Optimization of anodizing process of tantalum for Ta 2 O 5 -based capacitors
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ORIGINAL PAPER
Optimization of anodizing process of tantalum for Ta2O5-based capacitors Andrea Zaffora 1
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Francesco Di Franco 1
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Francesco Di Quarto 1
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Monica Santamaria 1
Received: 30 April 2020 / Revised: 2 June 2020 / Accepted: 2 June 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Anodic oxides were grown to 50 V on Ta in several organic ions containing anodizing baths. Their properties were compared with anodic Ta oxide film grown to the same formation voltage in 0.1 M NaOH. Anodizing process carried out in sodium citrate led to the growth of the anodic oxide with the best blocking properties whilst, when Ta is anodized in sodium adipate, a significant part of the circulated charge is wasted in side reactions, such as oxygen evolution. Photoelectrochemical measurements showed the presence of optical transitions at energy lower than the band gap for the anodic films grown in citrate and tartrate electrolytes, attributed to localized electronic states located close to the valence band mobility edge of the films generated by anions incorporation into the oxide. Differential capacitance measurements proved an increase by 17% in capacitance value for the oxide grown in citrate-containing solution with respect to that grown in NaOH electrolyte. A sketch of the energetic of the metal/oxide interface is provided. Keywords Anodizing . Band gap . Capacitor . Dielectric properties . Organic ions . Tantalum
Introduction Tantalum capacitors are currently used in modern electronics due to their peculiar characteristics, such as temperature and frequency stabilities, long-term reliability and high volumetric efficiency [1]. Capacitors core is the dielectric material that is Ta2O5 in the case of tantalum capacitors. The latter has exceptional dielectric properties, and for this reason, it is used for storage capacitors in DRAM and also as gate oxide in field effect transistors. More recently, anodic Ta2O5 has also been proposed as solid electrolyte for resistive switching memories [2–5]. Tantalum capacitor manufacturing process consists of several steps summarized in the Block Flow Diagram of Fig. 1. The forming step is an electrochemical oxidation, namely, anodizing, allowing the growth of Ta2O5 on the surface of Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-020-04704-0) contains supplementary material, which is available to authorized users. * Francesco Di Franco [email protected] 1
Dipartimento di Ingegneria, Università degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy
tantalum. The selection of the anodizing conditions is crucial for the overall manufacturing process since it determines the properties of the dielectric, i.e. the specific capacitance and the leakage current. The specific capacitance, Cox, of the oxide is given by the following relationship assuming a parallel plate capacitor model: C ox ¼
εε0 r d ox
ð1Þ
where ε is the oxide dielectric constant, ε0 is the vacuum permittivity, dox is
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