Most of industrial consumer products are coated by various thin films. Some of products may be covered with painting or plating films. Thin film coating is also seen in modern electronic products. In such cases, quite often, the quality of coating films decides the performance of the electronic products themselves. For instance, in liquid crystal display (LCD), transistors made by semiconductor thin films are working as a key device to control the brightness and colors of pictures. Ultra-large scale integrated circuits (ULSI), used in computers, contain so many thin films and the quality of such thin films decides the performance of ULSI. In solar cells, also, the quality of thin films determines their energy conversion efficiency.
　Thin films used in such electronic products are often required to be prepared at temperatures as low as 300℃ or less, keeping film quality as high as possible. The plasma enhanced chemical vapor deposition (PECVD) method, so far, appears used to obtain thin films satisfying such requirements. In PECVD, source gases are introduced into a vacuum chamber, where the molecules of source gases are decomposed by the physical collisions with energetic electrons generated in plasma, and the decomposed species are transported to the substrates to form films. This PECVD method is one of the most successful thin film technologies; however, because of the progress of industry society, the requirements are getting even severe. In PECVD, the surfaces of substrates or of previously deposited thin films always suffer from the damages by the collisions with ions generated in plasma along with energetic electrons. For the further improvement of performance of consumer electronic products, this plasma-induced damage becomes a serious problem in PECVD. Thus, the development of a new method obtaining high quality thin films at temperatures below 300℃ with no plasma-damages is strongly required.
　Our developed new CVD method, named as Catalytic Chemical Vapor Deposition (Cat-CVD) method, is to satisfy these requirements. In Cat-CVD, the molecules of source gases are decomposed in a vacuum chamber by the catalytic cracking reaction with heated catalyzers, and the cracked species are transported to the substrates to form films there. In the mechanism of gas decomposition in Cat-CVD, the films or substrates are free from plasma damages. Since metal wires are often used as catalyzers, this method is also known as “Hot-Wire CVD”, or “HWCVD”.

　Fig.1　Schematic view of PECVD and Cat-CVD apparatuses.

　Figure 2 demonstrates the images of the interface between amorphous-silicon (a-Si) and crystalline-silicon (c-Si) observed by the Scanning Transmission Electron Micro-scope (STEM) with a spatial resolution as high as 0.08 nm. The figure shows the images of both Cat-CVD a-Si/c-Si interface and PECVD a-Si/c-Si interface. The STEM machine used here, JEOL JEM-ARM200F, is one of the top grade experimental equipments possessed by JAIST. The machine was, used be, the world’s first operating machine. To keep the spatial resolution of 0.08 nm, the room is specially designed to avoid the vibration conducted from the land and the vibration made even by the sound or voice in the room. The room has a heavy concrete floor with a thickness more than 1 m and all walls are covered with the sound absorbers to be kept silent.
　In this case, the samples are observed from the direction of (110), and thus, many Si-Si pairs with an atom-distance of 0.15 nm are observed in c-Si regions as predicted by the crystalline model of c-Si.
　In case of Cat-CVD a-Si/c-Si interface, the c-Si image is fading into a-Si dark image with a transition layer of 0.6 nm, but in case of PECVD, it is 1.8 nm. In the observation by STEM, the image is drawn by electrons transmitted through the sample with a thickness of 10 to 20 nm. That is, the wider transition layer seen in the PECVD a-Si/c-Si interface means larger fluctuation or roughness at the interface than that in Cat-CVD a-Si/c-Si interface. The depth of damages created by collisions with Si atoms can be evaluated theoretically by using the formula of nuclear stopping power of Si atoms in c-Si. The depth of damages by Si atoms with energy of about 50 eV is estimated to be 1 to 2 nm. The collision energy of 50 eV is quite common as the energy often seen at the sheath of plasma. In case of PECVD, it is natural to think that Si atoms at the surface with a layer thickness of 1 to 2 nm are removed or scattered to make rough surface or interface. This is the real figure of plasma damages.
　This observation has been carried out several times by using PECVD samples made by outside of JAIST. As a result, it is found that the reproducibility is very high, and that the transition layer from c-Si to PECVD a-Si is always 1.8 to 2.0 nm.
　So far, the roughness at the interface between PECVD films and c-Si substrates has been accepted in industry or many cases. However, when really plasma-damage-less interface is required for achieving top grade device performance, this difference between Cat-CVD and PECVD interfaces would have serious meaning.

　Fig.2　STEM images of Cat-CVD a-Si/c-Si interface (left), and PECVD a-Si/c-Si interface (right). Intensities from crystalline signals are
　　　　　 also shown in the lower figures.

　In addition to this, 5) is also a great advantage in Cat-CVD. In PECVD, when the deposition area is expanded, as mentioned briefly above, the uniformity is lost because of generation of standing wave of RF wave. For instance, the wavelength of 13.56 MHz frequency is about 22 m in a vacuum. The length between the position of peak power and that of power 0 should be 1/4 of wavelength, and thus, it is about 5 m. The length is shortened when the chamber is filled by source gases, since the velocity of electromagnetic waves decreases in medium. When the size of sample tray is about 2 m in mass-production system, the value is no longer negligible for uniform deposition. If the plasma frequency is increased to 60 MHz for improving the film quality, for instance, it is even harder to realize uniform deposition on large area. For improvement of productivity for film deposition to reduce the film preparation cost, large area deposition is inevitable approach. This is a strong obstacle for PECVD, particularly, for VHF PECVD.
　 Here, we have to mention some of useful and effective attempts to overcome the demerit of PECVD. One idea is the introduction of “Hollow-Cathode” system, carried out in PECVD machine used in fabrication of thin film transistors of LCD production. In a large electrode-plate, small holes with a diameter less than 10 mm and a depth about also 10 mm are formed. In such a small hole, a small gas inlet hole and insulated small electrode are installed. Glow discharge is generated inside such a small holes. This type of system is called as “Hollow Cathode”. If such small Hollow Cathodes are arrayed on the electrode-plate keeping a distance of two Hollow Cathodes at 100 mm or less, large area deposition becomes possible without worry about standing wave problems, although high level technology is required to keep uniform glow discharges in many arrayed small Hollow Cathodes. By this technology, already, the apparatus for 3 m size deposition, so called “10th generation PECVD machine”, has been installed in LCD industry.
　 Even in this case, keeping plasma stably is still a big issue. When films are deposited near electrodes at Hollow Cathodes, plasma conditions are changed. Thus, the frequent cleaning of the chamber and also of the areas near electrodes are necessary. In the mass-production system, the etching gases such as nitrogen tri-fluoride (NF3) are excited by microwave-plasma at the outside of the chamber and such excited species are introduced into the Hollow Cathodes through gas inlet holes, in order to clean up the Hollow Cathodes and the area surrounding them. This production system is quite successful in LCD industry; however, it should be noticed that the cost of NF3 is not negligible. In addition, when we use halogen gas such as NF3, vacuum system should be prepared for it. All these things are reason for cost increase in this system. In case of LCD industry, the increase in cost can be absorbed by mixing with other costs. However, in solar cell industry, that should be difficult since further lowering the production cost is always required.
　 Please notice that there are no worries about constructing such complicated and expensive systems in Cat-CVD.

　Fig.3　Schematic view of vertical type Cat-CVD apparatus. This system is used in large area mass-production machines.

　Figure 3 shows schematic view of the Cat-CVD apparatus with catalyzers hung vertically in the deposition chamber, as mentioned at above 6). Here, two sample trays are set in parallel at the both sides of hanging catalyzing wires. After a-Si films are deposited for more than 1 month continuously, the hanging parts of catalyzing wires and the mechanical parts for moving trays are all slid out from the deposition chamber for chemical cleaning carried out at elsewhere outside of the chamber. The same parts which have been cleaned up are re-used and newly set at the top of the chamber. Since the most of areas inside chamber are all covered with trays, there is no place of chamber cleaning. We need not to use halogen gas at all and cleaning frequency is very low. This is one of the big advantages in Cat-CVD.

Fig.4　Collisions in PECVD (left) and in Cat-CVD (right)

　The data of gas efficiency in factory is not open in any companies. However, it is guessed that the gas efficiency in PECVD is only several % to obtain high quality films with a cost-effective deposition rate, but that the gas efficiency in Cat-CVD reaches to 30 % or more. In addition, the films with same quality to PECVD films are obtained with almost double the deposition rates in case of Cat-CVD.
　When the saline (SiH4) gas is used as a source gas particularly for large area deposition, this advantage of high efficiency of gas use becomes more remarkable. The volume of exhaust gas after deposition can be reduced and the amounts of dangerous SiH4 gas storage in factory can be also much reduced. These advantages make the film deposition cost low. The superiority of Cat-CVD to PECVD appears clear particularly when large area deposition is carried out without any plasma damages.

2. Reactions on Surface of Catalyzer

　When a gas molecule reaches at the surface of catalyzer, two types of reactions are expected. One is that the molecule connects with atoms of catalyzer surface by physical-adsorption. The other is that the molecule makes a bond with an atom at the catalyzer surface, by chemical adsorption. In some probabilities, physically adsorbed molecules are converted to chemical-adsorbed ones, although the most of physically adsorbed molecules are desorbed.
　Let’s consider the case when a molecule approaches into a certain distance from the surface of catalyzer. If the sum of the energy of a molecule and the energy of atoms at the catalyzer is larger than the total energy of bonds formed with decomposed molecules and catalyzer atoms, the molecule is likely to be decomposed to lower the total energy of the system and the dissociative adsorption is constructed with catalyzer atoms.
　Figure 5 shows illustration of this dissociative adsorption of SiH4 with W catalyzer for catalyzer temperatures at room temperature (RT) (left) and at over 1,000℃ (right).

Fig. 5 　Schematic illustration of chemical dissociative adsorption for SiH4 and W catalyzer system. The temperature of catalyzer is room temperature (RT) (left) and over 1,000℃ (right).