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IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 14, NO. 1, JANUARY 2015
Effect of Sputtering Parameters on the Morphology of TiO2 Nanotubes Synthesized From Thin Ti Film on Si Substrate Karumbaiah N. Chappanda, York R. Smith, Loren W. Rieth, Prashant Tathireddy, Mano Misra, and Swomitra K. Mohanty
Abstract—In this paper, we present the analysis of the properties of direct current (dc) magnetron sputtered Ti thin film that affect the morphology of TiO2 nanotubes synthesized by electrochemical anodization. Si wafer with thermally grown silicon dioxide was used as the substrate for deposition of Ti films. By varying the properties of the sputtered film, morphology of the anodized film can be varied from tubular to nanoporous TiO2 . Three sputtering parameters that affect the properties of the film were studied, which include sputtering power, process gas (argon) pressure, and substrate temperature. Anodization of these films was carried out at 30 V (dc) using an ethylene glycol-based electrolyte. We show that the properties of thin film such as grain size and residual stress (bi-axial) do not affect the morphology of the anodized film and density alone influences the morphology of the anodized film. Most of the applications demonstrated by TiO2 nanotubes require annealing at high temperatures (350–800 °C) for calcination. Low residual stress in the thin film is required to prevent delaminating of the nanotubes from the substrate when exposed to high temperatures. We demonstrate that by varying the sputtering parameters, Ti films with low stress can be deposited which is required to have stable TiO2 nanotubes or nanoporous structure, based on the requirement of the application. Index Terms—Nanoporous, nanotubes, residual stress, Si substrate, TiO2 , thin-film density.
I. INTRODUCTION N the recent decade, many applications such as sensors [1], [2], battery [3], [4], photocatalysis [5], photoelectrolysis [6], photovoltaics [6], [7], and capacitors [8], [9] using TiO2 nanotubes (T-NT) have been demonstrated. One of the main drawbacks is that these applications mainly have been demonstrated using T-NT synthesized from Ti foil. T-NT on foil limits the
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Manuscript received July 2, 2013; revised April 17, 2014; accepted August 13, 2014. Date of publication September 26, 2014; date of current version January 6, 2015. This work was supported by the Utah Science and Technology Research (USTAR) initiative. The review of this paper was arranged by Associate Editor G. Ramanath. K. N. Chappanda is with the Department of Electrical and Electronics Engineering, Birla Institute of Technology and Science, Pilani 333031, India (e-mail:
[email protected]). Y. R. Smith and M. Misra are with the Department of Metallurgical Engineering, University of Utah, Salt lake City, UT 84112 USA (e-mail:
[email protected];
[email protected]). L. W. Rieth and P. Tathireddy are with the Department of Electrical and Computer Engineering, University of Utah, Salt lake City, UT 84112 USA (e-mail:
[email protected];
[email protected]). S. K. Mohanty is with the Department of Chemical Engineering, University of Utah, Salt lake City, UT 84112 USA (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2014.2360501
ability to integrate with the current high precision microfabrication techniques to fabricate economical, sensitive, miniaturized, and low-power T-NT-based devices, which is required for commercialization. One of the ways to approach this problem is by synthesis of T-NT from thin Ti films deposited on stable/planar substrates such as Si. Synthesis and characterization of T-NT from thin film on substrates such as glass [10], plastic [11], quartz [12], and silicon [13] have been previously demonstrated. Thin Ti film deposited by different techniques such as dc sputtering [14], radio frequency sputtering [15], thermal evaporation [15], and e-beam evaporation [16] have been used for synthesis of T-NT. However, very little work to understand the properties of these thin films that affect the morphology of the T-NT has been reported, and most of the work has mainly focused on understanding the growth mechanism of T-NT. Moreover, little work demonstrating applications such as solar cells [10], [11] and sensors [17], [18] has been reported using thin-film-based T-NT. Also, well-developed micro-fabrication processes such as photolithography was not used to fabricate these thin-film T-NT-based devices, and hence, the advantages of microfabrication process such as miniaturization capability were not utilized, making them similar to T-NT synthesized on foil. In our previous work, we demonstrated integration of photolithography to achieve site-specific and patterned growth of T-NT on Si substrate with self-aligned electrodes establishing a platform for making miniaturized, highly sensitive, low-power, and economical T-NT-based devices [16]. Some of the issues with thin films such as stiction, residual stress, and density of the film need to be addressed in order to synthesize stable T-NT on planar substrates before including integrated circuits for making miniaturized low power devices. To our knowledge, no detailed study to understand the role of properties of thin film that influences the morphology of the nanotubes has been reported in the literature. In this paper, we report detailed analysis of deposition parameters of thin Ti film that affect the morphology of T-NT synthesized on Si substrate. A thin film was deposited on Si substrates via dc magnetron sputtering, which is one of the commonly used thin-film deposition technique in the current semiconductor industry. By changing the sputtering parameters, the properties of the thin film change and are related to the morphology of the synthesized T-NT. We also demonstrate the synthesis of T-NT, semi-TNT, and nanoporous TiO2 by simply varying the deposition parameters of the thin Ti film. Scanning electron microscopy (SEM) was used to study the morphology
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CHAPPANDA et al.: EFFECT OF SPUTTERING PARAMETERS ON THE MORPHOLOGY OF TIO2 NANOTUBES
the thin films before and after anodization. Laser interferometry was used to study the residual stress in the thin films. II. EXPERIMENTAL PROCEDURE
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odization voltage was ramped up at the rate of 1 V/s. For all the runs, the anodization was carried out at 30 V (dc). After anodization, the tubes were again cleaned in acetone and isopropanol alcohol followed by rinsing in DI water. The samples were then dried in air and used for characterization studies.
A. Thin-Film Deposition Clean n-type (1 0 0) Si wafer with resistivity of 1–5 Ω · cm and with RMS roughness of less than 1 nm were used as substrates to study the synthesis of T-NT from thin film. Wafers were subjected to wet thermal oxidation at 1000 °C to form ∼100 nm of silicon dioxide. The SiO2 was grown to electrically isolate the TNT from the substrate. The type of oxide (wet or dry) grown did not have any effect on the morphology of the T-NT. Ti films about 300–400 nm thick were then deposited on the wafer by dc magnetron sputtering in a Denton Discovery 18 sputter system. The sputter chamber was pumped down to pressures
