Atomic bond structure modification of ta-C films by Ar background gas in filtered vacuum arc process Tae-Young Kim*, Seung-Hyup Lee, Churl Seung Lee, Kwang-Ryeol Lee, Jun-Hee Han† and Kyuhwan Oh‡ Korea Institute of Science and Technology, Seoul, Korea *also at Seoul National University, Seoul, Korea †Korea Research Institute of Standard Science, Daejon, Korea ‡Seoul National University, Seoul, Korea

Tetrahedral Amorphous Carbon sp3 ta-C ta-C:H DAC PAC GAC No film sp2 H

Compressive Residual Stress Before deposition After deposition M.W.Moon, Acta Mater., 50 (2002) 1219.

Previous Approaches Third element addition into ta-C matrix Post annealing Multilayer of two different ta-C layers Diam. Rel. Mater., 11, 198-203 (2002).

Experimental Film deposition Analysis Buffer layer deposition Ar 8 sccm ( with gun 1 valve), -750 Vb ta-C deposition at GND substrate bias In various Ar gas pressures in the chamber Analysis Compressive residual stress Hardness – nano-indentor Film Composition – RBS Atomic structure – NEXAFS, ESR

Compressive Residual Stress

Hardness & Strain Modulus

Experimental Results & Questions As the Ar background pressure increased, Stress decreased Hardness didn’t changed significantly. Why did this phenomenon happen? Compositional change? Atomic Structural change?

RBS – film composition No Ar in the films – Pure Carbon system!!! Ar 6sccm treated ta-C film No Ar in the films – Pure Carbon system!!!

NEXAFS – sp2/sp3 bonding ratio sp2/sp3 bonding ratio was not changed at the different process condition.

ESR – Defect Density

Stress vs. Defect Density

Defect in ta-C π π* σ σ* A B A ESR detects the paramagnetic component (B) of the defects.

Defects in ta-C Dangling bond sp2 Dangling bond sp3 Distorted sp2 cluster

Decrease in Defect Density Structural relaxation of distorted sp2 clusters bonding/clusters supports the paramagnetic spin density measurement Distorted sp2 cluster Ordered sp2 cluster

What is the role of the background Ar gas?

I. Ar Massage Ar ion knock-on

II. Energy Dispersion Population Energy

MD Simulation Deposition Method Brenner potential for carbon atom Substrate Diamond substrate (6a0x5a0x6a0) 1512 Atoms Incident atom Reference energy = 75 eV Dispersion method Gaussian distribution (σ=0 ~ 10) Density of a-C structure ~ 3.14 [g/cm3] 54.4 A Fixed Layer Dynamics Layer

Residual Compressive Stress MD Simulation with Energy Dispersion ta-C film deposited by FVA with Ar background gas

Radial Distribution Function 2.02~2.17Å

Atomic Configuration 93.1° 94.2° 2.184 A 2.185 A Now, let me conclude the result. To understand the effects of silicon in reducing the residual stress of ta-C films, we performed molecular dynamics simulation. We combined Tersoff and Brenner force field to simulate the silicon incorporation into ta-C films more accurately. By the molecular dynamics simulation and statistical analyses of atomic structure, we could understand the residual compressive stress of ta-C films was originated from the decrease of bond distance and the distortion of bond angles. The left picture clearly shows the decreased bond length and the distorted bond angle in the pure ta-C film. The incorporation of silicon into ta-C film drastically changed the local atomic structure of ta-C film, as shown in the right side figure. The locally distorted bonds and the S-type atoms were hardly found in silicon incorporated ta-C film. Now, we can understand how silicon reduces the residual stress while maintaining the hardness high. Silicon affects on the relaxation of the locally distorted bonds only while maintaining the 3 dimensional interlink of sp3 bonds. Since S-type atoms in pure ta-C films were not abundant, only 0.5% of silicon could reduce the residual stress largely. We can understand why silicon atoms more than 0.5% was not effective for reducing the residual stress. Thank you for your attention.

sp3 Ratio & Density 53.7±1.7 [%] 3.14±0.03 [g/cm3]

Conclusions We demonstrate the possibility to reduce the atomic bond distortion without changing sp2/sp3 bond ratio by using Ar background gas in filtered vacuum arc process. This structural modification can reduce the residual stress of the film without deterioration of the mechanical properties. Suggested role of Ar background Low energy Ar ion massage Energy dispersion due to the scattering with the Ar atoms