1
Introduction

This chapter presents the outline of thin film technologies currently used and describes the relationship among catalytic chemical vapor deposition () and other conventional thin film technologies. In this chapter, the history of Cat-CVD and its related technologies is also briefly reviewed. Finally, the structure of this book is explained for easier reading.

1.1 Thin Film Technologies

Most industrial consumer products are coated with various thin films. Some products may be covered by painting or plating films. Thin film coating is also seen in modern electronic products. In such cases, quite often, the quality of the coating films determines the performance of the electronic products themselves. For instance, in liquid crystal displays (s) or organic electroluminescent displays, transistors made of semiconductor thin films are working as a key device to control the brightness and colors of pictures. In ultralarge-scale integrated circuits (), used in computers as a key device, many thin films are contained, and the quality of such thin films strongly determines the performance of ULSI. In solar cells, the quality of thin films also determines their energy conversion efficiency.

So far, many thin film technologies have been invented. Coating technology on various tools was invented at least more than 10?000?years ago. It is well known that the painted drawing on the walls of stone caves can have a history of more than 40?000?years.

Here, let us depict a family tree of thin film technologies. It is very useful to make a family tree of related research for taking a view point of our own research in the tree to judge the value of the research itself. It can sometimes lead to new ideas and helps to define what we should do in future for our own technology from a deeper understanding of the position of research. The family tree is shown in Figure 1.1. It demonstrates that the thin film technologies are divided to three major technologies.

Figure 1.1 A family tree of thin film technologies. and PECVD are abbreviations of silicon on insulator and plasma-enhanced chemical vapor deposition, respectively.

The first one is the pasting of thin films on solid substrates, where such thin films are prepared elsewhere. As a historical technology, pasting of gold leaf on solids is known. Because gold bullion can be stretched and transformed to a sheet by beating, a thin gold leaf with a thickness less than 100?nm can be made and attached strongly on the surface of solids. The second is the formation of thin films by converting the thin surface layer of a solid to a layer of different materials by chemical reactions of the solid surface with active gases. For instance, the thermal oxidation of crystalline silicon () wafers to form a silicon dioxide (SiO2) layer at the surface is a widely spread technique within this category. The third is the formation of thin films by deposition on solid substrates. Films are formed at the surface of substrates by deposition of species supplied from outside of the substrate. This technology is divided into two groups: one is the method in which the film forming process is completed on the surface of substrates and the other is the method in which molecules are decomposed in advance remotely from the substrates, and such activated species are used in the reactions of film formation at the surface of substrates. Usually, by using species activated in advance, the high-quality films can be obtained at relatively low substrate temperatures, typically lower than 300?°C.

The methods utilizing activated species created outside of the substrates are further divided into three groups. One of the methods uses plasma for the generation of active species, the second method uses catalytic cracking reactions, and the third one uses the energy of radiation, such as photochemical vapor deposition (). In Photo-CVD, there is discussion on the possibility of direct excitation of adsorbed molecules on the surface of substrates rather than in the gas phase outside of substrates. In this case, the method fits better in the group of 3-1-2.

Plasma-enhanced chemical vapor deposition () is the first method, in which molecules of source gases are decomposed by the collisions with energetic electrons generated in plasma. The method using catalytic cracking reactions to decompose molecules is Cat-CVD. In Cat-CVD, as heated metal wires are usually used, the method is often called hot-wire CVD or hot filament CVD. In Cat-CVD, no plasma is needed.

A similar technology using heated metal wires is known as initiated chemical vapor deposition (). In this newly developed method, activation of initiators is due to heated metal wires, and the activated species are inducing polymerization of adsorbed monomers at the surface of substrates, resulting in high-quality organic thin films. This method is suitable for the preparation of device quality organic polymer films.

Looking at the family tree, one may notice that there is plenty of spaces for the first divided groups in which the films are prepared in advance, outside of the substrates. For instance, in the family tree of creatures, at first, the creatures are divided into plants and animals, and there is a roughly equal number of branches in the two groups. However, in the case of the family tree of thin film technology, the first and second divided groups do not have as many branches or methods compared with the third method of thin film deposition. This means that there is a big potential to invent new thin film technologies in the first and second groups. For instance, if patterned thin films, made apart from the substrates, are pasted on substrates, the thin film formation process becomes much more cost effective. The prototype of this idea is already seen commercially for pasting painted sheets on walls of air planes, trains, and buses for advertisement. If thin films are more sophisticated, this idea will be used in industrial electronic devices, in particular, large area devices. By looking at the family tree, you can enjoy more to create new ideas.

1.2 Birth of Cat-CVD

Catalytic cracking is a well-known phenomenon. It was already known in the 1910s that hydrogen (H) atoms are simply generated by catalytic cracking reactions of a hydrogen molecule (H2) with heated tungsten (W) wires [1]. Since the 1970s, H. Matsumura et al. concentrated on making high-quality and thermally stable amorphous silicon () films by using fluorine (F) atoms as dangling bond terminators. Particularly, they used silicon difluoride (SiF2) molecules as a source for the formation of fluorinated a-Si (a-Si:F) films. By using SiF2, such a-Si:F films were formed by simple plasma-less thermal CVD, although the quality of the films was slightly less than that of hydrogenated a-Si (a-Si:H) films prepared by PECVD. Thus, to improve film quality, Matsumura attempted to introduce H atoms into such a-Si:F films. As the films were obtained without the use of plasma, he pursued the use of H atoms created by catalytic cracking reaction of H2 molecules at heated W wires to complete the development of a plasma damage-free deposition system. The work had been carried out from 1983 to 1985. The result was very encouraging. The property of such films appeared better than those of PECVD a-Si:H films. Matsumura named the method "Cat-CVD" in 1985.

The film quality was excellent; however, the results could not make the industry adopt this method, as they would not add halogen gases to their system because their systems preparing a-Si:H films had just been constructed in many companies. Then, he attempted to deposit a-Si:H by cracking of silane (SiH4) gas with heated W wires and succeeded in obtaining device quality a-Si:H films in the period from 1985 to 1986. This makes the birth of Cat-CVD technology. Matsumura succeeded in obtaining device quality a-Si films without the assistance of plasmas for the first time.

1.3 Research History of Cat-CVD and Related Technologies

The combination of thermal CVD with catalysis of metals was first reported in 1970 by S. Yamazaki et al. at Doshisha University, Japan. He installed catalysts such as platinum and nickel oxide just near the substrates in a quartz tube of a conventional atmospheric pressure thermal CVD apparatus for preparing silicon nitride (SiNx) films [2]. This catalyzer was just put near the substrates and it was unheated. He discovered that the temperature for forming SiNx films on the substrates could be reduced from 700 to 600?°C by the effect of the presence of catalysts nearby. The role of catalysts and the mechanism for lowering the deposition temperature were not clearly explained in the report. This also did not show the invention of a low pressure and low temperature deposition method, although metal wires were used as an example of catalysis.

Later, in 1979, H. Wiesmann et al., at the Brookhaven National Laboratory, USA, reported their discovery that a-Si films could be formed when heated W wires and carbon foils were exposed to silane (SiH4) gas in a low-pressure chamber [3]. This is...