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, (2002).

5. 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

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 No Ar in the films – Pure Carbon system!!!
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* A B A
ESR detects the paramagnetic component (B) of the defects.

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

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

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

17. I. Ar Massage
Ar ion
knock-on

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
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.

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