Scholarly article on topic 'Pulsed Direct liquid Injection ALD of TiO2 Films Using Titanium Tetraisopropoxide Precursor'

Pulsed Direct liquid Injection ALD of TiO2 Films Using Titanium Tetraisopropoxide Precursor Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — L. Avril, J.M. Decams, L. Imhoff

Abstract TiO2 thin films are grown by pulsed direct liquid injection atomic layer deposition with rapid thermal heating using titanium tetraisopropoxide and water vapor as precursors. The ALD growth rate is constant in the saturation zone range 35-47ms at the temperature deposition of 280°C. The TiO2 growth rate of 0.018nm/cycle was achieved in a self-limited ALD mode. SEM and AFM analysis showed the as-deposited films have a smooth surface with a low roughness. XPS analysis exhibited the stoichiometry of TiO2 in the homogenous depth composition.

Academic research paper on topic "Pulsed Direct liquid Injection ALD of TiO2 Films Using Titanium Tetraisopropoxide Precursor"

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Physics Procedia 46 (2013) 33 - 39

Nineteenth European Conference on Chemical Vapor Deposition, (EUROCVD 19)

Pulsed direct liquid injection ALD of TiO2 films using titanium tetraisopropoxide precursor

L. Avrila*, J.M. Decamsb, L. Imhoff

aLaboratoire Interdisciplinaire Carnot de Bourgogne (ICB), UMR 6303 CNRS-Université de Bourgogne, 9 Av. A. Savary, BP 47 870,

21078 Dijon Cedex, France

_bAnnealsys, rue de la Vieille Poste, 34055 Montpellier Cedex 1_

Abstract

TiO2 thin films are grown by pulsed direct liquid injection atomic layer deposition with rapid thermal heating using titanium tetraisopropoxide and water vapor as precursors. The ALD growth rate is constant in the saturation zone range 35-47 ms at the temperature deposition of 280°C. The TiO2 growth rate of 0.018 nm/cycle was achieved in a self-limited ALD mode. SEM and AFM analysis showed the as-deposited films have a smooth surface with a low roughness. XPS analysis exhibited the stoichiometry of TiO2 in the homogenous depth composition.

© 2013 TheAuthors.PublishedbyElsevierB.V.

Selection andpeer-review underresponsibilityofOrganizing Committee ofEUROCVD19.

Keywords: Pulsed liquid injection ALD ; Titanium oxides ; Titanium tetraisopropoxide precursor ; Thin films.

1. Introduction

In recent years, Atomic Layer Deposition (ALD) technique has been rapidly developed thanks to its attractive characteristics: ALD is a chemical deposition technique based on sequential self-terminating gassolid reactions suitable for the synthesis of ultra-thin films with thickness beneath the nanometer. ALD film growth is self-limited and based on surface reactions, which makes possible achieving atomic scale deposition control [Wang et al., 2002] [Leskela et al., 2002] [Ritala et al., 2003] [Steven et al., 2010]. Several different

* Corresponding author. Tel.: +33 3 80 39 63 32 ; fax: +33 3 80 39 38 19 . E-mail address: ludovic.avril@u-bourgogne.fr .

1875-3892 © 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Organizing Committee of EUROCVD 19. doi:10.1016/j.phpro.2013.07.063

metal oxide materials have been deposited by ALD. In particular, titanium dioxide (TiO2) has interesting applications in many research fields owing to its attractive physicochemical properties [Diebold et al., 2003].

The deposition of TiO2 films is currently obtained by ALD through different precursors. The most widely investigated precursors are titanium tetrachloride (TiCl4) and water [Kumagai et al., 2011] [Saleem et al., 2012]. However, the corrosive nature of TiCl4 and HCl, which is a reaction by-product, is considered as a drawback [Lim et al., 2006]. Thus, possible alternative precursors for TiO2 ALD have been tested in recent years, namely titanium tetraisopropoxide [Aarik et al., 2000], titanium ethoxide [Grigal et al., 2012], titanium methoxide [Pore et al., 2004] and tetrakis-dimethylamido titanium [Bontempi et al., 2010].

The direct liquid injection (DLI) system is a recent technology which differs fundamentally from bubbler systems. The DLI employment for the synthesis by metal organic chemical vapour deposition (MOCVD) and atomic layer deposition has been reported for vanadium oxide [Vernadou et al., 2004], titanium oxide [Awaluddin et al., 2001] and hafnium dioxide thin films [O'Kane et al., 2007]. The liquid injection permits to use the pure liquid and solid ones dissolved in an organic solvent. Moreover, it is able to handle most of solid and liquid compounds including low vapor pressure, thermally labile and viscous ones for the synthesis by ALD and CVD of thin films. In this paper, we investigated the TiO2 film growth behavior with the titanium tetraisopropoxide and water, as titanium and oxygen precursors, respectively. The pulsed direct liquid injection ALD is used to demonstrate the ability to grow TiO2 films.

2. Experimental details

2.1. Deposition procedure

Titanium dioxide thin films were grown on silicon (100) substrates using a pulsed direct liquid injection (DLI) system (Kemstream) and atomic layer deposition (ALD) process (Annealsys MC-050) with rapid thermal heating. Our DLI process is based on the atomized liquid injection of the precursor solution to generate the reactive vapor.

The substrate was laid on a silicon carbide coated graphite susceptor supported by a quartz holder inside a quartz reactor. The deposition temperature was varied between 250°C and 320°C using infrared (IR) lamps which were placed around the quartz tube. The susceptor temperature was monitored using a K-type thermocouple. The reactor was kept under vacuum (10-2 mbar) using a dry pump and a regulator valve, see Fig. 1.

To obtain a clean surface, the substrates were ultrasonically cleaned in organic solvent and in acetone. Then they were rinsed in ethanol and dried under N2 gas flow. For the TiO2 thin film elaboration, the precursors are titanium tetraisopropoxide (Ti(OCH(CH3)2)4, 99.99%, Strem) and water. The titanium precursor solution used in all experiments was a 5.10-3 M mixture of titanium tetraisopropoxide (TTIP) in anhydrous cyclohexane (C6H12, 99.99%, Sigma Aldrich). Cyclohexane was chosen as solvent because it meets the requirements for the DLI-ALD process: no interaction with the TTIP precursor, high vapor pressure, good thermal stability, low viscosity, low cost and no toxic. Moreover it can be employed to dissolve many metallic precursors such as trimethylaluminium [Eide et al., 2007]. The solvent was evaporated or decomposed by thermal effect and oxidant atmosphere. The precursor vessel was kept at room temperature and pressurized under 3.5 bar of N2. The second species used is water vapor (H2O) maintained in a vessel at room temperature. N2 gas with 99.999% purity was used as carrier and purge gas. The liquid precursor solution was introduced as micro-droplets by the pulsed liquid injection in the vaporizer chamber heated at 150°C where the precursor solution droplets were vaporized. The precursor solution vapors were transported from the vaporizer chamber to the ALD reaction chamber using N2 as gas carrier. The opening time of the injector and the pulse number are accurately controlled by computer monitoring.

Fig. 1. Schema of the DLI-ALD system. 2.2. Characterization of deposited films

The thickness of all TiO2 films was measured by a Veeco Dektak surface profilometer. The surface morphology of the films was observed by scanning electron microscopy (SEM) (JEOL 7600F Company) working at 15 kV.

The topography and surface roughness of the samples were studied by atomic force microscope (AFM) with a commercial microscopy (Nanoscope Ilia, Digital Instruments) operating in tapping mode, using a standard silicon cantilevers with spring constant ~ 42 N.m-1 and 369 kHz driving frequency. All images were recorded in ambient air at a scan frequency of 1-1.5 Hz. The background slope due to piezoelectric non-linearity was corrected using first or second orders polynomial functions.

X-ray photoelectron spectroscopy (XPS) depth profiles were performed using a PHI 5000 VersaProbe™ (ULVAC-PHI). Photoelectrons were excited by monochromatized Al Ka radiation (1486.6 eV) with a spot size of 200 |am in diameter and the spectrometer was equipped with a spherical capacitor energy analyzer with multi-channel detection. The X-ray beam was perpendicular to the surface and the analyzer axis was located at 45° with respect to the surface. Argon ion sputtering of 0.5 kV accelerating voltage was applied to obtain the relative element concentration depth. The sputtering rate was determined to be about 4 A/s.

3. Results and discussion

The main goal of our work was to optimize the deposition parameters in order to obtain a ALD self-limiting growth, avoiding CVD-like growth process due to the presence of residual TTIP during the H2O step as well as H2O during the TTIP step. With this aim, relatively long purging times for TTIP solution and water were used to ensure sufficient reactant purge for self-limiting film growth. In order to determine an appropriate TiO2 ALD zone, the growth rate was determined by measuring the thickness of films obtained with 2000 ALD cycles, by varying the deposition temperature, the pulse and purge times for TTIP and water, see Fig. 2 and 3.

40 60 80 100 TTIP pulse time (ms)

Fig. 2. TiO2 growth rate as a function of the TTIP pulse time at 280°C.

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Fig. 3. TiO2 growth rate as a function of the TTIP purge time at 280°C.

By optimizing the ALD reaction parameters, the TiO2 film growth was saturated to about 0.018 nm/cycle [Xie et al., 2007] [Lim et al., 2006], what means that the reaction is in a self-limited ALD growth mode. One cycle of the ALD regime consists of four steps: (i) 35 ms of TTIP solution pulse with 50 ms of N2 carrier gas

pulse, (ii) 300 ms of N2 purge gas pulse following by 10 s of pumping to effectively pump out the TTIP precursor and the reaction products, (iii) 50 ms of H2O pulse and (iv) 300 ms of N2 purge gas pulse following by 5 s of pumping to prepare the chamber for the next ALD cycle.

SEM surface image illustrated in Fig. 4 shows the as-deposited films on Si(100) substrates. The thickness of the film was about 20 nm, as measured by profilometer. The film grown at a temperature of 280°C has a smooth surface without any pinholes.

.. • i-V

• iVti '-»a . r.-j

Fig. 4. SEM surface micrographs of 20 nm TiO2 film.

AFM images of the films confirm the SEM results, see Fig. 5. The films cover homogeneously the Si substrate and no growth of pinholes within the thin film is observed.

0.0 Height 500.0 nm

Fig. 5. AFM image of 20 nm TiO2 film.

The in-depth composition of deposited films was studied by XPS, see Fig. 6. Firstly, C1s atomic fraction is small, below 1 atomic %. This indicates that the TiO2 film contains only O and Ti and no contamination from organic solvent is observed. Secondly the ratio of O1s/Ti2p is equal to 2. So a uniform distribution of constituent elements of TiO2 film is observed. These results indicate that a carbon species corresponding to the organic solvent are removed by the ALD reaction.

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 Sputtering time (s)

Fig. 6. XPS depth profiles of TiO2 thin films 4. Conclusion

The atomized liquid injection system has been used to elaborate thin films of titanium oxides on silicon substrates by using ALD process. Initially, the growth evolution of TiO2 from the TTIP and water precursors was investigated by changing the deposition parameters such as pulse and purge time of precursors, and the reaction temperature, in order to obtain the saturation zone and the ALD window. The AFM and SEM images indicate that the grains are uniform in size morphology and the as-deposited TiO2 films in the ALD regime exhibit smooth surface and a compact structure without pinholes. XPS composition depth profiling revealed the good stoichiometric TiO2 films with a low carbon contamination. The capability of the pulsed direct liquid injection for the TiO2 thin films synthesis by ALD has been demonstrated.

Acknowledgements

The authors acknowledge the financial support of the Conseil Regional de la Bourgogne (PARI INNOV).

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