Elsevier

Thin Solid Films

Volume 516, Issue 17, 1 July 2008, Pages 5864-5867
Thin Solid Films

Step coverage study of indium-tin-oxide thin films by spray CVD on non-flat substrates at different temperatures

https://doi.org/10.1016/j.tsf.2007.10.040Get rights and content

Abstract

Tin-doped In2O3 (indium-tin-oxide) films were deposited on non-flat Si substrates using an original spray chemical vapor deposition (CVD) system. When the deposition temperature was 300–350 °C, ITO films covered all surfaces (top surface, side walls, and bottom; opening size was 1 μm) after being sprayed 50 times. The step coverage decreased as the number of spray increased because the opening size became smaller due to the layered ITO films, and it therefore became increasingly difficult to supply raw materials. The ITO films could not be deposited when the grooves narrowed to less than 0.18 μm. The step coverage of ITO films clearly increased, although the deposition rate decreased due to the decreasing deposition temperature.

Introduction

Indium-tin-oxide (ITO; tin-doped In2O3) transparent conducting films are widely used for flat panel displays, transparent heaters, and infrared shielding of buildings and automobiles. Commercially produced ITO films are generally deposited by physical vapor deposition (PVD) processes, such as sputtering. Recently, however, ITO films have been successfully deposited by spray chemical vapor deposition (CVD) at a deposition temperature of 350 °C using an ethanol solution of indium (III) chloride and tin (II) chloride [1], [2], [3]. The advantages of this method are that it is simple, inexpensive, and can be done in air. The lowest resistivity of the as-deposited films (1.7 × 10 4 Ω cm) was compatible to those deposited using a sputtering process [4], [5], [6], [7], [8]. In the previous paper [2], the films were annealed in a reducing atmosphere (N2–0.2%H2; gas flow rate, 300 ml/min) at 600 °C for 120 min. The minimum resistivity (7.7 × 10 5 Ω cm) was the lowest among the films fabricated by a chemical process.

Deposition on submicron patterned substrates is indispensable for applications involving high density devices with three-dimensional structures. Generally, conformal deposition of patterned substrates is relatively easy with the CVD process, though it is difficult with PVD [9]. Deposition on submicron patterned substrates of metallic films [10], [11], [12], [13], oxide films [14], [15], [16], and ferroelectric films [17] by CVD process has been reported. A previously reported case of deposition of transparent conducting films on patterned substrates was actually aluminum-doped zinc oxide (AZO) film using the sputtering method [18], and this process has not been reported for ITO films. Furthermore, the report of this AZO film did not describe the step coverage and the opening size. In the present study, ITO films were deposited on submicron patterned substrates by spray CVD.

Section snippets

Experimental

Indium (III) chloride, InCl3 3.42H2O (purity, 99.99%) and tin (II) chloride, SnCl2 1.41H2O (purity, 99.9%) were dissolved in ethanol (purity, 99.5%) and stirred for 12 h or more. The concentration of the total metal ions in the solution was fixed at approximately 0.1 mol/l. The solutions were prepared with 7 at.% Sn. The solution was sprayed manually in air with an inexpensive atomizer used for cosmetic purposes. It was already reported that the average size of the drops prepared a similar

Deposition on the non-flat Si substrates at 300–350 °C

The X-ray diffraction spectra of all films corresponded to the peak of In2O3 powders (cubic bixbyite structure [19]). The (400) peak became stronger as the film thickness increased.

Fig. 1 shows cross-sectional FE-SEM images of ITO films with a different number of sprays deposited on non-flat Si substrates with an opening size of 1 μm. The ITO films sprayed 50 times already covered all surface areas including the top surface, side walls, and bottom, as shown in Fig. 1-a. The film thickness

Conclusion

An ethanol solution of indium (III) chloride and tin (II) chloride was sprayed onto non-flat Si substrates heated at 300–350 °C. The ITO films were sprayed onto the non-flat Si substrates with 1-μm openings, providing 93% step coverage. The opening size became smaller and smaller as the number of sprays increased, thus resulting in lower step coverage. Moreover, the ITO films could not be deposited in the grooves when the opening size was less than 0.18 μm. Step coverage can be improved by

Acknowledgments

The authors thank Prof. Y. Shirai of our Institute for the use of X-ray fluorescence analyzer and the Ministry of Education, Culture, Sports, Science and Technology (MEXT) for financial support of “High-Tech Research Center” Project (2005–2009).

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