Different stages during CVD deposition on porous substrates
Introduction
Chemical vapor (CVD) deposition has been exploited to produce dense as well as microporous films on porous supports by several groups of investigators [1], [2], [3]. The formation of a layer covering a substrate during CVD deposition processes takes place through a heterogeneous reaction on the growing surface, thus the surface plays a key role in the growth mechanism as well as in the properties of the deposited film [4]. As it is known, porous substrates are mainly used for membrane applications and thus the chemical reaction is usually favoured inside the pores for modification purposes. In this cases, infiltration techniques, named chemical vapor infiltration (CVI), are required and the deposit grows directly on the internal wall of the pores. The mechanisms for narrowing the pores have been widely studied, however the formation process of a layer deposited directly on a porous surface is poorly known.
On the other hand, the films deposited by CVD can grow in several ways depending on the conditions, so usually it is not precisely known which is the dominant growth mechanism. According to Pierson [5], the structure of a CVD material can be divided into three major types: columnar grains, faceted columnar growths or fine equiaxed grains. The microstructure varies depending on the material being deposited. Generally, CVD ceramic deposits as silicon oxide or nitride, alumina and most dielectric materials tend to be amorphous or, at least, have a very small grain microstructure. All these aspects are more complex when a porous surface instead of a polished one is used as substrate. In this case, it would be expected the flaws (irregularities) on the growing surface would strongly influence the structure of the deposited coating.
In this work, we show and compare the results obtained during the deposition of CVD silicon oxynitride on polished silicon and porous silicon nitride substrates for increasing deposition times.
Section snippets
Experimental
Silicon oxynitride films were deposited by plasma assisted chemical vapour deposition (PACVD) from silane (2% in N2), oxygen and ammonia gas mixtures (SiH4/O2/NH3, 2 : 5 : 55). The deposits were obtained at 100 Pa, 300°C and a radiofrequency power of 200 W in a conventional plasma-CVD apparatus radial types (STS, mod 310pc) operating at 13.56 MHz, the deposition time varying from 5 up to 180 min. The samples were deposited onto porous silicon nitride substrates (pore size 0.22 μm, after the supplier
Results
As expected, the deposition rate of the PACVD silicon oxynitride films, grown onto polished silicon substrates, remains almost constant along the process (see Fig. 3) [6]. Since the same conditions are used for all the experiments, the samples present exactly the same composition, as detected by infrared spectroscopy (IR spectra not shown). In addition, the SEM images of these deposits, obtained on polished silicon, show always a homogeneous silicon oxynitride film, likely with a very small
Discussion
Fig. 3 displays the time variation of the mean particle size (evaluated from the SEM top images) as well as the layer thickness of the samples deposited on a porous substrate. Also as comparison, the variation of the thickness of the smooth layers deposited on polished silicon wafers is included in the figure. As it can be observed, the particle size clearly increases with the deposition time, although its variation differs depending on the time range. Thus, for low deposition periods (⩽30 min),
Conclusions
Silicon oxynitride films have been deposited by CVD from SiH4/O2/NH3 gas mixtures, on polished silicon and porous silicon nitride substrates. On polished silicon substrates, smooth oxynitride layers were deposited at a high constant deposition rate (12.1 nm/min). However, when porous substrates are used, grainy appearance films were deposited at lower deposition rates (5.75 nm/min). The particle size of the grainy films increases with the deposition time. Therefore, during the silicon oxynitride
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