Local Oxide Growth Mechanisms on Nickel Films
Te-Hua Fang
*1and Kuan-Jen Chen
*2Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan
The oxidation characteristics of nickel thin films were investigated by atomic force microscopy (AFM) anodization. The anodization parameters, such as anodized voltages, oxidation times, pulse voltage periods and how they affected the creation and growth of the oxide nanostructures were explored. The results showed that the height of the nickel oxide nanodots grew as a result of either the anodization time or the anodized voltage being increased. The oxide growth rate was dependent on the anodized voltage and on the resulting electric field strength. Furthermore, as the electric field strength was at an order of 2 109Vm1, the anodization rate decreased quickly and the oxide dots stopped growing. [doi:10.2320/matertrans.48.471]
(Received October 13, 2006; Accepted January 15, 2007; Published February 25, 2007)
Keywords: atomic force microscopy, local electrochemical nanolithography, growth mechanism, nanostructure
1. Introduction
Scanning probe microscopy (SPM) such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have had a great impact on the development of the nanoscience and nanotechnology because of their demon-strated ability to manipulate atoms or molecules on the surface and to fabricate nanostructures.1,2) Scanning probe anodization is an electrochemical nanolithography based on the electrochemical oxidation of a specimen with an adsorbed water layer.3)In particular, an AFM-based local anodization process has been investigated for the fabrication of nano-structures and nanodevices.4,5)
The local electrochemical oxidation can be produced on nickel films using electric fields in an atomic force micro-scope. This technique can also be used to extend the application field of AFM-based lithography, when nano-patterning of the isolated layers is needed in silicon technology.6)Noncontact mode in AFM for the generation
of oxide patterns on silicon substrate clearly appears to be the best candidate due to the soft interaction between the surface and the tip.7) There are two advantages for using AFM
electrochemical lithography. First, the electric field govern-ing the exposure mechanism can be applied independently. Second, noncontact-mode AFM eliminates lateral shear forces and overcomes the tip-sample adhesion forces and capillarity, thereby avoiding damage to the surface and improving imaging and lithography resolution.8)
Nickel oxide (NiOx) films have been employed for ultra
large-scale integration (ULSI) because of its high thermal stability, low electrical resistivity and good diffusion barrier characteristics.9)Local oxidation on Ni thin films produced
NiOx oxide structures.10) However, there is still a lack of
information concerning the surface’s anodic kinetics, growth mechanisms, and the effect of the electric field strength using AFM with a conductive probe. In this study the AFM probe tip-induced local anodization on Ni surface is presented.
2. Experiments and Methodology
The Ni thin films were deposited on a p-type Si(100) substrate by ion beam sputtering method. Before the film’s deposition, the Si wafer was cleaned in a HF solution to remove the native oxide SiO2 layer. The average surface
roughness of the Ni film was approximately 1.0 nm, respectively. The typical thickness of Ni film is about 20 nm. The local oxidation experiments were performed using atomic force microscope (NT-MDT SPM, Russia) operated in a noncontact mode. A silicon cantilever with a TiN coated probe (3 107m) was used. The average force constant and the resonance frequency were 34 Nm1 and 350 kHz,
respectively. The cantilever was exercised at its resonance frequency. To perform the oxidation an additional circuit was used to apply a voltage between the tip and the substrate. The topography feedback was switched off during the nanolitho-graphic process.11) The process to oxidize the Ni surface
using AFM noncontact mode had three important steps: First, an oscillating probe is placed about 10 nm above the sample surface. Second, a voltage pulse is applied to form a liquid bridge between the tip and sample. The liquid bridge or meniscus is produced by the action of capillary forces at the AFM tip and the sample covered by an absorbed liquid film. Third, another voltage is applied to the silicon substrate to induce oxidation.
The process of AFM local anodic oxidation is depicted in the Fig. 1. The AFM probe was used as the cathode and the adsorbed water created from an ambient humidity of 55% was used as the electrolyte in non-contact mode. When the water meniscus adsorbed on the specimen surface provided the oxyanions oxide structures were formed on the surface. There is an adsorbed water layer on the surface, which provides the required electrolyte under ambient conditions. The thickness of the water layer depends on the relative humidity level in the surrounding air. Anodization by the water on the specimen’s surface will occur directly below the AFM probe tip when a bias voltage is applied to the probe. Using the AFM tip as a cathode, the surface of Ni will be oxidized, and the ions (including OHand O2) contribute to the formation of the surface oxide; then the oxidized structure will be made. The occurrence of the oxidation mechanism on
*1Correspondence author, E-mail: [email protected]; *2Graduate Student, National Formosa University
Materials Transactions, Vol. 48, No. 3 (2007) pp. 471 to 475 #2007 The Japan Institute of Metals
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