Si seed layer thickness effect on the structure of ultrathin tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) film is a promising materials serving as a protective coating layer due to its high sp3 content and its thickness up to 1-2 nm. In order to improve the adhesion of ta-C on metal, a Si film is usually pre-coated as seed layer. However, the Si film will increase the total thickness of protective layer. How to reduce the thickness of Si film is a requirement of industry applications. Here, we have studied the Si film thickness effect on the structure of ultrathin ta-C film. In our experiment, the thickness of Si film varies from 5 to 30 Å while the thickness of ta-C film is kept at about 2 nm. The Raman result shows that the sp3 fraction of ta-C film with pre-coated 5 Å Si film is a little lower than that of ta-C film without Si pre-coated. When the Si film becomes thicker from 5 to 30 Å, the sp3 fraction of Si/ta-C composite films is almost no changed. The XPS analysis shows that a part of Si bonding changes to SiC bonding which can enhance the adhesion of ta-C on metal. It also confirms that the SiC bonding fraction of Si film has no change when the thickness of Si film increases from 5 to 30 Å, which is consistent with the Raman analysis. Thus, an indication is that the fundamental limitation thickness of Si seed layer can be reduced to 5 Å.