Kwang Yong Eun, Ki Hyun Yoon b) Structure and Properties of Si Incorporated Tetrahedral Amorphous Carbon Films Prepared by Hybrid Filtered Vacuum Arc Process Thank you, Mr. chairman. My name is Churl Seung Lee from KIST. The title of my talk is structure and properties of silicon incorporated tetrahedral amorphous carbon films prepared by hybrid filtered vacuum arc process. Churl Seung Lee a), b) , Kwang –Ryeol Lee a) , Kwang Yong Eun, Ki Hyun Yoon b) a) Thin Film Research Center, Korea Institute of Science and Technology b) Department of Ceramic Engineering, Yonsei University

Introduction ta-C (Tetrahedral Amorphous Carbon) Advantages High ratio of sp3 hybridized carbon bonds Extreme hardness, smooth surface, thermal stability, chemical inertness…. Hardness (GPa) Ta-C is short for Tetrahedral amorphous carbon films. It denotes a uniform amorphous carbon structure where most of the carbon tetrahedrally bonded. It can be deposited by selected ion beams, pulsed laser ablation or filtered vacuum arc. ta-C films was well known as the high quality DLC films. As shown in this figure, the hardness of ta-C is close to that of Diamond. And more, smooth surface, low deposition temperature and excellent tribological properties are another advantages. For these reason, ta-C film has drawn much attention in both scientific and engineering aspects.

Introduction ta-C (Tetrahedral Amorphous Carbon) Disadvantage High residual compressive stress → poor adhesion Many attempts have been reported Substrate biasing , post-annealing, boron incorporation However, high residual compressive stress of ta-C films results in poor adhesion and the limitation of thickness. For example, like the left figure, the flat wafer strip was considerably bended after the deposition of ta-C film. As shown in the right figure, this high residual compressive stress makes ta-C film delaminate from the Si substrate and resulted in poor adhesion. It greatly limits the usefulness of ta-films for many applications. Many attempts have been reported to reduce the stress of the ta-C films without changing the other mechanical properties. In the present work, we attempt the study on the addition of silicon into ta-C to overcome the drawback of pure ta-C films. Si incorporation to ta-C film

Background Si addition to a-C:H Improved tribological properties in humid environment Improved the adhesion Enhanced the thermal stability On the other hand, silicon has been considered as an important alloying element to modify structure and properties of hydrogenated amorphous carbon films. Silicon incorporation to hydrogenated amorphous carbon films was reported as the effective method to improve the tribological properties(pointing left figure) and adhesion. Recently, it was also observed that silicon incorporation enhanced the mechanical (pointing the right figure) properties and thermal properties. K. Oguri et al., Surf. Coat. Tech., 47 (1991) 710 W.-J. Wu et al., Thin Solid Films, 307 (1997) 1

Motivation Si addition to ta-C To control the structure and the mechanical properties of ta-C Non-hydrogenated carbon source and solid type Si source Prevention of the confusion in the analysis of C-H-Si bonding configuration. Thus, Si addition to ta-C films would be also effective to control the structure and properties. However, the effect of Si addition on the structure and properties of ta-C films was not reported yet. In addition, due to the lack of hydrogen, the relationship between the atomic bond structure and the mechanical properties can be more readily addressed in ta-C films.

Synthesis of ta-C:Si Bias: Ground Control parameter Ar gas flow 10 ~ 20 SCCM Pressure B.P.= low 10-6 torr W.P.= mid 10-4 torr We synthesized silicon incorporated ta-C films by using filtered vacuum arc process with simultaneous silicon sputtering. This is the schematic of the deposition system used in the present work. Si concentration could be controlled by changing the flow rate of the Ar sputtering gas. Ar sputtering gas was supplied via gas feedthrough placed near the sputter gun. In order to minimize the Ar sputtering of films, the substrate was grounded. One hundred nanometer thick films were deposited on silicon wafer and thin silicon strip was also used for the stress measurement. Si was incorporated in the ta-C film by simultaneous magnetron sputtering of Si during the FVA deposition.

Si Incorporation C Si in substrate Si in the film This figure is the RBS spectrum of one of the Si incorporated ta-C films. As can be shown in this figure, we confirmed that silicon was uniformly incorporated in ta-C films. By this way, we analyzed the composition of films. Si in the film

Composition This result is the composition of the deposited films measured by RBS. The silicon concentration in the film could be controlled in a systematic way by changing the flow rate of the Ar sputtering gas. As shown in this figure, silicon concentration was strongly dependent on the Ar flow rate. When the Ar flow rate was less than 9 sccm, we could not obtain the Si incorporated ta-C film due to unstable ignition of the magnetron sputter source. However, as the Ar flow rate increased from 9 to 12 sccm, the Si concentration increased from 0.5 to 2.5 at.%. When the Ar flow rate was higher than 12 sccm, the significant increase in Si concentration was observed with increasing the Ar flow rate. When Ar flow rate was 18 sccm, the Si concentration of the film was 85 at.%. In all samples, small amount of oxygen was also incorporated with Si, which seems to be due to the surface oxide layer of the sputter target. However, the ratio of oxygen to silicon was less than 0.1 in most cases. The effect of oxygen on the structure of the film was assumed to be negligible in the present work.

Mechanical Properties These results are the changes in stress and mechanical properties with various silicon concentrations measured by nanoindentation. The Si incorporation significantly reduced the residual compressive stress. As shown in left figure, the stress sharply decreased from 6.0 to 3.3 GPa by adding only 1 at.% of silicon to the ta-C films. However, beyond 1 at.% of silicon, the stress gradually decreased to 0.8 GPa with the Si concentration. Hardness and plane strain modulus were summarized in the right figure. In contrast to the residual compressive stress, the mechanical properties did not sharply decrease with the Si addition. In the range of the Si concentration from 0 to 8.5 at.%, the hardness was reduced from 41 to 22 GPa, and the plane strain modulus from 354 to 200 GPa. The further increase of the Si concentration resulted in saturated values of the hardness and the plane strain modulus, which are comparable to those of nanocrystalline silicon carbide films.

Comparison I II III This figure clearly shows the different behavior between the reduction in stress and hardness. We can divide the variation of mechanical properties into three regions. The first region is where the Si concentration was lower than 1 at.%.(with pointing the figure) In the case of the stress, 52 % of the total reduction occurs in this region. But,in the case of the hardness, there is 20 % reduction in this region. In this second region, there are linearly decrease in the both stress and hardness. When the Si concentration was higher than 10 at.%, there are the continuous decrease in the stress, while the hardness show the saturated values.

Raman Spectra & G-peak Raman spectroscopy is a relatively simple and non-destructive analysis tool to characterize the structural change of carbon materials. The left figure is Raman spectra obtained from the silicon incorporated ta-C films. It is well known that the spectra of pure ta-C films are more symmetric and have lower intensity than those of hydrogen amorphous carbon films like this, because of much higher sp3 content. Right figure shows the fitting results of the G-peak position. When the residual stress of amorphous carbon film is larger than a few GPa, the effect of residual stress should be carefully considered in Raman spectrum analysis. Recently, we reported the effect of the residual stress on the Raman spectra.

The Effect of Stress on G-peak Position Stressed Stress-relieved This is our previous work about the effect of the stress on G-peak position. In the left figure, upper Raman spectrum is that of ta-C films with 7GPa stress. After stress was fully relieved by the substrate etching technique, G-peak position was shifted to lower wavenumber like this figure. As a result, we observed that the residual compressive stress shifted the G-peak to higher wavenumber by 4.1 kaiser per GPa. J.K.Shin et al., Appl. Phys. Lett., 78 (2001) 631

Raman Spectra & G-peak III II I Region I These open circles were the corrected G peak positions where the stress effect was excluded by previous stress data. Hence, the data of open circles reflect only the structural change due to the silicon incorporation. Now I will discuss the changes in the atomic bond structure in the previously mentioned region 1. That is, there is a significant reduction of the stress while the hardness gradually decrease by addition of a small amount of silicon. In this region, the shape and intensity of the Raman spectra were essentially same as those of pure ta-C films. Furthermore, as can be seen in the right figure, G-peak position did not vary significantly with silicon concentration. These results show that the ratio of sp3 over sp2 in the film was kept almost constant in this region. This implies that silicon atoms preferentially substituted the carbon atoms of sp3 bonds. Region I No significant changes in atomic bond structure. The stress effect on G-peak position

Atomic Bond Structure I II III Therefore, the behavior of the mechanical properties can be explained by the following model. When the silicon concentration is lower than 1 at.%, this silicon atom will form Si-C bonds like this figure, if silicon atoms preferentially substitute the carbon atoms of sp3 bonds This silicon incorporated site can play a role to compensate the distortion of the nearby C-C bonds. The relaxation of the residual stress would occur with large strains in the weaker Si-C bonds compared with C-C bonds. On the other hand, the hardness of the film is proportional to the degree of three dimensional interlinks of the atomic bond structure, which would be enhanced by sp3 bond. Because the incorporated silicon atoms substitutes the carbon of sp3 bond, the degree of three dimensional interlinks would not be reduced by the incorporated silicon atoms. Thus the decrease in the hardness is due to the weaker Si-C bonds rather than the decrease in the three dimensional interlinks. It caused the gradual change of the hardness in this concentration range.

Raman Spectra & G-peak III II I Region II Region III When the silicon concentration was higher than 2.5 at.%, the shape of Raman spectra become different from those of pure ta-C films. With increasing silicon concentration, the intensity of Raman peak increase and the position shifted to lower wavenumber. At 22%, single large Raman peak was observed at about 1450 kaiser. Some researchers reported that this Raman peak is related with the nanocrystalline silicon carbide. Then, when the Si concentration is higher than 22 at.%, the intensity of the Raman peak decreased and eventually disappeared at the silicon concentration of about 50 at.%. These result means that structural changes occurred in the different way from that of region I. In addition, the G-peak position was abruptly shifted to lower wavenumber. These changes can be understood in the following way. It was well known that the increase in the Raman intensity was related with the symmetry breaking of the aromatic sp2 cluster. From the increase in the Raman intensity in this region(with pointing), it can be judged that the incorporated silicon start to substitute the carbon atoms in sp2 clusters. Region II The initial stage of SiC phase appearance Nanocrystalline SiC related peak at 1450 cm-1 Region III SiC phase was dominant Si-Si bonding increased

The Changes of the Structure I II III FTIR XPS Si 2p Si-Si C-Si This is the result of FTIR spectra. Until the silicon concentration was 4 at.%, there was no change in the FTIR spectra. When Si concentration was higher than 8.5 at.%, the Silicon Carbon stretching absorption band started to appear around 750 cm-1. It can be thus said that the increase in the content of silicon carbide phase is the dominant structural change. Furthermore, from the result of XPS(pointing the XPS), we can see that silicon silicon bonds increased when the silicon concentration was higher than 22 at.%. Therefore, the changes of structure and properties in this region were mainly due to the increase of both the silicon carbon bonds and silicon silicon bonds. Consequently, the saturated behavior of the mechanical properties of which value are comparable to those of SiC, was the result of a large amount of silicon carbide phase formation in the region III. Region III SiC phase was dominant Si-Si bonding increased

Conclusions ta-C:Si films prepared by hybrid FVA Si concentration can be controlled by Ar gas flow The significant stress reduction by Si addition Hardness was reduced by 23 % ,while stress was reduced by 48 % in low Si concentration. Weaker Si-C bond sites relieved the stress without breaking the three dimensional interlink. When the Si concentration was higher than 22 at.%, the SiC phase strongly influenced on the structure and mechanical properties. Finally, I’d like to summarize our conclusion. Silicon incorporated tetrahedral amorphous carbon films were synthesized by using filtered vacuum arc process with simultaneous silicon sputtering. Si concentration can be controlled by Ar gas flow. The most important result in the present work is that. When the silicon concentration was lower than 2.5 at.%, the addition of silicon to the ta-C significantly reduced the stress, while the hardness gradually decreased. Hardness was reduced by 23 % ,while stress was reduced by 48 % in low Si concentration. Weaker Si-C bond sites relieved the stress without breaking the three dimensional interlink. When the silicon concentration was higher than 22 at.%, the silicon phase strongly influenced on the structure and mechanical properties.