Elsevier

Electrochimica Acta

Volume 54, Issue 13, 1 May 2009, Pages 3526-3531
Electrochimica Acta

Influence of chemical additives on the surface reactivity of Si in KOH solution

https://doi.org/10.1016/j.electacta.2008.12.044Get rights and content

Abstract

It is known that the electrochemistry of silicon in alkaline solution is closely linked to the anisotropic etching of the semiconductor. In this work the influence of two commonly used additives, hydrogen peroxide and isopropyl alcohol, on the surface chemistry of silicon in KOH solution was investigated by electrochemical methods. The results allow us to draw conclusions regarding the role of the additives in the chemical and electrochemical reactions.

Introduction

For most semiconductors, valence-band holes are required for anodic oxidation [1], [2]. Consequently, one expects the reaction to occur at a p-type semiconductor in the dark and at an n-type semiconductor only under illumination with (supra-)bandgap light. This is the case for compound semiconductors, such as the III–V and II–VI materials, and also for silicon carbide [1], [2], [3]. Surprisingly, both n-type Si(1 0 0) and (1 1 1) can be anodically oxidized in alkaline solution in the dark [4], [5], [6]; this is not the case for n-type Si in acidic fluoride solution [7]. It has been shown that electrochemical oxidation in alkaline solution occurs by electron injection into the conduction band from intermediates of the anisotropic chemical etching reaction [4], [8], [9]. Besides a strong dependence of the chemical etch rate on crystallographic orientation, there is also a considerable anisotropy in the electrochemistry [4], [5], [6], [10], [11].

Chemical etching of Si in KOH solution comprises two main steps: the OHcatalyzed conversion of Si–H surface bonds to give Si–OH (and H2) and the breaking of the polarized Si–Si back-bond by reaction with water [8], [9], [12]. The latter step regenerates the surface hydride. The analogy between silicon etching and organosilicon chemistry was noted by Schiffrin and co-workers and the etching anisotropy was explained on the basis of a penta-coordinated transition state that is formed during the nucleophilic attack of OHions on the Si–H surface bonds [12]. The atomic arrangement depends on the surface: (1 0 0) and (1 1 1) surface atoms have 2 and 3 Si–Si back-bonds, respectively. Therefore, for the (1 1 1) case the surface atoms are more firmly anchored in the lattice; it is more difficult to form the transition-state intermediate and the surface is more stable.

Previous work on (1 0 0) electrodes has shown that activated intermediates of the chemical reactions have energy levels high in the bandgap of the semiconductor [8], [9]. Such states can be further oxidized by electron injection into the conduction band, resulting in the anodic current that is measured in the external circuit. Electron injection in the second step can lead to oxygen insertion into the back-bonds, and thus to passivation of the surface [5], [8], [9]. Since the electrochemical reactions depend on the chemical reactivity of the surface, the kinetics of anodic oxidation are much more favourable for the (1 0 0) case [5]. For n-type Si(1 1 1), slow kinetics were especially evident in potential-step experiments [5]. Current transients were measured following an abrupt change in potential from open-circuit (Uoc) to a value in the passive range. For a wide range of temperatures, the initial current is low. After some time, the current increases and reaches a maximum, after which the electrode passivates. An increase in temperature markedly accelerates this activation process. For the (1 0 0) electrode, however, no induction period is observed after the potential is stepped. An initial high current is followed by a fast monotonous decrease to a steady-state value. This was explained by the higher chemical reactivity of the surface: the large dissolution rate leads to the production of a high surface density of reaction intermediates. Thus, measurement of anodization current transients can be very useful for obtaining information about the surface chemical reactions [5], [6].

For certain applications additives may be employed in anisotropic etchants. Isopropyl alcohol (2-propanol/isopropanol/IPA) in alkaline etchants such as KOH [11], [13], [14], [15] and tetramethyl ammonium hydroxide (TMAH) [16], and in 40% ammonium fluoride (NH4F) solutions [17], has been reported to influence the etch rate and the surface morphology. The alcohol is frequently used to reduce convex-corner undercutting and to change the surface morphology. It is also used in cleaning processes in micro-electromechanical systems (MEMS) technology [11], [18], [19], [20]. Despite the large range of applications, relatively little information is available on how the additive influences the surface chemistry under practical etching conditions. Considering the dependence of anodic oxidation on the chemical etching reaction described above, one might expect IPA to influence the electrochemical characteristics of Si in alkaline solution. A study of the electrochemistry should, therefore, give information about the role of the alcohol in etching. In this work we describe the use of potential step and electrical impedance measurements to study the effect of IPA on the surface chemistry of Si(1 1 1) under practical etching conditions in KOH solution. Since it has been suggested in the literature that the alcohol undergoes a chemical interaction with the surface [17] we compare the results with those obtained with a different additive, hydrogen peroxide (H2O2). This strong oxidizing agent is frequently used for wafer cleaning and is known to oxidize silicon in alkaline solution [21], [22], [23], [24]. Recent work on Si(1 1 0) etched in KOH solution has shown that H2O2strongly influences the surface morphology, a result explained by chemical oxidation of the surface [6], [25].

Section snippets

Experimental

Experiments were performed on both (1 0 0)- and (1 1 1)-oriented surfaces. Czochralski grown n-type (phosphorus doped, 1–10 Ω cm) and p-type (boron doped, 5–10 Ω cm) Si(1 0 0) wafers were supplied by Okmetic. For measurements on (1 0 0) surfaces, silicon wafers were diced into 2cm×2 cm samples. The exposed surface area for these samples was 1.50 cm2. Measurements on well-defined (1 1 1) surfaces were performed on V-groove electrodes exposing only (1 1 1) facets on masked (1 0 0) surfaces (mask opening width: 100 μ

Hydrogen peroxide

In a first set of measurements the influence of H2O2 on the general electrochemistry of Si(1 0 0) in KOH solution was studied by measuring cyclic voltammograms with a p-type electrode. Two issues were important. Does H2O2 affect the anodic current-potential characteristics of the system and, if so, could this be attributed to an electrochemical reaction of the oxidizing agent ? The advantage of the p-type electrode is the absence of a cathodic current due to hydrogen evolution, for which

Conclusions

Hydrogen peroxide and isopropyl alcohol chemically ‘activate’ Si surfaces, etching in KOH solution. Both additives influence the anodic oxidation process. In the case of hydrogen peroxide, the silicon is oxidized chemically and the oxide nuclei formed on the Si(1 1 1) surface accelerate anodic oxidation. Isopropyl alcohol also enhances surface oxidation, very likely by chemisorption, which then leads to an increased chemical reactivity. The proposed changes in surface termination under etching

Acknowledgements

The authors thank Duy Nguyen (MESA+ Research Institute, University of Twente, The Netherlands) and Ismail Shah (Radboud University Nijmegen, The Netherlands) for valuable discussions. This work was financially supported by the Dutch Technology Foundation (STW, TPC-5990).

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