Growth and characterization of TiO2 nanotubes from sputtered Ti film on Si substrate
© Chappanda et al.; licensee Springer. 2012
Received: 15 May 2012
Accepted: 12 July 2012
Published: 12 July 2012
In this paper, we present the synthesis of self-organized TiO2 nanotube arrays formed by anodization of thin Ti film deposited on Si wafers by direct current (D.C.) sputtering. Organic electrolyte was used to demonstrate the growth of stable nanotubes at room temperature with voltages varying from 10 to 60 V (D.C.). The tubes were about 1.4 times longer than the thickness of the sputtered Ti film, showing little undesired dissolution of the metal in the electrolyte during anodization. By varying the thickness of the deposited Ti film, the length of the nanotubes could be controlled precisely irrespective of longer anodization time and/or anodization voltage. Scanning electron microscopy, atomic force microscopy, diffuse-reflectance UV–vis spectroscopy, and X-ray diffraction were used to characterize the thin film nanotubes. The tubes exhibited good adhesion to the wafer and did not peel off after annealing in air at 350 °C to form anatase TiO2. With TiO2 nanotubes on planar/stable Si substrates, one can envision their integration with the current micro-fabrication technique large-scale fabrication of TiO2 nanotube-based devices.
TiO2 is a material of great interest due its low cost, stability, and interesting electrical and optical properties . TiO2 nanotubes (T-NT), compared to TiO2 films, have a large surface area-to-volume ratio. Synthesis of these tubes via electrochemical anodization has been studied extensively over the past decade . Due to their interesting electrical and optical properties, TiO2 nanotubes are shown to have various applications such as harvesting solar energy , charge storage , and sensors [5, 6]. However, these applications have been demonstrated by anodizing Ti foil, which is the limiting factor for commercialization since synthesis of T-NT on Ti foil limits the integration onto wafer-scale devices, which is required for large-scale production. Current semiconductor fabrication techniques use micro/nano-scale precision, which requires planar/stable substrates such as Si wafer for manufacturing compact and powerful devices. Commonly used titanium foils are mechanically flexible, rendering them prone to bending which inhibits the ability to achieve the micro/nano-scaled precision. Hence, stable/planar substrates are required for integrating T-NT with the current semiconductor fabrication techniques for manufacturing compact and powerful T-NT-based devices. Growth of T-NT on stable/planar substrates such as glass and Si deposited by sputtering has been previously demonstrated [7, 8]. However, in their synthesis of T-NT, low temperatures (0 °C) with a maximum voltage of 35 V were used, limiting the diameter of the tubes. Most of the micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) require a thin non-conductive layer to electrically isolate the device from the substrate and in order to define an electrical path to the device. Similarly, it is possible that a non-conductive layer isolating the tubes from the substrate is required in T-NT-based MEMS/NEMS. The T-NT synthesized per the method presented here generates nanotubes that are isolated from the substrate, which could prove advantageous while fabricating T-NT-based devices.
Here, we present for the first time, the synthesis of T-NT from thin Ti film on Si wafer at room temperature and at high anodization potentials up to 60 V. Direct current (D.C.)-sputtered thin Ti film, along with an organic electrolyte with approximately 6.5 pH, was used for the synthesis of the tubes. A 100-nm-thick layer of thermally grown SiO2 was used to electrically isolate the synthesized T-NT to demonstrate one of the suitability of the tubes for integration with the current MEMS/NEMS devices. The T-NTs were longer than the deposited film thickness, indicating selective electrochemical oxidation/dissolution of Ti by the organic electrolyte. The T-NT did not peel off from the substrate, exhibiting good adhesion even after annealing in air at high temperature. In order to extend the utility of T-NT to make compact and powerful strain gauges , sensors [5, 6], and drug delivery systems , integration of these devices with electronic circuits is required, which in turn is incumbent on a substrate being compatible with the current semiconductor fabrication techniques. With the use of Si as substrate, as typically exemplified in the current semiconductor industry, T-NT-based devices have a great potential for large-scale production and commercialization.
Thin film deposition
Clean n-type (100) Si wafer with resistivity of 1 to 5 Ω cm were used as the substrates for T-NT growth. Wafers were subjected to water vapor-free thermal oxidation at 1,000 °C to form 100 nm of dry SiO2. Ti film 650 nm thick was then deposited on the wafer by D.C. sputtering in a Denton Discovery 18 sputter system (Denton, Moorestown, NJ, USA). Ti target of 99.2% to 99.7% purity purchased from Kurt. J. Lesker (East Sussex, UK) was used for sputtering. The sputter chamber was pumped down to pressures < 2 μTorr. The Si substrate was heated to 200 °C, and the same temperature was maintained during sputtering . Argon gas was pumped into the chamber as the plasma source. The argon gas flow was set such that the pressure in the chamber during sputtering is about 2 mTorr. After sputtering, the wafer was cooled down to room temperature in vacuum to minimize the formation of native TiO2.
Scanning electron microscopy (SEM) micrographs from FEI NanoNova SEM (FEI Co., Hillsboro, OR, USA) were used to characterize the diameter and length of the tubes, along with its structural morphology. Bruker Dimension ICON-PT atomic force microscopy (AFM) micrographs (Bruker AXS, Madison, WI, USA), along with the SEM micrographs, were used to study the effect of anodization time on the tubes’ surface morphology. AFM micrographs using the PeakForce QNM (Bruker AXS) technique were obtained using ultra sharp AFM tips (10 nm diameter). The AFM micrographs were obtained at three different regions, and the average value was used for the roughness. Diffuse-reflectance UV–vis spectroscopy (UV-3600, Shimadzu, Kyoto, Japan) was used to characterize the band gap of the material. X-ray diffraction, utilizing the Philips X’Pert X-ray diffraction (XRD) system (Philips, Amsterdam, Netherlands), with incidence angles ranging from 20° to 80° was used for identification of crystalline phases and determination of stress in the nanotubes.
Results and discussions
TiO2 nanotube arrays were successfully synthesized on Si wafer by anodization of D.C.-sputtered Ti film in an organic electrolyte. Due to the presence of native titania and pronounced surface grains, formation of nano-porous TiO2 layer on top of self-aligned T-NT arrays was observed. From the SEM micrographs of T-NT viewed from the side and those of milled T-NT, the regular formation of nanotubes below the nano-porous layer was deduced. The diameter of the nanotubes varied linearly from 45 to 240 nm with an increase in anodization voltage from 10 to 60 V, respectively. SEM micrographs showed that the length of T-NT was about 1.4 times the thickness of the sputtered Ti film irrespective of the anodization voltage or longer anodization time. XRD characterization showed that unannealed tubes, irrespective of the anodization voltage/time, formed amorphous TiO2 nanotubes, and upon annealing in air at 350 °C, anatase phase TiO2 nanotubes was formed. AFM and SEM micrographs showed that on complete anodization of the film, the electrolyte etches TiO2, resulting in tubes with thinner walls. It can be concluded that the T-NT formed on Si wafer has similar properties to T-NT formed on foil via various characterizations tools, giving rise to potential applications demonstrated by T-NT synthesized on Ti foil. Upon combining the use of well-established semiconductor fabrication techniques used in the semiconductor industry, integration of T-NT in MEMS devices and subsequent large-scale commercialization looks promising.
The work presented here was supported by the Utah Science and Technology Research (USTAR) initiative.
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