Localized electrochemical deposition of metals using micropipettes

doi:10.1016/S0040-6090(99)01117-7

Thin Solid Films

Volume 366, Issues 1–2, 1 May 2000, Pages 32-36

https://doi.org/10.1016/S0040-6090(99)01117-7 Get rights and content

Abstract

The localized electrochemical deposition of metals using a pulled micropipette as tip (similar as in scanning ion-conductance microscopy) is a new scanning probe lithography method, which is still under development. This paper investigates the dependence of the deposited structures on the deposition parameters (electrode potentials, tipsample distance). A 2D numerical simulation is used to visualize the potential distribution in the present geometry. A linear copper structure (300 μm long and 100 μm broad) has been produced in a first experiment.

Introduction

For solid surface investigation and modification in an electrochemical environment, the electrochemical scanning probe methods (EC-SPM) electrochemical scanning-tunneling-microscopy (EC-STM) [1], [2], electrochemical atomic-force microscopy (ECAFM) [3], and the scanning electrochemical microscopy (SECM) [4], [5] are quite well established (for a review see, e.g. [6]). All of them make use of a metallic or metallized tip as one of the electrodes, which is coated with an insulating layer with the exception of the probe apex.

Scanning ion-conductance microscopy (SICM) [7], [8], [9], [10] is a quite rarely applied SPM technique, which makes use of the geometrical/hydrodynamic restriction (squeezing effect) of an ionic current, which flows in an electrolyte between the sample surface mounted on one electrode and another electrode hidden inside of a pulled-capillary tip probe, when the latter is approached close to the sample surface. It has been demonstrated that a lateral resolution of about 30 nm can be achieved by this technique with proper micropipettes [11]. Mainly for biological applications, such pulled micropipettes can be fabricated with inner aperture diameters of less than 100 nm [11], [12].

In connection with the scanning nearfield optical microscopy (SNOM), there has been accumulated knowledge about the preparation of fibre tips [13], [14] and about the tip-sample distance control by using the shear-force in a SPM setup [15].

The main idea of the present work is the use of a tip made from an insulating material (glass micropipette) for electrochemical scanning probe investigations. This pipette may be filled by an electrolyte solution and act as localized ion source. Furthermore, the glass pipettes are able to guide light, which may be used to observe the deposition process by detecting, simultaneously, the optical transmission through the sample in case of a transparent substrate. Such a new method of local electrochemical surface modification, which may be used to create structures from the mm down to less than 100 nm range should be of interest for a broad field of applications. This includes customized wiring on the 3D-structured surfaces of micro-electro-mechanical devices [16], well-defined under-potential deposition of crystalline patterns, which might be even developed till to the lithography of special nanoelectronic devices. Especially for 3D-structured surfaces, there exist no suitable technique for deposition of microscopic wires on mechanically sensitive devices. The presented method, could be the solution for this wiring problem.

The present work investigates in more detail the reasons for previous difficulties in performing a controlled deposition of linear metallic structures with the aim to optimize the parameters for further experiments. Our earlier experiments [17] were carried out with a oversimplified electrochemical setup consisting of only two electrodes, which had the serious drawback of the actual potential of the electrolyte outside the capillary to be undetermined.

In the first section of this paper we present the experimental set-up and discuss especially its electrode configuration. The second section contains a numerical simulation for this experiment, which helps to visualize the electrical potential distribution and supports that our experimental intentions can be really fulfilled. In the following section, we report on a first experiment to fabricate a linear metallic structure and on the influence of the deposition parameters. All detailed considerations in this paper will refer to the deposition of copper from a 0.5 M CuSO₄ electrolyte on a gold substrate. This gold substrate is prepared by evaporating a 100 nm thin Au film on glass and short annealing in a reducing butane flame (flame-annealing).

Section snippets

Micro-electrochemical deposition unit

In the following section, we discuss the electrochemical peculiarities of our approach. Precise electrochemical experiments are usually carried out with at least three electrodes [18]. The working electrode (WE) is the one on which the investigated reaction proceeds. The reference electrode (RE) is highly resistant (ideally non-polarizable) coupled to the electrolyte and detects the potential of the WE in the electrolyte. The counter electrode (CE) is used to adjust the potential of the

Numerical studies of the deposition conditions

Numerical simulations should give a possibility to get an impression of the potential distribution in the electrolyte right under the tip. In order to practice the numerical approach, several assumptions have to be made. The electrolyte was approximated by an ohmic resistance. The interface resistance was also assumed to be ohmic (voltage independent). Furthermore, it was assumed that no reaction takes place that means that no mass transport is included. The resistivity of the metal–electrolyte

Basic experiments

In order to check the parameter range for deposition, we started with macroscopic dimensions, because several downscaling problems will occur simultaneously in the final experiment. Some of these problems are already known. The resistance of the electrolyte inside is higher the smaller the inner diameter of the pipettes. Therefore, the potential decay over the electrolyte inside the capillary increases with decreasing inner diameter. To check the potential decay inside a pipette, experiments

Conclusion

The electrochemical deposition of metals using pulled glass micropipettes is a nearfield method that is still under development. It has been already shown that this deposition works with tip apex diameters of about some mm. This paper concentrates on the localization of electrochemical deposition due to a localized potential increase underneath the tip apex. Numerical simulation based on a very simple model as well as experimental examples confirm that the lateral extension of the deposited

Acknowledgements

Financial support from the Deutsche Forschungsgemeinschaft (Grant H/512/51 and Graduiertenkolleg ‘Thin Films and Non-Crystalline Materials’) is grateful acknowledged.

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Localized electrochemical deposition of metals using micropipettes

Abstract

Introduction

Section snippets

Micro-electrochemical deposition unit

Numerical studies of the deposition conditions

Basic experiments

Conclusion

Acknowledgements

Biophys. J.

Biophys. J.

Sensors and Actuators A

Appl. Phys. Lett.

J. Electrochem. Soc.

Science

J. Am. Chem. Soc.

Science

Science