35

33

Fig. 9. Combined AFM-SECM image recorded with a Parylene-C-insulated probe: electrode size: 1.2 ␮m, tip length: 650 nm, scan speed: 0.3 line/s, A: topography, B: electrochemistry, C: cross-section of topography, D: Cross-section of the electrochemical response recorded in 10 mM [Ru(NH3 )6 ]Cl3 /0.1 M KCl (Etip : −0.5 mV vs. AgQRE).

Fig. 10. A: SEM image of a diamond-insulated BDD-coated AFM probe; B: top-mill fabricated AFM-SECM probe with an integrated BDD electrode.

5. Conclusions Combined AFM-SECM probes can be fabricated with a recessed electrode using FIB milling based on an advanced patterning process from the top of the device at significantly reduced fabrication times. In addition, the presented fabrication process is suitable for wafer level production, as no further reorientation and stage alignment is required. However, the effective exposed electrode thickness, which is given by the thickness of the sputtered gold layer is slightly reduced from approximately 100 nm (compared

to a side-mill process (Kranz et al., 2001) to approximately 95 nm for Parylene-C-insulated probes, and further reduced to approximately 15–25 nm for Six Ny -insulated probes. Future research will focus on optimizing the exposed electrode area for Six Ny -insulated probes to maintain the thickness of the electrode layer. The VPD ratio of the used materials is predominantly responsible for this effect, thereby resulting in electrodes with low projected thicknesses (R > 1) and a steady-state current in the low nA region. In addition, the observed negative tilt of the exposed electroactive area renders the probe prone to degradation during imaging

34

A. Eifert et al. / Micron 68 (2015) 27–35

experiments. Using Parylene-C as insulation layer in addition to applying a gas-assisted FIB process resulted in a compensation of the negative tilt and promoted the exposure of an electroactive area with a thickness similar to the initially sputtered metal layer. As a result, combined AFM-SECM probes could be processed showing similar characteristics in respect to electrode thickness as reported for probes milled by conventional strategies (Kranz et al., 2001). Future research will focus on combined probes utilizing borondoped diamond (BDD) as conductive layer (Eifert et al., 2012), and intrinsic diamond (iD) as insulator material, which are ideally suitable for the developed top-milling process due to similar milling yields and VPD values (R ≈ 1). First experiments confirmed that this material composition with a VPD ratio of R ≈ 1 results in a surface tilt of 0◦ (Fig. 10). In addition, no gas-assisted milling is required for diamond probes. Milling of diamond leads to a smooth electrode surface evenly embedded into the surrounding insulating material (Fig. 10B). Utilizing BDD as electrode material significant improvements concerning the potential window (Bouamrane et al., 1996; Yano, 1998; Granger et al., 1999; Lim et al., 2008), chemical inertness (Zarechnaya et al., 2008) and signal-to-noise ratio (Strojek et al., 1996) can be achieved. This material properties gain access to new electroanalytical applications detecting molecules prone to electrode fouling (Spãtaru et al., 2001; Zhao et al., 2010) or measurements under harsh chemical conditions. Such measurements also benefit from a chemical robust insulating material, capable to withstand harsh conditions. Therefore intrinsic diamond should be perfectly suited as insulating material.

Acknowledgements The Focused Ion Beam Center UUlm supported by FEI Company (Eindhoven, Netherlands), the German Science Foundation (INST40/385-F1UG), and the Struktur- und Innovationsfonds Baden-Württemberg are acknowledged for support of this study. FEI is specifically thanked for providing the opportunity to participate in the NanoBuilder beta testing program. The work was supported by the program “Methoden für die Lebenswissenschaften P- LSMeth/23” of the Baden-Württemberg Stiftung. Furthermore C. Nebel, Fraunhofer Institute for Applied Solid State Physics in Freiburg/Germany is acknowledged for supplying borondoped diamond-coated cantilevers.

References Abbou, J., Demaille, C., Druet, M., Moiroux, J., 2002. Fabrication of submicrometersized gold electrodes of controlled geometry for scanning electrochemicalatomic force microscopy. Anal. Chem. 74, 6355–6363. Adams, D.P., Vasile, M.J., Mayer, T.M., Hodges, V.C., 2003. Focused ion beam milling of diamond: effects of H2 O on yield, surface morphology and microstructure. J. Vac. Sci. Technol. B 21, 2334–2343. Avdic, A., Lugstein, A., Wu, M., Gollas, B., Pobelov, I., Wandlowski, T., Leonhardt, K., Denuault, G., Bertagnolli, E., 2011. Fabrication of cone-shaped boron doped diamond and gold nanoelectrodes for AFM-SECM. Nanotechnology 22, 145306. Bard, A.J., Denuault, G., Lee, C., Mandler, D., Wipf, D.O., 1990. Scanning electrochemical microscopy-a new technique for the characterization and modification of surfaces. Accounts Chem. Res. 23, 357–363. Betzig, E., Finn, P.L., Weiner, J.S., 1992. Combined shear force and near-field scanning optical microscopy. Appl. Phys. Lett. 60, 2484. Bouamrane, F., Tadjeddine, A., Butler, J.E., Tenne, R., Lévy-Clément, C., 1996. Electrochemical study of diamond thin films in neutral and basic solutions of nitrate. J. Electroanal. Chem. 405, 95–99. Davoodi, A., Pan, J., Leygraf, C., Norgren, S., 2005. In situ investigation of localized corrosion of aluminum alloys in chloride solution using integrated EC-AFM/SECM techniques. Electrochem. Solid-State Lett. 8, B21–B24. Derylo, M.A., Morton, K.C., Baker, L.A., 2011. Parylene insulated probes for scanning electrochemical-atomic force microscopy. Langmuir 27, 13925–13930. Dobson, P.S., Weaver, J.M.R., Holder, M.N., Unwin, P.R., Macpherson, J.V., 2005. Characterization of batch-microfabricated scanning electrochemical-atomic force microscopy probes. Anal. Chem. 77, 424–434. Efremow, N.N., 1985. Ion-beam-assisted etching of diamond. J. Vac. Sci. Technol. B 3, 416–418.

Eifert, A., Smirnov, W., Frittmann, S., Nebel, C.E., Mizaikoff, B., Kranz, C., 2012. Atomic force microscopy probes with integrated boron doped diamond electrodes: Fabrication and application. Electrochem. Commun. 25, 30–34. Fan, F.-R.F., 2012. Scanning electrochemical microscopic imaging. In: Bard, A.J., Mirkin, M.V. (Eds.), Scanning Electrochemical Microscopy. CRC Press, pp. 53–74. Fasching, R.J., Bai, S.J., Fabian, T., Prinz, F.B., 2006. Nanoscale electrochemical probes for single cell analysis. Microelectron. Eng. 83, 1638–1641. Fasching, R.J., Tao, Y., Prinz, F.B., 2005. Cantilever tip probe arrays for simultaneous SECM and AFM analysis. Sensor. Actuat. B 108, 964–972. Granger, M.C., Xu, J., Strojek, J.W., Swain, G.M., 1999. Polycrystalline diamond electrodes: basic properties and applications as amperometric detectors in flow injection analysis and liquid chromatography. Anal. Chim. Acta 397, 145–161. Gullo, M.R., Fredrix, P.L.T.M., Akiyama, T., Engel, A., de Nico, F., Staufer, U., 2006. Characterization of microfabricated probes for combined atomic force and high-resolution scanning electrochemical microscopy. Anal. Chim. Acta 78, 5436–5442. Heintz, E., Kranz, C., Mizaikoff, B., Noh, H.-S., Hesketh, P., Lugstein, A., Bertagnolli, E., 2001. Characterization of parylene coated combined scanning probe tips for insitu electrochemical and topographical imaging. Proc. IEEE Nanotechnol. Conf., 346–351. Hengstenberg, A., Kranz, C., Schuhmann, W., 2000. Facilitated tip-positioning and applications of non-electrode tips in scanning electrochemical microscopy using a shear force based constant-distance mode. Chem. Eur. J. 6, 1547–1554. Hirata, Y., Yabuki, S., Mizutani, F., 2004. Application of integrated SECM ultra-microelectrode and AFM force probe to biosensor surfaces. Bioelectrochemistry 63, 217–224. James, P.I., Garfias-Mesias, L.F., Moyer, P.J., Smyrl, W.H., 1998. Scanning electrochemical microscopy with simultaneous independent topography. J. Electrochem. Soc. 145, L64–L66. Kranz, C., 2014. Recent advancements in nanoelectrodes and nanopipettes used in combined scanning electrochemical microscopy techniques. Analyst 139, 336–352. Kranz, C., Friedbacher, G., Mizaikoff, B., Lugstein, A., Smoliner, J., Bertagnolli, E., 2001. Integrating an ultramicroelectrode in an AFM cantilever: Combined technology for enhanced information. Anal. Chem. 73, 2491–2500. Kueng, A., Kranz, C., Lugstein, A., Bertagnolli, E., Mizaikoff, B., 2003. Integrated AFM–SECM in tapping mode: Simultaneous topographical and electrochemical imaging of enzyme activity. Angew. Chem. Int. Ed. 42, 3238–3240. Kwak, J., Bard, A.J., 1989. Scanning electrochemical microscopy. Theory of the feedback mode. Anal. Chem. 61, 1221–1227. Lee, Y., Amemiya, S., Bard, A.J., 2001. Scanning electrochemical microscopy. 41. Theory and characterization of ring electrodes. Anal. Chem. 73, 2261–2267. Lim, P.Y., Lin, F.Y., Shih, H.C., Ralchenko, V.G., Varnin, V.P., Pleskov, Y.V., Hsu, S.F., Chou, S.S., Hsu, P.L., 2008. Improved stability of titanium based boron-doped chemical vapor deposited diamond thin-film electrode by modifying titanium substrate surface. Thin Solid Films 516, 6125–6132. Ludwig, M., Kranz, C., Schuhmann, W., Gaub, H.E., 1995. Topography feedback mechanism for the scanning electrochemical microscope based on hydrodynamic forces between tip and sample. Rev. Sci. Instrum. 66, 2857–2860. Macpherson, J.V., Unwin, P.R., 2000. Combined scanning electrochemical-atomic force microscopy. Anal. Chem. 72, 276–285. Mirkin, M.V., Fan, F.-R.F., Bard, A.J., 1992. Scanning electrochemical microscopy part 13. Evaluation of the tip shapes of nanometer size microelectrodes. J. Electroanal. Chem. 328, 47–62. Moon, J.-S., Shin, H., Mizaikoff, B., Kranz, C., 2007. Bitmap-assisted focused ion beam fabrication of combined atomic force scanning electrochemical microscopy probes. J. Korean Phys. Soc. 51, 920–924. Piccirillo, A., Gobbi, A.L., 1990. Physical-electrical properties of silicon nitride deposited by PECVD on III–V semiconductors. J. Electrochem. Soc. 137, 3910–3917. Rodriguez, R.D., Anne, A., Cambril, E., Demaille, C., 2011. Optimized hand fabricated AFM probes for simultaneous topographical and electrochemical tapping mode imaging. Ultramicroscopy 111, 973–981. Shiku, H., Dunn, R.C., 1999. Near-field scanning optical microscopy. Anal. Chem. 71, 23A–29A. Shin, H., Hesketh, P.J., Mizaikoff, B., Kranz, C., 2007. Batch fabrication of atomic force microscopy probes with recessed integrated ring microelectrodes at a wafer level. Anal. Chem. 79, 4769–4777. Sklyar, O., Kueng, A., Kranz, C., Mizaikoff, B., Lugstein, A., Bertagnolli, E., Wittstock, G., 2005. Numerical simulation of scanning electrochemical microscopy experiments with frame-shaped integrated atomic force microscopy-SECM probes using the boundary element method. Anal. Chem. 77, 764–771. Smirnov, W., Kriele, A., Hoffmann, R., Sillero, E., Hees, J., Williams, O.A., Yang, N., Kranz, C., Nebel, C.E., 2011. Diamond-modified AFM probes: from diamond nanowires to atomic force microscopy-integrated boron-doped diamond electrodes. Anal. Chem. 83, 4936–4941. Spãtaru, N., Sarada, B.V., Popa, E., Tryk, D.A., Fujishima, A., 2001. Voltammetric determination of l-Cysteine at conductive diamond electrodes. Anal. Chem. 73, 514–519. Stark, T.J., Shedd, G.M., Vitarelli, J., Griffis, D.P., Russell, P.E., 1995. H2 O enhanced focused ion beam micromachining. J. Vac. Sci. Technol. B 13, 2565–2569. Strojek, J.W., Granger, M.C., Swain, G.M., Dallas, T., Holtz, M.W., 1996. Enhanced signal-to-background ratios in voltammetric measurements made at diamond thin-film electrochemical interfaces. Anal. Chem. 68, 2031–2037.

A. Eifert et al. / Micron 68 (2015) 27–35 Toledo-Crow, R., Yang, P.C., Chen, Y., Vaez-Iravani, M., 1992. Near-field differential scanning optical microscope with atomic force regulation. Appl. Phys. Lett. 60, 2957–2959. Wain, A.J., Cox, D., Zhou, S., Turnbull, A., 2011. High-aspect ratio needle probes for combined scanning electrochemical microscopy — atomic force microscopy. Electrochem. Commun. 13, 78–81. Wiedemair, J., Balu, B., Moon, J.-S., Hess, D.W., Mizaikoff, B., Kranz, C., 2008. Plasma-deposited fluorocarbon films: Insulation material for microelectrodes and combined atomic force microscopy-scanning electrochemical microscopy probes. Anal. Chem. 80, 5260–5265. Wiedemair, J., Menegazzo, N., Pikarsky, J., Booksh, K.S., Mizaikoff, B., Kranz, C., 2010. Novel electrode materials based on ion beam induced deposition of platinum carbon composites. Electrochim. Acta 55, 5725–5732.

35

Xu, Z.W., Fang, F.Z., Fu, Y.Q., Zhang, S.J., Han, T., Li, J.M., 2009. Fabrication of micro/nano-structures using focused ion beam implantation and XeF2 gasassisted etching. J. Micromech. Microeng. 19, 054003. Yano, T., 1998. Electrochemical behavior of highly conductive boron-doped diamond electrodes for oxygen reduction in alkaline solution. J. Electrochem. Soc. 145, 1870–1876. Zarechnaya, E.Y., Isaev, E.I., Simak, S.I., Vekilov, Y.K., Dubrovinsky, L.S., Dubrovinskaia, N.A., Abrikosov, I.A., 2008. Ground-state properties of boron-doped diamond. J. Exp. Theor. Phys. 106, 781–787. Zhao, H., Bian, X., Galligan, J.J., Swain, G.M., 2010. Electrochemical measurements of serotonin (5-HT) release from the guinea pig mucosa using continuous amperometry with a boron-doped diamond microelectrode. Diamond Relat. Mater. 19, 182–185.

Advanced fabrication process for combined atomic force-scanning electrochemical microscopy (AFM-SECM) probes.

An advanced software-controlled focused ion beam (FIB) patterning process for the fabrication of combined atomic force-scanning electrochemical micros...
2MB Sizes 0 Downloads 10 Views