Characterization of SiO2 thin films prepared by plasma-activated chemical vapour deposition

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Abstract

In this paper, we report on the preparation of SiO2 thin films onto different substrates using a direct plasma deposition process. The aim of this work was to investigate a variety of film properties depending on the deposition parameters, and to evaluate possible applications. The deposition process was based on a capacitively coupled r.f. low pressure plasma. The carrier and the monomer gases used for the silica deposition were air and hexamethyldisiloxane (HMDSO), respectively. The films deposited onto different substrates like silicon, brass and polyethylene terephthalate (PET) foils were characterized by FT-IR spectroscopy, ellipsometry, AFM, gas permeability measurements and corrosion test measurements. From ellipsometric and FT-IR measurements, it could be concluded that the films deposited at low r.f. input power and relatively high pressure contained carbohydroxyl groups indicating an incomplete reaction of the precursor material. It was found that the SiO2 film properties like refractive index, deposition rate, surface roughness and oxygen permeability were dependant on the deposition parameters. Otherwise, concerning the insulating properties and the corrosion tests strong dependencies were not found.

Introduction

Plasma-activated chemical vapour deposition is known as a key technology applied in a whole range of industries. An example are thin insulating SiO2 films produced by this technology which are the basic for applications in the semiconductor industry. Plasma-assisted chemical vapour deposition applications and techniques for dielectric thin films were reviewed in [1]. Furthermore, it was shown that HMDSO-based films can be used as corrosion protection layers on metallic surfaces [2]. The so-called plasma impulse chemical vapour deposition (PICVD) developed by Schott was transferred into industrial applications and is used now to produce scratch-resistant films and antireflective layers on plastic surfaces [3]. The potential of silica- or diamond-based coatings as barrier films for food packaging is pointed out in [4]. A current overview of further possible applications is given in [5].

With regard to atmospheric pressure CVD, it was shown that it is possible to create thin SiO2 films by microwave-induced remote PACVD [6], [7]. In this case, there exist problems to deposit the films homogeneously on a large scale. This disadvantage can be overcome by applying the so-called Pyrosil® technique, a combustion CVD method where a precursor-doped flame is used for thin film production. Thus, it was reported about the deposition of SiO2 thin films on float glass at atmospheric pressure over a width of 1.20 m for optical application or for improving adhesion properties [8] as well as for transmission enhancement [9]. Further investigations concentrate on the use of a flame-based hydrolyse process at atmospheric pressure which enables beside SiO2 the preparation of photocatalytical-active TiO2 thin films onto temperature-sensible substrates [10].

For all the mentioned applications, it is necessary to obtain special film properties. Thus, for the SiO2 thin film production in semiconducting processing there is an absolute necessity to deposit very smooth films with high insulating properties. For the creation of barriers on polymer foils, SiO2 films with a high packaging density and a low gas permeability are required. For scratch-resistant films, a good adhesion and a high hardness are essentially reached by changing the hydrocarbon content in the silica films [11].

During our work as a research partner of small and medium-sized enterprises we have produced SiO2 thin films onto different substrates by low pressure PACVD or by combustion CVD [8], [9] for a broad variety of applications. The aim of this article was to summarize our results concerning the low pressure PACVD film preparation and to show the influence of the deposition parameters on different film properties.

Section snippets

Experimental

The deposition procedures were carried out in a PACVD reactor as shown in Fig. 1. Inside the vacuum chamber, a plane parallel electrode arrangement was attached where the upper electrode was r.f. biased. The substrates were located on the lower, grounded electrode. The precursor gas was fed through a gas shower built in the upper electrode. The chamber was evacuated by using a rotary pump with an exhaustion rate of 4 m3/h down to a base pressure of about 10 2 mbar. The working pressure in a

Ellipsometrical measurements, refractive indices and deposition rates

For the determination of the deposition rates, ellipsometric and profilometric methods were used. For each parameter set (pressure and power), four SiO2 films with different thicknesses below 200 nm were deposited onto Si wafers. These four samples and an uncoated Si sample were investigated by ellipsometric measurements giving the ellipsometric data delta and psi. The values of delta and psi are dependent on the SiO2 film thickness. By determination of the refractive index of the substrate and

Conclusions

The influence of the preparation conditions on the properties of SiO2 thin films was investigated. For this, thin films were prepared using PACVD technique at different deposition conditions. From ellipsometric and FT-IR measurements, it could be concluded that the films deposited at low r.f. input power and relatively high pressure contained carbohydroxyl groups indicating an incomplete reaction of the precursor material. Additionally, it was found that the SiO2 film properties like refractive

Acknowledgement

The authors want to thank R. Sokoll for helpful discussions concerning the FT-IR spectroscopy results, K. Glock-Jäger for realizing the impedance spectroscopy measurements and B. Mäusezahl for the assistance during deposition and for measuring the gas permeability. This work was partially supported by the Federal Ministry of Economics and Labour under grant 129/03.

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