Tae-Young Kim * , Seung-Hyup Lee, Churl Seung Lee,

1. 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

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

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

4. 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).

5. Experimental • Film deposition • 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

6. Compressive Residual Stress

7. Hardness & Strain Modulus

8. 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?

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

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

11. ESR – Defect Density

12. Stress vs. Defect Density

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

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

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

16. What is the role of the background Ar gas?

17. Ar ion knock-on I. Ar Massage

18. II. Energy Dispersion Population Energy

19. 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

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

21. Radial Distribution Function 2.02~2.17Å

22. Atomic Configuration 93.1° 94.2° 2.184 A 2.185 A

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

24. 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