Dielectric properties of anodic films on sputter-deposited Ti–Si porous columnar films
Research highlights
► Isolated columnar Ti and Ti–7 at% Si films were successfully prepared by oblique angle magnetron sputtering. ► A uniform amorphous anodic oxide film is grown on the porous Ti–Si, but is limited to less than 6 V on the porous titanium. ► Electric properties of anodic oxide films are improved markedly by the addition of silicon due to suppression of crystallization.
Introduction
Titanium and its alloys are an important member of engineering materials with light weight, high weight-to-strength ratio, biocompatibility and high corrosion resistance even in very aggressive environments. The oxide formed on titanium is an n-type semiconductor with many attractive properties. The thickness of a native oxide film on titanium at ambient temperature is only a few nm, but can be increased greatly by anodizing in suitable electrolytes.
The anodic oxide films on valve metals are important dielectric materials. In fact, those on aluminum and tantalum have been used widely in electrolytic capacitors. Tantalum capacitors play a major role in passive components industry, due to their high reliability, excellent volumetric efficiency and low equivalent series resistance [1]. However, due to limitation of natural resources of tantalum and a strong demand of further increase in capacitance, the industry is seeking new materials with higher permittivity, which are composed of abundant elements.
Titanium dioxide has high permittivity (ɛox = 40–120) and is a promising dielectric oxide for electrolytic capacitors [2]. However, anodizing of titanium results readily in an amorphous-to-crystalline transition at low formation voltages [2], [3], [4], [5], [6], [7]. After crystallization of the anodic oxide, film growth accompanies oxygen gas generation, introducing a high density of flaws in the developed anodic oxide films [3], [7]. Thus, an amorphous-to-crystalline transition must be avoided for the formation of dielectric oxide suitable for capacitor application.
Recently, effective suppression of crystallization of anodic TiO2 has been demonstrated by incorporation of silicon species from metal substrate, i.e., a Ti–6 at% Si alloy [7], [8], [9]. Alloying of titanium with other metals, such as aluminum [7], molybdenum [10], [11], niobium [12], tungsten [13] and zirconium [14], [15], is also effective, although higher concentrations of alloying elements, compared with silicon addition, are required to form amorphous oxide without crystallization to voltages higher than 100 V.
For capacitor application of the Ti alloys that form flaw-free anodic oxide films, porous alloy films with high surface areas must be tailored to enhance the capacitance, since the capacitance, C, is proportional to surface area, S, as followsin which, ɛo is the permittivity of vacuum, ɛox is the relative permittivity of oxide and d is the thickness of oxide film. In tantalum electrolytic capacitors, a high-temperature sintering process has been utilized to produce porous tantalum anode with high surface area. However, such high-temperature process is not applicable for the Ti–Si alloy system, since the solubility of silicon to titanium is limited to less than 1 at% at equilibrium [16].
In the present study, oblique angle magnetron sputtering has been used as an alternative approach for fabrication of single-phase porous alloy films supersaturated with alloying element. Oblique angle deposition (OAD) is one of the useful and attractive physical vapor deposition techniques to tailor porous films with tilted isolated columnar structure [17]. The porosity is generated due to a self-shadowing effect and limited surface diffusion of adatoms. Recently, it has been reported that controlled rotation of substrate during OAD enables to create sculptured micro- and nano-structured thin films, such as nanopillars [18], [19], [20], [21], [22], [23], zig-zag [24], [25], [26], [27], nanospirals [28], [29] and Y-shape [30] columns. This advanced technique, often referred to as glancing angle deposition, allows us to develop a range of engineered micro- and nano-structures [17], [31]. However, complex substrate rotation during deposition is not suitable for the production of capacitor anode because it limits the production rate of the anode. The authors have recently reported that the utilization of a substrate with regular concave cell structure is effective in enhancing the shadow effect in OAD and thus in fabrication of isolated columnar films [32], [33], [34].
In this work, we have deposited Ti and Ti–7 at% Si isolated columnar films by oblique angle magnetron sputtering on to the substrate with a concave cell structure. The anodic oxide films are grown on the porous metallic films in ammonium pentaborate electrolyte, and their dielectric properties are examined. The composition of Ti–7 at% Si has been selected since further increase in silicon content decreases the capacitance as a consequence of incorporation of an increased amount of silicon species that reduces the permittivity of oxide [35].
Section snippets
Experimental
Thin porous films of Ti–7 at.% Si were prepared by magnetron sputtering at an oblique angle of 85°, with respect to the substrate normal, on to textured aluminum substrate for 3.6 ks, using a target consisting of 99.9% titanium disk of 100 mm in diameter and silicon pieces of 20 mm2, with the latter placed symmetrically on the erosion region of the former disk. The substrate holders were rotated along the chamber axis and its own axis to get alloy films of uniform thickness and composition. The
Morphology and phases of deposits
SEM images of the Ti and Ti–7 at% Si films deposited on textured substrate (Fig. 1) reveal porous nature with the size of pores between neighboring columns being 100–200 nm. The films have isolated columnar morphology with the column diameter of 300–400 nm and the column-to-column distance is ∼500 nm, which is in agreement with the cell size of substrate. The thickness of the columnar films is ∼600 nm. Due to an incidence of deposited atoms from the right-hand side of the micrographs, the columns
Conclusions
In summary, the present work demonstrates the high potential of the single phase porous Ti–Si alloy containing supersaturated silicon as a capacitor material. The porous isolated columnar alloy films are prepared successfully by oblique angle magnetron sputtering on to aluminum substrate with a regular concave cell structure. Crystallization of anodic oxide during anodizing, which is detrimental for capacitor application, is effectively suppressed by silicon addition, even on the porous alloy
Acknowledgement
The present work was supported in part by Global COE Program (Project no. B01: Catalysis as the Basis for Innovation in Materials Science).
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