A Leveling Method Based on Current Feedback Mode of Scanning

Technical Note pubs.acs.org/ac

A Leveling Method Based on Current Feedback Mode of Scanning Electrochemical Microscopy Lianhuan Han,† Ye Yuan,† Jie Zhang,† Xuesen Zhao,‡ Yongzhi Cao,*,‡ Zhenjiang Hu,‡ Yongda Yan,‡ Shen Dong,‡ Zhong-Qun Tian,† Zhao-Wu Tian,† and Dongping, Zhan*,† †

College of Chemistry and Chemical Engineering, and State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China ‡ Center for Precision Engineering, Harbin Institute of Technology, P.O. Box 413, Harbin 150001, China ABSTRACT: Substrate leveling is an essential but neglected instrumental technique of scanning electrochemical microscopy (SECM). In this technical note, we provide an effective substrate leveling method based on the current feedback mode of SECM. By using an air-bearing rotary stage as the supporter of an electrolytic cell, the current feedback presents a periodic waveform signal, which can be used to characterize the levelness of the substrate. Tuning the adjusting screws of the tilt stage, substrate leveling can be completed in minutes by observing the decreased current amplitude. The obtained high-quality SECM feedback curves and images prove that this leveling technique is valuable in not only SECM studies but also electrochemical machining.

he first report on scanning electrochemical technique can be traced back to 1972 in which a scanning reference electrode was used to study the pit corrosion on stainless steel.1 In the middle 1980s, Engstrom employed microelectrodes as the scanning probe to characterize the reactivity at graphite−epoxy surfaces and within the diffusion layer.2−4 In 1989, Bard’s group published a series of papers in which the fundamentals of scanning electrochemical microscopy (SECM) were proposed.5−7 Since then, SECM has become an important electrochemical technique to investigate the heterogeneous and homogeneous reactions, for high-resolution imaging of the chemical reactivity and topography of various interfaces, and for micro- and nanofabrications. The principles, instrumentations, and applications of SECM have been thoroughly covered in a monograph and also fully reviewed but not limited as referenced.8−13 As for SECM instrumentation, the main interests are put into tip fabrications, tip manipulation, and electrochemical modulations. Ultramicroelectrodes, nanoelectrodes, and micro- and nanopipettes are employed as SECM tips and their fabrications were described comprehensively.14 The basic configuration of tip manipulation includes a three-dimensional (3D) micropositioner system composed of X-Y-Z stepper motors, a Z piezoelectric motor, and their controllers. The feedback information for tip positioning can be tip current most frequently, shear force,15,16 or atomic force.17−19 Meantime, almost all kinds of traditional electrochemical techniques are applied in SECM investigations including chronoamperometry,20,21 chronopotentiometry,22 AC voltammetry,23−25 and electrochemical impedance.26,27 Moreover, many labmade SECM instruments have been developed for special usages except the commercially available ones.

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© 2013 American Chemical Society

However, little attention has been paid to substrate leveling, which is essential to obtain high-quality approaching curves, images, and micro- and nanopatterns. Although bubble level is often used to make the substrate horizontal, the error is so big that researchers have to use three additional wire-leveling methods.28 That means one performs approach curves at three different points on the substrate and tunes the adjusting screws alternately until the three approach curves are overlapped. In other words, the current feedbacks should be the same when the tip moves the same distance to the substrate from a constant height at all three points. The levelness can be high, but, the experimental operation is rather time-consuming. Substrate leveling might not be so important in the constantdistance scanning mode in which the tip is manipulated through constant current, shear force, atomic force (AFM), tunnel current (STM), or scanning near-field optic microscopy (SNOM), as mentioned above. Moreover, the “contact mode” of SECM is actually leveling-free, such as the soft stylus probes29−31 as well as the scanning electrochemical cell microscopy (SECCM).32−36 Some SECM workstations are claiming a capacity of obtaining the topography of substrate through SNOM or other SPMs and then scanning the SECM tip in a constant-distance-way according to the obtained topography. However, the cost of the special SECM instruments will be much higher, while the information process will become more complex. In this technical note, we proposed a highly efficient method for SECM substrate leveling. An air-bearing rotary stage is Received: October 25, 2012 Accepted: January 5, 2013 Published: January 5, 2013 1322

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Technical Note

Figure 1. Schematic diagrams of the SECM instrument: the air-bearing rotary stage is assembled on the cross of the X-Y stepper motor, the tilt platform with three adjusting screws, the electrolytic cell are planted onto the rotary stage in order, and the SECM tip is fixed on the Z manipulator, which is composed by the stepper moter Z1, piezoelectric motor Z2, and an implanted force displacement sensor (FS). During scanning or machining, the SECM tip is still, while the substrate moves in the X-Y directions or rotates.

be controlled either automatically or manually, are equipped into the stage to perform the leveling. The whole SECM system is controlled by a lab-designed program. The operational procedures are the same as conventional SECM investigations. To evaluate the quality of leveling, the current feedback is recorded and compared with the results obtained through a dial gauge (Harbin Measuring & Cutting Tool Group Company Ltd.). It should be noted that the SECM current feedback is distant sensitive, the precise leveling can be achieved through multiple strategies (i.e., a fine leveling will be performed after a coarse leveling by putting the tip closer to the substrate.

employed as the supporter of the electrolytic cell and the substrate. The levelness can be characterized through the amplitude of periodic current feedback obtained when the airbearing rotary stage is rotating. The leveling method shows advantages in the large-scale SECM imaging and also electrochemical machining.

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EXPERIMENTAL SECTION Chemicals and Materials. All chemicals used in the experiments (NaBr and H2SO4) are analytical grade and provided by Sinopharm Company. GaAs wafers with a surface roughness of 2 nm were purchased from China Crystal Technologies Company, Ltd. All aqueous solutions were prepared with deionized water (18.2 MΩ, Milli-Q, Millipore Corp.). Platinum wire with a diameter of 25 μm is obtained from Beijing Cuibolin Non-Ferrous Technology Developing Company, Ltd. The platinum wire is sealed in a glass tube (i.d.: 1 mm, o.d.: 2 mm). The obtained ultramicroelectrode is wellpolished to an RG value of 3 and used as the SECM tip. SECM Instrument. The diagram of the SECM instrument is shown in Figure 1. A CHI760 workstation is used to perform all the electrochemical experiments. The tip manipulation system is composed of X-Y-Z1 stepper motors, a piezoelectric motor Z2, and their controllers. A force displacement sensor FS (FAS-70-1, Joint Sensor Instruments Ltd., Hong Kong) is used to detect the contact force between the tip and the substrate, in order to protect the tip and the piezoelectric motor. The innovation of our SECM instrument is that an air-bearing rotary stage (ABRS150MP-M-X50, Aerotech. Inc.) is employed as the supporter for the electrolytic cell and the substrate. Its axial and radial error motions are less than 20 nm. During the leveling process, the rotating rate is 0.33 rad/min. Three adjusting screws, which can

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RESULTS AND DISCUSSIONS Although Br2/Br− is not the classic redox couple in electrochemistry, it is very practical in electrochemical patterning and machining.37−39 The following chemical system is employed for the testing experiment: Tip reaction: 2Br − → Br2 + 2e

(1)

Substrate reaction:Br2 + 1/3GaAs + H 2O → 2Br − + 1/3AsO33 − + 1/3Ga 3 + + 2H+

(2)

Note that the Br2 generated by the SECM tip can react with the GaAs substrate and produce Br−, a remarkable positive current feedback can be obtained when the tip is approaching the substrate (Figure 2). Suppose GaAs is a pure solid (αGaAs = 1), water is a pure liquid (αH2O = 1), and both are in large excess, reaction 2 can be simplified as Br2 + 2e → 2Br − 1323

(3)

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Technical Note

lowest point is −7.36 μm (Curve 2 in Figure 3a). The corresponding positive feedback current curve is recorded as curve 2 in Figure 3b. From Figure 3c, it can be observed that the positive feedback current curve is in harmonious accordance with the Z-θ curve obtained by the dial gauge. The results show that the SECM current feedback mode can be used for substrate leveling. To achieve more precise leveling, subsequent or multiple leveling processes can be performed. Actually, the precision obtained through SECM current feedback is much higher than the dial gauge because the latter can hardly sense the change in levelness in the subsequent leveling, due to its limitation of sensitivity. This point can be convinced through comparing Figure 3 (panels a and b). All the next experiments are performed with the leveling method based on the SECM current feedback mode. Figure 4a shows the approaching curves obtained on a GaAs substrate before and after leveling. Before leveling, the approach curve (curve 1) presents a shoulder and stops increasing when the tip is very close to the substrate. This phenomenon is caused by the wrong alignment between the tip and substrate in the case of bad levelness. After leveling, the quality of the approach curve is promoted, as shown as curve 2. Figure 4 (panels b and c) shows the corresponding SECM images of the GaAs substrate before and after leveling. It can be observed that the image quality after leveling becomes much better. As a further example, a circuit board with parallel copper wires (Figure 5a) is used as the substrate to detect the effect of this leveling method. Since the tip-generated Br2 can be reduced to Br− on the copper surface at the open-circuit potential, a positive feedback current can be observed on the conductive copper surface, while a negative feedback current can be observed on the insulated Bakelite surface. Figure 5b shows the lateral scanning curves before (curve 1) and after (curve 2) leveling. The feedback current steps after leveling become much more uniform and flat. Figure 5 (panels c and d) shows the 2D and 3D images of the circuit board before leveling, which are in accordance with curve 1 in Figure 5b. Figure 5 (panels e and f) show the 2D and 3D

Figure 2. Positive approach curve obtained on GaAs substrate in an aqueous solution containing 0.01 M NaBr and 2 M H2SO4. Tip: 25 μm diameter Pt disk electrode with an RG of 3, IT: the normalized current, L: the normalized distance.

From the SECM theory, kinetic rate can be derived as 1.2 × 10−2 cm/s with a diffusion coefficient of 1.8 × 10−5 cm2/s calculated from the limiting steady-state current.8,9 Here, the roughly estimated parameters just verifies that the redox couple Br2/Br− is qualified to be a testing system. The detailed investigation on the etching system, actually an ErCi process, is in process. Figure 3 (panels a and b) shows the leveling results obtained through the dial gauge and the SECM current feedback mode, respectively. During this leveling procedure with the dial gauge, the air-bearing rotary stage is still. Before leveling, the highest point of the GaAs substrate is 86.11 μm, while the lowest point is −7.64 μm (curve 1 in Figure 3a). Starting the air bearing rotary stage with a rotating rate of 0.33 rad/min, the positive feedback current curve is recorded as Curve 1 in Figure 3b. After leveling, the highest point of the GaAs substrate is −1.31 μm, while the

Figure 3. Leveling results of the (a) dial gauge and the (b) positive feedback current of SECM obtained on the GaAs substrate in an aqueous solution containing 0.1 M NaBr and 2 M H2SO4 with a 25 μm diameter Pt disk electrode (RG: 3); curve 1 and curve 2 are the corresponding records before and after leveling, respectively. (c) Comparison between dial gauge records (curve 1) and the SECM records (Curve 2) before leveling.

Figure 4. (a) Approach curves obtained before (curve 1) and after (curve 2) leveling on a GaAs substrate in an aqueous solution containing 0.1 M NaBr and 2 M H2SO4 with a 25 μm diameter Pt disk electrode (RG: 3) and corresponding SECM images of GaAs substrate (b) before and (c) after leveling. 1324

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Technical Note

Figure 5. (a) An optical image of the circuit board used as a substrate in the experiment. (b) The lateral scanning curves over the circuit board before and after leveling. (c and d) The 2D and 3D images of the circuit board before leveling, respectively. (e and f) The 2D and 3D images of the circuit board after leveling, respectively.

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images of the circuit board after leveling. With a comparison of the results, the effect of this leveling method is convincing.

AUTHOR INFORMATION

Corresponding Author

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*D.Z.: e-mail, [email protected]; tel, +865922185797; fax, +865922181906. Y.C.: e-mail, cfl[email protected]; tel, +8645186412924; fax, +8645186415244.

CONCLUSION A highly effective and low cost substrate leveling method was developed based on the current feedback mode of SECM. By employing an air-bearing rotary stage to support the substrate, the feedback current changes in a periodic waveform. The feedback current amplitude can be used to characterize the levelness of the substrate. Consequently, the substrate can be leveled by adjusting the tilt stage to decrease the amplitude of feedback current. Highquality approach curves and images were obtained through this leveling method. This method will be attractive to the SECM community but also those performing research in electrochemical machining. Further studies to expand this method into electrochemical machining are in process.

Author Contributions

L. Han and Y. Yuan have contributed equally to this paper. The authors declare no competing financial interest. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the National Science Foundation of China (Grant 91023006), the Natural Science Foundation of Fujian Province of China (Grant 2012J06004), the National 1325

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(36) Zhan, D.; Yang, D.; Yin, B.-s.; Zhang, J.; Tian, Z.-Q. Anal. Chem. 2012, 84, 9276. (37) Mandler, D.; Bard, A. J. J. Electrochem. Soc. 1990, 137, 2468. (38) Zhang, L.; Ma, X. Z.; Zhuang, J. L.; Qiu, C. K.; Du, C. L.; Tang, J.; Tian, Z. W. Adv. Mater. 2007, 19, 3912. (39) Lai, L.-J.; Zhou, H.; Du, Y.-J.; Zhang, J.; Jia, J.; Jiang, L.-M.; Zhu, L.-M.; Tian, Z.-W.; Tian, Z.-Q.; Zhan, D. P. Electrochem. Commun. In press, DIO: 10.1016/j.elecom.2012.12.017.

Science Foundation of China (Grants 91023047, 91023043, 21061120456 and 21021002), the Fundamental Research Funds for the Central Universities (Grant 2010121022), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry) are appreciated.

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