Local oxidation characteristics on titanium nitride film by electrochemical nanolithography with carbon nanotube tip

Local oxidation characteristics on titanium nitride film

by electrochemical nanolithography with carbon nanotube tip

Te-Hua Fang

_{, Kuan-Te Wu}

Institute of Mechanical and Electromechanical Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan Received 5 August 2005; received in revised form 22 October 2005; accepted 8 November 2005

Abstract

The oxidation characteristics of TiN thin films by atomic force microscopy (AFM) electrochemical nanolithography with multiwalled carbon nanotube tip was investigated. The TiN films were produced on silicon substrate by atomic layer chemical vapor deposition (ALCVD). The electrochemical parameters, such as anodized voltages, oxidation times, writing speeds, 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 TiN oxide dots 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· 107_V/cm,

the anodization rate decreased quickly and the oxide dots stopped growing. Auger electron spectroscopy (AES) measurements confirm the modified structures took the form of anodized TiN.

2005 Elsevier B.V. All rights reserved.

Keywords: Atomic force microscopy; Carbon nanotube; Nanolithography; TiN; ALCVD

1. Introduction

During the past decade, 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 nanotechnology because of their demonstrated ability to manipulate atoms on the surface and to fabricate nanostructures[1]. Scanning probe anod-ization is a nanolithographic technique based on the elec-trochemical oxidation of a specimen with an adsorbed water layer[2]. In particular, an AFM-based local anodiza-tion process has been investigated for the fabricaanodiza-tion of nanostructures and nanodevices[3,4].

The multiwalled carbon nanotube (MWNT) has been suggested as the ultimate high-resolution probe for AFM because of its excellent properties, such as its high aspect ratio, small diameter, selective chemical reactivity, excellent mechanical robustness and electrical conductivity, which

provide great advantages for imaging and writing tools compared to conventional silicon-based tips[5,6]. Conven-tional atomic force microscope tips attached with MWNT can offer many advantages in nanoscale imaging and lithography. Due to its geometric characteristics, structure, electronic and chemistry, the carbon nanotube has actually found to be quite useful in imaging surface structures at greater depths and softnesses as well as prolonging the life time of the probeÕs tip. Due to the high aspect ratio of the MWNT probe, it is able to produce better images of micro-circuits being deeper and narrower than those produced by silicon probes. Cooper et al. [7]has achieved Terabit-per-square-inch data storage on titanium surfaces by AFM nanolithography with a CNT probe. Moreover, CNT has good mechanical properties in terms of high YoungÕs mod-ulus and good flexibility, which make it more suitable to be the AFM probe than other materials presently[8,9].

Atomic layer chemical vapor deposition (ALCVD) is a promising technique for growing semiconductor materials with excellent step coverage and excellent thickness unifor-mity by means of atomic level layer-by-layer deposition on

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2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.11.005

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E-mail address:[email protected](T.-H. Fang).

www.elsevier.com/locate/elecom Electrochemistry Communications 8 (2006) 173–178

a large-area wafer [10]. Titanium nitride (TiN) films have been employed for ultra large-scale integration (ULSI) because of its high thermal stability, low electrical resistiv-ity and good diffusion barrier characteristics[11].

AFM-induced oxidation on TiN thin films produced TiOxNyoxide structures[12]. TiOxNyhas excellent

dielec-tric properties for microelectronics or high-density-storage applications. Also, TiNxOy films can be used for many

other useful applications, such as solar selective absorbers, wear-resistant coatings and as a low-adhesive surface mate-rial, to reduce the formation of bacterial-biofilms on bio-materials. However, there is still a lack of information concerning the surfaceÕs anodic kinetics, growth mecha-nisms, and the effect of the electric field strength using AFM with a MWNT tip. In this study the MWNT probe tip-induced local anodization on TiN surface is presented. 2. Experimental details

The TiN thin films were deposited on a p-type Si(1 0 0) substrate by ALCVD. Before the filmÕs deposition, the Si wafer was cleaned in a HF solution to remove the native oxide SiO2layer. TiCl4and NH3reaction gases were

sup-plied sequentially during the deposition process [13]. Ar was used as a purge gas to remove the chemical residue with no adsorption to the surface. The amount of time each gas was sequentially supplied for TiCl4, Ar purge, NH3and

Ar purge was 5, 4, 1 and 5 s, respectively. The possible reaction for the ALCVD using precursors of TiCl4 and

NH3is as follows:

6TiCl4+ 8NH3! 6TiN + 24HCl + N2 ð1Þ

A SPM was used in tapping mode to measure the surface morphology and surface roughness of the TiN films. The average surface roughness of the TiN films at the tempera-tures of 350, 400, 450 and 500C were approximately 1.7, 0.7, 0.6 and 0.4 nm, respectively. The typical thickness of the TiN films was about 10 nm. The filmÕs resistivity was measured using the four-point probe method at room tem-perature. The resistivity of the TiN films at the tempera-tures of 350, 400, 450 and 500C were about 710, 325, 250 and 145 lX cm, respectively. As the deposition

temper-ature was increased, the surface roughness and the resistiv-ity of the TiN films decreased. In this study the TiN film with the best surface behavior was at the deposition tem-perature of 500C and it was chosen to explore its oxida-tion characteristics. The substrate temperature during the deposition process was kept at 500C and the pressure was at about 133 N m
2.

The local oxidation experiments were performed using an AFM (NT-MDT Solver, Russia) with a carbon nano-tube probe (Nanoscience Instruments, USA) as shown in

Fig. 1. The carbon nanotube probe consisted the multi-wall nanotube tip that was mounted onto a commercial AFM etched silicon probe. The carbon nanotube diameter is about 30 nm. An Al reflective layer of 30 nm was coated on the AFM cantilever backside. The cantilever length and spring constant of the AFM probe were 125 lm and 40 N m
1, respectively. In this technique, the oxide struc-tures grew on the chemically reactive surfaces by the appli-cation of a bias voltage between the surface and the AFM probe tip. Composition of the oxide and the film was char-acterized by Auger electron spectroscopy (AES, VG Micro-lab 310D, USA) with incident electron energy of 10 keV. The AFM probe was used as the cathode and the adsorbed water created from an ambient humidity of 60% was used as the electrolyte in non-contact mode. The feedback sys-tem was turned off and the bias voltage was applied to per-form the local oxidation experiment in the non-contact mode. The thickness of the water film at 60% humidity was more than 1 nm and less than 10 nm and the capillary force and the surface tension between the AFM tip and the water layer affected the continuity of the water layer. The pulse duration was set as 20 s. When the electric field was greater than 2· 107

V/cm and the water meniscus adsorbed on the specimen surface provided the oxyanions oxide structures were formed on the surface.

3. Results and discussion

The mechanism of AFM local anodic oxidation of a TiN surface is depicted in theFig. 1. The AFM local ano-dic oxidation mechanism of a TiN surface is depicted in the

Fig. 1. There is an adsorbed water layer on the surface,

Fig. 1. Schematic of electrochemical principle of AFM with a carbon nanotube probe and SEM image of the carbon nanotube probe. 174 T.-H. Fang, K.-T. Wu / Electrochemistry Communications 8 (2006) 173–178

10 V and at a relative humidity of 60%. When a higher speed was used to induce the oxide, a slightly lower height of the nanowire was achieved at tip speeds of 0.1–10 lm/s. Comparing the height of the nanowires between that of nanodots under the same voltage. The height of the nanod-ots was found to be higher than that of the nanowire. The scanning tip produces nanowires with smaller heights due to both a decreased oxidation time and a reduced residence time.

In order to understand the composition of the local oxi-dized structure, Auger electron microscopy analysis was conducted on an oxidized zone of 10· 10 lm2. The Auger spectra of the local oxidized region and the unmodified region are shown inFig. 6. It can find that the clear emis-sion peak of O with kinetic energy of about 502 eV. The oxygen to Ti intensity ratio is greater in the local oxidized region than that in the unmodified region. This is consis-tent with the result of the previous study [18]. They sug-gested that an enhanced incorporated process of oxygen occurred and the weaker oxygen occurred in the unmodi-fied region originated from the native oxide[18].

Chemical analysis of the TiN is complicated by the fact the KL23L23 Auger electron emission from nitrogen

occurred at energy that overlaps the L3M23M23 transition

from Ti at the kinetic energy of about 385 eV [19]. The Auger electron microscopy results were similar to Gwo et al.[12]pervious study. Since there is no N Auger transi-tions kinetic energy above 400 eV, the L3M23M45transition

of Ti occurs at the kinetic energy of about 418 eV[12]. This result showed that the anodic oxidized TiN films contains the TiNxOytransition layer.

4. Conclusion

In summary, the mechanism of TiN anodization process was investigated by applying an anodized voltage to the AFM MWNT tip. The TiN oxides were created by AFM oxidation nanolithography. The oxide thickness was found to increase as the anodized voltage or the anodized time was increased. The greater the oxide structures thickness became the weaker the electric field strength became, which also limited the oxide structures growth. The oxide

thick-ness was governed by the electric field strength. The TiN-oxide nanostructures were successfully fabricated and the mechanisms studied during the anodization process. These results show that AFM nanooxidation with a MWNT tip is to be a promising method for fabricating TiN thin films and has great potential for use in the fabrication of future nanodevices applications.

Acknowledgement

This work was partially supported by National Science Council of Taiwan, under Grant No. NSC94-2212-E150-046.

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